Comparing herbivory effects of stream macroinvertebrates on microalgal patch structure and recovery

Abstract

We studied short‐term herbivory and recovery of microalgal communities grazed by snails (Potamopyrgus antipodarum), mayflies (Deleatidium spp.), and caddisflies (Pycnocentrodes aeris) in microcosms containing low algal food supplies characteristic of grazer‐controlled streams. Herbivore biomass was standardised across single‐species treatments to assess grazing effects owing to taxon‐specific differences in feeding modalities. Potamopyrgus decreased algal biovolumes, on average, by 56% in 4 days, and multidimensional scaling (MDS) showed that snail‐grazed microalgal communities were distinct. The erect diatoms Synedra ulna and Staurosirella leptostauron were less abundant, and basal cells of the chlorophyte Stigeoclonium lubricum were more abundant, in snail‐containing treatments than in ungrazed controls and other species treatments. In contrast, more mobile caddisfly and mayfly larvae failed to significantly alter algal biomass and assemblage, suggesting radular mouthparts of snails were more effective at removing algae from the diatom‐dominated communities than the bladelike or brushing mouthparts of caddisflies and mayflies, respectively. Microalgal communities recovered to pre‐grazed conditions in all treatments after 4 days in a grazer‐free, outdoor flume. Diatoms also dominated the recovery phase, but the erect forms S. ulna and S. leptostauron were more abundant during recovery than grazing. Wilhm's diversity decreased after recovery, suggesting grazing had a positive effect on algal diversity. Thus, herbivory primarily by Potamopyrgus affected microalgal patch structure, and hence the starting successional sere, but had no detectable effect on short‐term successional direction in these fast‐recovering communities.