August 1, 2013.
Declines in the abundance of several pelagic fish species in the upper San Francisco Estuary have prompted investigation into food web interactions within the estuary and delta (the lower Sacramento and San Joaquin Rivers). This area is characterized by low primary production and pelagic food webs much longer and reticulated than previously thought, implying low efficiency in the energy transfers from primary producers to planktivorous fish. We determined the carbon and nitrogen stable isotope (SI) composition of zooplankton samples collected monthly between June 2012 and February 2013 at eight stations along the salinity gradient. As consumers SI composition reflects that of their food, we use it to identify the sources of organic matter and to describe trophic relationships among the different species. Our preliminary results indicate that most species have SI compositions consistent with a diet based on phytoplankton in June and July and that they do not seem to consume Microcystis (toxic cyanobacteria developing in large blooms in the low salinity/freshwater part of the system). Nevertheless, the overlap between phytoplankton SI composition and the one expected for vascular/C3 plants prevents us from definitely ruling out organic detritus-bacteria-microzooplankton-zooplankton as an alternative trophic pathway.
Biogeochemistry | Marine Biology | Other Environmental Sciences | Terrestrial and Aquatic Ecology
Romberg Tiburon Center for Environmental Studies (RTC)
This material is based upon work supported by the S.D. Bechtel, Jr. Foundation and by the National Science Foundation under Grant No. 0952013 and Grant No. 0833353. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the S.D. Bechtel, Jr. Foundation or the National Science Foundation. This project has also been made possible with support of the National Marine Sanctuary Foundation. The STAR program is administered by the Cal Poly Center for Excellence in Science and Mathematics Education (CESaME) on behalf of the California State University (CSU).