Effects of Detritus Loading and Fish Predation on Leafpack Breakdown and Benthic Macroinvertebrates in a Woodland Stream

Abstract

To test the roles of coarse particulate organic matter (CPOM) loading and fish predation on benthic community structure and leafpack breakdown, CPOM and fish density were manipulated in a stream. Mt. Sinai Creek, a 3rd-order stream in Piedmont North Carolina, USA, was divided into twelve 30-m² segments with 0.5-m high weirs. CPOM was loaded and maintained at 0.01 ambient, ambient and 4 × ambient levels for 19 mo. Fish were excluded from half the segments and maintained at natural densities in the others, in a fully cross-classified design. Neither CPOM loading nor fish predation had significant effects on leafpack breakdown and macrobenthic species richness or diversity in leafpack and sediment samples on all dates. The total abundance of invertebrates was reduced by fish in sediment samples but not in leafpacks. Almost all species were not significantly affected by the CPOM or fish treatments. Only two taxa responded to each treatment in sediments throughout the study, and none responded in leafpacks, i.e., less than 5% of the taxa tested. Interactions between fish and CPOM were rarely significant. This work shows that CPOM loading and fish predation, and their interactions, were not major forces in determining the diversity, richness, or litter breakdown rates of this lotic community. The absence of a fish effect on the animals in leafpacks suggests that leafpacks function as a refuge from fish predation for macrobenthic invertebrates. The effect of fish predation on total individuals but not on populations in sediments suggests that fish feeding has a significant impact on the benthos, but that natural variability in population sizes can obscure this effect.