variability in the seasonally and interannual patterns of abun¬ 
dance that are most easily ascribed to natural causes; (2) the 
apparently hardy nature of many of the species; and (3) the lack 
of appropriate studies to prove or disprove cause and effect 
(Nichols et. al .. 1986). 
Effects of the Benthos on the Pelagic Food Web 
Because the estuary is shallow and the water in it well 
mixed, phytoplankton (microscopic single-celled plants growing 
in the water column that form the base of aquatic food webs) are 
directly available to benthic animals that filter food particles 
out of the water. Because of their great abundance, benthic fil¬ 
ter feeders may act as a natural biological control on eutrophi¬ 
cation -- the growth of nuisance phytoplankton blooms in aquatic 
systems in response to enrichment with nutrients such as nitro¬ 
gen and phosphorus (Cloern, 1982). Eutrophication in estuaries 
often leads to the depletion of oxygen in the water and the sub¬ 
sequent death of aquatic animals. By removing phytoplankton as 
fast as they grow, benthic invertebrates in San Francisco Bay 
convert sewage-derived nutrients directly into animal biomass. 
Thus, the Bay is not subject to noxious accumulations of excess 
phytoplankton (Nichols et. al. 1986). We can conclude from this 
finding that processes that could selectively disturb the ben¬ 
thos, such as severe contamination with pollutants, might permit 
the development of nuisance blooms and anoxia in San Francisco 
Bay. Occasionally, localized, thick mats of decaying macroalgae 
become deposited on intertidal mudflats and smother resident 
benthic animals (Nichols and Thompson, 1985). These occurrences 
are unpredictable and only partially understood. 
Whether the direct removal of phytoplankton by benthic 
filter feeders actually inhibit overall productivity of the 
estuary is not clear. However, during the 1976-77 drought, the 
high salinity of northern San Francisco Bay during two 
successive winters permitted the establishment of large 
populations of benthic animals that, in turn, may have been 
responsible for the unusual declines in phytoplankton, 
zooplankton, shrimp, and larval striped bass (Nichols, 1985). 
The implication from these observations is that during any 
future periods of persistently low flows when winter salinity 
levels remain high, the food web in northern San Francisco Bay 
may shift from pelagic type, culminating in the striped bass, to 
a benthic type, culminating perhaps, in less important demersal 
fish species. 
67 
