Climate Change and Eutrophication in Lakes
With their large surface-to-volume ratios, shallow lakes are prone to environmental change, and clear, biodiverse waters can become turbid and less biodiverse relatively easily. It has long been known that shallow lakes might therefore be vulnerable to global warming trends, but few studies have been conducted until now.
As part of the EU Euro-limpacs project (Evaluating the Impacts of Global Change on European Freshwater Ecosystems), a team of British researchers used 48 tanks to simulate shallow lake communities. They studied the effects of warming by 4°C (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 reached 21°C in unheated shallow lakes and 25°C in heated lakes, whilst neither type of lake ever dropped below 3°C.
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, which was 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.
The Water Framework Directive suggests that nitrogen concentrations should be considered when assessing the ecological quality of water. The limited evidence of the effects of nitrogen concentrations (compared to the evidence of the effects of phosphorus in fresh-waters) has made many government agencies reluctant to apply standards for nitrogen. This research indicates that nitrogen concentrations should be assessed however and that managing nitrogen loadings is an important part of maintaining good water quality, especially under a changing climate.
For more information, see:
Feuchtmayr, H., Moran, R., Hatton, K. et al. (2009). Global warming and eutrophication: effects on water chemistry and autotrophic communities in experimental hypertrophic shallow lake mesocosms. Journal of Applied Ecology. 46: 713-723.
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