Regime Shifts DataBase

Large persistent changes in ecosystem services

The Chesapeake Bay is an estuary into which more than 150 rivers and streams drain. Hypoxia was observed already in the 1930s. By the 1970s there was consistent anoxia in the summer months. In the 1980s the hypoxic and anoxic conditions covered most of the bay bottom with year-to-year variation. To some extent, hypoxia in the Chesapeake Bay is natural since the bay has a large catchment area, seasonally stratified water mass and isolated basins. Opinions differ on the degree to which hypoxia has worsened due to eutrophication, but it is clear that hypoxia intensified greatly between the mid-1950s and mid-1980s, which is the period when human population in the Chesapeake Bay watershed nearly doubled and the use of inorganic fertilizers nearly tripled.

Chesapeake Bay is particularly susceptible to dysfunction from eutrophication. Compared with other marine ecosystems, the bay has higher primary production than would be predicted from known nutrient inputs. The size of the bay, material residence times, and tidal and non-tidal circulation lead to a greater recycling and reuse of nutrients. Large quantities of sessile benthic biota die during summer hypoxia and anoxia. Although some species migrate, fall recolonization may fail and cause changes in communal dominance. Migration of fish may also cause fish declines as available food supply and space decline. Species changes in phytoplankton communities have been observed and timing, quality and size of the blooms have changed.

Chesapeake Bay is particularly important as a spawning and nursery ground for many species. Many species that play fundamental ecosystem roles in Chesapeake Bay are in decline, as are several species of key economic importance to the region. Chesapeake Bay is used for commercial shipping, generation of electricity, waste disposal, commercial harvesting of wildlife, recreation and research. In 1987 a commitment was made to reduce controllable sources of nitrogen and phosphorus to combat eutrophication. Nutrient inputs have decreased. 

The East China Sea has faced a huge stress from population growth in the Changjiang river (Yangtze River) drainage basin and the areas along the coasts. Hypoxia was first documented in the early 1980s. In the past two decades, the anthropogenic nutrient load from the Changjiang River has increased over 10-fold and continuous growth is expected in the future. The formation and maintenance of the hypoxia is due to anthropogenic nutrient load through the river and strong stratification. The major source of nutrients is the use of fertilizers in agriculture. It has been suggested that there have been episodic hypoxia for the past 50 years but not every year, and that all events with large size of affected area occurred after the late 1990s.

The East China Sea hypoxia is episodic and sensitive to weather conditions. The Changjiang River is dominated by the East Asia Monsoon causing high flows with large sediment loads (decomposition consumes large quantities of dissolved oxygen) to the sea during summers. Reoccurring typhoons can mix the water and decrease the hypoxic volume. The cold air southward intrusion in the summer can change the wind direction and break the hypoxia. Bottom topography of the East China Sea and inflow of Taiwan Warm Current saline water may also be additional drivers for the hypoxic areas.

East China Sea is one of the world's major fishing grounds.

Guanabara Bay is a semi-enclosed eutrophic, tropical estuarine system surrounded by large urban areas (a.o. Rio de Janeiro) and over 12 million people living in its immediate surroundings. Water exchange is mainly tidally driven through a deep channel. Nutrient inputs to the bay are caused by mostly untreated domestic sewage and industries. Untreated or only a little treated waste together with limited vertical and horizontal mixing have caused extreme eutrophication in Guanabara Bay.

Seasonal hypoxic and anoxic conditions have led to total collapse of coastal bottom ecosystems in several inshore stretches of Guanabara Bay. Anoxic bottom waters in heavily polluted coastal systems also allow heavy metals to be incorporated into bottom sediments (sediment trapping). The large size of Guanabara Bay and the several processes acting at different scales in various parts of the bay cause a lot of spatial and temporal variation, but in general the lowest oxygen values have been found at the western part of the bay probably due to the large input of domestic sewage in this area.

Guanabara Bay has extensive mangrove ecosystems and considerable fisheries for crabs, fish and mollusks. The industrial, semi-industrial and artisanal fishery in and off Guanabara Bay has great local socio-economic importance. Pollution control plan was created already in 1979 but still in 1991 only 15% of the sewage was subjected to any treatment. Water quality situation in Guanabara Bay has become critical. Heavy metals in anoxic sediments may become a health risk if the water quality improves: oxidation of the sediments would release the heavy metals into the food web and thus contaminate sea food for humans.