Effects of multiple stressors on macroinvertebrate communities

Impacts from mixtures of pesticides and nutrients are prevalent and widespread in aquatic ecosystems. Our ability to predict and manage changes in real systems however, is uncertain. My doctoral dissertation evaluates the toxic effects of these insecticide mixtures on field-collected, aquatic invertebrate communities using outdoor, artificial streams. I examined the effects of mixtures of three of the most commonly used insecticides in North America namely, chlorpyrifos, dimethoate, and imidacloprid. Macroinvertebrate community responses to the insecticide mixture treatments were examined with respect to nutrient gradients and increased intensity of predation by a stonefly (Agnetina capitata). I found that mixtures containing dimethoate were less toxic than expected (Chapter 2). Low insecticide doses combined with moderate nutrients were also less toxic than expected (Chapter 3). In contrast, nutrient enrichment combined with higher insecticide doses made some treatments more toxic (Chapter 3). Interestingly, the intensity of predation also exacerbated the effects of nutrient-insecticide gradients but only in mesotrophic treatments (Chapter 4). My results suggest that managers may be better informed by regionally focused study of common mixtures of multiple compounds on relevant assemblages of organisms than by the systematic misuse of toxicity models. Community responses may not be predictable from laboratory-based, single-species bioassays because nutrient gradients and biotic factors, such as the intensity of predation, may be more important to the structure of aquatic communities than insecticide dose. The reality of even more complex stressor patterns than those examined here warrants further investigation.