Effects of Discharge Reduction on Diatom Colonization below a Large Hydroelectric Dam

Abstract

Diatom community development was monitored in the Colorado River below Lake Mead over two successive 8-wk periods that differed in discharge patterns to examine the effect of discharge reduction on diatom community dynamics. Flow varies on both a daily and seasonal basis in this system, but other physicochemical parameters show little seasonal change. During each discharge period, samples were collected from clay tiles incubated in either exposed or sheltered habitats on the river bed. The initial 8-wk colonization period was characterized by relatively high mean daily discharge (HMD). A seasonal reduction in daily discharge minima resulted in low mean discharge (LMD) conditions in the final 8 wk. Biomass accumulation in sheltered habitats was lower in LMD than in HMD, while exposed habitats supported greater biomass in LMD conditions. A greater number of diatom taxa colonized exposed substrata under low mean discharge conditions than in HMD. Additionally, high densities of colonial-unattached taxa occurred on exposed-LMD tiles, suggesting that reduction in minimum daily discharge enhanced immigration success of species that were poor immigrants in higher flows. Densities of benthic diatoms did not differ between discharge treatments on exposed substrata, but were significantly lower in sheltered habitats during LMD. This response may have stemmed from decreased growth rates of benthic taxa in sheltered environments brought about by lower reproductive rates in slow flow. The manner in which diatom communities were affected by large-scale discharge reductions below Lake Mead depended upon differences in the localized current environment, and in the growth-form and autecologies of the diatom taxa involved.