Multiple stressor responses of stream benthos to nutrient enrichment and inorganic sedimentation

Abstract

There is a need to investigate freshwater ecosystem degradation as a function of multiple, often coinciding factors (stressors). Artificial stream systems known as mesocosms provide a useful means of disentangling the contributions of individual stressors acting in a multiple stressor environment while also investigating interactions between these factors. This thesis examines the impacts of locally relevant levels of nutrient loading and inorganic sedimentation on benthic invertebrate assemblages characteristic of northern temperate streams. Experimental manipulations were performed using a mesocosm system consisting of 96 self-contained artificial streams. Key questions addressed included: (1) what are the individual contributions of sedimentation and nutrient loading when simultaneously acting on benthic environments; and (2) do multiple-stressor impacts simply match the effect of the most deleterious stressor or are stressors worse in combination than alone? Complex multiple-stressor relationships were observed between nutrients and fine sediment, and combined effects could not be inferred from information on single-stressor effects. Several individual invertebrate taxa, such as Lepidostomatidae caddisflies, responded positively to additions of nutrients and fine sediment, although the apparent subsidy from single factors was often suppressed under multiple-stressor conditions. Sediment functioned exclusively as a habitat variable, and the effects on individual taxa were dependant on their specific habitat requirements (niches). Nutrients drove shifts in periphytic algal composition, from low-profile diatom dominated communities to stands of high-profile diatoms with an overstory of mostly filamentous green algae. Moderate nutrient enrichment was related to increased organism density, particularly among herbivores, although densities of several taxa declined with high enrichment, especially when coupled with sediment addition. This suggests that in highly enriched systems the food resource subsidy is confounded by habitat related stress, as excessive algal accumulation smothers the substrate and clogs interstitial spaces. Therefore, in highly impacted streams stressor mechanism overlap is probable, as both algal accumulation and sedimentation contribute to deterioration of benthic/hyporheic habitat. Given that these stressors strongly co-vary along agricultural land use gradients, it should be anticipated that with increased agricultural intensity the cooperative action of nutrients and sediment will amplify, leading to ecological impacts that far exceed that of either driver individually. Pollution standards and management practices should reflect this precept and the independent management of common non-point source pollutants should be phased out in favour of comprehensive, multiple-stressor strategies.