Regime Shifts DataBase

Large persistent changes in ecosystem services

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.