Concurrent grazing effects of two stream insects on periphyton1

Abstract

The grazing effects of caddisfly larvae (Neophylax) and mayfly nymphs (Ameletus) were measured in Barnwell Creek, a second‐order stream in northern California. The densities of these grazers were manipulated in situ with flow‐through Plexiglas channels that contained natural cobble substrate. After 13 d, log‐transformed measures of periphyton standing crop (ash‐free dry matter, chlorophyll a, and algal biovolume) were inversely proportional to Neophylax biomass. The negative effect of caddisfly larvae on algal biomass resulted primarily from depletion of Epithemia sp. 1, an undescribed diatom species that accounted for most of the algal biovolume in the periphyton. Neophylax guts contained disproportionately large numbers of Epithemia sp. 1. Ameletus had no significant effect on overall Chl a or algal biovolume but substantially decreased the biovolume densities of Nitzschia spp., Synedra spp., Amphipleura pellucida, and Melosira varians. Ameletus guts contained disproportionately large numbers of a small Nitzschia species. Neophylax growth in the channels was density‐dependent, implicating intraspecific competition for food. Periphyton from the unmanipulated streambed resembled periphyton in experimental channels stocked with natural densities of grazers, allowing extrapolation of experimental results to the unmodified stream.