Summer oxygen levels are declining in some parts of the North Sea, probably because of ocean warming and the decay of photosynthetic blooms that form as a result of nutrient influx.

Bastien Queste at the University of East Anglia in Norwich, UK, and his team compared the results of an oceanographic field survey conducted in August 2010 with twentieth-century records of oxygen concentrations in the North Sea. They found that the intensity of seasonal oxygen depletion in highly stratified regions — where there is little mixing between layers of water of different temperature — has increased markedly since 1990.

In 2010, dissolved oxygen in the central North Sea and in an area known as the Oyster Grounds near the Dutch coast came close to ecologically critical values that, if reached, would require management action under the European Union's Water Framework Directive, the team reports.

Biogeochemistry http://dx.doi.org/10.1007/s10533-012-9729-9 (2012)