Nutrient loads from watersheds, atmospheric deposition, and cyanobacterial nitrogen fixation have led to eutrophication in the Baltic Sea. Here we give the historical evolution of this, detail some of the specific eutrophication features of the Baltic Sea, and examine future scenarios from climate related changes in the Baltic Sea region.
We distinguish northern and southern regions of the Baltic Sea. The northern watersheds have sub-polar climate, are covered by boreal forest and wetlands, are sparsely populated, and the rivers drain into the Gulf of Bothnia.
The southern watersheds have a marine influenced temperate climate, are more densely populated and are industrially highly developed. The southern areas are drained by several large rivers, including the representative Oder River. We compare these regions to better understand the present, and future changes in Baltic Sea eutrophication.
Comparing the future projections for the two regions, we suggest that in addition to changes in nutrient inputs, increased temperature and precipitation are likely to become important forcings. Rising temperature may increase release of dissolved organic matter (DOM) from soils and may alter the vegetation cover which may in turn lead to changed nutrient and organic matter input to the Baltic Sea. For the southern Oder River catchment a model study of nutrient input is evaluated, MONERIS (Modelling Nutrient Emissions in River Systems).
The strong correlation between precipitation, flow and nutrient discharge indicates a likely increase in nutrient concentrations from diffuse sources in future. The nutrients from the Oder River are modified in a lagoon, where removal processes change the stoichiometry, but have only minor effects on the productivity.
We suggest that the lagoon and other nearshore areas fulfil important ecological services, especially the removal of large quantities of riverine nitrogen but at the same time are threatened systems due to increasing coastal hypoxia.