Food Web
It is difficult to characterize the historic food web of the San Francisco Estuary because of the dramatic changes in geography, hydrology, and species composition that have occurred in the past century. However, monitoring begun in the 1970s gives some information about the historic dynamics of the foodweb. Prior to the 1980s the LSZ was dominated by a phytoplankton-driven foodweb, a stable mesoplankton population dominated by E. affinis, and large macrozooplankton typified by San Francisco bay shrimp and mysids These provided nutrition and energy to native filter feeders such as the northern anchovy (Engraulis mordax), and planktivores such as delta smelt and juvenile salmon.
Food web change has been driven historically by increased turbidity, and more recently by introduced species, as described in the sections on primary and secondary production.
Notably, the high clearance rate of the introduced Amur River clam population has produced a ten-fold decline in plankton density, resulting in a carbon trap in the benthos and an assumed increase in waste detrital production. This waste is hypothesized to fuel the microbial loop, resulting in an increase in microzooplankton such as L. tetraspina, which utilize rotifers and ciliates.
These changes are one cause for declining fish stocks. For example, the northern anchovy, Engraulis mordax, was until the 1980s quite abundant in the Low Salinity Zone, until its range in the Estuary became restricted to the Central and South Bays. This is probably due to a behavioral response following the introduction of the Amur River clam and the subsequent decline in plankton availability.
More recently, a general Pelagic Organism Decline (POD) was described, and this has been the source of much concern within the scientific, managerial, and political communities.
Several key species, including delta smelt, longfin smelt, striped bass, and threadfin shad have been declared "species of interest" because of a stepwise decline in abundance beginning in 2001. This was attended by a similar decline in secondary productivity and is currently the source of much research. A number of hypotheses have been proposed to explain the POD, including food web decline, water exports from the Delta, and toxics from urban, industrial, or agricultural sources.
Read more about this topic: Ecology Of The San Francisco Estuary
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