Eutrophication

  • Eutrophication is a process that occurs in bodies of water like lakes, ponds and rivers due to excessive nutrients, specifically nitrogen and phosphorus.
  • These excess nutrients may originate from factors such as runoff of fertilisers from agricultural land, or sewage pollution.
  • Nutrient enrichment leads to rapid growth of algae and other plant species in the water, a condition also known as an algal bloom .

Consequences of Eutrophication

  • The rapid growth of algae and plants can cause the water surface to become blocked, preventing light from reaching deeper parts of the water body.
  • This lack of light limits the growth of other photosynthetic organisms and disrupts the normal trophic relationships .
  • As these algae and plant species die, they are broken down by decomposers .
  • Decomposition uses up the available dissolved oxygen in the water, creating an anoxic, or oxygen-depleted, condition.
  • The oxygen depletion can lead to the death of many aquatic species, particularly fish and other animals that require dissolved oxygen for respiration.
  • The result is a significant decrease in biodiversity in the affected water body.

Prevention and Control

  • Eutrophication can be prevented or controlled through measures such as using less or more targeted application of fertilisers in agriculture.
  • Better disposal and treatment of human and animal waste can also help in preventing the enrichment of water bodies with excess nutrients.
  • Constructed wetlands and buffer strips along water bodies can filter out and absorb excess nutrients before they can enter the water.
  • Restoration of affected ecosystems involves measures like oxygenation of the water, removal of built-up sediment, or even addition of chemical agents to precipitate out excess nutrients.
  • It is important to note that prevention is always more effective and less costly than trying to restore an ecosystem after eutrophication has occurred. Hence, sustainable practices and regulation are key to managing the issue.

Revision Centre

Exam Revision Notes and Online Learning Resources

Revision Notes

Leaching and Eutrophication

Leaching describes the “washing out” from soils any soluble chemicals that are not “bound” to the soil particles. It occurs as excess rain (or flood) waters drain through the soil.

Farmers are frequently at fault by adding excess fertilisers (usually nitrates or ammonium compounds, containing nitrogen, which plants use for making proteins). It is these salts that are most likely to be leached and so pass into streams, rivers, lakes etc…

These salts are normally in short supply and hence act as “limiting factors” restricting the growth of plants in such bodies of water. In the presence of leachate, however, the algae grow in huge numbers causing an “algal bloom” . The salts are soon used up and so most of the algae die and sink to the bottom.

Here they are decayed by saprophytic bacteria (and fungi). The bacteria therefore increase in numbers and, as the decay (digestive) processes are energy requiring, the bacteria use “dissolved oxygen” for their respiration.

This may result in the death of all other oxygen requiring organisms, such as fish, water fleas etc… Also, even large flowering plants growing in the water may die.

Thus the bodies of water become lifeless except for the bacteria which can survive the anaerobic conditions. The whole sequence of events caused by the extra salts leading to death of organisms is called “Eutrophication”.

Eutrophication may also be caused by sewage / slurry etc… which gets into the water supply because these too contain nitrate rich chemicals. (Washing powders containing phosphates can have the same effect as phosphorus, also a limiting factor).

For the Instructor

  • Mississippi River Case Study

Dead Zone in the Gulf of Mexico

Agricultural runoff can contribute pollutants to natural waters, such as rivers, lakes, and the ocean, that can have serious ecological and economic impacts, such as the creation of areas with low levels of dissolved oxygen called dead zones caused by pollution from fertilizers. Nutrients , such as nitrogen and phosphorus, are elements that are essential for plant growth and are applied on farmland as fertilizers to increase the productivity of agricultural crops. The runoff of nutrients (nitrogen and phosphorus) from fertilizers and manure applied to farmland contributes to the development of hypoxic zones or dead zones in the receiving waters through the process of eutrophication (Figure 4.2.7).

Schematic of Eutrophication

Watch the following videos from NOAA's National Ocean Service that show how dead zones are formed and explain the dead zone in the Gulf of Mexico:

  • Video: Happening Now: Dead Zones in the Gulf 2017 (2:33)
  • Video: Hypoxia (3:51)

The nutrients that make our crops grow better also fertilize phytoplankton in lakes and the ocean. Phytoplankton are microscopic organisms that photosynthesize just like our food crops. With more nitrogen and phosphorus available to them, they grow and multiply. When the phytoplankton dies, decomposers eat them. The decomposers also grow and multiply. As they're eating all of the abundant phytoplankton, they use up the available oxygen in the water. The lack of oxygen forces mobile organisms to leave the area and kills the organisms that can't leave and need oxygen. The zone of low oxygen levels is called a hypoxic or dead zone. Streams flowing through watersheds where agriculture is the primary land use exhibit the highest concentrations of nitrogen (Figure 4.2.8).

graph of Nitrogen concentrations in streams draining watersheds with different land uses

The Mississippi River is the largest river basin in North America (Figure 4.2.9), the third largest in the world, and drains more than 40 percent of the land area of the conterminous U.S., 58 percent of which is very productive farmland (Goolsby and Battaglin, 2000). Nutrient concentrations in the lower Mississippi River have increased markedly since the 1950s along with increased use of nitrogen and phosphorus fertilizers (Figure 4.2.10). When the Mississippi River's nutrient-laden water reaches the Gulf of Mexico, it fertilizes the marine phytoplankton. These microscopic photosynthesizing organisms reproduce and grow vigorously. When the phytoplankton die, they decompose. The organisms that eat the dead phytoplankton use up much of the oxygen in the Gulf's water resulting in hypoxic conditions. The resulting region of low oxygen content is referred to as a dead zone or hypoxic zone. The dead zone in the Gulf of Mexico at the mouth of the Mississippi River has grown dramatically and in some years encompasses an area the size of the state of Connecticut (~5,500 square miles) or larger. Hypoxic waters can cause stress and even cause the death of marine organisms, which in turn can affect commercial fishery harvests and the health of ecosystems.

Map of Mississippi and Atchafalaya River Basin and hypoxic zone in Gulf of Mexico

Figure 4.2.9. The Mississippi and Atchafalaya River Basin and the hypoxic zone in the Gulf of Mexico

Credit: USGS Factbook - Nitrogen in the Mississippi Basin-Estimating Sources and Predicting Flux to the Gulf of Mexico

graph of Nitrogen inputs and population from 1940-2010

Optional Reading

Additional resources about the dead zone in the gulf of mexico.

  • NOAA sponsored program out of LSU runs the hypoxia net website with great information ( Hypoxia Research Program')
  • Hypoxia and Eutrophication from the National Centers for Coastal Ocean Science of the NOAA Nation Ocean Service
  • USGS Fact Sheet 105-03, 2003, Nutrients in the Upper Mississippi River: Scientific Information to Support Management Decisions

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Case studies eutrophication

  • 1.1 Introduction
  • 1.2 Causes and consequences
  • 1.3 Solutions
  • 2 Human health impacts of eutrophication (Case study: Humans at the top of the food web)
  • 3 Economic impacts of eutrophication (Case study: Shellfish flavour)
  • 4 Recreational and aesthetic impacts of eutrophication (Case study: Foam on the beach)
  • 5 References

Ecological impacts of eutrophication (Case study: Eutrophication and dead zones)

eutrophication case study gcse

Introduction

Dead zones are very low oxygen areas ( hypoxic ) in the ocean where marine life including fish, crabs and clams cannot survive. In the 1970s oceanographers began noting increased instances of dead zones. A 2008 study counted 405 dead zones worldwide [1] . Hypoxia is a natural phenomenon that occurs periodically in coastal waters around the world. During the last 50 years however, increases in key pollutants from human activities on land have thrown many coastal ecosystems out of balance, resulting in expanded dead zone regions.

Causes and consequences

Aquatic and marine dead zones can be caused by an increase in nutrients (mainly nitrates and phosphates ) in the water known as eutrophication . Major nutrient sources come from human activities such as the use of fertilizers in agriculture and the burning of fossil fuels. These nutrients lead to a rapid increase of the density of certain types of phytoplankton resulting in algal blooms . The organic matter produced by these phytoplankton species at the surface of the ocean sinks to the bottom (the benthic zone) where bacteria break it down. The bacteria use oxygen and give off carbon dioxide during this breakdown. Fish and mobile invertebrates can migrate out of hypoxic areas. Plants and animals that are slow moving or attached to the bottom (sea grass, worms and clams) cannot escape from the dangers of hypoxic waters and will die. The dead zone of the Baltic Sea is probably the largest worldwide [1] . Overfishing of Baltic cod has greatly intensified the problem. Cod eat sprats, a small, herring-like species that eat microscopic zooplankton that in turn eat the algae. So, fewer cods and an explosion of zooplankton-eating sprats means more algae and less oxygen- a vicious cycle develops [2] .

The main goal in reducing dead zones is to keep fertilizers on the land and out of coastal waters. The Black Sea dead zone largely disappeared between 1991 and 2001 after fertilizers became too costly to use following the collapse of the Sovjet Union and the demise of eastern European economies. Nutrients loads entering the sea where therefore considerably reduced. Fishing has again become a major economic activity in the region. However, our ocean ecosystems are fragile and the combined threats of climate change, overexploitation, pollution and habitat loss,all mostly caused by human activity, are undermining the sustainability. Expanded dead zones caused by global warming will remain for thousands of years and have harmful long-term effects on ocean ecosystems.

Human health impacts of eutrophication (Case study: Humans at the top of the food web)

eutrophication case study gcse

The consumption of shellfish (e.g. mussels, clams) is one of the most common ways for algal toxins to impact human health. Marketable shellfish are generally considered to be safe, but in spite of these precautions, there are known illnesses. One dramatic incident occurred in 1990 when six fishermen almost died from eating mussels during a fishing trip on Georges Bank, a productive offshore finfish and shellfish area. The fishermen became ill after eating a pot of mussels they had caught in their nets. The Captain, who had joined the meal later than the rest of the crew, witnessed his fellow fishermen become incapacitated due to the paralytic effects of the toxin. He himself also became ill, but was capable of sending an urgent radio message to the US Coast Guard. In the hospital they were treated using respiratory therapy to sustain their breathing and prevent them from dying due to paralysis of the lungs. The event, presumably caused by a massive Alexandrium [3] bloom transported offshore from areas along the northeast coast, closed the surf clam industry on Georges Bank to further harvest. Source: WHOI [4]

Economic impacts of eutrophication (Case study: Shellfish flavour)

Some algae and diatoms impart off-flavours or bitter taints to shellfish, rendering them unpalatable and unmarketable. In 1987 in Port Phillip Bay, Melbourne, Australia, a bloom of the diatom Rhizosolenia chunii [5] [6] occurred and 3 species of shellfish within the bay, mussels, oysters and scallops, developed a powerful bitter taint. The taint was so persistent and unpleasant that the mussels from the bay were unmarketable for 7 months, causing a revenue loss of approximately $1 million.

Recreational and aesthetic impacts of eutrophication (Case study: Foam on the beach)

eutrophication case study gcse

Some algae, particularly of the taxa Phaeocystis [7] , produce a mucus, which when disturbed produce a foam. These algae are more prone to develop when there is little competition. It seems that in areas such as the south-east coast of the North sea, where all the silica has been captured by diatoms in estuarine regions, the residual nitrogen is used by Phaeocystis to bloom. They produce large amount of mucus which, if the weather is windy, will in turn be transformed into large amounts of foam covering extensive areas of beach and lake shores. Besides the impact on the landscape and the nuisance it represents for tourists, this foam is suspected of disturbing flat fish larvae development. This phenomenon is frequently observed at the Belgian and Dutch coasts, and appears from time to time in Germany.

