ORGANIZATION OF MICROBENTHIC COMMUNITIES IN INTERTIDAL ESTUARINE FLATS, A CASE-STUDY FROM THE MOLENPLAAT (WESTERSCHELDE ESTUARY, THE NETHERLANDS)

The microbenthic communities of a tidal flat in the Westerschelde estu ary were studied at 4 stations in late spring and early autumn 1996. A dditional information on the diatom component of these communities was obtained from a one-year survey of these organisms. Total biomass of pigmented (PIG) protists greatly exceeded that of non-pigmented (NPIG) protists in late spring, especially at the more silty stations. Howev er, in autumn, the ratio of PIG/NPIG protists was much lower and is <1 in the most sandy station. Epipelic diatoms generally comprised the b ulk of primary producers, whereas epipsammic diatoms and flagellates o nly significantly contribute to PIG biomass in the more sandy sediment s. NPIG biomass was dominated by flagellates and ciliates. Sandy sedim ents had the most diverse ciliate communities and the highest biomass. The increase in ciliate biomass and the greater importance of herbivo rous versus bacterivorous ciliates from June to September might be att ributed to selective grazing by metazoa on the generally larger herbiv orous ciliates in June. Preliminary estimates indicate that apart from episodic blooms of herbivorous taxa, ciliate grazing does not seem to have an important impact on epipelic diatom populations. In general, silty sediments appear to be characterized by considerable temporal ch anges in microbenthic biomass and composition, related to predictable seasonal changes in environmental conditions as well as episodic and s tochastic events resulting in severe disturbance and resuspension. San dy sediments may have more complex and resilient microbenthic communit ies, adapted to a continuous regime of disturbance in the top layers o f sediments and with a less pronounced seasonality.