Trophic cascades in streams: effects of nutrient enrichment on autotrophicand consumer benthic communities under two different fish predation regimes

We tested the hypothesis that differences in top-level predators could medi ate the importance of top-down versus bottom-up forces in stream food chain s using three streams dominated by Galaxias (fish native to New Zealand) an d three dominated by brown trout (Salmo trutta) (an introduced species). Th ese two fish species have quite different predation strategies and energeti c requirements. Periphyton in the Galaxias streams formed relatively low bi omass (indicative of high invertebrate grazing), whereas periphyton in two of the three trout streams formed relatively high biomass (indicative of lo wer grazing pressure). Periphyton biomass response to inorganic nutrient en richment varied by fish type. Invertebrate densities on artificial substrat es were higher in the Galaxias streams, with the exception of chironomids. Nutrient enrichment of periphyton led to increases in total invertebrate de nsities, but these increases were greater in the Galaxias streams where inv ertebrate predation was thought to be lower. Our results suggest that the s pecific feeding behavior traits of the consumers should be considered when investigating trophic cascades or predicting effects of bottom-up nutrient enrichment on autotrophy.