Ciliate grazing on bacteria, flagellates, and microalgae in a temperate zone sandy tidal flat: ingestion rates and food niche partitioning

The trophic role of benthic ciliates was studied in a temperate zone sandy tidal flat community during three summers. The contents of digestive vacuoles of ciliates taken directly from the field were examined by light microscopy after ciliates had been stained with hematoxylin. Most ciliate species were herbivorous, preferentially consuming several dinoflagellate, heterotrophic flagellate, and diatom species. Ciliate-feeding was further examined under experimental conditions employing the prey items indigenous to the tidal flat. The latter experiments confirmed that ciliates specialized on microphytobenthic species, and additionally revealed bacterivory by some ciliates. On the basis of distinct prey items and their frequencies in ciliate digestive vacuoles (i.e. ciliate food spectra), we calculated the degree of overlapping of ciliate food niches on a pair-wise basis. The degree of ciliate food spectrum overlapping correlated negatively with ciliate population densities. Ciliate species with the least degree of overlapping were the most abundant, and, in contrast, the least abundant ciliates had the highest degrees of overlapping of their food spectrum with the food spectrum of other ciliates. Therefore, ciliate abundance and the diversity of ciliate assemblages appeared to be controlled, among other factors, by competition for prey. The entire ciliate community consumed 10% of the total daily primary production as from literature data on unicellular alga growth rates. Ciliates grazed 93 % of the estimated daily dinoflagellate production. The diatom daily production was consumed at much lower rates (6%), and only four of the 42 extant diatom species were extensively grazed. Ciliate grazing on diatoms was thus likely to influence diatom species frequencies but not the total diatom abundance. Heterotrophic flagellates constituted 17% of the entire ciliate assemblage food spectrum. Therefore, the influence of ciliates on bacterial dynamics was both direct, through moderate grazing (11% of bacterial production), and indirect by grazing on flagellates that were assumed to be bacterivores.