Microzooplankton grazing in northern San Francisco Bay measured by the dilution method

Abstract

Mlcrozooplankton (defined hereln as phagotrophs <200 pm) grazing was studied in northern San Francisco Bay, California, USA, from September 1993 to July 1994 using the dilution method The focus of the study was the estuarine tui bldity maxilnum zone of northern San Francisco Bay (Suisun Bay), though some experments were also perfoimed In southern San Francisco Bay a n d Tolnales Bay Three microzooplankton prey Items were e x a m ~n e d phytoplankton (as chlorophyll a), cyanobactena and bactenoplankton (l e non-chlorophyll containing bactena) Stat~stically s ~g n i f ~c a n t grazing rates were observed in 5 of 31 northern San Francisco Bay expenments over 7 dates spanning 10 mo Average grazing rates on phytoplankton, cyanobacteria a n d bactenoplankton were 0 06, 0 00, and 0 22 d l respectively Grazlng rates were statistically significant In 5 of 7 southern San Franc~sco Bay expenments on 3 dates Average grazing lates on phytoplankton and cyanobactena were 0 41 and 1 84 d I respectively Grazing iates were statistically significant In 4 of 4 Tomales Bay experiments performed on 1 date, averaging 0 69 and 0 75 d.' on phytoplankton and bacterioplankton respectively The low grazlng ~a t e s in northern San Francisco Bay suggest that microzooplankton may not play a pivotal role in controlling pley biomass although this interpretation is coinplicated by the possibility that one or more 01 the dilutlon method assumptions may not hold ~n this environment We hypothesize that the introduced Asian clam Potamocolbula a m u r e n s ~s may b e controlling the blomass of microzooplankton, their prey or both A review of the literature Indicated that the ddution method regularly yields statistically non-significant grazing rates, suggesting that our results are not unique