Research Product

Bacterioplankton abundance and metabolic characteristics were observed 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 b-D-glucosidase), and bacterial abundance using 1-µm filters to separate free from particle-associated bacteria. A seasonal decline in all bacterial metabolic measures was observed 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. A sharp decline in particle-associated bacteria was also observed in Suisun Bay and the Sacramento River between July and August. This decline may have been due to combined effects of declining nutritive value of the aging particles and increasing grazing pressure by filter feeders. Aminopeptidase activity was positively related with increasing salinity, and b-D-glucosidase was negatively correlated with 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 mean proportion of particle-associated bacteria (49%), followed by Sacramento River (36%) and Central Bay (11%). Particles were the sites of enhanced ecotoenzyme activity but not amino acid incorporation. Bacteria may be actively dissolving the particulate organic matter, but their growth rates on particles are not significantly enhanced.