Distribution patterns of zooplankton in Tomales Bay,California

Spatial patterns of abundance of the zooplankton of Tomales Bay, California, were studied over one year from August 1987 to September 1988. Samples were taken on six transects up the long axis of the bay, and the species composition and abundance of common species were determined. Distribution patterns were similar to those observed in other estuaries and bays, with species from nearby neritic waters occurring in the outer bay and a few resident species in the inner bay. This pattern may be best explained by size-selective predation within the bay. Most alternative explanations can be ruled out for Tomales Bay, except for possible temperature effects on cool-temperate neritic species. The four common species ofAcartia in Tomales Bay were in two subgenera, each of which included a neritic species and a smaller inner-bay species. The occurrence of the smaller of each pair in the inner bay, which has been observed forAcartia and other species in other estuaries and bays, may also be a result of size-selective predation.