Less is more: reduced nitrogen has greater potential than originally thought.
For decades, experts have debated whether reducing the amount of nitrogen flowing into lakes can improve water quality in the long-term, even though blue-green algae can bind nitrogen from the air. However, no lakes with decreased nitrogen inputs have been monitored for long enough to clarify this – until now: scientists from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) have analysed long-term data to prove that decreasing nitrogen in Berlin’s Lake Müggelsee is the key to reducing algal blooms in summer. They showed that the amount of atmospheric nitrogen bound by blue-green algae is far too small to be used as an argument against the ecologically necessary reduction of nitrogen inputs.
In the 1970s, scientists discovered that nutrient inputs – mainly phosphorus and nitrogen – from agriculture and wastewater discharge were the main cause of excessive plant and algal growth in lakes and rivers. Since then, water management experts have concentrated on reducing phosphorus inputs. “Although this strategy often works, it is by no means always successful. In shallow lakes, the sediment releases large quantities of phosphorus in summer. In these cases, reducing nitrogen input may help to control algal blooms because algae need both phosphorus and nitrogen to grow. Until now, however, there has been no convincing evidence that decreasing nitrogen inputs, which is more complex and costly than decreasing phosphorus, works in the long term,” stated IGB freshwater ecologist Dr. Tom Shatwell, explaining the starting point of the study.
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Image via Tom Shatwell, IGB
