The effect of global warming on eutrophication in lakes

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The effect of global warming on eutrophication in lakes
Source: European Commission, Environment DG
Jul. 24, 2009

Shallow lakes are an important type of ecosystem that may be vulnerable to current warming trends. A recent study examines just how vulnerable they are. It indicates that climate change combined with nutrient pollution could exacerbate eutrophication and suggests nitrogen levels should be monitored.

The researchers used 48 tanks in north-western England which simulated shallow lake communities. They studied the effects of warming by 4°C (which is the high emission scenario for the temperature increase during a hundred years period) and the effects of two levels of nutrient loading relevant to current degrees of eutrophication.

Levels of nutrients, oxygen and pH, as well as phytoplankton, fish and plants, were also studied. During the experiment the highest temperatures at noon reached 21°C in unheated shallow lakes and 25°C in heated lakes. They did not drop below about 3°C in either.

The study demonstrated that warming increased the concentration of soluble phosphate in the water. It also increased total plant biomass, but surprisingly reduced the amount of phytoplankton. The fall in phytoplankton is thought to be caused by shading from increased floating plants, which may be linked to a warming-induced release of soluble phosphate from the sediment. Warming also reduced fish biomass, probably the result of oxygen stress. Perhaps more importantly, high nitrogen loading as well as warming reduced the number of plant species.

Although temperature rises alone are unlikely to cause a switch in water conditions, they could intensify signs of eutrophication in shallow lakes. For example, increased temperature together with increased nutrient loading may cause nuisance growths of floating plants which may affect biodiversity.