Central San Francisco Bay suspended-sediment transport processes study and comparison of continuous and discrete measurements of suspended-solids concentrations

Sediments are an important component of the San Francisco Bay estuarine system. Potentially toxic substances, such as metals and pesticides, adsorb to sediment particles (Kuwabara and others, 1989; Domagalski and Kuivila, 1993). Sediments on the bottom of the bay provide the habitat for benthic communities that can ingest these substances and introduce them into the food web (Luoma and others, 1985). Nutrients, metals, and other substances are stored in bottom sediments and pore water in which chemical reactions occur and which provide an important source andor sink to the water column (Hammond and others, 1985; Flegal and others, 199 1). The transport and fate of suspended sediment is an important factor in determining the transport and fate of the constituents adsorbed on the sediment. Seasonal changes in sediment erosion and deposition patterns contribute to seasonal changes in the abundance of benthic macroinvertebrates (Nichols and Thompson, 1985). Tidal marshes are an ecologically important habitat that were created and are maintained by sedimentation processes (Atwater and others, 1979). In Suisun Bay, the maximum suspended-sediment concentration marks the position of the turbidity maximum, which is a crucial ecological region in which suspended sediment, nutrients, phytoplankton, zooplankton, larvae, and juvenile fish accumulate (Peterson and others, 1975; Arthur and Ball, 1979; Kimmerer, 1992; Jassby and Powell, 1994). Suspended sediments confine the photic zone to the upper part of the water column, and this limitation on light availability is a major control on phytoplankton production in San Francisco Bay (Cloern, 1987; Cole and Cloern, 1987). Suspended sediments also deposit in ports and shipping channels, which must be dredged to maintain navigation (U.S. Environmental Protection Agency, 1992). The objectives of the Central San Francisco Bay suspended-sediment transport-processes study are to estimate which factors determine suspended-solids concentrations (SSC) in Central Bay and to collect time series of SSC data that are appropriate for (1) continuous monitoring of SSC and (2) calibration and validation of numerical models. Potentially important factors include semidiurnal and diurnal tides, the springheap cycle, delta discharge, dredging and dredged material disposal, and wind waves. SSC monitoring sites were established at Point San Pablo in December 1992 and at San Francisco Pier 24 in May 1993 (Figure 1) (Buchanan and Schoellhamer, 1995). At each site, optical backscatterance (OBS) sensors are positioned at mid-depth and near the bottom. The OBS sensors optically measure the amount of material in the water every 15 minutes, and the output of the sensors is converted to SSC with calibration curves developed from analysis of water samples. The sites are serviced every 1 to 5 Lveeks to clean the sensors, which are susceptible to biological fouling, and to collect water samples for sensor calibration. About half the data collected is invalid, primarily because of sensor fouling. SSC monitoring sites also are located in South San Francisco Bay and Suisun Bay (Figure 1). The sites are operated in cooperation with the U.S. Army Corps of Engineers (Central Bay): the California Regional Water Quality Control Board. San Francisco Bay Region, as part of the Bay Protection and Toxic Cleanup Program (South Bay); and the Interagency Ecological Program (Suisun Bay). Continuous SSC data can be used to help place the discrete water-quality data collected as part of the Regional Monitoring Program (RMP) into a proper context. Vertical profiles of SSC were collected with an