The Changing Baltic Sea: Between Nutrient Load Reduction and a Warming Climate

Eutrophication of the Baltic Sea was recognized more than half a century ago, but it remains a major threat to the sea's ecosystem. Requirements developed by the Baltic Marine Environment Protection Commission (formed in 1974) and subsequently implemented in national and European Union law have led to reductions of phosphorus river load by approximately 50% and nitrogen river load by approximately 30% since the 1980s, but so far, the measures have failed to significantly improve the surface water quality. A decades-long accumulation of phosphate and oxygen-sapping substances appeared to reduce the efficiency of the lateral supply of oxygen from intrusions and major Baltic inflows via the narrow Baltic Straits. The dynamic change of, in particular, phosphate cycling in deep waters during these inflows contrasts with the sluggish response to river load reduction measures. Seasonal phosphate recycling in surface water results mainly from exchange with the large deep-water phosphate pool, and this key exchange can be better interpreted based on an improved understanding of its physical drivers.