Big river benthos : linking year-round biological response to altered hydrological regimes

Abstract

Abstract : The Mississippi River is heavily relied upon for commercial navigation, but it is also home to a diversity of organisms adapted to large river habitats. Macroinvertebrates have long been used as habitat/water quality indicators in wadeable streams, but because of sampling difficulty, large rivers have been understudied. Herein, the authors focused on the benthic macroinvertebrate fauna as indicators of biological response to anthropogenic alterations to flow particularly to closure dikes within naturally occurring secondary channels of the Lower Mississippi River. During mid to high river stages, water flows over dikes into secondary channels, but during low river stages, many side channels become disconnected from the main channel at the upstream end. Nine secondary channels spanning a gradient of hydrological connectivity to the main river channel at low river stages were assessed. Main and secondary channels were sampled for benthic macroinvertebrates in May and June 2014 using a Ponar grab and benthic sled, respectively. The objective was to determine whether temporary hydrological disconnection from the main channel during low river stages had a legacy effect on benthic community structure in the succeeding year. There was a significant positive relationship between taxonomic richness in 2014 and hydrological connection of secondary channels in the previous year (2013), indicating a legacy effect of connectivity on macroinvertebrate diversity of secondary channels. These findings contribute to a better understanding of ecological response to altered flow regimes and help document benefits of restoring connectivity between secondary channels and the Mississippi River main channel.