  • ↑ Diaz R. J, Rosenberg R. (2008): Spreading dead zones and consequences for marine ecosystems. Science 321, 629.
  • ↑ Westman, 2010; cited in Owen 2010. World's Largest Dead Zone Suffocating Sea, National Geographic News.
  • ↑ WoRMS (2012). Alexandrium Halim, 1960 emend. Balech, 1989. In: Guiry, M.D. & Guiry, G.M. (2012). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=109470
  • ↑ http://www.whoi.edu/science/B/redtide/foodweb/shellwedolunch.html
  • ↑ Kraberg, A. (2011). Rhizosolenia chunii Karsten, 1905. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=341502
  • ↑ Parry, G.D., Langdon, J.S. & Huisman, J.M. (1989). Toxic effects of a bloom of the diatom Rhizosolenia chunii on shellfish in Port Phillip Bay, southeastern Australia. Marine Biology, Berlin 102: 25-41.
  • ↑ Guiry, M.D. (2011). Phaeocystis. In: Guiry, M.D. & Guiry, G.M. (2011). AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=115088
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The Issue of Eutrophication: A Case Study of Lake Erie

Have you ever encountered a green water body with an abundance of algae floating on the surface? That is a significant symptom of eutrophication. An increasing number of lakes and water bodies look like this globally, including some of our most crucial bodies of water. 

What is Eutrophication?

Eutrophication is a process by which excess nutrients such as phosphorus (P) and nitrogen (N) deposit into a body of water and become concentrated in particular areas. This process disturbs natural systems and often causes adverse effects. While eutrophication is a natural process in aging lakes, human activities accelerate this process in younger waters by depositing excess nutrients (Castro & Freitas, 2010). Some human activities that drive this process include population growth; urbanization; industrial expansion; agricultural pollution; water supply development; and changes in land use. 

These areas of high nutrients often experience accelerated algae growth, resulting in algae blooms . These blooms disturb the aesthetic nature of the water bodies and impact water oxygen levels. As these algae blooms begin to spread, they quickly take in the oxygen from the water, decreasing oxygen levels. With lower oxygen levels, the fish populations become trapped in hypoxic (deficient levels of oxygen) waters leading to their mortality, also known as fish kills . These hypoxic areas in the waterbody are also called dead zones . These dead zones are areas where the eutrophication process is causing such low oxygen levels that nothing can survive in these areas. 

In addition to causing biodiversity loss, eutrophication also impacts human health and the economy. This process reduces water quality, causing it to become undrinkable. Some toxins are irritants, and some are suspected to be carcinogens . The fish in these lakes can also become inedible and sometimes toxic, thus affecting the livelihoods of the local communities. 

ub9Utv9KeZPZx1oeZz7i4f5O59N1 2K TYXotnEK37RbvgJz7VibFaX6J DP7awqENRSj9UrNs4lPoUNfc8U89k Tr8aaZfhsMmp0HxxDlRCV4dUsEG3sWa52IX7OaIqA6XjD9kXB 2W1Bw2U2ZhhViku5JUEYP6Xs The Issue of Eutrophication: A Case Study of Lake Erie

Eutrophication and Climate Change

Climate change has also had a hand in accelerating eutrophication. Changes in climates and temperatures impact the natural cycle and processes of water bodies, leading to “ significant changes in the physical structure and the biological configuration of the waters .” Eutrophication processes often thrive in warmer, nutrient-rich areas with prolonged ice-free seasons. So as climate change prevails, the warmer season lengthens in places with colder winters, such as Canada. As the warmer periods in a year extend, so does the growing period for these plants and toxins.

A Case Study of Lake Erie

Lake Erie is the world’s eleventh-largest lake in terms of surface area and is situated on the international boundary between Canada and the United States. It “ supplies drinking water to 11 million [people, contains] 50% of the fish found in all of the Great Lakes combined ,” and is home to numerous aquatic species. 

It is the southernmost, warmest, and shallowest of the Great Lakes, making it the perfect breeding ground for eutrophication. Because of this, eutrophication is nothing new to Lake Erie, as concerns began in the 1960s and 1970s when increased phosphorus inputs from various human activities resulted in a notable degradation in water quality. In response, in 1972, phosphorus abatement programs were initiated as part of the Great Lakes Water Quality Agreement , which had quick success in Erie and reigned till 1987. But the declaration of the ‘restoration’ of Lake Erie quickly reversed , and algae populations have continued to increase and affect the lake since the mid-1990s and have caused excessive oxygen depletion.

The current eutrophication of Lake Erie threatens all the services provided by this lake. Consequently, gaining a handle on this issue will not only help sustain the services currently offered by the lake but will also enhance the potential for future services.

Solutions to Eutrophication

As part of any remediation project, further research into reliable indicators of eutrophication and direct indicators of the sources of these nutrients is necessary to manage these ecosystems. 

Understanding which nutrients is essential for mitigation is also crucial to successful management. Research has concluded a unanimous agreement that reducing phosphorus inputs will directly benefit eutrophication reduction. However, there is a gap in research about nitrogen’s role in eutrophication, as reducing nitrogen alone will not aid mitigation. Research into reducing phosphorus and nitrogen inputs together is still needed before understanding the optimal course-of-action. 

There are many innovative approaches to solving eutrophication. An example of a solution to the source of of the issue is vertical farming . Vertical farming is an excellent alternative to intensive agriculture with less input of nutrients. Vertical farming is a closed system that does not deposit nutrients into earth systems. 

Another innovative solution to eutrophication is duckweed. Duckweed can be used as a remediation for the aftermath of he process. Duckweed is a plant that has taken nutrients used in agricultural practices and provides oxygen back into the water source. The use of duckweed has the potential to reverse the adverse effects of eutrophication and restore natural ecosystems.

Canada has begun their remediation process for Lake Erie. It recognized the eutrophication of the lake as an ecological risk. Their action plan was published in February 2018 and will start in 2023, with revision every five years after. The plan outlines the importance ; their actions to achieve phosphorus reduction targets ; their efforts to improve policies ; and areas needing more research .

Canada, E. and C. C. (2018, March 13). Government of Canada. Canada.ca. Retrieved December 8, 2022, from https://www.canada.ca/en/environment-climate-change/services/great-lakes-protection/action-plan-reduce-phosphorus-lake-erie.html 

Castro, P., & Freitas, H. (2010). Linking anthropogenic activities and eutrophication in estuaries: The need of reliable indicators. Eutrophication: Causes, Consequences and Control, 265–284. https://doi.org/10.1007/978-90-481-9625-8_13 

Dokulil, M. T., & Teubner, K. (2010). Eutrophication and climate change: Present situation and future scenarios. Eutrophication: Causes, Consequences and Control, 1–16. https://doi.org/10.1007/978-90-481-9625-8_1 

Kane, D. D., Conroy, J. D., Richards, R. P., Baker, D. B., & Culver, D. A. (2014). Re-eutrophication of Lake Erie: Correlations between tributary nutrient loads and phytoplankton biomass. Journal of Great Lakes Research , 40 (3), 496-501. Retrieved from: [PDF] researchgate.net

Landesman, L., Fedler, C., & Duan, R. (2010). Plant nutrient phytoremediation using duckweed. Eutrophication: Causes, Consequences and Control, 341–354. https://doi.org/10.1007/978-90-481-9625-8_17 

Sayer, C.D., Davidson, T.A., Rawcliffe, R. et al. Consequences of Fish Kills for Long-Term Trophic Structure in Shallow Lakes: Implications for Theory and Restoration. Ecosystems 19, 1289–1309 (2016). https://doi.org/10.1007/s10021-016-0005-z  

Scavia, D., Allan, J. D., Arend, K. K., Bartell, S., Beletsky, D., Bosch, N. S., … & Zhou, Y. (2014). Assessing and addressing the re-eutrophication of Lake Erie: Central basin hypoxia. Journal of Great Lakes Research , 40 (2), 226-246. Retrieved from: Assessing and addressing the re-eutrophication of Lake Erie: Central basin hypoxia – ScienceDirect  

Watson, S. B., Miller, C., Arhonditsis, G., Boyer, G. L., Carmichael, W., Charlton, M. N., … & Wilhelm, S. W. (2016). The re-eutrophication of Lake Erie: Harmful algal blooms and hypoxia. Harmful algae , 56 , 44-66. Retrieved from: https://ciglr.seas.umich.edu/wp-content/uploads/2017/09/Watson_etal.pdf.pd f

1663106210638 The Issue of Eutrophication: A Case Study of Lake Erie

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  • Published: 22 October 2021

Globally consistent assessment of coastal eutrophication

  • Elígio de Raús Maúre   ORCID: orcid.org/0000-0003-0505-3796 1 ,
  • Genki Terauchi 1 ,
  • Joji Ishizaka   ORCID: orcid.org/0000-0003-0398-1572 2 ,
  • Nicholas Clinton 3 &
  • Michael DeWitt 3  

Nature Communications volume  12 , Article number:  6142 ( 2021 ) Cite this article

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This article has been updated

Eutrophication is an emerging global issue associated with increasing anthropogenic nutrient loading. The impacts and extent of eutrophication are often limited to regions with dedicated monitoring programmes. Here we introduce the first global and Google Earth Engine-based interactive assessment tool of coastal eutrophication potential (CEP). The tool evaluates trends in satellite-derived chlorophyll- a (CHL) to devise a global map of CEP. Our analyses suggest that, globally, coastal waters (depth ≤200 m) covering ∼ 1.15 million km 2 are eutrophic potential. Also, waters associated with CHL increasing trends—eutrophication potential—are twofold higher than those showing signs of recovery. The tool effectively identified areas of known eutrophication with severe symptoms, like dead zones, as well as those with limited to no information of the eutrophication. Our tool introduces the prospect for a consistent global assessment of eutrophication trends with major implications for monitoring Sustainable Development Goals (SDGs) and the application of Earth Observations in support of SDGs.

Introduction

Coastal ecosystems provide innumerable services that make them major environmental and economic assets globally 1 . However, their integrity is increasingly threatened by impacts of human activities 2 . Nutrient enrichment, for instance, is known to stimulate phytoplankton productivity. While this growth of phytoplankton can initially be beneficial to the ecosystem, continuous accumulation of organic matter can lead to eutrophication of the system with a series of undesirable ecological effects that can also be harmful to humans. Defined as the increase in the rate of organic matter supply to water bodies 3 the definition of eutrophication has been expanded to meet both scientific and legal requirements 4 . This cultural eutrophication associated with excessive or disproportionate nutrient loading is known for its modifications of nutrient levels and structures including a selective magnification of nitrogen and phosphorus supply but a reduction of silica 5 . These conditions can trigger a chain of biogeochemical feedback including shifts in phytoplankton composition, formation and persistence of harmful algal blooms (HABs) and the consequent occurrence of hypoxic waters 6 , 7 , 8 , 9 . The increased incidence of oxygen deficit waters (hypoxic waters), in turn, can stimulate the proliferation of hypoxia-tolerant species such as Noctiluca scintillans 10 , 11 . Furthermore, eutrophication can also increase the possibility of jellyfish outbreaks 12 , can contribute to ocean acidification 7 and to degradation of shallow water habitats 13 , 14 . In the case of submerged vegetation, they can display an array of direct and indirect responses to nutrient loadings that ultimately may lead to their loss, as seen in some seagrass meadows 13 . Therefore, monitoring and/or assessment of eutrophication is important in providing the extent and context of eutrophication 15 , 16 , 17 . Such information is especially relevant for coastal managers to take the required management interventions.

Many coastal regions worldwide experience some level of eutrophication despite that only a few regions with dedicated monitoring programmes have information of eutrophication status. Existing tools for eutrophication assessment 15 , 18 , 19 , 20 , although vital for the identification of eutrophication patterns as well as for understanding the eutrophication causes and consequences, their application entails prohibitively expensive and intensive field monitoring programmes. Alternatively, water quality parameters from satellite imagery are often introduced as effective tools for a synoptic eutrophication assessment 21 and to overcome the spatiotemporal limitations of in situ observations. Chlorophyll- a (CHL, mg m −3 ) concentration, a proxy for phytoplankton biomass, is a commonly used indicator of eutrophication as it links nutrient enrichment and the stimulated phytoplankton productivity 22 , 23 , 24 , 25 . CHL is recognised by the Global Climate Observing System as an Essential Climate Variable 26 , 27 . It is an important parameter in the study of the climate system and associated changes, as well as in the study of different factors affecting the dynamics of marine ecosystems including those of anthropogenic origin. In fact, CHL is listed as one of the parameters for the index of coastal eutrophication potential in the Global Manual on Measuring Sustainable Development Goals (SDGs) 14.1.1, 14.2.1 and 14.5.1 28 . To assess coastal eutrophication trends globally, both levels and trends of satellite derived CHL are essential. While the few existing assessment approaches based on satellite data are generally based solely on CHL levels 21 , 29 , the Northwest Pacific Action Plan Eutrophication Assessment Tool (NEAT) considers both the levels and trends of satellite-derived CHL 17 , 30 . The NEAT was developed by the Special Monitoring and Coastal Environment Assessment Regional Activity Centre (CEARAC) of the Northwest Pacific Action Plan (NOWPAP), a part of the Regional Seas Programme of the United Nations Environment Programme, for the preliminary eutrophication assessment based solely on satellite-derived CHL. It is effective in discriminating both eutrophication potential (see Methods for definitions of eutrophic and eutrophication) waters as well as those in recovery 17 .

