Ecosystem responses to multiple-stressor gradients: Nutrient and sediment addition to experimental stream channels

Agricultural land use can strongly affect stream ecosystems by increasing levels of nutrients and fine sediment cover on the streambed. Knowledge of patterns in ecological responses along gradients of these two stressors will help define thresholds of harm. Leaf breakdown rates, algal and bacterial communities are directly and/or indirectly influenced by nutrient supply and fine sediment cover. To investigate the effects of stressor gradients and their interactions on these biological response parameters, we designed a full-factorial experiment in circular stream-side channels with eight levels each of nutrients (36 to 6900 μg·l^-1 DIN, 1.4 to 450 μg·l^-1 DRP) and fine sediment (0 to 100 % cover). Algal biomass, bacterial diversity and leaf pack decomposition were determined after three weeks of exposure to both stressors. Algal biomass was significantly higher in channels with lower levels of fine sediment. Bacterial diversity generally increased with increasing nutrient concentrations up to an intermediate nutrient level but then decreased again with the exception of reaching the highest diversity overall at the top nutrient level. Thus, increased levels of nutrients and fine sediment caused major changes to the algal and bacterial communities. In turn, these changes affect other food-web components as well as ecosystem functioning, including decomposition.