Multiple threats to the health of the Baltic Sea

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Multiple threats to the health of the Baltic Sea

Monday, 15 June 2009 19:32

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Baltic Sea. Credits: Wikipedia The Baltic Sea is one of many marine areas facing manmade and natural pressures. Recent changes in the ecosystem have caused serious losses in ecological and economic resources. According to a new study, improved management is needed to restore a healthy ecosystem in the Baltic Sea. This includes closing fisheries to allow cod stocks to recover.

This research suggests that both climate and manmade changes triggered a change in ecological state, or regime shift, in the Baltic Sea in the late 1980s and early 1990s.

The deepwater of the Central Baltic is heavily influenced by the inflow of saline (salty) and oxygenated water from the North Sea. In the 1980s, these inflows were reduced, possibly through increased rainfall and land runoff caused by rising temperatures. Consequently, salinity and oxygen levels dropped.

During the same period, low levels of oxygen at increasingly larger seabed areas increased nutrient levels through the natural release of accumulated nutrients from seabed sediment. In addition, manmade nutrient inflows to the sea elevated these levels, causing severe eutrophication problems in the Central Baltic Sea.

Excessive fishing during the 1980s had weakened the Baltic ecosystem and cod populations diminished. Low salinity and oxygen conditions reduced the number of plankton, necessary for the survival of young cod. In response, sprat came to be the dominant fish species. In the absence of predatory cod, sprat populations flourished.

A strong inflow of water from the North Sea, together with reduced temperatures due to more favourable atmospheric forcing, improved conditions in 1993. While physical and chemical conditions in the Baltic Sea are returning to their original state, the biological state continues to decline. This has been mainly caused by continued overfishing, which has not allowed cod to take advantage of the more favourable conditions. Sprats are also effective predators of zooplankton, including cod eggs, which may make it even harder for cod to recover.

Nutrient inputs need to be reduced to keep the Baltic deepwater healthy and fisheries need to be closed to give cod stocks time to recover.

(From environmental-expert.com)