Resource depression by a trophic generalist in an acid stream

The occurrence and impact of algal grazing by invertebrates is widespread and well known in circumneutral streams but poorly understood in acidified streams, where specialist grazing taxa are absent. We examined the ability of the generalist stonefly, Nemurella pictetii (Plecoptera: Nemouridae), maintained at different densities using in-stream experimental enclosures, to depress the biofilm resources coating substrata in an acid stream. Nymphs were reared on the epilithic biofilm coating cobbles in clear aquaria in Lone Oak at 0, 0.5, 1 and 4 times ambient density and their growth rate compared. The biomass and algal taxonomic composition of the biofilm was also compared among treatments and contrasted with stones from the stream. After 23 d, biofilm resources, evidenced by descriptors such as diatom abundance, chlorophyll, carbohydrate and protein content, had been depressed by the nymphs. The acidobiontic diatom Eunotia exigua was harvested more efficiently than other algal taxa when nymphal densities were high. Growth of N. pictetii nymphs in the experimental channels was density dependent and the decline in growth of the nymphs with increasing density was probably due to food limitation. The ability of N. pictetii, often classed as a shredder of leafy detritus, to affect the biomass and composition of epilithic biofilm suggests that this abundant species is an important consumer of algae in acidified systems, where specialist herbivores are often absent. Grazing can clearly be important in acidified streams.