Threshold Effects in Semi-Enclosed Seas

VAN DE KOPPEL, J.; TETT, P.; NAQVI, W; OGUZ, T.; PERILLO, G.M.E.; RABALAIS, N.; RIBERA DALCALÀ, M.; JILAN, S.; ZHANG, J.

Watersheds, Bays, and Bounded Seas: The Science and Management of Semi-Enclosed Marine Systems

Island Press

Lugar: Washington, DC; Año: 2009; p. 31 - 47

Th e natural history of the twentieth century documents dramatic changes in many ecologicalsystems, mostly due to the infl uence of the expansion of human civilization.Semi-enclosed marine systems (SEMSs) are no exception to this. Here is an example,from Jannson and Dahlberg (1999):In the 1940s, the Baltic Sea was a nutrient-poor sea with low biological production,clear water, and rocky shores with dense growths of the brown seaweed bladderwrack,providing food and shelter for many species, including spawning andnursery grounds for many fi sh. Th ere was suffi cient oxygen in the bottom waterfor cod to spawn in the deep areas of the Baltic Proper, except for periods of oxygendepletion in the Gotland Deep. Top consumers like seal and sea eagle werecommon and people living around the Baltic Sea could eat fi sh without riskingtheir health. Th e Baltic Sea of today is diff erent. Eutrophication and toxic substancesnow aff ect the entire Baltic Sea ecosystem, even the off shore areas. Filamentousgreen and brown algae shade the bladderwrack and may even totallyreplace it. Increased plankton blooms and organic particle production [have] loweredlight penetration by 3 m and oxygen depletion and hydrogen sulfi de formationsometimes dominate as much as one third of the total bottom area. To reducethe nutrient load to the levels of the 1940s, a reduction by 65% for phosphorus and80% for nitrogen is needed. . . .