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

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