Our study, therefore, introduces the NEAT as an app constructed on Google Earth Engine (GEE) cloud environment 31 for the global screening of coastal eutrophication potential (CEP). To the best of our knowledge, our app (the Global Eutrophication Watch) is the first of its kind to provide coastal eutrophication trends globally. It classifies CEP based on temporal and spatial patterns of CHL levels as well as trends in annual bloom magnitude allowing for a globally consistent assessment in a way never done before. Although it neither differentiates the bloom forming algae nor determines the frequency or duration of the bloom, it does, however, provide a synoptic view of eutrophic potential waters (those with high levels of CHL) or waters under high risk of eutrophication (those with increasing CHL trends) for prioritised management interventions. The findings not only are pertinent for management and mitigations of eutrophication, but also for monitoring SDGs, specifically indicator 14.1.1a “Index of coastal eutrophication of the SDG 14: Life Below Water”. This is  to conserve and sustainably use the oceans, seas and marine resources. Further, in addition to putting in-situ obtained results into a wider context, the findings of this study can be put into practice by contrasting them with those obtained from in-situ measurements, model simulations, etc. On the other hand, this study introduces the first global map of CEP for many regions lacking routine water quality monitoring. Accordingly, the information obtained will be vital in guiding the development of monitoring programmes regionally. This study contributes towards the use of Earth Observations in support of the SDGs and the results emphasize the importance of the Global Eutrophication Watch as a global framework for eutrophication monitoring.

Results and discussion

We first introduce a case study in Bohai Sea (Fig.  1a )—a semi-enclosed marginal sea, one of the China seas, that has been severely impacted by human activities in the last half century—to demonstrate the value of the eutrophication screening tool (cf. 2.1). The Bohai Sea has become eutrophic and suffers from symptoms of eutrophication that are well-documented 8 , 9 , 32 . Second, we introduce the global screening of CEP in section 2.2 using the satellite data from the Moderate Resolution Imaging Spectroradiometer on Aqua (MODISA), reprocessing 2018, with a spatial resolution of 4 km, obtained using the standard ocean colour index algorithm (OCI 33 ; https://oceancolor.gsfc.nasa.gov/atbd/chlor_a/ ). The CHL time series from MODISA are the longest among ocean colour sensors and are used as the default data for the global assessment.

The OCI algorithm provides adequate CHL retrieval in the global open ocean. In optically complex coastal waters, like in the Bohai Sea, however, the optically active constituents (e.g., coloured dissolved organic matter) and phytoplankton may vary independently 34 , so reliable CHL retrievals may not be achieved 35 , 36 . Therefore, in 2.1 we adopted a CHL product that uses a regional algorithm. This regional product was obtained using the Yellow Sea Large Marine Ecosystem Ocean Color Project (YOC) algorithm, an empirical algorithm appropriate for the Bohai Sea as it alleviates the impacts of suspended sediments and coloured dissolved organic matter on CHL retrievals 35 . The YOC CHL data span a 22-year period (1998-2019) and have a spatial resolution of 1 km. Further details of the datasets are given in Methods, 3.1.

The definitions adopted for the terms eutrophic and eutrophication potential, as discussed in the following sections, are given in the Methods section (3.3). Briefly, eutrophic potential will refer to any productive system characterised by high CHL, whereas eutrophication potential refers to the process of becoming eutrophic or a progression of an already eutrophic water body. Oligotrophication potential will be the reverse of eutrophication potential.

figure 1

LD, LN, and LI depict the status as being low CHL ( α < 5 mg m −3 ) with decreasing trend, no trend and increasing trend, respectively. HD, HN and HI indicate high CHL ( α ≥ 5 mg m −3 ) with the three above-mentioned trends, respectively. a Preliminary assessment of CEP for the period 1998–2015. b Same as a but for the period 1998–2019. The rectangle in magenta ( b ) shows the location of Bohai Sea.

Assessment of coastal eutrophication potential: a case study in the Bohai Sea

The assessment results obtained from the global eutrophication watch (Fig.  1 ) comparing two assessment periods, 1998–2015 and 1998-2019, revealed that some coastal waters associated with high CHL (≥5 mg m −3 ) and increasing temporal trends (HI) have significantly shrunk. In contrast, low CHL (<5 mg m −3 ) waters with no trends associated (LN) and low CHL waters with decreasing trends (LD) have expanded in the Bohai Sea. Overall, the area covered by pixels associated with increasing trends, that is, eutrophication potential (LI and HI) shrank ~27%, whereas for those associated with oligotrophication potential (LD and HD) had a threefold increase between the two assessment periods (Fig.  1 ). This, in part, may be indicative of improving water quality. Reports suggest that there have been a series of control measures implemented in China to reduce nutrient emissions from terrestrial sources. These measures curbed the worsening trend of coastal eutrophication in the China seas 25 , and possibly contributed to the gradual decrease in red tides annual frequency and dramatic decrease in the red tides affected area since 2003 9 , 25 . Besides the human interventions discussed above, natural climate variability also plays a role in the variations of CHL 37 . Zhai et al. 37 discussed the influence of sea surface temperature and rainfall on CHL long-term changes. Warmer temperature anomalies, present during the positive phase of the Pacific Decadal Oscillation (PDO), and concurrent with negative rainfall anomalies in the Bohai Sea, were suggested to be conducive to negative CHL anomalies through their negative impacts on nutrient fluxes into the sea. Transient factors such as water exchange between the Bohai Sea and the Yellow Sea also modulate the variations of CHL 38 . Additionally, the observed changes in CHL trends between the two assessment periods could also be an indication of the sensitivity of trend detection to the data length. This study highlights the usefulness of our tool, which identifies the spatial patterns of eutrophication potential using CHL levels and trends over a larger spatial and temporal scales and condensed in a single map.

The patterns identified in Fig.  1 are further corroborated by reports of water quality of the Bohai Sea. As already stated, most of the bays in this sea have been severely impacted by human activities. Particularly, high inputs of dissolved inorganic nitrogen have been observed in recent decades 5 , 8 , 9 . Interestingly, the increase in nitrogen inputs continued even when the total river discharge consistently decreased in relation to that of the 1960s 9 . As a result, several ecological disasters such as the incidence of red-tides and the occurrence of hypoxia and/or anoxia intensified 9 , 39 . In the case of the assessment map in Fig.  1 , a patch of HI was effectively identified in the coastal waters off Qinhuangdao. This region has been found to be an oxygen minimum zone in the Bohai Sea. Further, the waters adjacent to this patch constitute a hypoxia hotspot 8 . The above illustrates the suitability of our tool in identifying the spatial distribution of CEP with the application of CHL from satellite ocean colour remote sensing.

The number of pixels associated with CHL increasing trends significantly decreased between the two assessment periods (Fig.  1 ) indicating that there might be some large-scale phenomena driving this shrinkage of increasing trends in the whole Bohai Sea. Atmospheric deposition, which acts on a much larger scale, can be an important nutrient source to the ocean 40 . In the Bohai Sea, the influence of atmospheric deposition is also significant. Observations and simulation results suggest that the atmospheric contribution to dissolved inorganic nitrogen can range from ~25% to 54% of the total 39 , 41 . As for the flux of particulate phosphorus entering the sea through windblown dust storms, it can be >500 times greater than on normal days 39 . In recent years, however, declines in the frequencies of dust storms and the volume of China’s emissions of major anthropogenic air pollutants have been observed 39 , 42 . The decline in emissions results from the introduction of China’s clean air policies in 2010, driving significant reductions in pollutant emissions in the first seven years of its inception 42 . As shown in a modelling study assessing the effects of atmospheric nitrogen deposition on the marine ecosystem in the Bohai Sea 41 , the inclusion of the atmospheric deposition can cause an average increase in phytoplankton biomass of >50%. It naturally follows that our latter assessment (Fig.  1b ), which includes recent years when both dust storms and anthropogenic emissions have markedly reduced, might reflect the long-term changes in atmospheric nutrient deposition.

Other large-scale climate processes such as El Niño (La Niña) and PDO have also been implicated in the dynamics of the Bohai Sea ecosystem 37 , 38 . Fan et al. 38 analysed the spatial and temporal variations of particulate organic carbon (POC) in the Yellow-Bohai Sea over the period 2002–2016. They suggested that the above climate indices impact the surface POC through their influence on water exchange between the Yellow-Bohai Sea and the East China Sea. This water exchange is controlled by the East Asian winter monsoon and its influence on the Yellow Sea Warm Current. The fact that these factors appear to have an indirect influence 38 suggest that atmospheric deposition might be a major driver of the observed large-scale decrease in CHL levels and trends.

As introduced above, the study by Zhai et al. 37 used a 16-year record of MODISA CHL and observed spatially coherent increasing CHL trends from 2003 to 2011 and decreasing trends from 2012 to 2018 in the Bohai Sea. They suggested that these changes were mainly controlled by variations in sea surface temperature and rainfall, which are linked to the PDO. In positive PDO phases, positive temperature (negative rainfall) anomalies prevail in the Bohai Sea. These conditions lead to decreased dissolved inorganic nitrogen content in the surface layers due to suppressed vertical nutrient diffusion and reduced land-sourced nutrient fluxes 37 . Factors such as changes in nutrient levels and structures have major impacts on CHL long-term changes. Wang et al. 39 showed that the summer concentration of dissolved inorganic nitrogen in the Bohai Sea continuously increased from the 1990s, while that of phosphorus exhibited a decreasing trend in the period 1978–2016. So, the nitrogen/phosphorus ratio mostly followed that of nitrogen content 39 . As a result, the nutrient regime of the Bohai Sea has shifted from nitrogen-limitation before the 1990s to potential phosphorus-limitation thereafter 8 , 9 , 39 .

Although we speculate about the possible factors driving the CHL variability observed in the Bohai Sea, the changes in nutrients levels and structures as well as the CHL response in a eutrophic environment are complex 5 , 9 . At this point, we emphasize that our procedure is simply meant for the screening of CEP. The mechanisms behind the identified patterns are beyond the scope of the tool and that should be supplemented by follow-up studies. Here, we stress the use of CHL estimates from ocean colour remote sensing as the preliminary parameter for a rapid and a consistent assessment of CEP globally. The significance of this approach is in the use of a single parameter that condenses the spatial and temporal information which allows the identification of areas in potential need of preventive management or eutrophication mitigation efforts.

