MICROZOOPLANKTON GRAZING IN NORTHERN SAN-FRANCISCO BAY MEASURED BY THE DILUTION METHOD

Microzooplankton (defined herein as phagotrophs <200 mu m) grazing was studied in northern San Francisco Bay, California, USA, from Septembe r 1993 to July 1994 using the dilution method. The focus of the study was the estuarine turbidity maximum zone of northern San Francisco Bay (Suisun Bay), though some experiments were also performed in southern San Francisco Bay and Tomales Bay. Three microzooplankton prey items were examined: phytoplankton las chlorophyll a), cyanobacteria, and ba cterioplankton (i.e. non-chlorophyll containing bacteria). Statistical ly significant grazing rates were observed in 5 of 31 northern San Fra ncisco Bay experiments over 7 dates spanning 10 mo. Average grazing ra tes on phytoplankton, cyanobacteria and bacterioplankton were 0.06, 0. 00, and 0.22 d(-1) respectively. Grazing rates were statistically sign ificant in 5 of 7 southern San Francisco Bay experiments on 3 dates. A verage grazing rates on phytoplankton and cyanobacteria were 0.41 and 1.84 d(-1) respectively. Grazing rates were statistically significant in 4 of 4 Tomales Bay experiments performed on 1 date, averaging 0.69 and 0.75 d(-1) on phytoplankton and bacterioplankton respectively. The low grazing rates in northern San Francisco Bay suggest that microzoo plankton may not play a pivotal role in controlling prey biomass, alth ough this interpretation is complicated by the possibility that one or more of the dilution method assumptions may not hold in this environm ent. We hypothesize that the introduced Asian clam Potamocorbula amure nsis may be controlling the biomass of microzooplankton, their prey, o r both. A review of the literature indicated that the dilution method regularly yields statistically non-significant grazing rates, suggesti ng that our results are not unique.