23 July 2009
The effect of global warming on eutrophication in lakes
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.
Shallow lakes are important in their support of biodiversity. However, their large surface-to-volume ratios mean that
they are prone to environmental change and can switch between clear waters, with a high biodiversity, to turbid waters,
due to suspended sediment or increased amount of phytoplankton, with a lower biodiversity.
Such switches are induced by changes in nutrient loadings (mainly nitrogen and phosphorus), but the effect of climate
change together with changes in nutrient concentrations is poorly understood.
One of the objectives of the EU Water Framework Directive1 is to ensure that European surface waters have a good
ecological and chemical status. This involves assessing water quality in the present and predicting what it will be in the
future. This study forms part of the EU Euro-limpacs project, which aims to assess the effects of future climate change on
Europe’s freshwater systems2.
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.
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 must be assessed and that managing nitrogen loadings to European
waters is an important part of maintaining good water quality, especially under a changing climate. However, it should be
remembered that the research was undertaken in only one location and outcomes may be different in areas with different
starting conditions.
1. See: http://ec.europa.eu/environment/water/water-framework/info/intro_en.htm
2. Euro-limpacs (Evaluating the Impacts of Global Change on European Freshwater Ecosystems) was supported by the European Commission under
the Sixth Framework Programme. See www.eurolimpacs.ucl.ac.uk
Source: 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.
Contact: [email protected]
Theme(s): Biodiversity, Climate change and energy, Water
Opinions expressed in this News Alert do not necessarily reflect those of the European Commission
To cite this article/service: "Science for Environment Policy": European Commission DG Environment News Alert Service, edited by
SCU, The University of the West of England, Bristol.
European Commission DG ENV
News Alert Issue 161
July 2009
1