Epsilon Archive for Student Projects

Abstract

Streams and rivers are highly susceptible to environmental degradation from agricultural activities, including the clearance of riparian vegetation and the runoff of chemical fertilizers and pesticides. These impacts are likely to increase in the future as agricultural practices intensify to meet the needs of an expanding human population. For example, pesticide application has considerably increased in the last 35 years, with an increased runoff to aquatic ecosystems. Importantly, intensive agriculture often entails the use of multiple pesticides for different purposes (e.g. control of different bacterial, fungal or insect pests). Prediction of the ecosystem effects of the application of multiple pesticides is complicated by the potential both for interactions among the pesticides themselves, and for the pesticides to alter interactions among different organism groups within trophic webs. I investigated the effects of two contrasting pesticides targeting two different organism groups (the insecticide Lindane and fungicide Azoxystrobin) on a stream detrital food web consisting of detritivores (Ispoda: Asellus aquaticus) - and microbes (an assemblage of fungal hyphomycetes) consuming leaf litter. I assessed effects of the stressors on ecosystem functioning, quantified as multiple ecosystem process rates. These included leaf decomposition, leaf processing efficiency and detritivore growth rate. Leaf decomposition is a key ecosystem process in the nutrient and energy budgets of forested streams worldwide. Additionally I quantified detritivore mortality and moulting characteristics (frequency and moulting period). Standardized discs of black alder leaves (Alnus glutinosa L.) were colonized with a fungal assemblage for use in a microcosm experiment. The fungal assemblage was sourced from a forested catchment characterized by mixed agricultural and forest landuse. Each microcosm contained 20 colonized leaf discs, and 50 mL of standardized artificial fresh water (“M7”). Four pesticide treatments were varied among the microcosms: (i) no presence of pesticides (i.e. controls), (ii) Lindane 5 μg/l (single stressor), (iii) Azoxystrobin 2600 μg/l (single stressor), and (iv) a mixture of Lindane 5 μg/l and Azoxystrobin 2600 μg/l (multiple stressors). Additionally, the presence and absence of the detritivore Asellus aquaticus (Isopoda) was varied among the microcosms, to assess the effect of pesticides across multiple trophic levels. I hypothesized that the fungicide and insecticide applied as single stressors will both negatively affect leaf decomposition through negative effects on microbe and detritivore-mediated
6 decomposition respectively, with additional “knock-on” effects of the fungicide on detritivore leaf processing efficiency and growth due to negative effects on microbial conditioning (microbial “softening” of the litter necessary for detritivore feeding). Consequently, I further hypothesized that two pesticides will interact synergistically negatively to affect leaf processing by the full detrital foodweb, with the strongest effects likely in the pesticide mixture treatment when the detritivores are present.

Pesticides affected ecosystem functioning in my laboratory microcosms, but these effects did not always correspond with expectations based on their target trophic level. The fungicide little affected decomposition mediated by microbes, and the insecticide did not have an overall affect on decomposition mediated by detritivores. However, an important interaction was apparent between the detritivore and pesticide treatments, with the fungicide and mixture treatments reducing decomposition only when the detritivore was present. This indicates the fungicide had significant knock-on effects on the performance of the detritivores, most likely reflecting the importance of microbial “conditioning” (leaf softening) of the detritus for the participation of A. aquaticus in the decomposition process. Synergistic interactions between the pesticides were also apparent, with detritivore leaf processing efficiency depressed most strongly when both pesticides were applied together. These effects were not reflected in identical responses for detritivore growth, which may be a consequence of the relatively short experimental period. The mortality rate was higher under the fungicide and mixture treatments, which may reflect reduced resource intake due to fungicide effects on microbial conditioning, toxic effects of the pesticide, or both. Finally, there was evidence that detritivore moulting period (time to first moult) was shortened under the pesticide treatments, which may indicate that detritivores have some capacity to adjust their moulting time to shed exoskeletons contaminated with toxins, particularly under repeated pulses of exposure. My results indicate that changed interactions within food webs can complicate prediction of pesticides effects on ecosystem functioning in streams, and highlight the potential for pesticides to disturb ecosystem structure and function in agricultural areas.