The importance of algal and detrital food supplies to the planktonic food web of a highly disturbed, estuarine ecosystem was evaluated in response to declining zooplankton and fish populations. We assessed organic matter bioavailability among a diversity of habitats and hydrologic inputs over 2 years in San Francisco Estuary's Sacramento-San Joaquin River Delta. Results show that bioavailable dissolved organic carbon from external riverine sources supports a large component of ecosystem metabolism. However, bioavailable particulate organic carbon derived primarily from internal phytoplankton production is the dominant food supply to the planktonic food web. The relative importance of phytoplankton as a food source is surprising because phytoplankton production is a small component of the ecosystem's organicmatter mass balance. Our results indicate that management plans aimed at modifying the supply of organic matter to riverine, estuarine, and coastal food webs need to incorporate the potentially wide nutritional range represented by different organic matter sources.

We examined the relative nutritional values of natural phytoplankton and particulate detritus for zooplankton growth in a detritus‐rich environment. Seston was collected seasonally from four different habitat types in a tidal freshwater system and fed to juvenile Daphnia magna under controlled culture conditions by use of a flow‐through design. Seston particulate organic carbon (POC) and chlorophyll a contents ranged from ~330 to 3,800 µg L-1 POC and 1.4 to 45 µg L-1 Chl a. A partial residual analysis revealed that detrital carbon concentrations were only weakly related to Daphnia growth, whereas Chl a proved to be highly predictive of Daphnia growth rates across all investigated habitat types. Overall, habitat type had a strong effect on growth rates, whereas season of seston collection did not, but differences among habitats could be attributed to differing Chl a concentrations. The results from this study imply that, even in systems with overwhelming amounts of detrital carbon from a variety of sources, nutritional factors associated with phytoplankton can be dominant in regulating zooplankton growth.

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We analyzed a 42‐yr record of primary productivity in small, subalpine Castle Lake to determine how climatic variability might influence lake primary productivity. A Pacific Decadal Oscillation (PDO) polarity reversal in 1977 significantly affected winter air and summer water temperatures in Castle Lake. The timing of lake ice‐out was explained by spring air temperature and winter total precipitation (r2 = 0.72) and significantly affected water temperature (r2 = 0.74). Primary productivity was negatively correlated with ice‐out date and positively correlated with primary productivity during the previous year (r2 = 0.47). Alternatively, primary productivity was positively correlated with water temperature and primary productivity during the previous year (r2 = 0.49). Ammonium availability immediately after ice‐out was significantly related to primary productivity from the previous and the current year, suggesting that nutrient availability is an important mechanism for the serial correlation. Daphnia and cyanobacteria biomass also increased during warmer years. Our results suggest that variability in air temperature and precipitation from global warming, PDO, and the El Niño Southern Oscillation (ENSO) influence primary productivity and plankton communities in North American dimictic lakes.