Benthic-pelagic coupling: a comparison of the community structure of benthic and planktonic heterotrophic protists in shallow inlets of the southern Baltic

1. The taxonomic composition, abundance and biomass of heterotrophic protis ts (ciliates, heterotrophic flagellates (HF), rhizopods and actinopods) in the sediment and water column of shallow inlets of the Southern Baltic was studied under a variety of environmental conditions during 1996-1997. A sha llow, highly eutrophic station and a deeper, less eutrophic station were co mpared. 2. Community biomass ranged from 0.12 to 0.34 mug C cm(-3) in the water col umn and from 1.5 to 105 mug C cm(-3) in the sediment. Heterotrophic protist s dominated zooplankton biomass at both stations (73% and 84% mean contribu tion), while they were of minor importance within the zoobenthos. Expressed per unit area, benthic biomass contributed a significant part (44% and 49% ) to the total heterotrophic protistan community at both stations. 3. Although the methodology for counting ciliates and HF was focussed on a high taxonomic resolution, the results reveal some general trends in the di stribution of heterotrophic protists: protozooplankton biomass was dominate d by flagellates (80% mean biomass contribution) at the shallow station and by ciliates (73% mean biomass contribution) at the deep station. In the be nthos at both stations, ciliates were the dominant protozoans, followed by the hitherto little-studied rhizopods (25% and 35% mean biomass contributio n) and flagellates. 4. The degree of benthic-pelagic coupling differed between taxonomic groups . Benthic and pelagic communities of ciliates showed little taxonomic overl ap. In contrast, many heterotrophic flagellate species were found both in t he benthos and in the pelagic. These benthic-pelagic species contributed si gnificantly to the biomass of HF in the water column. The planktonic rhizop od community consisted of a subset of those species found in the benthos. 5. The abundance of benthic and pelagic protists was positively correlated at the shallow station, but taxonomic data indicate that the direct exchang e between benthic and pelagic communities was only partly responsible.