Detritus fuels ecosystem metabolism but not metazoan food webs in San Francisco estuary's freshwater delta |
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| Authors: | William V Sobczak James E Cloern Alan D Jassby Brian E Cole Tara S Schraga Andrew Arnsberg |
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| Institution: | 1. U.S. Geological Survey, 94025, Menlo Park, California
2. Department of Environmental Science and Policy, University of California, 95616, Davis, California
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| Abstract: | Detritus from terrestrial ecosystems is the major source of organic matter in many streams, rivers, and estuaries, yet the
role of detritus in supporting pelagic food webs is debated. We examined the importance of detritus to secondary productivity
in the Sacramento and San Joaquin River Delta (California, United States), a large complex of tidal freshwater habitats. The
Delta ecosystem has low primary productivity but large detrital inputs, so we hypothesized that de tritus is the primary energy
source fueling production in pelagic food webs. We assessed the sources, quantity, composition, and bioavailability of organic
matter among a diversity of habitats (e.g., marsh sloughs, floodplains, tidal lakes, and deep river channels) over two years
to test this hypothesis. Our results support the emerging principle that detritus dominates riverine and estuarine organic
matter supply and supports the majority of ecosystem metabolism. Yet in contrast to prevailing ideas, we found that detritus
was weakly coupled to the Delta's pelagic food web. Results from independent approaches showed that phytoplankton production
was the dominant source of organic matter for the Delta's pelagic food web, even though primary production accounts for a
small fraction of the Delta's organic matter supply. If these results are general, they suggest that the value of organic
matter to higher trophic levels, including species targeted by programs of ecosystem restoration, is a function of phytoplankton
production. |
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