Can predation by net-spinning caddis larvae (Trichoptera: Hydropsyche siltalai) cause longitudinal changes in zooplankton species composition in lake-outlet streams?

Abstract

In lake-outlet streams, large zooplankton taxa are often reduced at a higher rate than smaller taxa. Laboratory experiments and a field study were performed to test the importance of abiotic factors and predation by fifth instar Hydropsyche siltalai (Trichoptera: Hydropsychidae) larvae, in creating this pattern. Mortality caused by hydraulic stress was low and not significantly higher in Cyclops than in Bosmina, and no evidence of sedimentation was found. However, traps were used as flow refugia by copepods and Latona setifera (Sididae). No differences among taxa in the probability of entering a hydropsychid net were found. Once in a net, though, the risk of being attacked was higher for Cyclops and Daphnia than for Bosmina when all taxa were present. Zooplankton trapped in a hydropsychid net sometimes escaped, and in Bosmina, the proportion of successful escapes increased in the presence of alternative prey. These results suggest that predation by hydropsychids has the potential to cause longitudinal changes in zooplankton species composition, whereas the impact of abiotic factors for zooplankton reduction in lake-outlet streams needs further investigation.