Ecological degradation of streams and rivers as a result of agricultural land-use intensification is a major concern in New Zealand and other parts of the world. Agriculture introduces multiple stressors to streams, presenting a challenge for freshwater managers who must understand the relative strengths of each individual stressor and their combined multiple-stressor effects if they are to implement the most effective management actions and avoid ;;ecological surprises’ that arise from complex interactions between stressors. To investigate patterns of ecological response variables across broad gradients of two major stressors, augmented levels of dissolved inorganic nutrients and deposited fine sediment, I designed a streamside mesocosm experiment with eight levels each of nutrients (36 to 6900 micrograms per litre of dissolved inorganic nitrogen plus 1.4 to 450 micrograms per litre of dissolved reactive phosphorus) and deposited fine sediment (0 to 100 % cover of the streambed), and conducted a field survey in a regional set of 43 streams ranging from 2nd to 6th order. I used multiple linear regression and an information-theoretic approach to select the best predictive models for a series of ecological response variables, including algal, invertebrate and ecosystem variables, and tested (1) the subsidy-stress hypothesis for each stressor (where at low stressor levels an ecological variable responds positively until an inflection point beyond which the effect is negative), (2) whether sediment and nutrients operated as single or multiple stressors and whether they interacted with each other, and (3) whether sediment effects were more pervasive than those of augmented nutrient concentrations. In the 21-day long experiment, subsidy-stress patterns across the nutrient gradient were frequently found for algal and invertebrate taxa and communities, but consistently negative response shapes were more prevalent across the sediment gradient. The subsidy-stress hypothesis was not supported by the response of an ecosystem variable (organic matter breakdown, measured using cotton strips and fresh mahoe leaves). Overall, nutrients and fine sediment acted predominantly as multiple stressors and sometimes in complex interactive ways. The relative strengths of fine sediment and nutrient effects were similar for algal response variables but sediment was the more pervasive stressor for invertebrates, a finding that was also supported by the field survey. My field survey further suggested that nutrients and sediment commonly interact in synergistic ways to affect invertebrate variables, with fine sediment overwhelming any subsidy effects that nutrients may have in isolation. The combined experimental and survey results indicate that augmented levels of fine sediment and dissolved inorganic nutrient concentrations need to be managed together because they mostly act as multiple stressors in their effects on algal, invertebrate and ecosystem response variables. While managers should seek to control both nutrient and fine sediment inputs to streams to achieve good ecological stream condition, measures to reduce or avoid further sedimentation are particularly likely to be effective in mitigating ecological impairment and preventing further harm. Finally, in order to best assess the likely causes of decline in stream health, it will be highly desirable for managers to routinely monitor both nutrients (as currently done) and fine sediment in the future.
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Multiple-stressor effects along gradients of deposited fine sediment and dissolved nutrients in streams
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