Relationships between invertebrate assemblage structure, 2 trout species, and habitat structure in Utah mountain lakes

We tested 4 current hypotheses about how trout predation and habitat structure affect lentic invertebrate assemblages. The hypotheses were 1) trout predation affects larger-bodied species and individuals more strongly than small ones, 2) brook and cutthroat trout affect invertebrates differently because of species-specific feeding preferences, 3) the effects of predation by both trout species are less pronounced in structurally complex habitats than in simpler ones, and 4) physical habitat is more important than predation in structuring benthic communities. These hypotheses were tested by comparing how abundance and individual size of both benthic and pelagic invertebrates varied among 47 mountain lakes that differed in both the presence of trout and bottom substrate complexity. We created a 3 x 3 factor, replicated natural experiment by sampling lakes that differed in their trout stocking history and bottom type. Trout treatments included no fish, cutthroat trout only, and brook trout only. Bottom types included sand, cobble, and vegetation. Densities of large benthic (e.g., caddis larvae, Hemiptera, and amphipods) and planktonic (Chaoborus and some Diaptomidae) taxa were 3- to 7-times less abundant in lakes with trout than without. Trout also appeared to reduce the mean body size of the 2 most common zooplankton taxa (Daphnia rosea and diaptomid copepods) by similar to 25%, although mean size of benthic taxa was unrelated to the presence of trout. Three taxa (Lumbriculus, Daphnia rosea, and cyclopoid copepods) had substantially higher abundances in the presence of trout, implying that trout benefit some taxa by altering the flow of resources to different components of the food web. Differences in benthic and pelagic assemblages between lakes with brook and cutthroat trout were subtle. Abundances of benthic invertebrates varied up to 54-fold among the 3 habitat treatments, with highest abundances in macrophytes. However, differences in habitat did not appear to mediate effects of trout predation on benthic invertebrate assemblages.