Four guilds from a lake sediment-water interface microbial community were isolated and tested for sensitivity to cycloheximide (0.1 to 200 mg liter-1). Field experiments were conducted to compare the inhibition, dilution, and filtration methods for determining grazing rates. Cycloheximide inhibited anaerobic bacteria at 50 mg liter-', and inhibition of bacterial growth was observed in the grazing experiments. The results show that the assumption of selective inhibition of heterotrophic eucaryotes was violated and preclude the use of cycloheximide in grazing experiments. Chemical inhibition of protozoan bacterivores has been used by many researchers to quantify grazing mortality of bacteria (4, 5, 9, 14, 19-23, 25). The most common inhibitor is cycloheximide used at a concentration of 100 mg-liter-' (9, 20-22). It was initially reported that cycloheximide had no effect on bacteria (7); however, Taylor and Pace (25) reported the inhibition of bacterial growth in the epipelagic region of the ocean. We examined the effects of cycloheximide on the microbial community from the sediment-water interface of a lake. The results of the experiments show that cycloheximide inhibited nontarget, anaerobic bacteria, violating the assumption of selective inhibition of heterotrophic eucaryotes.

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The critical assumptions of the dilution method for estimating grazing rates of microzooplankton were tested by using a community from the sediment-water interface of Lake Anna, Va. Determination of the appropriate computational model was achieved by regression analysis; the exponential model was appropriate for bacterial growth at Lake Anna. The assumption that the change in grazing pressure is linearly proportional to the dilution factor was tested by analysis of variance with a lack-of-fit test. There was a significant (P < 0.0001) linear (P > 0.05) relationship between the dilution factor and time-dependent change in In bacterial abundance. The assumption that bacterial growth is not altered by possible substrate enrichment in the dilution treatment was tested by amending diluted water with various amounts of dissolved organic carbon (either yeast extract or extracted carbon from lake sediments). Additions of carbon did not significantly alter bacterial growth rates during the incubation period (24 h). On the basis of these results, the assumptions of the dilution method proved to be valid for the system examined.

Although the impact of acidification on planktonic grazer food webs has been extensively studied, little is known about microbial food webs either in the water column or in the sediments. Protozoon-bacterium interactions were investigated in a chronically acidified (acid mine drainage) portion of a lake in Virginia. We determined the distribution, abundance, apparent specific grazing rate, and growth rate of protozoa over a pH range of 3.6 to 6.5. Protozoan abundance was lower at the most acidified site, while abundance, in general, was high compared with other systems. Specific grazing rates were uncorrelated with pH and ranged between 0.02 and 0.23 h-1, values similar to those in unacidified systems. The protozoan community from an acidified station was not better adapted (P = 0.95) to low-pH conditions than a community from an unacidified site (multivariate analysis of variance on growth rates for each community incubated at pHs 4, 5, and 6). Both communities had significantly lower (P < 0.05) growth rates at pHs 4 and 5 than at pH 6. Reduced protozoan growth rates coupled with high grazing rates and relatively higher bacterial yields (ratio of bacterial-protozoan standing stock) at low pH indicate reduced net protozoan growth efficiency and a metabolic cost of acidification to the protozoan community. However, the presence of an abundant, neutrophilic protozoan community and high bacterial grazing rates indicates that acidification of Lake Anna has not inhibited the bacterium-protozoon link of the sediment microbial food web.

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