Assessment of coastal eutrophication potential: global ocean

The global map of CEP (Fig.  2a ) is composed mostly of LN and HI (Table  1 ). Pixels associated with high CHL are mostly found in coastal and inland waters. Here, we only focus on the coastal waters (depth ≤200 m). To get an intuition of the global distribution of area covered by each eutrophication potential waters, the area estimate was obtained through the combined use of bathymetry data and the marine biogeochemical provinces 43 . Our analysis suggested that globally LI and HI (~799,305 km 2 ) occupy a larger fraction of coastal waters than LD and HD (~602,406 km 2 ). The major fraction of both LD-HD and LI-HI combined was found in coastal provinces of Asia (SUND, Table  1 ). However, the HI class was predominant in the Atlantic Ocean where some of the well-known dead zones, the Gulf of Mexico and the Baltic Sea, are found 16 , 44 . Besides the above cases, there are many other coastal seas which were flagged as eutrophication potential (both LI and HI) and are distributed across the globe (Table  1 ). Although Table  1 also includes coastal upwelling regions, their contribution is relatively smaller than non-upwelling regions. These examples emphasize the utility of the introduced tool in preliminary eutrophication assessment. Not only was the tool able to identify known areas of eutrophication, but also those potentially suffering from the effects of eutrophication in addition to non-reported locations experiencing some level of eutrophication 6 , 45 . Therefore, the introduction of our Global Eutrophication Watch, a rapid and consistent preliminary assessment of CEP is now globally feasible. This tool should instigate a concerted action against the proliferation of coastal eutrophication.

figure 2

a Preliminary assessment of CEP in the global ocean for the period 2003–2019 based on MODISA global dataset. The CHL threshold is same as in Fig.  1 . b Preliminary assessment of CEP in the Bohai Sea based on the YOC algorithm for the same period as in a but with spatial resolution of 1 km. c Same as b but for MODISA 1 km spatial resolution. d Same as b but for MODISA 4 km spatial resolution. The southern and northern regions with few observations (<70% in the 17-year period) were masked. The GEE App is accessible through the link  https://eutrophicationwatch.users.earthengine.app/view/global-eutrophication-watch .

In addition to the global map of eutrophication potential, we also compared the assessment results based on our improved CHL introduced in 2.1 vs. the standard MODISA CHL product for the period 2003–2019. Overall, we found the CEP waters identified with YOC CHL (Fig.  2b ) were also apparent in the standard MODISA product (Fig.  2c, d ). However, LD waters appeared more than LI in the map generated using the standard CHL. The retrievals of CHL in highly dynamic and optically complex waters such as in coastal waters are challenging. The existing algorithms for atmospheric correction are robust in the open ocean where the ocean colour covaries with phytoplankton concentration 46 . In the case of Bohai Sea, we have the YOC and some other statistically based CHL retrieval algorithms 36 that best represent the phytoplankton variability. We believe that different regions may also have a CHL product that more accurately suits the characteristics of the designated area. The global application of our methodology in preliminary assessment of CEP should not be contingent on the global standard CHL product. In our GEE-based tool, the Global Eutrophication Watch, there is an option for users to enter the path to their asset (dataset in the GEE) of monthly CHL time series. This monthly CHL data can then be used in the assessment instead of the default datasets.

While the focus is on the preliminary assessment of eutrophication potential, oligotrophication potential (LD, HD) are equally worthy of mention. Under the warming climate, the tropics and subtropics are likely to experience enhanced stratification and reduced nutrient supply to the euphotic layer. As a result, phytoplankton growth will be limited with long-term decline (Fig.  2a ) associated with decreasing primary production 47 . In coastal and enclosed seas, measures to reduce nutrient loading can lead to decreased phytoplankton concentration or reduce the eutrophication and associated ecological disasters such as the incidence of hypoxic events, though other issues like oligotrophication can emerge 48 . The Seto Inland Sea of Japan experienced severe eutrophication during the high economic growth period of the 1960s and 1970s 49 , but now is reported to be undergoing oligotrophication 48 . Significant reductions in nutrient loading along with loss in biodiversity are reported to be the precursors of oligotrophication. Moreover, in the oligotrophication process, changes in the food web structure are suggested to have caused a decrease in fishery production of the Seto Inland Sea 50 .

In this study, we introduced the Global Eutrophication Watch, a tool for a preliminary eutrophication assessment solely based on satellite-derived CHL. Although different eutrophication assessment methods exist, especially comprehensive eutrophication assessment methods 15 , 19 , 20 , their global application is complicated by the need for extensive and intensive field observation campaigns. So, the significance of our introduced tool is in its simplicity and scale. It only uses satellite derived CHL to provide a systematic assessment of CEP at a macroscopic (global) and microscopic (regional) levels as well as with sufficient temporal information to allow coastal water managers make informed decisions on where to focus their eutrophication management efforts. In this method, we stress the importance of CHL levels and trends. This combination provides a simple but robust assessment scheme. For instance, low CHL but increasing trends (LI) may inform managers about required management actions to prevent future ecological disasters. This warning might go missing in case only CHL levels 21 are considered. On the other hand, with the sole use of CHL trends, high CHL but no trend waters (HN) can be overlooked. CHL levels are often linked to phytoplankton biomass, which is also linked to the health of the ecosystem. So, our methodology is inexpensive and robust for a global assessment of CEP.

Overall, we expect this contribution to aid in the many global efforts acting to counter the impacts of nutrient pollution and eutrophication. It is well known that management planning efforts should also incorporate available knowledge, and adapt to changing environmental conditions, while evaluating the effectiveness of implemented measures. Thus, our Global Eutrophication Watch tool, with its ready-to-use map of up-to-date information of the status of CEP, provides the required scientific knowledge to support monitoring programmes, adaptive management, and decision-making. It is also useful for educational purposes and in raising awareness, as it is simple and uses very few resources. A simple internet connection, either on a smartphone or computer, allows one to evaluate eutrophication trends worldwide.

CHL datasets

For the global detection of CEP, we used the currently available 17-year record of daily CHL data from the Moderate Resolution Imaging Spectroradiometer on Aqua (MODISA), reprocessing 2018 ( https://oceancolor.gsfc.nasa.gov/reprocessing/r2018/aqua/ ), and with a spatial resolution of 4 km. The data set is stored in the App’s asset (see 3.4 below) and its temporal extension is updated on a yearly basis. In addition to these yearly updates, the data sets will also be updated following NASA (National Aeronautics and Space Administration, U.S.) periodic reprocessings that improve product quality with advances in algorithms or sensor calibration knowledge.

Besides the global screening of CEP, a case study was developed in the Bohai Sea (cf. 2.1) to demonstrate the usefulness of the introduced tool. In coastal regions, like the Bohai Sea, CHL estimates based on the standard ocean colour algorithms—like the MODISA OCI algorithm ( https://oceancolor.gsfc.nasa.gov/atbd/chlor_a/ )—often fail. The reason being that the optical properties of these complex waters (also denoted case 2 waters) are influenced not just by phytoplankton but other optically active constituents (including coloured dissolved organic matter), and these may vary independently of one another 34 . So, for this case study, we used a regionally tuned CHL dataset obtained as monthly composites from the Marine Environmental Watch of the NOWPAP ( https://ocean.nowpap3.go.jp/ ) with spatial resolution of 1 km. These CHL data were only available in the NOWPAP region.

The above regional CHL data were obtained using the YOC algorithm, designed to alleviate the impacts of coloured dissolved organic matter and suspended sediments on CHL retrievals 35 . The YOC product, as used in this study, was a blending of YOC CHL and CHL based on OC algorithm combined with the colour index (CI) algorithm, that is, the OCI algorithm 33 . The switching between the two was determined by the values of normalised water leaving radiance (nLw, mWcm −2 mm −1 sr −1 ) at 555 nm 51 . The YOC algorithm was applied in waters with high nLw555 (>2.5), whereas the OCIs were applied in waters with low nLw555 (<1.5). A smooth transition between the two extremes was ensured by a linear combination in the mid-range of nLw555 (2.5>nLw555 > 1.5). Accordingly, adequate CHL estimates could be obtained in waters with high nLw that otherwise would be overestimated 35 , 51 , and in such cases the YOC CHL had superior quality with better spatial and temporal variations relative to the standard products 51 , 52 , 53 . Therefore, this improved CHL is of critical importance to the case study presented here. The YOC algorithm was originally developed using the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) sensor bands 35 . Its application to MODISA and to Medium Resolution Imaging Spectrometer (MERIS) data was based on the regression between SeaWiFS and MODISA (MERIS) bands and band ratios. In this study we used the CHL data based on the OCI algorithm. Please refer to Terauchi et al. 17 for additional details on the computation of regression coefficients.

Given that the global level 3 data constitute our default asset for the global eutrophication assessment, following the case study in the Bohai Sea (2.1), in 2.2 we briefly compare the trends estimated using the YOC CHL with those obtained from the global data readily obtainable from NASA. This comparison is essential given that the NASA global standard products are more accessible than any other lower-level (such as level 2) data to non-expert users, including water quality managers and decision makers. In addition, it is the least expensive way for a rapid eutrophication assessment before a thorough, in-situ based investigation can follow.

Trend analysis

The estimation of trends at pixel level is based on the Sen’s slope method 54 —a non-parametric trend estimation method—which detects the presence of monotonic trends in a yearly data record at the 90% significance level. Nonparametric tests provide higher statistical power in the case of nonnormality, as is the case with CHL, and are robust against outliers and large data gaps. Trends estimated below a critical threshold are treated as N (no trend). Moreover, as the focus is on the detection of eutrophication potential with consideration of it being a process occurring over a long-time scale (on the order of years), the temporal trends in CHL are estimated from annual maximum obtained from monthly composites of each considered year. The choice is partly motivated by the fact that the evaluation of existence of monotonic trends can also be statistically challenged by short-term variability in CHL. So, by using CHL annual maximum from monthly composites, we effectively remove the seasonal and short-term variabilities. Doing so, we focus on the CHL peak season. Consequently, the obtained trends reflect the interannual behaviour of the phytoplankton bloom season, assuming that the bloom is manifested as high biomass.

NEAT methodology as a global screening tool of coastal eutrophication

In this study we used the NEAT methodology to develop a GEE-based tool for the global detection of CEP (the Global Eutrophication Watch) using satellite-derived CHL. In its screening procedure, the NEAT—a robust satellite-based preliminary assessment tool of eutrophication potential—unifies, in a single map, the temporal and spatial information of the area under consideration. It combines the levels and trends of CHL to generate six patterns of water quality 17 . The CHL levels generate two patterns based on the CHL concentration ( α [mg m −3 ]), the first being composed by CHL lower than the threshold α, CHL < α (L), and the other by CHL ≥ α (H). The trends have three patterns, namely: waters with decreasing trend (D), with no trend (N), or with increasing (I) trend. In this way, a composite map of six classes can be generated, viz. LD, LN, LI, and HD, HN, HI. Before moving on to the explanation of the meaning of each class, it is worth defining the terms adopted in this paper for clarity. Eutrophic potential will be used to indicate a productive system with high CHL, whereas eutrophication potential refers to the process of becoming eutrophic or a progression of an already eutrophic water body. In addition to the above definitions, we also introduce oligotrophication potential which is associated with the progression to a least productive water body. Hence, pixels flagged HD, HN and HI are eutrophic potential with HD indicative of systems under recovery, whereas in LI and HI are eutrophication potential. In HI, the conditions may worsen as the water body is already eutrophic potential. Moreover, LD is suggestive of reversed eutrophication, that is, further oligotrophication. LN and HN are indicative of L and H CHL but stable conditions over the analysis period.

It is important to note that classification of waters as being L or H is subject to the consideration of the threshold α , which will vary depending on the conditions of each region. However, the same is not the case for D, N or I. Trends will most probably be impacted by the length of the analysis period and/or other environmental factors controlling the variability of CHL rather than a given α . As such, both LD, LI and HD, HI provide critical information about the eutrophication of the system under scrutiny. The global eutrophication watch, therefore, not only provides important information of areas potentially in need of preventive management efforts, but also helps in evaluating the impacts of measures taken to reduce the effects of eutrophication. The NEAT procedure uses a threshold of 5 mg m −3 , and this threshold is computed based on the most recent 3-year mean data of the analysis period. Nevertheless, this threshold is not fixed, and users are able to adjust the level and the composite period to area specific values as different regions may have different thresholds according to the region’s background.

