Effects of Flow Reductions on Aquatic Biota of the Colorado River below Glen Canyon Dam, Arizona

Abstract

We examined influences of 3 d of reduced flows on biotic assemblages in the Colorado River below Glen Canyon Dam, Arizona. Exposure in the varial zone reduced standing crops of periphyton, pondweed Potamogeton pectinatus, and benthic macroinvertebrates, and losses following the drawdown were not explained by expected seasonal trends. Losses in macroinvertebrate densities and mass and in periphyton mass and chlorophyll a suggest that effects of a sudden reduction in dam discharge of moderate duration and magnitude are comparable with losses associated with the varial zone of fluctuating daily flows. Compared with expected seasonal changes, abundance of rainbow trout Oncorhynchus mykiss increased during and following reduced flows. Proportional composition of fish less than 152 mm total length (TL) was greater after the drawdown, whereas the percentage of trout 406–558 mm declined. Relative condition factor before, during, and after the drawdown did not differ significantly for all fish combined, following a long-term pattern. However, among length-classes, condition of fish 305–405 mm TL declined following reduced flows. Feeding and composition of dietary items generally followed expected patterns of seasonal change and failed to correspond with changes in periphyton, macrophyte, and macroinvertebrate benthic densities after the drawdown. However, fewer rainbow trout ate Cladophora glomerata, and they ate less of this alga. In comparison, fish ate more gastropods following the drawdown than accounted for by long-term patterns, but frequency of occurrence was unchanged. Sudden reduction in flows of moderate duration and magnitude, although significantly reducing benthic assemblages, had little apparent negative short-term consequence for rainbow trout.