BENTHIC ECOLOGY AND HEAVY METAL ACCUMULATION 
Frederick H. Nichols 
U.S. Geological Survey 
Abstract 
The benthos of San Francisco Bay (the comrftunity of inverte¬ 
brates living in bottom sediments) is an important source of 
food for fish, birds, and humans, and is dominated by exotic 
species introduced during the past 130 years. These species are 
largely small, hardy, short-lived, rapidly-reproducing species 
(much like weeds) whose distributions and abundances vary widely 
in both space and time. As a result, they appear resilient in 
the face of both natural and human-induced disturbances. 
The Bay's benthic organisms are contaminated to varying 
degrees and, in some cases, physiologically affected by wastes. 
Contaminant concentrations vary seasonally, annually, and with 
proximity to contaminant sources. There is an apparent but not 
clearly understood relationship between river flow and the 
accumulation of wastes in the estuary's sediments and organisms. 
However, effects at the community level are not easily distin¬ 
guished from natural variability. 
Variations in river flow can have a marked effect on the 
distributions and abundance of benthic animals, thereby affect¬ 
ing local food web dynamics. During periods of persistently low 
flow, for example, benthic invertebrates can become relatively 
more important in the northern part of San Francisco Bay and, as 
a result, compete with the small pelagic animals (that are food 
for fish) for the phytoplankton produced there. The benthos is 
apparently important in preventing eutrophication in San 
Francisco Bay by consuming phytoplankton before it can grow to 
nuisance levels. 
Introduction 
The "benthos" is the community of invertebrate animals 
(worms, clams, shrimps, etc.) living on the bottom of aquatic 
environments. These animals consume organic matter that grows 
on or settles to the bottom and, in turn, become food for fish 
and other consumers, including humans. They are often sessile, 
living most of their life in the same location. Thus, they 
provide a continuing record, through changes in species composi¬ 
tion or abundance or the effects of both short- and long-term 
changes in the environment. This feature had lead to their use 
as indicators of water pollution. 
Introduced Species 
Today, the benthos of San Francisco Bay is composed largely 
of introduced exotic species, many having arrived with the 
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