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Photo by Cheryl Reynolds
The unique ecological characteristics of estuaries support exceptional wildlife communities. Estuary “mixing zones,” the location in the estuary at which fresh and salt water meet and mix, produce abundant tiny aquatic plants call phytoplankton. These phytoplankton form the foundation of a complex food chain. Clams, oysters, and other invertebrates in the Bay bottom feed on the phytoplankton, as do small fish. These creatures are in turn preyed upon by larger fish, as well as by birds and mammals. The San Francisco Bay Estuary was so large that the richness of the ecosystem it supported was staggering. Tens of millions of migratory, or anadromous, fish passed through the Estuary’s waters each year on their journey from river to sea and back again. These included several species of salmon that we still see today. Similarly, because of the Estuary’s location along the Pacific Flyway, millions of migrating ducks, geese, and shorebirds spent winters along the shores, having journeyed south from arctic breeding grounds.
Because of the exceptional conditions created by the Estuary’s setting, plants and animals that are found nowhere else on earth evolved here. The California clapper rail and the salt marsh harvest mouse are two examples. Although the rail and mouse are endangered species, some creatures that were extirpated from the Bay's edges are beginning to return. Thanks to the efforts of groups like the Friends of Alhambra Creek and Worth A Dam, beavers are making a comeback in the watershed near the edge of the Estuary on the Carquinez Strait. These interesting mammals are considered a keystone species and often attract other animals to an area. Although it was once thought that they only lived along flowing streams, we now know that some use tidal marshes.
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