Dead zones are hypoxic (low-oxygen) areas in the world’s oceans and large lakes, caused by “excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom water. (NOAA).” In the 1970s oceanographers began noting increased instances of dead zones. These occur near inhabited coastlines, where aquatic life is most concentrated. (The vast middle portions of the oceans, which naturally have little life, are not considered “dead zones”.)
Baltic Sea hypoxia
Baltic Sea hypoxia refers to low levels of oxygen in bottom waters, also known as hypoxia, occurring regularly in the Baltic Sea. The total area of bottom covered with hypoxic waters with oxygen concentrations less than 2 mg/l in the Baltic Sea has averaged 49,000 km2 over the last 40 years. The ultimate cause of hypoxia is excess nutrient loading from human activities causing algal blooms. The blooms sink to the bottom and use oxygen to decompose at a rate faster than it can be added back into the system through the physical processes of mixing. The lack of oxygen (anoxia) kills bottom-living organisms and creates dead zones.
Figure of hypoxic area from 2007. Red shows the areas that experience hypoxia (O2 < 2 ml/l) and black is the anoxic area with no oxygen left in the water. Figure from SMHI. (lost link to figure ?)
The countries surrounding the Baltic Sea have established the HELCOM Baltic Marine Environment Protection Commission to protect and improve the environmental health of the Baltic Sea. In 2007, the Member States accepted the Baltic Sea Action Plan to reduce nutrients. Because the public and media are frustrated by the lack of progress in improving the environmental status of the Baltic Sea, there have been calls for large-scale engineering solutions to add oxygen back into bottom waters and bring life back to the dead zones. An international committee evaluated different ideas and came to the conclusion that large-scale engineering approaches are not able to add oxygen to the extremely large dead zones in the Baltic Sea without completely changing the Baltic Sea ecosystem. The best long-term solution is to implement policies and measures to reduce the load of nutrients to the Baltic Sea.
the Baltic is now home to seven of the of the world’s ten largest marine “dead zones”—areas where the sea’s oxygen has been used up by seabed bacteria that decompose the raining mass of dead algae.