The effects of floods on the invertebrate fauna of a large, unstable braided river

Abstract

The abundance and species composition of benthic invertebrates were studied at three sites in the Lower Rakaia River for one year, and continued at one of these sites for a further year. A total of 33 taxa were identified. Trichoptera (15 species) and Diptera (7 species) were the most common species. Larvae of mayfly Deleatidium were, however, numerically dominant, comprising up to 96% of samples. Species abundance and diversity were low, probably because of the frequent floods. Abundance was inversely related to antecedent discharge and was least following severe floods of about 550 m³ s ‘. Species diversity and standing crop were greater during the more stable winter flows than during the flood‐prone spring, summer, and autumn flows. The similarity of species composition decreased with increasing distance between sites but was least between years at one site, probably as a result of changed frequency of floods. Increases in species abundance and diversity were recorded even during short periods of relatively low stable flows. Floods which cause a significant increase in bed load movement are, however, likely to affect specific browsers and sedentary species more than collector‐browsers, filter feeders, predators, and free‐living species. It is concluded that low species diversity is likely to characterise unstable, braided rivers.