Microbial Food Webs in Marine Sediments. II. Seasonal Changes in Trophic Interactions in a Sandy Tidal Flat Community

Abstract

The role of grazing by marine sediment flagellates, ciliates, and meiobenthic animals in controlling production of their bacterial and diatom prey was investigated. At six selected time points, over the year, bacterial production and diatom standing stock were compared to grazing pressure exercised by proto- and micrometazoan consumers. The intensity of prey-predator relations showed pronounced yearly dynamics in which two stages could be distinguished. During the first phase, from the end of winter to mid-summer, the consumption of diatoms gradually increased, with possible overgrazing at the end of the period. This was followed by a collapse of diatom abundance, to the winter level. During the first stage, no appreciable bacterial consumption was observed in spite of the high abundance and production of bacteria. The second stage started in mid-summer and continued through the fall. During this period, the grazing on bacteria increased and reached the year's maximum. For at least a brief period (October), micrograzers removed the majority of bacterial production. In contrast, herbivory stayed at the year's lowest level, and diatoms appeared to be controlled by factors other than grazing. The observed ingestion rates seem to support the apparent energy requirements of flagellates and some ciliates (scuticociliates and hypotrichids). Other ciliates (pleurostomatids and karyorelictids) could not subsist on the observed diet and might have to complement it with other energy sources, possibly via dissolved organic matter absorption.