Short-Term and Interannual Variability in Primary Production in the Low-Salinity Zone of the San Francisco Estuary

We measured primary production during spring???\ud summer 2006???2007 to determine the carbon supply to the\ud low-salinity pelagic food web of the San Francisco Estuary\ud (SFE). Weekly or biweekly samples were taken at three\ud stations of fixed salinity for size-fractionated primary production\ud and biomass, both as chlorophyll and from biovolume\ud based on counts. Error variance in productivity\ud estimates arose mainly from the depth integration of 14C\ud uptake, showing the importance of productivity measurements\ud at high light levels for estimates of depth-integrated\ud production. Temporal and spatial variability in production\ud were surprisingly small. Combining data from this study with\ud long-term monitoring data, productivity and biomass were\ud variable in time and salinity but without persistent patterns\ud and with infrequent blooms. Production within the lowsalinity\ud zone was unresponsive to variation in freshwater flow,\ud in contrast to findings in other estuaries where nutrient loading\ud drives variability in production and other regions of the SFE\ud where production responds to residence time or to stratification.\ud Estimated annual primary production was only 25\ud and 31 gC m???2 year???1 during 2006 and 2007, only half of it in\ud cells >5 ??m. These results imply that phytoplankton provided\ud poor food web support for higher trophic levels, probably\ud contributing to the long-term decline in fish abundance in\ud the brackish to freshwater region of the estuary