Abstract-In recent years, populations of resident aquatic species in California's Sacramento-San Joaquin Delta, USA, have declined appreciably in numbers. The cause of these declines is not known, but has been attributed to a number of factors including water diversions, loss of habitat, introduced exotic organisms, and toxic compounds. To detect and characterize the spatial extent, severity, frequency, and causes of potential toxicity caused by anthropogenic pollutants, a monitoring study was conducted over a period of two years (1993)(1994)(1995). Sites were monitored on a monthly basis using the standardized U.S. Environmental Protection Agency freshwater toxicity test with the zooplankton species Ceriodaphnia dubia. Twenty-four sites were sampled in 1993 to 1994. During the 1994 to 1995 sampling season, the number of sampling sites was restricted to 20, with special emphasis placed on back sloughs, delta island agricultural drains, and main-stem river sites. Significant mortality or reproductive toxicity in C. dubia was detected in 9.8% of 400 water samples tested. Ecologically important back sloughs had the largest percentage of toxic samples. Of 71 and 103 samples collected from back sloughs during 1993 to 1994 and 1994 to 1995, respectively, 14.1% and 19.6% were toxic. To determine the causative chemical(s), toxicity identification evaluations (TIEs) were conducted on 23 toxic samples. These included eight follow-up samples taken to determine whether toxicity at the respective site persisted. Organophosphate (chlorpyrifos, diazinon, malathion) and carbamate (carbofuran, carbaryl) pesticides were identified as primary toxicants. Chlorpyrifos was present at toxic concentrations in 87% of samples tested by TIE. Analysis of data from the follow-up samples suggested that toxicity may have persisted over periods of several days to weeks.

This study compared clearance and assimilation of natural bacterioplankton (< 1.2 pm) and cultured phytoplankton by an Asian bivalve, Potamocorbula amurensis. The average clearance rate for bacterioplankton was 45 ml h-l clam-' and was independent of the size (shell length, wet wt including shell, or dry tissue wt) of the clam. The clearance rate for phytoplankton is given byf = 162 + 166 x WW orf = -40 + 199 x L whereJ; WW, and L are clearance rate (ml h-l), wet weight including shell (g), and shell length (cm).Bacteria were readily assimilated by P. amurensis. Gross assimilation was 73% after 49 h compared to 90% for Isochrysis galbana. Net assimilation was 45 and 53% for bacterioplankton and I. galbana, respectively. Bacterial carbon appeared to be respired faster than algal carbon. As seen in other bivalves, feces production increased and assimilation efficiency decreased at higher food concentrations.At the mean bacterioplankton and phytoplankton standing stocks found in northern San Francisco Bay, bacteria supplied -13 and 16% of the sum of bacteria and phytoplankton C and N, respectively, consumed by a 1 -cm P. amurensis. We calculate that a 1 -cm clam could double its C biomass in 22 1 d by feeding on bacterioplankton and in 26 d by feeding on phytoplankton.

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