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ABSTRACT: Data collected in the San Francisco Bay estuary over the last 65 years show that numbers of macrofaunal species are greatest in the marine environment of the central region near San Francisco, decreasing toward the north and south. This distribution has traditionally been attributed to differences in absolute values of salinity and sediment texture. Recent studies of both the benthos and the physicochemical environment near the substrate suggest that species distribution is more related to temporal variation in salinity and to intermittent disturbance of bottom sediments by storm-generated and seasonal wind waves and by the seasonally alternating high and low river inflow. Physical disturbance of the substrate apparently contributes to a state of non-equilibrium in the benthic community especially in the shallow reaches: the community, dominated by colonizers, reflects an early stage of species succession. Some of the most successful species under these conditions are those introduced from other estuaries.
Maximum values of total benthic biomass, in contrast to numbers of species, are found in South Bay, probably reflecting reduced salinity variability, somewhat greater stability of subtidal sediments, and the large quantities of potential food (high sewage-waste loadings, high concentrations of suspended particulate matter, and moderate to high standing stock of primary producers) resulting from shallow depth and the absence of strong water circulation. High biomass can also be attributed to the successful establishment of several large and abundant introduced species that thrive in South Bay.
Although once apparent as a reduction of numbers of species, the effect of waste disposal on the benthos is now often masked by natural perturbations resulting from biotic and abiotic disturbances of surficial sediments and by inhomogeneous distribution of the animals. Anthropogenic influences on benthic community structure other than that resulting from the introduction of exotic species will become increasingly difficult to quantify and therefore to predict. Future changes in the biota may be expected with continued reduction in fresh water flow into the estuary.
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