This is how eutrophication works. Marine areas receive large quantities of nitrogen and phosphorus, mainly from agriculture. Nitrogen and phosphorus are rich nutrients that act as fertilizers, enhancing plant growth. This especially applies to algae that experience a significant population increase (so called algal bloom). Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water. When algae die and sink to the bottom bacteria that feed on the increasing mass of dead algae, sucks up all the available oxygen, creating anaerobic conditions.
Baltic Sea has serious problem with marine dead zones since seven of the world's 10 largest marine dead zones are located in the Baltic. WWF issued in its statement: "WWF demands quick and decisive action to reduce emissions, not least from agriculture around the Baltic Sea". The good news is that marine dead zones are reversible. For instance the Black Sea marine dead zone, that used to be the world's biggest marine dead zone in period from 1991 to 2001, largely disappeared after fertilizers became too expensive to use following the collapse of Soviet Union and the demise of centrally planned economies in Eastern and central Europe.
Marine dead zones are areas where the water at the sea floor has very low, or is completely without dissolved oxygen. Among the top 10 largest marine dead zones, seven is located in Baltic.