Influence of Reservoir Transit on Riverine Algal Transport and Abundance

Abstract

The Des Moines River lost 65–75% of its algal standing crop (chlorophyll a) in passing through each of two impoundments (mean retention times 11 and 16 d), and chlorophyll concentrations within both impoundments were 50–90% below the predictions of empirical chlorophyll–nutrient models. Sedimentation of river-borne algae and light limitation within the impoundments were identified as major loss processes. A reduction in algal size from upstream to downstream in one reservoir paralleled the loss of algal biomass. Algal losses in each impoundment increased with both increasing retention time and water temperature so that chlorophyll concentration below the dams was uncoupled from the temperature and flow dependence seen in river reaches not influenced by impoundments. The reduction in riverine algal transport associated with reservoir transit was cumulative over the two-reservoir series; this reduction can be interpreted as a "reset" to river headwater conditions.