Seminar: The Baltic Sea: why do we care?

06/12/2017 - 16:00
Baltic Sea

The Baltic Sea: why do we care?

Coastal zone hypoxia has dramatically increased worldwide in recent decades, having devastating effects on ecosystems. These oxygen deficient zones are commonly caused by increased input of nutrients from land, this leads to eutrophication and increased consumption of oxygen when organic matter is degraded. Once established, these hypoxic conditions are difficult to reverse. Oxygen free sediments release phosphate and ammonium, which sustains the eutrophication. The largest hypoxic zone in the world is found in the Baltic Sea. Oceanographic samplings have been going on in the area for over a century, making it a valuable site for studying mechanisms behind nutrient cycling, eutrophication and hypoxia.

Natural hypoxia has been a recurring phenomenon in the deeper parts of the Baltic Sea for 8000 years, due to a strong vertical stratification and limited water exchange with the North Sea. However, since the 1960s the hypoxic area has increased fourfold caused by human activities increasing nutrient input and eutrophication. In the last decades, considerable effort has been put into decreasing the nutrient input to the Baltic Sea. Despite this, the Baltic Sea is recovering very slowly, this has been attributed to the large release of phosphorus from anoxic bottoms. The excess phosphorus in turn supports the growth of nitrogen fixing cyanobacteria, which bloom in the Baltic Sea each summer.

In the beginning of 2015, a large inflow of water from the North Sea oxygenated the long term hypoxic bottoms in the Baltic Sea. Before and after the oxygenation, we measured in situ sediment-water fluxes of nutrients with benthic landers. We showed that phosphorus was retained in newly oxygenated sediments, and initiated denitrification which removed nitrogen from the system. Despite the first inflow being followed by smaller inflows, the oxygen was quickly consumed. As oxygen concentrations decreased, the beneficial effects on nutrient sediment-water fluxes disappeared. Our study clearly shows how difficult it is for a system to recover from established, long term hypoxia.