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Published: 2009-11-06

Elevated atmospheric nitrogen affects aquatic ecosystem in alpine lakes

NEWS The amount of nitrogen in the atmosphere and different ecosystems has increased dramatically as a result of human use of fertilizers and burning of fossil fuels. One example is the elevated nitrogen levels in many lakes in areas with high nitrogen deposition. This affects the availability of nutrients for phytoplankton, according to a new study published in the journal Science.

Today, the concentration of nitrogen in the atmosphere is significantly greater than 50 to 100 years ago, including an increased input from agriculture and automobile traffic. The more nitrogen in a lake, the more polluted algae becomes. As the algae becomes less nutritious for fish and other organisms, the lakes become less biodiverse. The nitrogen can be transported long distances in the atmosphere and lead to an increased deposition over large parts of the globe.

“So far, the consequences of increased nitrogen input via deposition to the lakes have been given little attention. Our results show that nitrogen deposition has great importance," says Ann-Kristin Bergström, Associate Professor at the Department of Ecology and Environmental Science, Umeå University.

Berström’s collaborators in the study, published in the 6 November issue in Science, were colleague Professor Mats Jansson at Umeå University, scientists at Arizona State University and Colorado State University in the United States, and University of Oslo in Norway.

The article is based on data from more than 2000 lakes in Sweden, Norway and North America located in areas with both low and high nitrogen deposition. In addition, experiments were conducted in more than 90 of these lakes to determine how nitrogen deposition affects phytoplankton. The results show that the lakes located in areas with high nitrogen deposition rates have increased levels of nitrogen compared to lakes located in areas with low nitrogen deposition. Increased nitrogen deposition therefore alters balance between the two substances, nitrogen and phosphorus, which control plankton plant growth in lakes. In all regions, the experimental findings displayed a correlation between the volume ratio of nitrogen and phosphorus in lakes. In areas with low nitrogen deposition, plankton growth was primarily limited by nitrogen deficiency, while the plant plankton growth in regions with high nitrogen deposition was primarily limited by lack of phosphorus.

The new findings are challenging an established "truth" in freshwater ecosystems research - that the plankton plant growth in lakes is limited by lack of phosphorus. “Phosphorus-limited phytoplankton growth should be seen rather as a condition of lakes in the Upstate because we have radically increased nitrogen supply in the last century, and especially through the atmosphere," says Ann-Kristin Bergström.

Umeå scientists have previously published studies which demonstrated that the increased nitrogen deposition has resulted from increased biomass of phytoplankton, known as eutrophication, in many lakes in Sweden, Europe and North America. Other likely ecological effects are changes in species and the succession of phytoplankton. “Phosphorus-limited phytoplankton is also a poor food source for zooplankton, which in turn can lead to production changes at higher levels in the food chain," explains Ann-Kristin Bergström.

The global transport of atmospheric nitrogen deposition is expected to increase in the coming decades. “Therefore, there is a risk that lakes which today are located in sparsely populated areas with relatively low nitrogen deposition will be affected in the future,” emphasises Ann-Kristin Bergström.

The Sweden-based studies were funded by the Research Council Formas with funds allocated to Ann-Kristin Bergström, and the ‘strong research environment’ LEREC (Lake Ecosystems Response to Environmental Change) at the Department of Ecology and Environmental Science, Umeå University.

For further information please contact: Ann-Kristin Bergström, Associate Professor at the Department of Ecology and Environmental Science, Umeå University Phone: + 46 (0)70-274 77 34
E-mail: ann-kristin.bergstrom@emg.umu.se

Editor: Karin Wikman