Differences in benthic macroinvertebrate assemblages associated with a bloom ofDidymosphenia geminatain the Lower American River, California

Abstract

Didymo (Didymosphenia geminata) is a freshwater colonial diatom native to northern latitude lakes and streams and, until recently, was considered indicative of high-altitude, low-temperature, nutrient-poor habitats experiencing little anthropogenic impact. In recent years, however, this diatom has expanded its geographic distribution into lower latitudes and altitudes. In early 2012, we discovered a bloom of didymo in the Lower American River below Nimbus Dam in Sacramento County, California. Due to production of large masses of extracellular stalks that can smother stream substrates, and its proximity to a salmonid habitat restoration project, discovery of didymo prompted us to conduct a study of its effects on benthic macroinvertebrates, an important food source for juvenile salmonids. In May and June 2012, we sampled benthic macroinvertebrates in a reach of the main channel where didymo was abundant and in a nearby reach on a side channel where environmental conditions were similar but didymo was seemingly rare or absent. We found significant differences in taxonomic composition and abundance of macroinvertebrates inhabiting the two reaches. Higher numbers of Chironomidae (midges), Gammaridae (scuds), Hirudinidae (leeches), Hydroptilidae (microcaddisflies), and total macroinvertebrates were recorded in the main channel. In contrast, higher numbers of Baetidae (baetid mayflies), Glossosomatidae (saddlecase caddisflies), and Tipulidae (crane flies), along with a higher percentage of combined Ephemeroptera, Plecoptera, and Trichoptera (%EPT), were recorded in the side channel. We also recorded higher values for three common measures of taxonomic diversity (Margalef richness, Pielou's evenness, Shannon-Wiener diversity) in the side channel. From these initial observations, we tentatively concluded that the didymo bloom has influenced benthic macroinvertebrate composition in the Lower American River.