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. A sudden increase in surface temperature caused by changes in atmospheric forcing (corresponding to a warming effect) in the late 1980s combined with depleted cod stocks, the result of unsustainable fishing practices, were sufficient to tip the ecosystem over to another regime or ecological state.
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.
Restoring the Baltic Sea to the state that supports commercially important cod stocks will be expensive. It needs to be restored to beyond the point were the system flipped from one state to another. A resilient ecosystem is more stable and better able to absorb and recover from shocks. A better understanding of the system's dynamics can help us understand how it can be managed to maintain its strength.
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. In addition, the researchers suggest early warning systems could be set up to monitor changes in the physical state of the Baltic Sea as well as changes in the food web.