Nitrogen runoff has been associated with poor water quality in lakes for many years, but it remained unclear whether reducing the amount of nitrogen that ended up in lakes would improve the quality of their water over a long period. That's because blue-green algae called cyanobacteria is known to live in lakes, where it can gather nitrogen, or fix it, from the air. Now, researchers at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) have collected long-term data from Berlin's Lake Müggelsee; they found that reducing nitrogen in the lake is critical to lowering summer algal blooms; the amount of nitrogen binding to blue-green algae is insignificant compared to the nitrogen coming from runoff.
We have known for decades that nutrients like nitrogen and phosphorus were plentiful in water coming from agriculture and wastewater discharge, and they were contributing to the excessive growth of plants and algae in rivers and lakes. Water management has been primarily focused, however, on phosphorous reduction.
"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," explained IGB freshwater ecologist Dr. Tom Shatwell.
For this work, the researchers assessed 38 years of data taken from 1979 to 2016 at Lake Müggelsee. Beginning in the 1970s, weekly samples were taken from the lake and its tributaries to learn more about the long-term impact of phosphorous and nitrogen; they looked at concentrations of the chemicals and the composition of algae growth.
An excess of phosphorous appeared every summer in the lake, which is one of the few in the world in which phosphorous and nitrogen pollutants have significantly decreased. The researchers found that the algae blooms went down because the nitrogen was low. Water clarity also increased. They determined that the cyanobacteria did not fix more nitrogen to replace what had been lost from the water, contrary to prevailing views. Very little nitrogen was gathered from the air by the cyanobacteria, in fact.
Video courtesy of Leibniz-Institut für Gewässerökologie und Binnenfischerei (IGB)
"It takes much more energy to fix atmospheric nitrogen than it does to use nitrogen compounds present in the water. Blue-green algae obviously only use this method when absolutely necessary and when there is sufficient solar energy," explained study co-author and research group leader Dr. Jan Köhler.
This long-term monitoring of Lake Müggelsee has been enabled the researchers to draw solid conclusions. This work may apply to many other shallow lakes too.
"In any case, the results should be a sufficient incentive to test the targeted reduction of nitrogen for other lakes, too. Our study is a significant step towards achieving more effective water management," said Shatwell.