In the above-introduced approach we model the interannual changes in phytoplankton bloom magnitude (CHL annual maximum) to assess the eutrophication of a coastal ecosystem. The main purpose is the identification of waters with symptoms of coastal eutrophication, which may include the incidence of HABs or other related issues 5 , 14 , 25 . Although HABs can be a direct or indirect manifestation of eutrophication, the interactions between the two are complex 5 , 9 , 24 . HABs, often, associate with specific types of algal blooms such as cyanobacteria, Karenia spp., etc 29 , 55 , 56 . But CHL, which we used for our index of eutrophication, is present in both HAB and non-HAB blooms. Although satellite-derived CHL has been found, in some cases, sufficient to detect HABs 29 , 56 , often additional information (such as the knowledge of local CHL patterns) is necessary to make the link between the two. So, while our approach can identify patterns of algal bloom magnitude over the years and relate them to eutrophication potential, neither does it discriminate the bloom forming algae nor does it determine the frequency or duration of the bloom. As such, this approach can only provide a context of areas with symptoms of water quality deterioration, and therefore with potential for HAB occurrence (cf. 2.1) without a priori information of the considered ecosystem.

Successful HAB detection or prediction often goes beyond the sheer use of CHL, and in most cases in-situ observations or more complex approaches are required. For example, Stumpf et al. 29 used CHL anomaly to flag waters with potential for Karenia brevis blooms in the Gulf of Mexico. Their CHL anomaly was computed as a difference between a single image of satellite-derived CHL and a two-month average image taken two weeks prior to the image being considered. While their procedure was successful in Karenia brevis identification in the Gulf of Mexico, such methodology would be limited in other environments with different background or with a different Karenia species 29 , 55 .

The merits of our eutrophication screening approach are in its use of CHL levels and trends. If only the CHL trends are considered, the magnitude of the problem would be overlooked. On the other hand, if only the levels are considered, only the spatial dimension of the problem would be captured 21 , 29 , 57 and thereby overlooking, for instance, LI waters. So, here we reemphasise the importance of the spatial and temporal dimensions provided by satellite derived CHL and condensed in a single map by this approach, which retains both space-time information. Thus, a synoptic view of eutrophication potential is gained prior to any expensive field sampling, although vital to complementing satellite information.

The GEE Global Eutrophication Watch App

The Global Eutrophication Watch (Fig.  3 ) on the GEE is composed of three main fields: (1) the data-set specification panel, (2) the panel for selection of trend detection intervals and (3) the specification of the CHL composite interval and the threshold selection panels. The data panel allows the selection of two default data sets, that is, MODISA and YOC CHL. In practice, only YOC CHL can be checked as MODISA is the de facto default. Moreover, this panel also includes a box for users to enter the path to an Earth Engine asset of monthly composites of CHL for the tool to read and use for the assessment. The option is especially important given the challenges associated with CHL retrievals in the coastal waters. Unlike in the open ocean, where phytoplankton dominate the optical properties or co-vary with other optically active constituents, in coastal waters phytoplankton may vary independently of the optical constituents, and thus the global CHL product may fail to resolve phytoplankton variations 58 . So, this option can be understood as a plug-in that allows users around the globe to conduct the eutrophication assessment based on their own datasets. This feature enables users to incorporate regionally improved CHL data while keeping the assessment procedure consistent. This has the immediate result of allowing consistent results to be obtained from a spectrum of ecosystems with different characteristics. The next panel is used to specify the trend detection interval, the start and end years. This panel also includes a button to toggle views, that is, to split the map into two windows providing a capability for comparative assessment. The impact of inclusion of more years in the trend detection analysis, for instance, can be verified by simply using two different year intervals. Finally, the last user defined parameters are for the CHL threshold. Controls for start and end dates are available for users to indicate the time interval to be used to compute the mean CHL. This is used in conjunction with the cut-off level (threshold) to split L vs. H CHL waters.

figure 3

The left panel shows the control panel of the app. The map of CEP in the NOWPAP region based on MODISA global dataset is shown in the middle. The right panel shows the CHL max time series of a select point on the map. The CEP classes are also shown.

Data availability

The satellite derived CHL data used in (2.2) are available from the NOWPAP Marine Environmental Watch website at https://ocean.nowpap3.go.jp/ . The data used in (2.3) are available from the website of the NASA’s Ocean Biology Processing Group at https://oceancolor.gsfc.nasa.gov/ . The eutrophication potential maps in Fig.  1 through 3 can be obtained via the Google Earth Engine global eutrophication watch app at https://eutrophicationwatch.users.earthengine.app/view/global-eutrophication-watch The data of coastal biogeochemical provinces are available from the Marine Regions at https://www.marineregions.org/sources.php#longhurst . The bathymetry map used in combination with biogeochemical provinces was created using Windows Image Manager ( https://www.wimsoft.com/ ).

Code availability

The Earth Engine code used for trend analysis based on the Sen’s slope method is available at the Google Earth Engine community tutorials ( https://developers.google.com/earth-engine/tutorials/community/nonparametric-trends ).

Change history

06 december 2021.

The original version of this Article contained errors in the caption of Fig. 2 and the Methods, where an incorrect hyperlink to the Global Eutrophication App was provided. The link has been corrected ( https://eutrophicationwatch.users.earthengine.app/view/global-eutrophication-watch ) in both the PDF and HTML versions of the Article. In the original version of this article, the given and family names of Elígio de Raús Maúre were incorrectly structured. The name was displayed correctly in all versions at the time of publication. The original article has been corrected.

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Acknowledgements

This work was made possible through the support received from the Ministry of the Environment of Japan, the Toyama Prefectural Government, and the Northwest Pacific Region Environmental Cooperation Center established to promote the Action Plan for the Protection, Management and Development of the Marine and Coastal Environment of the Northwest Pacific Region as a part of the Regional Seas Programme of the United Nations Environment Programme. E.R.M. thanks Tak Amaru for improving the language quality of the introduction.

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E.R.M. and G.T. conceived the study. G.T. and J.I. developed the remote sensing-based eutrophication assessment method. E.R.M. processed and ingested the satellite derived CHL datasets into Earth Engine. N.C. implemented the Sen’s slope detection method on Earth Engine. E.R.M. led the Earth Engine App development with support from M.D. E.R.M led the writing of the manuscript with contributions from all authors.

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Maúre, E.d.R., Terauchi, G., Ishizaka, J. et al. Globally consistent assessment of coastal eutrophication. Nat Commun 12 , 6142 (2021). https://doi.org/10.1038/s41467-021-26391-9

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Mechanisms and assessment of water eutrophication *

Xiao-e yang.

1 MOE Key Laboratory of Polluted Environment Remediation and Ecological Health, Zhejiang University, Hangzhou 310029, China

2 Zhejiang Provincial Key Laboratory of Subtropic Soil and Plant Nutrition, Zhejiang University, Hangzhou 310029, China

3 Institute of Food and Agricultural Sciences, Indian River Research and Education Center, University of Florida, Fort Pierce, FL 34945, USA

Water eutrophication has become a worldwide environmental problem in recent years, and understanding the mechanisms of water eutrophication will help for prevention and remediation of water eutrophication. In this paper, recent advances in current status and major mechanisms of water eutrophication, assessment and evaluation criteria, and the influencing factors were reviewed. Water eutrophication in lakes, reservoirs, estuaries and rivers is widespread all over the world and the severity is increasing, especially in the developing countries like China. The assessment of water eutrophication has been advanced from simple individual parameters like total phosphorus, total nitrogen, etc., to comprehensive indexes like total nutrient status index. The major influencing factors on water eutrophication include nutrient enrichment, hydrodynamics, environmental factors such as temperature, salinity, carbon dioxide, element balance, etc., and microbial and biodiversity. The occurrence of water eutrophication is actually a complex function of all the possible influencing factors. The mechanisms of algal blooming are not fully understood and need to be further investigated.

INTRODUCTION

Water eutrophication is one of the most challenging environmental problems in the world. The increasing severity of water eutrophication has been brought to the attention of both the governments and the public in recent years. The mechanisms of water eutrophication are not fully understood, but excessive nutrient loading into surface water system is considered to be one of the major factors (Fang et al., 2004 ; Tong et al., 2003 ). The nutrient level of many lakes and rivers has increased dramatically over the past 50 years in response to increased discharge of domestic wastes and non-point pollution from agricultural practices and urban development (Mainstone and Parr, 2002 ). For more than 30 years, nutrient enrichment, especially phosphorus (P) and nitrogen (N), has been considered as a major threat to the health of coastal marine waters (Andersen et al., 2004 ). Once a water body is eutrophicated, it will lose its primary functions and subsequently influence sustainable development of economy and society. Therefore, nowadays the solution of water eutrophication and recovery of the multiple functions of the water system have become the key issues for environmental biologists. The main purpose of this paper is to provide a brief review on recent advances on understanding the mechanisms of water eutrophication and progresses in identifying the influence factors inducing water eutrophication.

DEFINITION AND OCCURRENCE OF WATER EUTROPHICATION

Definition of water eutrophication.

Lakes and estuaries accumulating large amounts of plant nutrients are called “eutrophic” (from the Greek words eu meaning “well” and trophe meaning “nourishment”). Eutrophication can be defined as the sum of the effects of the excessive growth of phytoplanktons leading to imbalanced primary and secondary productivity and a faster rate of succession from existence to higher serial stage, as caused by nutrient enrichment through runoffs that carry down overused fertilizers from agroecosystems and/or discharged human waste from settlements (Khan and Ansari, 2005 ). Water eutrophication can be greatly accelerated by human activities that increase the rate of nutrient input in a water body, due to rapid urbanization, industrialization and intensifying agricultural production. For lake aquatic ecosystems, human activities in the watershed can lead to loss of dominant species and functional groups, high nutrient turnover, low resistance, high porosity of nutrients and sediments, and the loss of productivity (Liu and Qiu, 2007 ). For example, aquaculture is one of many human activities contributing to the environmental decline of coastal waters and the collapse of fisheries stocks worldwide (Alongi et al., 2003 ). Because the influence of the human activities, excessive nitrogen, phosphorus and other nutrients are loaded into water bodies like lake, reservoirs, embouchure and bay, which could cause negative ecological consequences on aquatic ecosystem structures, processes and functions, result in the fast growth of algae and other plankton, and deteriorate water quality (Western, 2001 ). Generally speaking, water eutrophication is caused by the autotrophy algae blooming in water, which composes its bioplasm by sunlight energy and inorganic substances through photosynthesis—the process of eutrophication is described as follows:

equation M1

According to above equation, it can be concluded that inorganic nitrogen and phosphorus are the major control factors for the propagation of algae, especially phosphorus. The Florida Everglades, a wetland of international importance, has been undergoing a significant shift in its native flora and fauna due to excessive total phosphorus (TP) loadings (an average of 147 ton per annum from 1995 to 2004) and an elevated mean TP concentration (69 μg/L of TP in 2004) from agricultural runoff and Lake Okeechobee outflow despite the use of over 17 000 ha of stormwater treatment areas (Richardson et al., 2007 ).

Assessment of water eutrophication

Surface water quality guidelines have been improved in recent years. The parameters to assess the ambient surface water quality have been increased. In China, the parameters for assessing environmental quality of surface water have been increased to over 30 (CNEPA, 2002 ). Five classes of surface water quality have been set up, and some selected parameters for assessing water quality of lakes or reservoirs are shown in Table ​ Table1. 1 . However, there are no perfect evaluation criteria for assessing water eutrophication. Generally, the physical and chemical evaluation parameters were used to assess water eutrophication, mainly nutrient concentration (N and P), algal chlorophyll, water transparency and dissolved oxygen. Although there are many different assessment parameters, the concentrations of total nitrogen and phosphorus are the two basic ones. Cheng and Li ( 2006 ) used total nutrient status index (TNI) to assess eutrophication status of lakes. The calculation of total nutrient status index is as follows:

equation M2

where, TNI is the sum of indexes of all nutrient parameters, TNI j is the TNI of j parameter, W j is the proportion of j parameter in the TNI, and r ij is the relation of chlorophyll a (Chla) to other parameters. The available parameters concerned include total nitrogen (TN), total phosphorus (TP), Chla, dissolved oxygen (DO), chemical oxygen demand by K 2 MnO 4 oxidation method (COD Mn ), biological oxygen demand (BOD 5 ), etc., and TN, TP and Chla are selected for calculating the TNI (Cheng and Li, 2006 ). Table ​ Table2 2 shows the burthen values of TN, TP and TNI in various eutrophicated water. It has been shown that the eutrophication or red tide occurs when N concentration in water reaches 300 μg/L and P concentration reaches 20 μg/L. Richardson et al.( 2007 ) reported that exceeding a surface water mean TP threshold concentration of 15 μg/L causes an ecological imbalance in algal, macrophyte and macroinvertebrate assemblages as well as slough community structure in the Everglades areas. Therefore, it is considered that a threshold zone (12~15 μg/L) of TP may be more realistic and protective for all trophic levels.

