LONG-TERM DYNAMICS OF AN INVERTEBRATE ASSEMBLAGE DOWNSTREAM FROM A LARGE DAM

Abstract

A century of hydrologic data (1895–1999) and 50 yr of aquatic macroinvertebrate assemblage data (1947–1999) were examined for two tailwater reaches of the Green River downstream from Flaming Gorge Dam in northeastern Utah, USA (40°54′ N, 109°25′ W). One reach was located upstream of an intermittent tributary, and the other downstream. The purpose of the study was to chronicle long-term dynamics and the effect of partial thermal restoration on invertebrate assemblages. The immediate hydrologic effect of the dam was a large decrease in annual maximum daily discharges, water temperatures, and sediment transport. Upstream of the intermittent tributary, macroinvertebrate genera declined from >70 to <30, and the mean macroinvertebrate density increased from 1000 to 10 000/m2 after dam closure. In 1978, a multilevel water intake structure was installed on the dam. Mean summer water temperatures increased from 6° to 12°C, and the number of annual degree days warmed from 2340 to 3200, which was similar to pre-dam conditions; but the rate and timing of warming remained different than before the dam. In contrast to an expected increase in taxon richness, the number of taxa routinely collected after partial thermal restoration was similar to or lower than that observed before thermal restoration. Downstream of the intermittent tributary, taxon richness was about twice that found upstream after dam closure, and the mean annual per-sample taxon richness increased from 7.6 to 11.0 following partial thermal restoration. In both reaches, invertebrate densities were near 10 000/m2 before and after thermal restoration. The lack of an appreciable increase in insect taxon richness upstream from an intermittent tributary following partial thermal restoration is likely due to the combined effects of three factors: (1) the competitive dominance of insect taxa by amphipods, (2) low rates of immigration and colonization, and (3) low reproductive success of insects due to a few degrees difference in the water temperatures between the regulated river and natural streams in the area. These results suggest that we should not only evaluate traditional habitat attributes, but biological interactions as well, when determining or monitoring the effects of river regulation on aquatic biota.