Microbial Processes in the San Francisco Bay Estuarine Turbidity Maximum

Abstract

We examined microbial processes and the distribution of particulate materials in the estuarine turbidity maximum (ETM, salinity 2–10 PSS) of northern San Francisco Bay on three cruises during the late spring of 1994 (low flow: April 19, April 28, May 17) and two cruises during the early summer of 1995 (high flow; June 13, July 18). Under low flow conditions, chlorophyll concentrations decreased by a factor of 2–4, bacterial abundance decreased by 20%, and L-leucine incorporation rate decreased by a factor of about 2 over a salinity range of 0–2 PSS, then remained relatively constant at higher salinities. Over this same salinity range under high flow conditions, chlorophyll concentration was c. twofold lower, bacterial abundance was c. threefold higher, and L-leucine incorporation rate was in the same range as during low flow. Under high flow conditions, chlorophyll concentration increased by 20%., bacterial abundance decreased by a factor of 2, and L-leucine incorporation rate decreased by half (June 13) or remained unchanged (July 19) with increasing salinity. Under low flow conditions the concentration of suspended particulate material (SPM), particulate organic carbon (POC), and particulate organic nitrogen (PON) increased 3–10 fold with salinity, to a maximum at intermediate salinities (c. 6 PSS). As a result, the contribution of phytoplankton to POC decreased from a maximum of 32% in fresh water to c. 6% in the ETM. The contribution of bacterial biomass similarly decreased from 5% in fresh water to 0.8% in the ETM. The C:N ratio of particulate material increased from 12 in the ETM. High variability in abundance estimates confounded analysis of patterns in bacterial biomass partitioning between particle-associated and free-living fractions along the salinity gradient. However, the partitioning of L-leucine incorporation shifted dramatically from being predominantly by free-living cells in fresh water to being predominantly by particleassociated populations in the ETM. The metabolic fate of thymidine taken up differed, between particle-associated and free-living bacteria, suggesting some metabolic divergence of these assemblages.