The criteria of surface water quality for lakes or reservoir (CNEPA, 2002 )

DO: dissolved-oxygen; COD Mn : Chemical oxygen demand by K 2 MnO 4 oxidation method; COD Cr : Chemical oxygen demand by chromium oxidation method; BOD 5 : Biological oxygen demand; TN: Total nitrogen; TP: Total phosphorus

The burden values of N and P in various eutrophicated water

TN: Total nitrogen; TP: Total phosphorus, TNI: Total nutrient status index

Inglett and Reddy ( 2006 ) reported evidences to support the use of stable C (delta C-13) and N (delta N-15) isotopic ratios as indicators for eutrophication and shifts between N and P limitation. Lin et al.( 2006 ) compared the stable isotopes from dissolved nutrients and plants and water column nutrient parameters and integration of multiple proxies in a sediment core from Meiliang Bay of Taihu Lake, and found that differences in aquatic plant species and trophic status between East Taihu Bay and Meiliang Bay are indicated by their variations in delta C-13 and delta N-15 of aquatic plants and delta N-15 of NH 4 + -N. A significant influence of external nutrient inputs on water quality of Meiliang Bay is reflected in temporal changes in delta N-15 of NH 4 + -N and hydro-environmental parameters. The synchronous change between delta C-13 and delta N-15 values of sedimented organic matter (OM) has been attributed to elevated primary production at the beginning of eutrophication between 1950 and 1990, and then recent inverse correlation between them has been caused by the uptake of N-15-enriched inorganic nitrogen by phytoplankton grown under eutrophication and subsequent OM decomposition and denitrification in surface sediments, indicating that the lake has suffered from progressive eutrophication since 1990.

More sensitive biological indicators for assessing water eutrophication are needed to further study. Water eutrophication caused a degradation of healthy aquatic ecosystem, so the assessment methods and parameters should reflect the extents of aquatic ecosystem health. A set of ecological indicators including structural, functional and system-level aspects were proposed for a lake ecosystem health assessment, according to the structural, functional and system-level responses of lake ecosystems to chemical stresses including acidification, eutrophication, and copper, oil and pesticide contamination. The structural indicators included phytoplankton cell size and biomass, zooplankton body size and biomass, species diversity, macro- and micro-zooplankton biomass, the zooplankton/phytoplankton ratio, and the macrozooplankton/microzooplankton ratio. The functional indicators encompassed the algal C assimilation ratio, resource use efficiency, community production, gross production/respiration (i.e., P/R) ratio, gross production/standing crop biomass (i.e., P/B) ratio, and standing crop biomass/unit energy flow (i.e., B/E) ratio. The ecosystem-level indicators consisted of ecological buffer capacities, energy, and structural energy. Based on these indicators, a direct measurement method (DMM) and an ecological modeling method (EMM) for lake ecosystem health assessment were developed (Xu et al., 2001 ). The results of a case study demonstrate that both methods provided similar results which corresponded with the lake’s actual trophic state.

Occurrence of water eutrophication

The investigation from the UNEP (United Nation Environmental Protection) indicates that about 30%~40% of the lakes and reservoirs have been affected more or less by water eutrophication all over the world. Table ​ Table3 3 cites selected samples of water eutrophication occurrence in lake, reservoir, estuary and river in the world. Erie Lake is excessively rich in nutrients (Reutter, 1989 ), which has resulted in huge blooms of floating blue-green algae and the attached green alga, Cladophora spp. These blooms have rolled onto beaches in large mats resembling green steel wool. Water eutrophication has been reported in USA for Washington Lake (Welch and Crooke, 1987 ), Okeechobee Lake (Schelske, 1989 ), City Park Lake (Ruley and Rusch, 2002 ), etc. In Lugano Lake, between Italy and Switzerland, a faster rate of eutrophication was reported due to excessive discharges from human settlements around the lake, owing to population increase and immigration (Barbieri and Simona, 2001 ). The majority of Danish lakes are highly eutrophic due to high nutrient input from domestic sources and agricultural activities (Jeppesen et al., 1999 ). Garg et al.( 2002 ) studied three lakes of Bhopal (Upper Lake, Lower Lake and Mansarovar Lake) in India, to assess the potential fertility of lentic waters and analyze the floral ecology. The highest level of eutrophication was found in Mansarovar Lake. The nutrient loading into the lake initially promoted the growth of phytoplanktons. Eutrophication constitutes a serious threat to many European lakes (Søndergaard et al., 2007 ), such as Pamvotis Lake in Northwest Greece (Romero et al., 2002 ), which has undergone cultural eutrophication over the past 40 years and is currently eutrophic. In South Africa, de Villiers ( 2007 ) reported that hypertrophic conditions indicated by TP levels prevail at least episodically at all of the Berg River monitoring stations; additionally, river water phosphate levels show a dramatic increase by a factor of more than 10 over the past 20 years, mainly due to anthropogenic inputs. Chivero Lake, Zimbabwe was reported to be hypertrophic and not sustainable (Nhapi, 2004 ). Sewage effluent is the major source of nutrients in the lake.

Selected samples of water eutrophication occurrence in lake, reservoir, estuary and river in the world

In China, water eutrophication occurred in 67 lakes (51.2% of the total lakes). Although the Boyanghu Lake and the Dongtinghu Lake are still mesotrophic at present, Dianchi Lake in Yunnan is possibly the most hypertropic lake in the world. In the early 1970s the water of Dianchi Lake was graded as Class III, now declined to the more inferior Class V (Lu et al., 2005 ). Taihu Lake, in China, has similar eutrophication issue. It is the third largest freshwater lake in China, located in the Yangtze River delta, one of the more developed areas of eastern China. In recent decades, because of severe pollution, water quality in Taihu Lake degraded from Class I/II in the early 1960s to Class II/III in the early 1980s and then to Class IV by the mid-1990s. At present, 83.5% of the lake area is eutrophic with an inferior Class V ranking (Liu and Qiu, 2007 ). The increasingly wider occurrence of excessive algae growth also begins earlier and lasts longer each year in Taihu Lake, and in the summer of 2007 an outbreak of blue algal bloom caused many drinking water treatment plants shut down and created a severe “water crisis event” in Wuxi City. Chaohu Lake is the fifth largest lake in China, located in central Anhui Province, and has a population of 2.3 million and more than 3000 factories in its basin. Since the 1990s, massive and rapid nutrient loading has made it one of the most eutrophic freshwater lakes in China. Jin et al.( 2005 ) reported that eutrophic trend of Taihu Lake, Chaohu Lake and Xuanwu Lake in the region of the middle and lower valleys of Yangtze River was predicated using the ecological stress model. Provided the pollution water treatment rate is 60% in 2030, approximately 30 billion ton of polluted water would still be discharged directly into the lakes. Therefore, by 2030, all the urban lakes and most of the medium-sized lakes at the urban-rural fringe areas in China may be eutrophicated or hypertrophicated.

In the region of Yangtze River delta, 80% of the rivers have been polluted and the water quality cannot meet the standards of drinking water source. The degraded water quality mainly due to eutrophication in this region has resulted in extremely serious problems for drinking water supply. In Zhejiang Province about 36 out of 88 counties are suffering from the short supply of good drinkable water sources. In 2004, water eutrophication and algae blooming even occurred in the Qiantang River, which has the highest water flow velocity in China. High concentration of phosphorus and nitrogen is gradually causing eutrophication.

Water eutrophication in rivers occurs worldwide. During the past several decades, catastrophic losses in seagrass meadows have occurred worldwide, especially in flushed estuaries, coastal embayments and lagoons where nutrient loads are both large and frequent (Burkholder et al., 2007 ; Ralph et al., 2006 ). Coastal marine ecosystems of Northern Europe are under pressure from global change (e.g., nutrient enrichment), which threatens these resources (Gowen and Stewart, 2005 ). There are many statutory obligations and strong political pressures for greatly increased emphasis on the control of nutrients levels in UK rivers because of serious problem of water eutrophication (Mainstone and Parr, 2002 ). Within Europe, many national and international initiatives have been implemented in order to reduce the inputs and effects of nutrients in waters, e.g., the European Union’s Water Framework Directive (Andersen et al., 2004 ).

Harmfulness of water eutrophication

Generally speaking, the main harmfulness of water eutrophication is that it can break out the intrinsic equilibrium of the aquatic ecosystem and lead to the damage of the water ecosystem and the gradual degeneration of its functions. As a result, it can affect water quality and make transparency of water become worse than ever. Thus, little sunlight can penetrate water body and photosynthesis of plants under the water will be weakened or even stopped. Water eutrophication can also cause the supersaturation or lack of dissolved oxygen in water, which will be dangerous to aquatic animals and cause great death to them. Eutrophic systems tend to accumulate large amounts of organic carbon causing a shift in organic matter biochemical composition (Dell′Anno et al., 2002 ). Meanwhile, because of water eutrophication, a mass of algae, mainly Cyanophyta and green algae, bloom and form a thick layer of “green scum” on water surface. Algae can release toxins and render the organic matters in water to be decomposed into harmful gases, which will poison the fish and seashell.

The harmfulness of eutrophication also includes causing the shortage supply of drinking water source by degrading water quality. When the blooming algae die, they can produce lots of algae’s toxin which is harmful to human health. Cyanobacteria toxins (cyanotoxins) including cytotoxins and biotoxins are responsible for acute lethal, acute, chronic and sub-chronic poisonings of wild/domestic animals and humans. The biotoxins include the neurotoxins; anatoxin-a, anatoxin-a(s) and saxitoxins plus the hepatotoxins; microcystins, nodularins and cylindrospermopsins (Carmichael, 2001 ). Recent investigation showed that the algae produced toxins, which are the metabolized production of Cyanotoxins, were detected in the Yangtze River, as well as many reservoirs and lakes of Yellow River valleys, apart from Dianchi Lake, Taihu Lake and Chaohu Lakes (Yu and Len, 2004 ). Besides, increased nitrite concentration in the eutrophic water will be dangerous to human health, too, as products of nitrite nitrification process is a strong carcinogen. Thus, the exacerbation of water eutrophication with the increased severity of algae blooming in surface water system has attracted great attention of both public and private sections.

FACTORS INFLUENCING WATER EUTROPHICATION

Water eutrophication is mainly caused by excessive loading of nutrients into water bodies like N and P. Excessive nutrients come from both point pollution such as waste water from industry and municipal sewage, and non-point pollution like irrigation water, surface run water containing fertilizer from farmland, etc. Increased nutrient load to water body is now recognized as a major threat to the structure and functions of near shore coastal ecosystems, and severe eutrophication problems associated with harmful algal bloom is a major manifestation. Although related to nutrient enrichment in general, the basic cause of water eutrophication is more connected to an imbalance in the load of nitrogen and phosphorus with respect to silica (Dauvin et al., 2007 ). At present, excessive TN and TP in water are considered as the only factors inducing water eutrophication, but nutrient enrichment is only the necessary but not the sufficient condition for algal boom. Eutrophication is not likely to occur if both TN and TP in water are low, but eutrophication may not occur in water high in TN and TP if other conditions such as temperature and current speed are not favorable. The influencing factors of water eutrophication include: (1) excessive TN and TP, (2) slow current velocity, (3) adequate temperature and favorable other environmental factors, and (4) microbial activity and biodiversity (Li and Liao, 2002 ). Water eutrophication may occur rapidly when all of these conditions are favorable.

