In situ measurement of meiobenthic grazing rates on sediment bacteria and edaphic diatoms

Abstract

A radioactive tracer technique was used to measure meiofaunal grazing on bacteria and diatom communities in natural sediments. Radioactive 14C-glucose and 14C-bicarbonate were used to label heterotrophs (bacteria) and autotrophs (diatoms), respectively. The labeled compounds were added to undisturbed sediment cores, and they were 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, where microbial uptake is linear and where 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 activity 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. Turnover times of .apprx. 30 h for bacteria and .apprx. 6.5 days for diatoms are apparently sufficient to maintain the microbial community in steady state under the meiofaunal grazing regime. Meiofaunal grazing pressure (60 .mu.g bacterial C and 27 .mu.g diatom C ingested 10 cm-2 h-1) probably represents a significant stimulatory effect on the microbial community.