THE EFFECTS OF COMPLEX TROPHIC INTERACTIONS ON A MARINE MICROBENTHIC COMMUNITY

The consequences of complex trophic interactions for the dynamics of a natural microbenthic community from an intertidal Australian seagrass bed were examined during experiments in January, March, April, and Ju ne 1989. Effects of meiofauna and protist density manipulations on bac terial 'numbers and division rates and microalgal biomass were determi ned over 6 d each month. Results varied among experiments, microbial g roups, and vertical positions within the sediment. Meiofaunal effects increased bacterial division rates but not densities in January subsur face and March surface sediments. In June, enhanced meiofaunal densiti es decreased subsurface but increased surface bacterial densities. A c omplex, tri-trophic-level interaction among meiofauna, protists, and b acteria probably caused the increased bacterial densities in June surf ace sediments. Meiofauna had no discernible effect on bacterial densit ies and division rates in January surface and March subsurface sedimen ts even though relatively small (< 50%) differences between treatments could be detected with reasonable statistical power. Changes in micro algal biomass could not be attributed to experimental treatments, and meiofauna likely have a limited effect on microalgae in this intertida l seagrass habitat. Results indicate that the combined effects of meio fauna can alter microbial densities and growth rates and significantly affect microbenthic community structure. Spatial and temporal variabi lity in the nature of trophic interactions will determine the cumulati ve effects of meiofauna on microbial communities.