Shifts in Zooplankton Community Structure: Implications for Food Web Processes in the Upper San Francisco Estuary |
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| Authors: | Monika Winder Alan D Jassby |
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| Institution: | 1.John Muir Institute of the Environment, Tahoe Environmental Research Center,University of California,Davis,USA;2.Leibniz-Institute of Marine Sciences at Kiel University (IFM-GEOMAR),Kiel,Germany;3.Department of Environmental Science and Policy,University of California,Davis,USA |
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| Abstract: | Zooplankton are an important trophic link and a key food source for many larval fish species in estuarine ecosystems. The
present study documents temporal and spatial zooplankton dynamics in Suisun Bay and the Sacramento–San Joaquin Delta—the landward
portion of the San Francisco Estuary (California, USA)—over a 37-year period (1972–2008). The zooplankton community experienced
major changes in species composition, largely associated with direct and indirect effects of introductions of non-native bivalve
and zooplankton species. A major clam invasion and many subsequent changes in zooplankton abundance and composition coincided
with an extended drought and accompanying low-flow/high-salinity conditions during 1987–1994. In the downstream mesohaline
region, the historically abundant calanoid copepods and rotifers have declined significantly, but their biomass has been compensated
to some extent by the introduced cyclopoid Limnothoina tetraspina. The more upstream estuary has also experienced long-term declining biomass trends, particularly of cladocerans and rotifers,
although calanoid copepods have increased since the early 1990s due to the introduced Pseudodiaptomus spp. In addition, mysid biomass has dropped significantly throughout the estuary. Shifts in zooplankton species composition
have also been accompanied by an observed decrease in mean zooplankton size and an inferred decrease in zooplankton food quality.
These changes in the biomass, size, and possibly chemical composition of the zooplankton community imply major alterations
in pelagic food web processes, including a drop in prey quantity and quality for foraging fish and an increase in the importance
of the microbial food web for higher trophic levels. |
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