For decades it has been discussed whether a reduced input of the nitrogen compounds nitrate and ammonium sustainably improves the water quality, although nitrogen can also be bound by blue-green algae from the air. To clarify this, long-term observations of lakes were reduced in which nitrogen was reduced - so far: Scientists from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) have shown with the help of long-term data that a reduction of nitrogen in Berlin Müggelsee the key to avoid algal blooms in the summer. And that the binding of atmospheric nitrogen by blue-green algae is much too small to be considered as a counter-argument for the ecologically necessary reduction of nitrogen inputs.
Already in the 1970s, nutrient inputs were recognized by agriculture and wastewater discharges as the main cause of excessive plant and algae growth in lakes and rivers. Since then, water management has focused on reducing phosphorus inputs. "This strategy is often successful, but by no means in all waters: in shallow lakes, the sediment releases large amounts of phosphorus in summer. Then a reduction in nitrogen supply could help because the algae need both phosphorus and nitrogen to grow. So far, however, there has been strong evidence that the more costly and expensive reduction of nitrogen inputs is successful in the long term. Tom Shatwell, aquatic ecologist at the IGB, the starting point of the study.
Long-term data let you look deeply
For their study, the scientists have statistically evaluated data from 38 years (1979-2016). As part of a long-term program, the Müggelsee and its tributaries have been sampled weekly since the 1970s, and the concentrations of phosphorus and nitrogen as well as the species composition of the algal communities have been investigated. The Müggelsee is one of the few lakes in the world that has significantly decreased its exposure to phosphorus and nitrogen and has been studied long enough to draw conclusions about the effects of reduced nitrogen inputs.
Phosphorus abounded in the Müggelsee every summer because it was released from the lake sediments. The amount of algae decreased only by the reduction of nitrogen supply - and the clarity of the water. The assumption that certain blue-green algae would replace the missing nitrogen from the inflows in the long term by atmospheric nitrogen has not been confirmed in the Müggelsee. In fact, the blue-green algae did not multiply and hardly bound nitrogen from the air. "The fixation of atmospheric nitrogen requires much more energy than the use of nitrate or ammonium from the water. The blue-green algae apparently use this method only when it is absolutely necessary and when sufficient solar energy is available, "explains Dr. med. Jan Köhler,
Learning from the Müggelsee
So far, the Müggelsee is the only case study on a large scale. However, the strong release of phosphorus from the sediment and nitrogen from the water into the air is typical of shallow lakes in the summer, so that many other shallow lakes are likely to behave similarly. "In any case, the results should be incentive enough to test a targeted reduction of nitrogen for other lakes as well. Our study is a building block for more effective water management, "says Tom Shatwell, summarizing the relevance of the results.
This article has been republished from materials provided by Forschungsverbund Berlin. Note: material may have been edited for length and content. For further information, please contact the cited source.
Decreased nitrogen loading controls summer cyanobacterial blooms without promoting nitrogen‐fixing taxa: Long‐term response of a shallow lake. T. Shatwell J. Köhler. Limnology and Oceanography, 03 September 2018, https://doi.org/10.1002/lno.11002.