Food quality and hydropsychid caddisfly density in a lake outlet stream in Glacier National Park, Montana, USA.

Hydropsychid caddisfly larval density averaged 6500/m² in the lake outlet, but decreased to 1427 and 209/m² 200 and 3400m downstream. Food quantity differed significantly between sites on only 3 of 13 sampling dates, but food quality variables (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. 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 and by a concomitant decrease in seston food quality. -from Authors