ABUNDANCE PATTERNS OF FILTER-FEEDING CADDISFLIES (TRICHOPTERA, HYDROPSYCHIDAE) AND SESTON IN A MONTANA (USA) LAKE OUTLET

The abundance patterns of four species of net-spinning caddisflies (Trichoptera : Hydropsychidae) were studied in a Montana (U.S.A.) lake outlet stream. Total Hydropsychidae reached very high densities (X > 1400 / 0.2 m2) near the lake outfall but declined precipitously downstream. Abundance patterns generally agree with a published model of filter-feeder / seston interactions in lake outlets. Larval densities were greater on moss-covered substrates and one species occupied habitats of lower water velocity compared to other species. Seston concentrations increased downstream contrary to expectation. Nutrient quality of seston may be important in predicting abundance of stream filter-feeders. © 1979 Dr. W. Junk b.v. Publishers.