Lotic Protistan Herbivore Selectivity and Its Potential Impact on Benthic Algal Assemblages

Despite the ubiquitous presence of microscopic herbivores such as protozoa in aquatic ecosystems, research on benthic herbivory has largely focused on the role of macroscopic grazers. The present study shows the potential for protozoan herbivores to affect the structure of benthic algal assemblages through selective grazing. The abundance and feeding preferences of three common algivorous protozoans, the ciliates Chilodonella sp. and Trithigmostoma cucullulus and the sarcodine Pelomyxa sp., were determined in a productive, third-order stream and in stream-side channels which contained different densities of two macrograzers and algal assemblages with different physiognomies and species composition. Although the species composition of the ingested algal contents of individual micrograzer cells varied, all three micrograzer populations exhibited selectivity in their feeding on dominant diatom species. Selectivity varied among microherbivore species and with the abundance of indigenous macrograzers. Although the smallest of the three microherbivores (Chilodonella) appeared to be limited to grazing smaller diatom species, selectivity by the two larger species was not strongly related to size or growth form of diatom prey. Selective feeding was still prevalent when the three microherbivore species were considered together as a guild, indicating the potential for an impact on diatom assemblage structure. Although initial predictions of grazing rates of herbivorous protozoa on benthic diatoms suggest that such impacts can be substantial, more detailed studies of protozoan production potentials and microherbivore-algal interactions will be required to confirm or reject these preliminary estimates.