WARNEMUENDE, August 6 (WNM/Frontiers in Earth Science/Barbara Hentzsch) - Between 1995 and 2014, river discharges of the two main drivers of eutrophication, nitrogen and phosphorus, were significantly reduced in the western Baltic Sea. But are these measures also having an effect in the open Baltic Sea? The marine chemists of the Leibniz Institute for Baltic Sea Research have not yet found any clearly discernible changes there. In a recently published study, they report on a method they used to track the fate of nutrients from river mouths into the Baltic Sea. They show: yes, the reductions in river discharges are also detectable in the Baltic Sea, but they are compensated by discharges from other sources.
Let us imagine the Baltic Sea as one big pot of soup, to which many cooks have contributed. The soup is spoiled, but which cook is responsible? Once the spice is in the pot, it mixes with all the ingredients, so that it is no longer possible to tell, who the culprit was. This is also the problem facing the marine chemists at the Leibniz Institute for Baltic Sea Research Warnemünde, when they want to find out whether measures to reduce nutrient discharges via the rivers in the open Baltic Sea are effective. A total of 40,000 tonnes less nitrogen and 1,000 tonnes less phosphorus were discharged into the western Baltic Sea between 1995 and 2014. However, the measured concentrations in the open Baltic Sea still show no improvement.
Recently, a team led by Joachim Kuss, a marine chemist from Warnemünde, Germany, evaluated for the first time a dataset of more than half a million data in order to trace the effects of the reduction measures. They took advantage of the fact that the rivers ultimately leave their mark on the sea by reducing the salinity of the Baltic Sea water. As expected, the concentrations of nutrients generally decreased with the distance from the estuary with increasing salinity. Moreover, the large amount of data available to the scientists for the period 1995 - 2016 also enabled them to work out that the changes in the ratio of nutrient concentration to salinity on the route between the coast and the open Baltic Sea did not always remain the same, but changed during the period under investigation: the reduction in nutrients was now visible!
And the applied methodology revealed more information: while there was a particularly good correlation between salinity and nitrogen components, the phosphorus content showed only a low dependency on salinity. This underlines the fact that nitrogen is significantly linked to river water inputs, whereas phosphorus is clearly also derived from internal sources in the Baltic Sea.
In order to obtain significant information on the temporal decrease of phosphorus components, which are undoubtedly also discharged in significant amounts via rivers, and to improve the data basis for nitrogen, the IOW data set was extended by the measurement results of the state authorities from the coastal area, State Agency for Environment, Nature Conservation and Geology, Mecklenburg-Western Pomerania, and the State Agency for Agriculture, Environment and Rural Areas, Schleswig-Holstein.
With the help of this enlarged data pool, the effects of the manifold processes affecting nutrients in the transition region between fresh and salt water could be well worked out: Organisms take advantage of the favourable nutrient situation and form blooms. As a result, nutrients are removed from the water and converted into organic matter. The dying bloom sinks to the sea floor, where it is decomposed by microorganisms. These make an ambivalent contribution to nutrient reduction, as they convert reactive nitrogen components into inactive elemental nitrogen. This is an important positive achievement for the nitrogen balance, but can be harmful for the phosphorus balance: During the decomposition of organic matter, oxygen is consumed. If the shallow water of the transition area is well mixed by wind and waves, this is not critical. In calm weather conditions, however, "dead zones" can form on the ground. Then phosphorus compounds, which are stored in the sediment under good oxygen conditions, are dissolved and increase the phosphorus content in the water. "At present, oxygen deficiency situations are becoming more frequent in coastal waters," explains the leading author Joachim Kuss, "This means that phosphate pollution of the western Baltic Sea currently appears to be the primary problem. However, this should not hide the fact that it is also the nitrogen compounds introduced that ultimately lead to oxygen deficiency and the reactivation of old phosphorus deposits on the sea floor. There is still a need for action to reduce both nutrients."