Effects of nutrient enrichment on stream benthos override those of fine sediment addition : a flume experiment

Abstract

1. Agricultural runoff greatly contributes to stream eutrophication and sedimentation, which in turn can severely impact the benthic algal and invertebrate communities. Our aim was to determine the combined and individual effects, of both nutrient and sediment stressors, on benthic algae and invertebrates, at the taxon and community level, as well as leaf litter degradation, a measure of ecosystem functioning. 2. Eight experimental stream channels (10×0.5×0.3 m, LWH), supplied with water from a nearby nutrient- and sediment-rich stream, were subjected for 43 days, to combined levels (low versus high) of phosphorus and clay in a controlled experiment. Invertebrates were sampled from leaf bags filled with alder leaves, to determine their composition and abundance. Benthic algae were allowed to grow on unglazed tiles, and algal composition was analyzed weekly for 6 weeks. Rates of leaf litter degradation were also determined from leaf bags. 3. Measured response variables included the biomass and density of invertebrate and algal taxa; diversity, evenness and taxon richness of both invertebrate and algal communities; percentage abundance of algal growth forms; invertebrate consumption and microbial decomposition of alder leaves; and various physicochemical properties of water. Nutrient enrichment was the most pervasive stressor, directly impacting 37% of all biological response variables, while fine sediment addition impacted only 3%. Nutrient enrichment increased algal biomass and rates of leaf litter degradation and decreased invertebrate densities, while increasing invertebrate richness, diversity and evenness. Sediment addition on the other hand, exerted a very limited effect at the taxon level of both algal and invertebrate communities. 4. Interactions between stressors occurred rarely and were antagonistic. The river biota showed mixed responses to combined levels of clay and phosphorus. The negative effect of clay on the invertebrate community, was weakened under phosphorus enrichment. Moreover, the positive effect of added phosphorus on rates of leaf litter degradation, was masked by high levels of added sediment. 5. Our main findings show that phosphorus exerted a strong separate effect on the stream biota when added in combination with fine sediment, which had barely any effect. Thus, P enrichment from agricultural runoff, can severely impact streams that are already rich in P and fine sediments. It is therefore critical to control agricultural runoff, in order to mitigate its impacts on the water quality and ecological integrity of rivers and streams. Also, interactions between nutrients, sediments and multiple other anthropogenic stressors, need to be addressed in future research, to fully comprehend consequences of agricultural practices in lotic ecosystems.