Individual and combined responses of stream ecosystems to multiple stressors

1. Managers must understand the effects of stressors on ecosystems in order to identify thresholds of harm but, to be meaningful, thresholds will usually need to be defined for situations where multiple stressors are operating. 2. We investigated the individual and combined effects of the principal stressors (nutrient concentration and streambed fine sediment cover) operating in native grassland streams converted to pasture in New Zealand, using two different approaches: a survey of 32 small streams and an experiment involving nine streams where the stressors were manipulated in a factorial design. We investigated the consequences for populations of benthic invertebrates and for the structure of communities, including taxon richness and the representation of species traits. 3. Up to half the taxa and most community metrics responded to at least one stressor. Our results suggest that in these streams, an increase in fine sediment loading from anthropogenic causes had more widespread effects than augmented nutrient concentrations. Of most significance is our finding, both from the survey and, in particular, the experiment, of a variety of complex interactions among the stressors. 4. Synthesis and applications. The development of indices of stream health that distinguish the effects of sediment from those of nutrients should help prioritize catchment management actions. Of more general importance is our finding that the consequences of stressors are often unpredictable on the basis of knowledge of single effects; if managers only consider the effects of individual stressors, their assessment of risk may be higher or lower than reality.