Spatially varying trophic effects of reservoir-derived plankton on stream macroinvertebrates among heterogeneous habitats within reaches

Using a stable isotope two- and three-source mixing model, we evaluated trophic contributions of reservoir-derived plankton, epilithon, and terrestrial leaves to stream macroinvertebrates at two dam upstream and four dam outlet reaches. We compared four different habitat types co-occurring within the reaches: lotic habitat, bar-head lentic habitat, bar-tail lentic habitat, and isolated pond on sandy bars. Filter-feeders, which provide the highest trophic contributions of reservoir-derived plankton, did not significantly differ among habitats, whereas collector-gatherers and predators differed significantly. Thus, the three-source model analysis for collector-gatherers and predators indicated a lower proportion of reservoir-derived plankton in lentic habitat than in lotic habitat. This difference in food source composition was reflected in the lower trophic contribution of plankton to lentic species. Macroinvertebrates in lentic habitats fed on sources less mixed with reservoir-derived plankton; therefore, the trophic impact of reservoirs was indistinctive at the community level, indicating that lentic habitats can function as trophic refugia to mitigate the trophic impact of reservoirs. Our findings demonstrate that habitat heterogeneity potentially provides a foundation for determining the trophic contribution of food webs while affecting communities when there is anthropogenic input of food resources to the habitat.