Effects of water clarity and other environmental factors on trophic niches of two sympatric piscivores

Water clarity can have a profound influence on aquatic ecosystem structure and processes via its effects on physical habitat (e.g., thermal regime, macrophyte density) and behavioural responses of biota (e.g., predator avoidance, reaction distances, foraging efficiency). Changes in foraging efficiencies under varying water clarity conditions are well documented for many freshwater piscivores in laboratory studies, but the influence of visual foraging conditions on interspecific trophic dynamics is poorly understood in wild populations, especially within water clarity ranges that are realistic for north-temperate boreal lakes. Here, we used stable isotopes of nitrogen (N-15/N-14) and carbon (C-13/C-12) in fish muscle tissue to investigate how resource partitioning between two sympatric visual piscivores is related to water clarity and other habitat variables in 28 small (100-200ha) Boreal Shield lakes. One of the species is adapted for foraging in low subsurface illumination (walleye, Sander vitreus) and the other in high subsurface illumination (smallmouth bass, Micropterus dolomieu). Trophic niche dimensions of the dark-adapted predator did not respond significantly to differences in water clarity. In contrast, total isotopic niche space of the light-adapted predator decreased significantly with increasing water clarity through greater use of pelagic resources and a narrower range of trophic levels, although these relationships were weak. Niche overlap ranged from 0 to 65%, but was not significantly related to water clarity. Rather, indices of prey availability appeared to be much stronger predictors of trophic interactions. Both species occupied more similar food-web positions when yellow perch (Perca flavescens) abundance was higher, and had more similar niche size and trophic evenness with decreasing prey fish species richness. Results indicate that the trophic ecology of predators adapted to foraging in low light conditions is less influenced by water clarity than that of predators adapted to foraging in high light conditions. However, prey availability, rather than the environmental conditions in which foraging occurs, may be a more important driver of resource partitioning among generalists even when light conditions favour one species' foraging strategy over another.