Organisation 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 estuary were studied at 4 stations in late spring and early autumn 1996. Additional 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. However, in autumn, the ratio of PIG/NPIG protists was much lower and is <1 in the most sandy station. Epipelic diatoms generally comprised the bulk of primary producers, whereas epipsammic diatoms and flagellates only significantly contribute to PIG biomass in the more sandy sediments. NPIG biomass was dominated by flagellates and ciliates. Sandy sediments had the most diverse ciliate communities and the highest biomass. The increase in ciliate biomass and the greater importance of herbivorous versus bacterivorous ciliates from June to September might be attributed to selective grazing by metazoa on the generally larger herbivorous 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 changes in microbenthic biomass and composition, related to predictable seasonal changes in environmental conditions as well as episodic and stochastic events resulting in severe disturbance and resuspension. Sandy sediments may have more complex and resilient microbenthic communities, adapted to a continuous regime of disturbance in the top layers of sediments and with a less pronounced seasonality.