Spatial variation in the invertebrate macrobenthos of three large Missouri River reservoirs

Abstract

Although benthic macroinvertebrates have been used as indicators of ecological condition for a variety of aquatic ecosystems, large reservoirs present challenges for assessment design. Reservoir embayments may have unique characteristics relevant for assessment, and the reservoirs may include zones that combine the characteristics of lotic and lentic systems. The purpose of this study was to characterize spatial variation in benthic macroinvertebrate assemblages of three large upper Missouri River reservoirs in the USA. A probability design was used to select sample sites from Lake Oahe (North and South Dakota), Lake Sakakawea (North Dakota), and Fort Peck Lake (Montana). Sites were chosen to represent the main stem of each reservoir, as well as small, medium, and large bays. The assemblages in all lakes were dominated by oligochaetes (46―60 % of total organisms) and chironomids (29―49 %). Small and medium bays generally had greater total densities, particularly of chironomids and the burrowing mayfly Hexagenia. Composition and density of the benthos were strongly influenced by depth. Information on the density, distribution, and variability of benthic macroinvertebrates will be used to further refine assessment strategies for large reservoirs.