A radioactive tracer technique was used to measure meiofaunal grazing on bacteria and diatom communities in natural sediments. Radioactive '4C-glucose and 14C-bicarbonate were used to label heterotrophs (bacteria) and autotrophs (diatoms), respectively. The labeled compounds were added to undisturbed sediment cores and incubated for 4 h at in situ temperatures. After incubation, radioactivity was determined for the sediment (microbes) and the major meiofaunal taxa. To quantify meiofaunal grazing on microbes, a 3-compartment model was used where: available label is not limiting, microbial uptake is linear, and meiofaunal uptake is hyperbolic. The formula to calculate meiofaunal grazing rate (k) is: k = 2f/t, where f = fraction of meiofaunal radioactivity (DPM) relative to microbial radioactivity at time t. Although microbial actlvity was greater in summer than winter, there were no differences between meiofaunal grazing rates in winter and summer Total meiofaunal ingestion of microbes was dominated by polychaetes. Preliminary results suggest that, on the average, 3 % of the bacteria and 1 % of the diatom communities were removed per hour Thus turnover times of approximately 30 h for bacteria and 6.5 d for diatoms are apparently sufficient to maintain the microbial community in steady state under the meiofaunal grazing regime. This meiofaunal grazing pressure (60 pg bacterial C and 27 pg diatom C ingested 10 cm-2 h-') probably represents a significant stimulatory effect on the microbial community.

Disruption of grazer populations, or mismatches between phytoplankton blooms and population dynamics of their planktonic and benthic grazers, may play an important role in the initiation of some harmful algal blooms. The Laguna Madre of Texas has experienced a persistent, nearly monospecific phytoplankton bloom since January 1990 commonly referred to as the "brown tide." Whole-water samples collected in the area where the bloom began reveal the pattern of bloom initiation and dispersion during the first few months of the bloom. Enumeration of microzooplankton from these same samples also reveals that protozoan grazer populations were depressed and nearly eliminated before the bloom began. Benthic biomass and species diversity also declined before the onset of the bloom. The disruption of planktonic and benthic grazers may have been due to extreme hypersaline conditions caused by an extended period of drought (salinities > 60%0). A massive fish kill caused by an abnormal period of below freezing temperatures released a pulse of nutrients into an already disturbed environment allowing this nuisance bloom to become established. Planktonic and benthic grazers have been unable to contribute to the demise of this bloom, which has persisted for >7 yr without interruption.

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