Production of filter-feeding Trichoptera in an impounded and a free-flowing river

Abstract

Production was estimated (size frequency method) for five species of Hydropsychidae and two species of Philopotamidae at two sites on the impounded North Anna River and two sites on the free-flowing South Anna River. Total production was highest (3250.0 g dry weight (DW)∙ha−1∙year−1 at the site immediately below the dam. At the other sites total production ranged from 94.0 to 515.1 g DW∙ha−1∙year−1. An attempt was made to determine the influence of various factors on production. From an analysis of the trophic dynamics of production, seston energetics, and rates of consumption we conclude that the zooplankton released from the reservoir greatly enriches the seston in a narrow size range (105–864 μm) that corresponds to the size of the zooplankton released from the reservoir and the capture net mesh size of the hydropsychids. The extremely high density of filter feeders below the dam reduces the food quality of the seston and limits production downstream. Ecological interactions also were found to influence production, requiring detailed knowledge of the biology of individual species to understand the dynamics of a system.