Effects of Didymosphenia geminata removal on river macroinvertebrate communities

Abstract

Periphyton blooms dominated by Didymosphenia geminata have been reported from hundreds of rivers. D. geminata blooms consist of thick mats that cover kilometers of riverbed. Benthic habitat conditions are altered when large areas of sediment and native periphyton are replaced by D. geminata mats and these habitat changes may lead to changes in the composition and abundance of benthic invertebrates. Mensurative studies have provided circumstantial evidence of changes in invertebrate communities in response to D. geminata blooms. However, those studies may have confounded the effects of D. geminata and other environmental variables. To identify direct effects of D. geminata on invertebrate communities, we conducted a paired before-after, control-impact (BACIP) experiment in three D. geminata-infested rivers. The experimental impact was the removal of D. geminata mats from 35-50 m2 areas of riverbed. We tested for effects of D. geminata removal on macroinvertebrate community metrics, including total density, taxon richness, and the prevalence of Ephemeroptera, Plecoptera, and Trichoptera (EPT) and trophic groups. The BACIP analysis indicated that D. geminata removal caused a decrease in total density at two rivers, a decrease in the density of dipterans and an increase in % EPT invertebrates at one river, and an increase in % algivorous invertebrates at another river. A multivariate analysis indicated that there was more variation in invertebrate community structure among rivers than between treatments within rivers. The limited effects of D. geminata removal on invertebrate communities were probably related to the composition of colonist pools and the relatively small cleared areas; cleared areas were surrounded by large areas of D. geminata-covered habitat and the colonist pools were dominated by taxa that inhabit D. geminata mats. © 2014 Taylor & Francis.