Flow Refugia for Benthic Macroinvertebrates during Flooding of a Large River

Abstract

Benthic macroinvertebrates shifted from deep water to shallow water of the shore zone during annual flooding of a large, gravel-bed river. The shore zone is seasonally inundated during flooding for up to 4 mo of the year, and is a geomorphic feature of many large rivers in northern latitudes with limited floodplain habitat. We collected samples of invertebrates along with measures of several hydraulic and sedimentary variables 5 times through the flood cycle from 4 fixed water depths; stations shifted laterally over a distance of 30 m between baseflow and peak flow. In deep water (1.5 and 3.0 m), shear velocity and substrate mobility increased as a result of flooding. At shallow depths (0.2 and 0.5 m) within the shore zone, stable substrate and low hydraulic stress remained throughout the flood cycle, providing flow refugia to benthic invertebrates. Invertebrate density and species richness were similar during months on the rising limb of the hydrograph while the wetted area of the channel markedly increased. Persistence of the invertebrate community appears to have been facilitated by a shift in a large proportion of organisms from deep water to shallow depths of the shore zone during flooding. Total density and the density of most collector-gatherers such as Rhithrogena and Baetis was highest at 1.5 m prior to flooding but shifted to depths of 0.5 and 0.2 m at peak flow. The filter-feeding caddisfly Hydropsyche was most abundant at 1.5 m in all months of the study while the location of 1.5 m samples shifted laterally over a distance of 30 m through the flood cycle. The ecological importance of the shore zone as a flow refugium was demonstrated by the broad diversity of species with varying feeding behaviors and morphologies that concentrated in this zone during flooding.