Food Quality and Hydropsychid Caddisfly Density in a Lake Outlet Stream in Glacier National Park, Montana, USA

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Abstract
Hydropsychid caddisfly larval density averaged 6500/m2in the lake outlet, but decreased to 1427 and 209/m2at a distance of 200 and 3400 m downstream. Food quantity, measured as the concentration of particulate organic carbon in grab samples of seston, differed significantly between sites on only 3 of 13 sampling dates. But food quality variables (i.e. protein and ATP content of seston) were significantly higher in the lake outlet during months of higher lake productivity; monthly values of particulate protein regressed negatively with distance downstream. We concluded that stable substrate, discharge, and temperature regimes, combined with a plentiful supply of high-quality food, maintained high densities of filter-feeding hydropsychids in the lake outlet. Downstream populations were limited by comparatively greater abiotic variation (i.e. discharge, substratum, diel temperature) and by a concomitant decrease in seston food quality.