Direct and indirect effects of increased bedload on algal and detrital-based stream food webs.

Anthropogenic sedimentation poses a significant threat to stream ecosystems throughout the world. Increases in bedload (sediment transported and deposited on the stream bottom) can be especially detrimental for benthic communities. To examine how increased bedload directly and indirectly affects stream communities, we simultaneously manipulated sediment and top-down effects of macroconsumers (fishes and crayfish) in situ in two factorial experiments, one using tiles and one using leaf packs as sampling substrates. Bedload was increased by adding small amounts of sediment (2.5 x normal levels) to localized areas (0.25 m2) of an otherwise unimpacted stream. This increase in bedload had direct effects on basal resources in both the tile and leaf pack experiments. In the tile experiment algal composition was altered by sediment addition, while in the leaf pack experiment fungal biomass declined with sediment. The only direct effect of sediment on insect assemblages occurred in the leaf pack experiment, where sediment reduced predatory stonefly (Plecoptera) and Atherix (Diptera) biomass. However, sediment addition significantly altered top-down effects of macroconsumers in both experiments. In the tile experiment, macroconsumer reductions of total insect biomass, hydropsychid (Trichoptera) biomass, and predatory stonefly biomass were eliminated when sediment was added. The only impact of macroconsumers that was not eliminated in sediment treatments was their reduction of chironomid (Diptera) biomass. Increased bedload did not have as great an impact on top-down effects in the leaf pack experiment, although macroconsumer reductions of tanypodid (Diptera) abundance and biomass were eliminated in sediment treatments. These experiments show that small, environmentally realistic increases in bedload can directly and indirectly affect biota in both algal and detrital-based food webs. While indirect impacts of sedimentation have been examined less frequently than direct biotic responses, this study demonstrates that alteration of top-down impacts can be an important effect of increased sedimentation. In these experiments, small amounts of sediment were added to localized areas of an otherwise unimpacted stream. Even these relatively small-scale increases had significant impacts on stream communities, suggesting that the influence of larger scale sedimentation will be even greater. Study_Dataset_Keywords: bedload, Coweeta Hydrologic Laboratory, electric exclusion, indirect effects, macroconsumers, predator-prey, sedimentation, species interactions, stream, top-downWe conducted two electric exclusion experiments. One experiment used ceramic tiles as sampling substrates, and was conducted from 7/29/97 to 9/8/97. The other experiment used sugar maple leaf packs as sampling substrates, and was conducted from 10/9/99 to 12/4/99. In each experiment, sampling substrates were secured in 0.25 m^2 PVC frames; ten pairs of these frames were placed in the given reach. One frame in each pair was hooked to an electric fence charger. This electric charge excluded macroconsumers (i.e., fishes and crayfishes) from these areas, creating ten replicate pairs of control (or access) and electrified (or exclusion) frames (this was true for both the tile experiment and the leaf pack experiment). In each experiment, five pairs served as sediment controls (i.e., ambient sediment conditions). The remaining five pairs served as sediment addition treatments, with each frame receiving daily additions of 250 g of sand. In each experiment, we thus had four treatments: ambient sediment/macroconsumer access, ambient sediment/macroconsumer exclusion, sediment addition/macroconsumer access, sediment addition/macroconsumer exclusion. In the tile experiment, we examined insect abundance and biomass, chlorophyll a concentration, AFDM, inorganic sediment, and algal abundance and biomass across treatments. In the leaf pack experiment, we examined leaf breakdown, fungal biomass, and insect abundance and biomass across treatments