University of California Water Resources Center

We examined bacterioplankton abundance and metabolic characteristics in northern San Francisco Bay, California during spring and summer 1996 at three sites: Central Bay, Suisun Bay and the Sacramento River. These sites spanned a salinity gradient from marine to freshwater, and sampling occurred during a period of seasonally declining river flow. The microbial measures included radio-labeled amino acid uptake (L-leucine, L-proline, L-serine), ectoenzyme activity (aminopeptidase and 13-D-glucosidase), and bacterial abundance using lum filters to separate free from particle-associated bacteria. We observed a seasonal decline in all bacterial metabolic measures at all stations, suggesting that a system-wide variable may be important in controlling bacterial activity. One such variable is freshwater flow into the Bay (as a proxy for organic matter flux), which positively covaried with all metabolic measures. We also observed a sharp decline in particle-associated bacteria in Suisun Bay and the Sacramento River between July and August. This may be due to combined effects of declining nutritive value of the aging particles, and increasing grazing pressure by benthic filter feeders. Aminopeptidase activity was positively related to increasing salinity and 13-D-glucosidase was negatively related to increasing salinity, indicating a gradient in the relative quality of organic matter from carbohydrate-rich riverine to protein-rich oceanic materiaL Overall, Suisun Bay had the highest proportion of particle-associated bacteria (avg. 49%), followed by Sacramento River (avg, 36%) and Central Bay (avg. 11%), Particles were the sites of enhanced ectoenzyme activity, but not amino acid incorporation. Therefore bacteria may be actively dissolving the particulate organic matter but their growth rates are not significantly enhanced on particles.