Nutrient enrichment

There is clear evidence that nutrient loading to lakes, estuaries and coastal oceans has greatly increased through human activities over the past few decades and that this has caused or enhanced many of the symptoms of the aquatic ecosystem transformation known as eutrophication (Bishop et al., 2006 ). There are different opinions on the relationship of nutrient enrichment to water eutrophication and algal bloom: (1) When P concentration in water is low, it may be the limiting factor for inducing water eutrophication and algal bloom; (2) When P concentration in water increases rapidly, other may become a new limiting factor, such as pH, water depth, temperature, light, wave, wind or other biological factors; (3) The influence of N and P still lasts for a longer time because of the high development level of our society (Zhao, 2004 ).

N and P input and enrichment in water are the most primary factors to induce water eutrophication. The “experienced molecular formula” of alga is as “C 106 H 263 O 110 N 16 P” based on the chemical components of algae. N and P are the two elements which account for least proportion in the molecular formula of algae, especially P, it is the main limiting factor to control the growth of alga in water (Mainstone and Parr, 2002 ). It was reported that 80% lake and reservoir eutrophication is restricted by phosphorus, about 10% lake and reservoir eutrophication is relative to nitrogen, and the rest 10% lake and reservoir eutrophication is relative to other factors (Zhao, 2004 ). In many ecosystems, phytoplankton biomass is correlated with the availability of N or P (Cloern 2001 ; Bledsoe et al., 2004 ). The composition of phytoplankton species is also affected by the concentrations of N and P (Reynolds, 2006 ). The ratio of N:P in the water body (referred to as the “Redfield ratio”) is an important indicator of which nutrient is limiting eutrophication. If the Redfield ratio is 16:1, P is most likely the limiting factor for algal growth; lower ratios indicate that N is of great importance (Redfield et al., 1963 ; Hodgkiss and Lu, 2004 ). P has been shown to be the principal limiting nutrient for primary production of phytoplankton in many freshwater environments (Phlips, 2002 ), while N is commonly limiting in marine ecosystems (Cloern, 2001 ). However, there are many exceptions to this general pattern. In some freshwater environments, particularly in the tropics and subtropics, N has been found to be the primary limiting nutrient for phytoplankton production, due in large part to excessive P load and long growing seasons. For instances, in the Ten Mile Creek of Indian River Lagoon, where TP is >0.2 mg/L, chlorophyll a and turbidity sharply increased with addition of available N (0.2~6.0 mg/L), but not affected by addition of reactive P (Lin et al., 2008 ). The results indicate that available N is the limiting nutrient for the growth of phytoplankton at water bodies with high P. In phosphate-deficient water bodies or those having reasonably good growth of blue-green algae, which fix enough of the atmospheric nitrogen, phosphorus becomes the limiting element, because a portion of P is used to counterbalance high nitrate content (Reynolds, 2006 ). Such circumstances can be seen that no paroxysmal algal boom may break out in heavily eutrophicated water bodies with both high N and P. Thus, it is the key point to control the concentrations of both N and P reasonably for solving the problem of water eutrophication.

The variations in the chemical composition of natural waters are believed to be an important factor in regulating the abundance, composition and geographical and periodic distribution of phytoplankton. It has been considered that the growth of phytoplankton is influenced by dissolved silicate-Si (DSi) concentration in water and its ratio to nitrate. When the DSi:nitrate-N atomic ratio is near 1:1, aquatic food webs leading from diatoms which require silicate to fish may be compromised and the frequency or size of harmful or noxious algal blooms may increase. Used together, the DSi:nitrate-N ratio and nitrate-N concentration are the robust comparative indicators of eutrophication in large rivers (Turner et al., 2003 ).

Hydrodynamics

There is no relationship between water disturbance and diatom alga occurrence or its scale, but water disturbing can influence the growth of Pyrrophyta alga because Pyrrophyta alga blooms when it is grown in relatively stable water. Cai et al.( 2007 ) found that when there is no water to dilute, disturbing water itself can influence the process of eutrophication and species succession, which, however, is not related to disturbing water itself but is influenced indirectly by changing light and nutrient status. In shallow water, increased frequency of disturbance could increased the P release from the sediment, especially at high temperature (Cai et al., 2007 ). This is an instructional point to maintain beneficial alga in water. Also, tide not only can urge alga assembling but can also influence the multiplication of alga bloom through changing the concentration of nutrition in water. Zhu et al.( 2007 ) studied the effects of hydrodynamics on phosphorus concentrations in water of Taihu Lake, a large, shallow and eutrophic lake of China. They found that hydrodynamical disturbance had no significant relationship with water quality at the top layer when significant wave height was smaller than 30 cm, but it significantly increased suspended solids (SS) concentration of the bottom water layer. Concentrations of nutrients showed no positive correlation with SS concentration in the water body. Intensive sediment resuspension may not have occurred when the hydrodynamic stress on sediment was only a little higher than the critical stress for sediment resuspension. A new method for confirming the critical stress for intensive sediment resuspension and nutrient release still needs to be developed. Le Pape and Menesguen ( 1997 ) studied hydrodynamic prevention of eutrophication in the Brest Bay (France). The Brest Bay is a semi-enclosed coastal ecosystem where primary production is nutrient-limited, even if huge nutrients loading from tributaries are present. The most striking feature of the bay is the semi-diurnal tidal influence, resulting in large water exchange with the continental shelf. A historical study of the available data has shown the steadiness of this ecosystem during the last two decades inspite of increasing eutrophic conditions.

Environmental factors

A range of factors are related to water eutrophication, but the mechanisms of their influencing algal bloom are not fully understood. In many moderately eutrophicated water bodies, algal bloom occurs in some seasons or some years, when the environmental conditions are favorable. The algal bloom caused by phosphorus inputs also modifies several abiotic factors of the water body. These factors directly govern the growth, diversity and density of the biotic components. The impact of algal bloom on any one or some of these factors indirectly influences the structure and characteristics of the water bodies. The influence of nutrient inputs on some of these factors is discussed as follows:

1. Temperature and salinity are the two important factors to induce alga bloom. Alga bloom always occurs at temperature between 23 °C and 28 °C, salinity between 23% and 28%. The variation of temperature and salinity also affect algal bloom, and an important condition for algal bloom is that temperature increases and salinity decreases faster than ever in short time. From the conception of ecology, exquisite change of temperature may cause the subrogation of biological communities, thus leading to algal bloom when other environment conditions are adequate (Wang et al., 1996 ). Statistical analysis shows that the influence of temperature on algal growth rate is the largest, followed by salinity and their interaction. The process of sporangium pullulating is hypersensitive to temperature. When under adequate temperature, it can bourgeon largely and alga bloom will form very fast. Change of salinity is also influenced by the concentration of nutrition. Research shows that salinity is negatively related with NO 3 − -N, and PO 4 3− -P, but positively related with NH 4 + -N, and however, it is not very related with NO 2 − -N. In addition, average temperature in winter is highly relative with the beginning growth time of Gymnodinium, but whether it has universality to all algae still needs to be studied. In the Vistula Lagoon, salinity gradient was determined as an important factor (along with water temperature and predation by young herring) that defined the dynamics of zooplankton abundance and biomass in this estuary (Telesh, 2004 ).

2. Carbon dioxide level is one of major factors controlling water eutrophication. Cyanophytes are more capable of utilizing low levels of carbon dioxide and become more buoyant at low levels of carbon dioxide and high pH. It keeps them in the upper layers of the water column with abundant sunlight. In addition, some species produce dense mats of vegetation, inhibit the growth of other phytoplankton, and also limit the swimming of zooplankton. These factors together mean that a slow-moving freshwater ecosystem can rapidly become dominated by blue-green algae, displacing not only members of the phytoplankton but some of the animal community as well. The reduction of light reaching the lake floor also inhibits submerged and rooted macrophytes, and sediments become anoxic as large amounts of planktonic biomass are added to them (Kant and Raina, 1990 ). The fluctuations in free carbon dioxide values correspond directly with the fluctuation in the standing crop of phytoplankton. As the diversity and density of phytoplanktons increase through various months, the amount of free carbon dioxide for photosynthetic activity becomes limiting. The pH changes in these ponds are governed by the amount of free carbon dioxide, carbon trioxide, and bicarbonate (Kant and Raina, 1990 ). Inflow nutrient concentration, inflow volume and inflow water temperature show very regular and reasonable impacts on the quality of lake water (Imteaz et al., 2003 ). Yin ( 2002 ) reported that monsoons served as a flushing mechanism in two ways: (1) They reduced seasonal eutrophication by nutrient enrichment in summer, and (2) they prevented long-term (annual) accumulation of organic matter in the sediments due to nutrient enrichment in the region. Because of the monsoon-influenced processes and low phosphorus in the Pearl River estuary, the estuary and adjacent coastal waters of Hong Kong appeared to be more resilient to enrichment of nitrogen.

3. Light plays an important role in the growth, diversity and density of aquatic flora. Algal growth has been reported to increase with light intensity, and luminescence of 4000 lux was found most favorable (Shen, 2002 ). As eutrophication progresses, a decline of submerged macrophytes occurs in many shallow water bodies, probably due to low light intensity caused by algal blooming. It is suggested that the adaptation strategy of Potamogeton maackianus under a certain range of low light stress is to accelerate the elongation of the main and lateral shoots and to increase their density (Ni et al., 1999 ). The light has been almost completely absorbed by the plankton of the top few meters, so that too little light penetrates to the thermocline and beyond to support photosynthesis. However, there is a rain of corpses into the deep water, whose decomposition requires oxygen. Since the deep water is cut off from the air until fall overturn, an oxygen deficit develops in the deep water, and the bottom mud is reduced. Eutrophication in an estuary is a complex process, and climate change is likely to affect each estuary differently due to interactions with nutrient loading and physical circulation. Hence, it is essential to consider the effects of climate change on the context of individual estuarine function to successfully manage eutrophication (Howarth et al., 2000 ).

There are other factors like pH and dissolved oxygen affecting water eutrophication (Khan and Ansari, 2005 ). The minima and maxima in the concentration of dissolved oxygen are found to be directly related to the maxima and minima of the phytoplankton, The direct relationship between phytoplankton and dissolved oxygen content has been observed by a number of researchers (Khan and Ansari, 2005 ). pH is a plant growth limiting factor. The change in pH is directly related to the availability and absorption of nutrients from solution. Ionization of electrolytes or the valence numbers of different ion species are influenced by changes in pH. An acidic pH has been reported to promote growth of Spirodelapolyrrhiza at a faster rate, but high pH values promote the growth of phytoplankton and result in bloom. It must be pointed out that many factors influencing eutrophication are relative and affect each other.

Microbial and biodiversity

Microbial activity is the inducement factor to alga bloom (Paerl, 1998 ; Paerl et al., 2003 ). It can enhance abundant breeding of alga bloom. Nutrient-enhanced microbial production of organic matter, or eutrophication, is frequently accompanied by altered microbial community structure and function (Paerl, 1998 ). The amount of microbial biomass is positively related to the content of organic matter and the amount of plankton in eutrophicated water. There exists certain intrinsic relationship between the amount of bacteria and the occurrence of eutrophication. The decomposition of organic matter by bacteria activities can produce nutrients and organic substances, which may promote algal bloom breaking out. Of course, it may also produce some toxic substances, which are harmful to other algal species, so that it will selectively enhance the bloom of some algae to become preponderant species and subsequently eutrophication will occur. It may be relative with the decomposing of bacteria biomass, which can promote effective circulation of nutrients when alga bloom and eutrophication occur under lower concentrations of nutrients. Chang et al.( 2005 ) demonstrated that impose of submerged macrophyte in combination of immobilized nitrogen cycling bacteria could effectively reduce chlorophyll a concentration and increase water transparency. Marshland drainage channels (=ditches) in the UK are relicts of a once extensive habitat whose management requires quantitative information on the ecology of marshland organisms. Distribution of these organisms in wetlands worldwide can reflect natural water quality, vegetation and anthropogenic factors (Watson and Ormerod, 2004 ). Acrophyte-specific richness and abundance increased along an upstream-to-downstream zonation, which was characterized by an increase in mineralization and nutrient level (Thiébaut and Muller, 1998 ). In hyper-eutrophicated water body, remarkable improvement in water quality and inhibition on algal growth was obtained by introducing nutrient cycling bacteria in proper combination with floating hydrophyte (Chang et al., 2006 ).

