Functional structure of benthic ciliate communities along salinity gradient in two contrasting estuaries: testing paradigms in estuarine ecology

Ciliates are essential components of estuarine ecosystems. However, compared with metazoans, it remains largely unclear whether changes in ciliate assemblages across a marineriverine gradient fit conceptual models in estuarine ecology. To address this question, benthic ciliates were studied in the intertidal zone of two estuaries with contrasting conditions. The Chernaya (CH) is a small (2km long) subArctic estuary in the White Sea; the Vellar (VE) is a large (20km long) tropical estuary in the Bay of Bengal. At both sites, the average abundance and biomass of ciliates were similar and quite typical of other marine interstitial habitats. The diversity was high (114 species in CH; 39 taxa, identified to genus or putative species level, in VE), not supporting the thesis about high abundance but low diversity of estuarine communities. Abundance, biomass and taxonomic-based diversity decreased gradually from the sea upstream in CH, while had several peaks in VE. Contrary to the ‘critical salinity’ concept, no diversity minima were found in the mid-estuary oligohaline zones. In contrast, most functional traits followed similar trends in both estuaries, in accordance with the r-K strategy continuum model. In particular, the estuarine assemblages in the upper reaches were dominated by small-bodied, free-swimming bacterio- and detritivore forms, compared to the outermost coastal areas. The patterns in species composition and functional structure complied with the ecotone model in the smaller CH estuary, but better corresponded the ecocline model in the larger VE estuary. Our findings demonstrated that benthic ciliate assemblages fitted with some, but not all, conceptual models in estuarine ecology. Further research is needed to understand the potential mechanisms shaping microeukaryotic communities in transition zones.