Toxic Substances Hydrology Program
Climate-Driven Ocean Changes Affect Estuaries
Pacific Ocean Cooling Triggers Phytoplankton Blooms in San Francisco Bay
Long-term studies by U.S. Geological Survey (USGS) scientists have found that a cooling in ocean temperatures led to increased phytoplankton blooms and red tides in San Francisco Bay, California. The declining temperatures took place off the coast of California between 1999 and 2004.
This is a surprising result because scientists and water-resource managers normally associate phytoplankton blooms with increases in the amount of nutrients, such as nitrogen and phosphorus, entering estuaries from such sources as wastewater treatment plants and runoff from agricultural fields. In this case, the phytoplankton blooms in the Bay occurred during a period of decreasing nutrient concentrations and inputs.
The scientists discovered the effects of the cold Pacific temperatures by using water-quality and biological data collected over 25 years. The colder temperatures caused changes in the types, abundance, and migration patterns of marine life in the San Francisco Bay and costal ocean waters. The drop in temperature caused marine life, such as fish, shrimp, and crabs, to migrate to warmer waters, like San Francisco Bay.
The migrations caused an increase in the numbers of predators, such as Bay shrimp and Dungeness cabs, that eat filter feeders, such as clams. Clams can filter large quantities of phytoplankton from the Bay’s water, which can prevent phytoplankton blooms. With the increase in predators, there was a corresponding decrease in clam populations and an increase in the amount of phytoplankton.
Results of this study show that:
Water-resource managers tasked with implementing programs to restore or protect estuaries can use these results to help develop more effective policies concerning phytoplankton blooms and nutrient enrichment in estuaries.
Cloern, J.E., Jassby, A.D., Thompson, J.K., and Hieb, K.A., 2007, A cold phase of the east pacific triggers new phytoplankton blooms in San Francisco Bay: Proceedings of the National Academy of Sciences of the United States of America, v. 104, no. 47, p. 18561-18565, doi:10.1073/pnas.0706151104.
Cloern, J.E., Jassby, A.D., Schraga, T.S., and Dallas, K.L., 2006, What is causing the phytoplankton increase in San Francisco Bay?, in The Pulse of the Estuary -- Monitoring and managing water quality in the San Francisco Estuary: San Francisco Estuary Institute Annual Report 2006, p. 62-70.