A Loss of Bivalves

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Science  07 Dec 2007:
Vol. 318, Issue 5856, pp. 1525-1527
DOI: 10.1126/science.318.5856.1525d

The potential for ecosystems to shift abruptly from one state to another is becoming increasingly recognized, especially in aquatic environments. Cloern et al. document an unusual and instructive example in one ecosystem, brought about by changes in another, neighboring system. In 1999, the San Francisco Bay—a large lagoon-like estuary on the west coast of the United States—began to experience massive algal blooms for the first time since monitoring had begun more than two decades earlier. Such blooms are normally associated with eutrophication: the enrichment of waters by runoff of excessive nutrients (especially N and P) from agricultural land. However, in this case the nutrient loading of the estuary had been decreasing before the bloom. It appears that the bloom was instead the result of a collapse in the population of the bivalve consumers of the algae. This collapse was brought about by an influx of flatfish and crustacean predators of the bivalves into the estuary from the coastal ocean, which itself had resulted from a physical oceanographic change in the California Current System. Increased coastal upwelling of cold, nutrient-laden waters led to increased oceanic primary production and a bonanza for consumers and their predators, which were recruited in substantial numbers into the neighboring estuary. Hence, the state change in the estuary was caused indirectly by hithertounsuspected connectivity with the ocean. — AMS

Proc. Natl. Acad. Sci. U.S.A. 104, 18561 (2007).

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