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 s tructure affect lentic invertebrate assemblages. The hypotheses were 1 ) trout predation affects larger-bodied species and individuals more s trongly than small ones, 2) brook and cutthroat trout affect invertebr ates differently because of species-specific feeding preferences, 3) t he effects of predation by both trout species are less pronounced in s tructurally complex habitats than in simpler ones, and 4) physical hab itat is more important than predation in structuring benthic communiti es. These hypotheses were tested by comparing how abundance and indivi dual size of both benthic and pelagic invertebrates varied among 47 mo untain lakes that differed in both the presence of trout and bottom su bstrate complexity. We created a 3 x 3 factor, replicated natural expe riment by sampling lakes that differed in their trout stocking history and bottom type. Trout treatments included no fish, cutthroat trout o nly, and brook trout only. Bottom types included sand, cobble, and veg etation. Densities of large benthic (e.g., caddis larvae, Hemiptera, a nd amphipods) and planktonic (Chaoborus and some Diaptomidae) taxa wer e 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 zoopla nkton taxa (Daphnia rosea and diaptomid copepods) by similar to 25%, a lthough mean size of benthic taxa was unrelated to the presence of tro ut. Three taxa (Lumbriculus, Daphnia rosea, and cyclopoid copepods) ha d substantially higher abundances in the presence of trout, implying t hat trout benefit some taxa by altering the flow of resources to diffe rent components of the food web. Differences in benthic and pelagic as semblages between lakes with brook and cutthroat trout were subtle. Ab undances of benthic invertebrates varied up to 54-fold among the 3 hab itat treatments, with highest abundances in macrophytes. However, diff erences in habitat did not appear to mediate effects of trout predatio n on benthic invertebrate assemblages.