A comparison of aquatic macrophyte diversity of two streams reflected the impact of human-induced perturbations (fish farms, domestic sewage) in such weakly mineralized and poorly buffered waters. Disturbed sites with very high nutrient loading were characterized by low vascular plant richness and by the absence of filamentous algae (Thiébaut and Muller, 1998 ). Vădineanu et al.( 1992 ) studied the phytoplankton and submerged macrophytes in the aquatic ecosystems of the Danube Delta and found that the species changes were linked to accelerated eutrophication of the lakes, with increased phosphorus loading and a reduction in the N/P ratio. Distinct changes were observed in the macrophyte species composition in response to phosphorus enrichment (Vaithiyanathan and Richardson, 1999 ). Marshes in the unenriched and enriched areas were dominated by Ladiumjamaicense and Typha domingensis , respectively. Open-water areas were characterized by Eleocharis spp., Utricularia spp., Chara zeylanica and Nymphaea odorata in ligotrophic areas and by floating plants and Polygonum spp. in eutrophic areas. A shift in primary producers from eelgrass to macroalgae in response to increased nutrient loading altered the habitat, physicochemical structure and food webs. The nitrogen decreased shoot density and biomass of the eelgrass and promoted a record increase in the algal biomass (Deegan et al., 2002 ). Enhanced nutrient concentrations and loading have been observed in several coastal areas of the North Sea, resulting in increased production and changes in the species composition of phytoplankton (Colijn et al., 2002 ). Garg et al.( 2002 ) studied aquatic flora in three lakes of Bhopal (Upper Lake, Lower Lake and Mansarovar Lake) in India and assessed the potential fertility of the lentic water and its aquatic flora. Eutrophication was highest in Mansarovar Lake. The observations of Garg et al.( 2002 ) indicated that different species of phytoplankton could subsist up to a certain nutrient level, beyond which competition between cyanophytes and other algae enhanced and eliminated the sensitive plankton flora.

RESEARCH PERSPECTIVES

The problem of water eutrophication has become more and more severe worldwide, but the mechanism of its occurrence has not been fully understood. The limited knowledge of water eutrophication processes will add difficulties for the prevention and remediation of water eutrophication. Therefore, more researches should be turned to the mechanisms of water eutrophication under different watershed conditions. For example, the mechanisms of the adsorption and release of the contaminants in sediments should be clarified, which named inner pollution converging in water bodies, especially the absorption and release of P in sediments; the mechanism of the excessive production of algae and Cyanobacteria, especially excessive production of blue-green algae in water should be further studied, which is the key for the prevention of algae and Cyanobacteria growth. Also, the guidelines for estimating eutrophication are still very incomplete. Comprehensive guidelines for assessing eutrophication should be established by considering various factors in combination with the development of economy and society, especially in modern society ecology and health are paid more and more attention in order to avoid adverse influence on the sustainable ecological development and human health to the best of our abilities. In view of the high level of nutrients already polluted into lakes, reservoirs, estuaries, etc., understanding the functions of the factors influencing algal growth and bloom will certainly help controlling algal bloom even at high nutrient burden in surface water bodies.

* Project supported by the Key Project from the Ministry of Education of China (No. 705824), the Project from Science and Technology Bureau of Zhejiang Province (No. 2006C13059), and a grant from the St. Lucie River Water Initiative (SFWMD contract No. OT060162), USA, in part

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GCSE Biology – Eutrophication

Learning Objectives

-I can describe how human activity can cause eutrophication -I can describe the process of eutrophication

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1 . Question

What industry causes eutrophication?

2 . Question

What is the definition of eutrophication?

  • Chemicals polluting streams and rivers
  • Noise pollution from mining
  • The act of releasing greenhouse gases into the atmosphere
  • A form of global dimming

3 . Question

What happens to ecosystems during eutrophication?

  • Ecosystem dies
  • Ecosystem grows
  • Ecosystem become complex

4 . Question

How is eutrophication caused?

  • The washing away of fertilizers
  • The overflow of rivers
  • The use of rivers for boating
  • The increased pollution in the air

5 . Question

What is it called when fertilisers are washed off of land by rainwater?

  • Distillation
  • Chromatography

6 . Question

What does eutrophication cause a growth of?

7 . Question

What type of organism dies due to eutrophication?

  • Water plants

8 . Question

Why do water plants die from eutrophication?

  • Sunlight is blocked
  • No carbon dioxide
  • They stop breathing

9 . Question

What process stops in water plants during eutrophication?

  • Photosynthesis
  • Respiration
  • Active transport

10 . Question

What organisms decompose these plants?

11 . Question

Why do oxygen levels drop when eutrophication takes place?

  • The respiration of bacteria
  • The respiration of the water plants
  • The photosynthesis of the water plants
  • The photosynthesis of the bacteria

12 . Question

What does the lower levels of oxygen during eutrophication cause to die?

  • Insects and fish
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IMAGES

  1. GCSE Eutrophication

    eutrophication case study gcse

  2. GCSE Science

    eutrophication case study gcse

  3. Eutrophication Activity

    eutrophication case study gcse

  4. Eutrophication (GCSE biology

    eutrophication case study gcse

  5. How Does Eutrophication Work? Causes, Process and Examples

    eutrophication case study gcse

  6. Process of Eutrophication Storyboard by dima1234

    eutrophication case study gcse

VIDEO

  1. EUTROPHICATION concept 8 EXPLAINED

  2. #162 @ What is Eutrophication ?

  3. Eutrophication क्या है? #biologywings देखिए Live Example के साथ

  4. Eutrophication and how it happened

  5. EUTROPHICATION. #conservationbiology #eutrophication #waterpollution #ncertchemistry #pollution

  6. Eutrophication class 12th ncert #neet #neet2023 #neetexam #ncert #neetmotivation #neet2024 #biology

COMMENTS

  1. Ecosystems

    WJEC Ecosystems - pollution and nutrient cycles - WJEC Eutrophication Pollution can have a significant effect on the environment. Humans introduce chemicals to the environment which can enter...

  2. 4.17 Biological Consequences of Eutrophication

    Biological Consequences of Eutrophication Sequence of events causing eutrophication in lakes and rivers Exam Tip

  3. Eutrophication

    Eutrophication is a process that occurs in bodies of water like lakes, ponds and rivers due to excessive nutrients, specifically nitrogen and phosphorus. These excess nutrients may originate from factors such as runoff of fertilisers from agricultural land, or sewage pollution.

  4. The Organism in the Environment

    9.1 The Organism in the Environment. Easy. Medium. Hard. Model Answers. 1a 1 mark. Figure 1 shows some apparatus set up to test the effect of an abiotic factor on the rate of photosynthesis in pondweed. Figure 1.

  5. Leaching and Eutrophication

    The whole sequence of events caused by the extra salts leading to death of organisms is called "Eutrophication". Eutrophication may also be caused by sewage / slurry etc… which gets into the water supply because these too contain nitrate rich chemicals.

  6. 9.4.1 Human Impact on Biodiversity

    Revision notes on 9.4.1 Human Impact on Biodiversity for the Edexcel GCSE Biology syllabus, written by the Biology experts at Save My Exams. ... GCSE Religious Studies. AQA. Revision Notes; Past Papers A (8062) Past Papers B (8063) AQA (Short Course) ... Eutrophication can occur; Fish Farming Methods Table.

  7. Pollution

    This video is for Edexcel IGCSE Biology 9-1 but is relevant for many GCSE Biology courses. It covers the following objectives from the syllabus 4.16 Understa...

  8. B18.2

    This video covers Water pollution and Eutrophication for the AQA Biology GCSE 9-1 Specification. Also covered is sewage treatment and indicator species.

  9. Mississippi River Case Study

    Figure 4.2.7. Eutrophication Credit: EPA: Mississippi River/Gulf of Mexico Hypoxia Task Force Watch the following videos from NOAA's National Ocean Service that show how dead zones are formed and explain the dead zone in the Gulf of Mexico: Video: Happening Now: Dead Zones in the Gulf 2017 (2:33) Video: Hypoxia (3:51)

  10. Eutrophication

    In this video you will learn all the science for this topic to get a grade 9 or A* in your science exams! All content, music, images, worksheets are the pro...

  11. What is eutrophication?

    Eutrophication (also called algal bloom) is a massive growth of green plants in lakes and rivers, caused by the use of fertilisers in industry, washed off the land by rain.The fertilisers cause the green plants to grow, and as they do they cover the surface of the lake, preventing sunlight from reaching plants so they can no longer photosynthesise.

  12. Describe the process of Eutrophication.

    Answered by Grace K. What is the difference between an ectotherm and an endotherm? Answered by Ed S. Eutrophication occurs when mineral ions from the soil such as nitrates are leached (washed away by the rain) and they run into rivers, lakes and ponds. This incre...

  13. Case studies eutrophication

    Solutions The main goal in reducing dead zones is to keep fertilizers on the land and out of coastal waters. The Black Sea dead zone largely disappeared between 1991 and 2001 after fertilizers became too costly to use following the collapse of the Sovjet Union and the demise of eastern European economies.

  14. The Issue of Eutrophication: A Case Study of Lake Erie

    Eutrophication is a process by which excess nutrients such as phosphorus (P) and nitrogen (N) deposit into a body of water and become concentrated in particular areas. This process disturbs natural systems and often causes adverse effects.

  15. Globally consistent assessment of coastal eutrophication

    25 Altmetric Metrics This article has been updated Abstract Eutrophication is an emerging global issue associated with increasing anthropogenic nutrient loading. The impacts and extent of...

  16. Mechanisms and assessment of water eutrophication

    Eutrophication can be defined as the sum of the effects of the excessive growth of phytoplanktons leading to imbalanced primary and secondary productivity and a faster rate of succession from existence to higher serial stage, as caused by nutrient enrichment through runoffs that carry down overused fertilizers from agroecosystems and/or discharg...

  17. Assessment and management of lake eutrophication: A case study in Lake

    A case study was conducted using Lake Erhai, and the results of the TSI methods indicated that Lake Erhai was in a mesotrophic state, and exhibited N and P co-limitation before 2006, and P limitation after 2006. (2) Lake eutrophication mechanisms were explained and specific measures for managing eutrophication were provided.

  18. 7.3.2 Waste Management

    Sequence of events causing eutrophication in lakes and rivers Land pollution Sources of land pollution and their effects Air pollution Sources of air pollution and their effects How air pollution leads to acid rain Exam Tip

  19. GCSE Biology

    What is the definition of eutrophication? The act of releasing greenhouse gases into the atmosphere Chemicals polluting streams and rivers A form of global dimming Noise pollution from mining What happens to ecosystems during eutrophication? Nothing Ecosystem become complex Ecosystem grows Ecosystem dies How is eutrophication caused?

  20. Underestimated nutrient from aquaculture ponds to Lake Eutrophication

    Nitrogen and phosphorus discharges from freshwater pond aquaculture have become an important source of eutrophication in lakes. Based on multi-source remote sensing and Google Earth Engine, we extracted aquaculture ponds nationwide and calculated their quantities and area density distributions. Experimental results reveal that regions with densely distributed aquaculture ponds are also ...

  21. Cost-effective management of coastal eutrophication: A case study for

    Our study shows that the costs of reducing coastal eutrophication caused by the Yangtze in 2050 amount to 1-3 billion $ (range for the cases of Scenarios 1 and 2). These are the costs of a gap closure of 80-90 % in river export of both TDN and TDP ( Figs. 2 , D.1-D.10, Tables D.1-D.2).

  22. 2.3.4 Coastal Opportunities & Hazards

    Coastal hazards can be either natural or human induced. Natural hazards include storms, flooding and tsunamis. Human actions cause a variety of issues as shown in the table below: Opportunities. Consequences. Impacts. Urbanisation and transport. Dredging and disposal of harbour sediments; changes in land use - ports, harbours and airports; road ...