SPATIAL VARIATION IN ZOOPLANKTON COMMUNITY STRUCTURE IS RELATED TO HYDROLOGIC FLOW UNITS IN THE MISSOURI RIVER, USA

Zooplankton are a vital link in the food webs of large rivers, and their communities are shaped by both local environmental features and advection. In the Missouri River, flow characteristics naturally change along its length, but human modifications to facilitate commercial transport have altered natural flow in many sections of the river. We evaluated the effect of flow on zooplankton community structure at multiple spatial scales, and used multivariate analyses to evaluate the relative importance of flow and local abiotic environment on these communities. During July September 2005, zooplankton samples and physicochemical measures were collected from the Missouri River main channel at 78 sites over a 2831 km range (Montana to Missouri). We identified a total of 30 cladoceran species, 22 copepod species and 27 rotifer genera, and we detected highly significant differences in zooplankton community structure among hydrologically distinct flow units and larger spatial zones. At the local scale, crustacean zooplankton and rotifers responded differently in the analyses. For copepods and cladocerans, distance from the nearest upstream reservoir explained more of the overall community pattern of the river than any other combination of environmental factors, reflecting the influence of dams on the zooplankton community of the Missouri River. For rotifers, a combination of flow characteristics due to impoundment and channelization and local environment (temperature) was important. Our study indicates that, because of the overwhelming effect of flow on zooplankton communities, hydrology must first be taken into account before zooplankton can be used as bioindicators of other environmental stresses. Copyright (C) 2009 John Wiley & Sons, Ltd.