Threshold effects in semi-enclosed marine systems

摘要: Th e natural history of the twentieth century documents dramatic changes in many eco-logical systems, mostly due to infl uence expansion human civilization. Semi-enclosed marine systems (SEMSs) are no exception this. Here is an example, from Jannson and Dahlberg (1999):In 1940s, Baltic Sea was a nutrient-poor sea with low biological produc-tion, clear water, rocky shores dense growths brown seaweed blad-derwrack, providing food shelter for species, including spawning nursery grounds fi sh. ere suffi cient oxygen bottom water cod spawn deep areas Proper, except periods oxy-gen depletion Gotland Deep. Top consumers like seal eagle were common people living around could eat sh without risking their health. today diff erent. Eutrophication toxic sub-stances now aff ect entire ecosystem, even off shore areas. Fila-mentous green algae shade bladderwrack may totally replace it. Increased plankton blooms organic particle production [have] low-ered light penetration by 3 m hydrogen sulfi de forma-tion sometimes dominate as much one third total area. To reduce nutrient load levels reduction 65% phosphorus 80% nitrogen needed.