Sediment addition reduces the importance of predation on ecosystem functions in experimental stream channels

Abstract

Sedimentation is a pervasive cause of biological impairment in streams, and predation exerts strong control over lower trophic levels. However, studies combining these two factors are lacking. In a factorial experiment in flow-through channels, addition of sand (<0.5 mm) and predatory stoneflies (Perlidae) caused independent effects on benthic invertebrates, algal biomass, and leaf breakdown. Sand reduced invertebrate density by 55% and also reduced leaf breakdown and algal biomass. Predators reduced invertebrate densities by 40%, with the strongest impact on algal-feeding invertebrates. Predators also decreased densities of leaf-shredding invertebrates and reduced leaf breakdown, thereby inducing a trophic cascade via detritus-based food web. The two treatments exhibited an antagonistic interaction whereby sand obscured any effect of predators on algae, indicating that an abiotic stress may modify a trophic cascade. By contrast, we found no support for synergistic interactions between sand and predation. The strong effects of sedimentation on key ecosystem processes illustrate that stream management needs to exploit riparian and in-stream measures to reduce sediment inputs to headwater streams.