BALANCE BETWEEN AUTOTROPHIC AND HETEROTROPHIC COMPONENTS AND PROCESSES IN MICROBENTHIC COMMUNITIES OF SANDY SEDIMENTS - A FIELD-STUDY

The microscopic community of a microtidal sandy sediment on the Swedis h west coast was studied in situ at two depths (0.5 and 4 m) on four o ccasions (January, April, August and October). Biomass of microalgae, bacteria, ciliates and meiofauna, as well as primary and bacterial pro ductivity, were quantified. (Meiofaunal grazing on algae and bacteria was measured simultaneously by radiolabelling intact sediment cores. A utotrophic biomass dominated the microbial community at both depths an d on all sampling occasions, accounting for 47-87% of the microbial bi omass. Meiofauna contributed 10-47%, while bacteria and ciliates toget her made up less than 6%. The microflora was dominated by attached (ep ipsammic) diatoms, but occasional 'blooms' of motile species occurred. Vital cells of planktonic diatoms contributed to benthic algal biomas s in spring. Primary productivity exceeded bacterial productivity in A pril and August at both depths, while the balance was reversed in Octo ber and January. Meiofauna grazed between 2 and 12% of the algal bioma ss per day, and between 0.3 and 37% of the bacterial biomass. Almost a n order of magnitude more algal (17-138 mg C m(-2)) than bacterial (0. 1-33 mg C m(-2)) carbon was grazed daily. At the shallow site, primary productivity always exceeded grazing rates on algae, whereas at the d eeper site, grazing exceeded primary productivity in October and Janua ry. Bacterial productivity exceeded grazing at both depths on all four occasions. Thus, meiofaunal grazing seasonally controlled microalgal, but not bacterial, biomass. These results suggest that, during summer , only a minor fraction (<10%) of the daily microbenthic primary produ ction appears to enter the 'small food web' through meiofauna. During spring and autumn, however, a much larger fraction (approximate to 30- 60%) of primary production may pass through meiofauna. During winter, meiofaunal grazing is a less important link in the shallow zone, but a t sublittoral depths, algal productivity may be limiting, and meiofaun a depend on other food sources, such as bacteria and detritus. (C) 199 6 Academic Press Limited