Eutrophication, dark side of ‘greenness’

Mohammed El-Said
10 Min Read

Scientists and environmentalists are trying to transform our climate-threatened planet into a green planet by conserving forests and natural habitats as well as increasing afforestation, but green may not always be a good indicator, according to a recent study by researchers from the Protecting Agency Environment in the United States.

The study, published recently in the Nature Communications journal, concluded that the greening of lakes known as ‘eutrophication,’ would cause methane emissions to increase in the atmosphere by 30% to 90% over the next 100 years.

Methane is one of the greenhouse gases that are 34 times more effective than carbon dioxide (CO2). It is produced after algae die and lakes get covered with green spots rich in toxins which degrade in drinking water and contaminate it.

Greenhouse gases include a number of chemical compounds, most notably CO2, methane, and ozone, and increases in the concentrations of greenhouse gases in the atmosphere pose serious risks, most notably climate change, and global warming.

Researchers estimate that two-thirds of the amount of methane emitted daily in the summer months of Lake Erie-one of the Great Lakes of North America-is the result of algal bloom.

What does eutrophication mean?

The word ‘eutrophic’ stems from (originally) the Greek language, meaning ‘well-nourished’. So eutrophication is essentially a high level of nutrients (for instance, phosphorus and nitrogen) in lakes, which typically results in a high biomass and can lead to toxic algae blooms. Eutrophic lakes are typically greener/darker in colour. These colour differences have an impact on light penetration and essentially the functioning of the lake, in terms of which species dominate and where they hang out in the water column.

Also, when these typically big algae blooms die they accumulate in the lake bottom sediments where microbes degrade this carbon input. This degradation by microbes often consumes oxygen leading to anoxic (oxygen-free) bottom waters. This has obvious consequences for the species in the lake which require oxygen. This oxygen-free environment in the sediments and water column are also ideal locations for methane production.

Co-author of the study, Tonya DelSontro of the Department F-A Forel for environmental and aquatic science, University of Geneva, Switzerland, told Daily News Egypt that this study came about following the publication of a study last year, where she and the same the authors of the current study collected literature data of all greenhouse gas emissions from lakes and reservoirs (CO2, methane, and nitrous oxide) and looked for environmental variables which could be related to these gas emissions.

The team back then had found that ‘chlorophyll a’ (the green pigment in algae and other plankton) correlated strongly and positively with methane emissions. This delivered a clear message to the team that there is likely a link between the amount of algae (and hence the health of the lake) and methane emissions.

They also know that there is a link between the amount of nutrients in a lake and the amount of algae such that when these levels become too high, it is the process of eutrophication, and is an indicator of the lake’s poor health.

Climatic feedback

Authors of the paper believe that their findings are important because it provides-for the first time-a prediction for the climatic feedback of freshwaters due to global environmental change. The increase in nutrients in lakes ultimately cause the eutrophication is anthropogenic, and a result of increasing population (for instance, more agricultural and urban waste runoff).

But this increase will be exacerbated by climate change as well–for instance; more precipitation leading to more runoff, increases in temperature which causes more microbial degradation producing methane, and larger lake surface area. These global environmental changes will ultimately induce more methane emissions from freshwaters, which is a stronger greenhouse gas than CO2, and thus be a positive feedback on climate change.

The authors used a statistical model they created in 2018 which correlates methane emissions with lake size and chlorophyll–which is a measure of high algal biomass stimulated by phosphorus. By using the global distribution of lake size and total lake area, the climatic heating of lakes, future phosphorus concentrations and storm-driven nutrient runoff, they were able to estimate future lake methane emissions, which the authors say has not been previously conducted.

The optimistic outcome is that improved nutrient management practices could reverse the greening or eutrophication of lakes and thereby reduce methane emissions. Additionally, local action to improve water quality could have important global consequences, according to the paper.

In terms of biodiversity, DelSontro told DNE that the paper does not approach that subject but there is a lot of work going on now about how climate change affects planktonic species in lakes. Also, she is working at the University of Geneva on how these species’ changes affect the mixing and light penetration in a lake, which will have consequences for how lakes react to climate change too.

Future predictions  

DelSontro noted that the objective of this second publication was to make some predictions regarding future eutrophication of freshwaters on a global scale, and the potential increase in methane emissions because of that eutrophication.

The researchers used several pieces of literature regarding potential future increases in fertiliser production and nutrient runoff, as well as increases in water temperature and lake area due to climate change, and found that eutrophication could increase almost fivefold in the future.

“We used a more conservative value of threefold as our maximum, and our model from the previous publication to estimate that lake and reservoir methane emissions could double by 2100 if this eutrophication increase were to happen,” said DelSontro.

The study showed that eutrophication in lakes will increase over the next 100 years because of three factors. First, the population is expected to increase by 50% in 2100, resulting in an increase in wastewater and fertilisers used in agriculture.

More fertilisers mean that there are more nutrients in the water and so water borne organisms increasingly grow, which reduces the amount of oxygen in the water, and promotes the production of methane. Eutrophication is sometimes thought of as ‘choking’ a lake, because of the oxygen loss.

The second factor is the increased occurrence of storms and rain and the resulting water runoff that transfers nitrogen from the soil to inland lakes. Thirdly, with the steady rise in temperature, the temperature of the lakes and the warm water will be favourable for the growth of algae.

Researchers predict that eutrophication in lakes will be increased by between 25% and 200% by 2050, according to current population growth rates and climate change.

Reducing impacts

According to the paper eutrophication is bad for many reasons; the biggest reason is the potential for toxic cyanobacteria (algae) blooms. These are toxic to animals and humans, thus creating poor recreational and drinking water quality of freshwaters. The green colour induces changes in the lake in terms of species, functioning, and physics, which we do not yet fully understand, but it has an impact on how we use our freshwaters.

Then the degradation of these algae blooms leads to oxygen consumption in lake bottoms, which causes problems for fish and other aquatic species. The researchers found that green lakes will likely emit more methane-a potent greenhouse gas-and promote further climate change.

In order to reduce the occurrence of eutrophication as individuals, we can be careful with or simply not use fertilisers in our gardens. We can also reduce the amount of water we use, which will reduce sewage runoff. We can also decide to buy only agricultural products which are fertiliser-free, according to the researcher.

She further explained that because much of the runoff will increase because of climate change, anything we do which helps reduce climate change (for instance, use less fossil fuels) will help reduce eutrophication.

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Mohammed El-Said is the Science Editor for the Daily News Egypt with over 8 years of experience as a journalist. His work appeared in the Science Magazine, Nature Middle East, Scientific American Arabic Edition, SciDev and other regional and international media outlets. El-Said graduated with a bachelor's degree and MSc in Human Geography, and he is a PhD candidate in Human Geography at Cairo University. He also had a diploma in media translation from the American University in Cairo.