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Initially the L. peregra bioassay developed for use under laboratory conditions demonstrated reduced ingestion and egestion rates of L. peregra at sublethal exposure concentrations to aqueous solutions of cadmium and copper. The two organic compounds tested, the polyaromatic hydrocarbon pyrene and the pyrethroid insecticide lambda-cyhalothrin, did not inhibit ingestion or egestion at sublethal concentrations. Post-exposure median ingestion and egestion values (ECso) for copper were 15 and 13 J.lg/L and for cadmium were 0.05 and 5.6 J.lg/L respectively. The lethal effect values for L. peregra exposed to copper and cadmium were 115 J.lg/L (estimated laboratory derived LCso) and 2500 J.lg/L (LCso from literature) respectively were both higher than the estimated ECso values. This indicates the relevance of using these post-exposure sublethal endpoints for L. peregra exposed to these heavy metals. Investigation of the uptake of cadmium by L. peregra revealed that the major route was through aqueous solution with only minor contributions from contaminated food. The in situ bioassay was developed further under semi-natural (in microcosms) and field conditions. L. peregra showed reduced egestion following aqueous exposure to zinc in microcosm studies (at 0.5 J.lg/L), but again did not show a response to either of the organic contaminants lambda-cyhalothrin and the herbicide linuron. The L. peregra field bioassay was deployed at 9 historically contaminated sites throughout Scotland and Northern England. Post-exposure egestion rates were shown to be significantly reduced two metal contaminated sites. The inconsistency ofresults gained from the remaining sites may have been due to decreased sensitivity brought about by wide variation in egestion rates. The L. peregra bioassay is useful tool for the investigation of the effects of single stressors such as heavy metals. However, the lack of consistent results gained from the in situ deployments suggests that the L. peregra bioassay should be used, at this time, as one of a suite of tests to investigate the quality of freshwaters rather than as a discrete test.

The conditions found in stream ecosystems are an integration of watershed characteristics and processes. Anthropogenic disturbances are part of this integration and include direct inputs to streams, alteration of riparian areas, and modification of catchment properties which affect material inputs. Impacts to discrete portions of terrestrial watersheds combine as water moves down gradient, transporting the byproducts of catchment disturbances to streams including, nutrients, organic materials, particulates, and toxins. This dissertation explores the effects of disturbance on nutrient cycling in stream ecosystems at three spatial scales: the patch scale includes localized processes and assemblages, the reach scale encompasses tens to hundreds of meters of stream length, and the watershed scale consists of the hierarchical network of stream orders found in catchments. Additionally, the gut bacterial communities of three freshwater snail species were investigated to better understand the ecology and physiology of this important group of aquatic grazers. Snails are important regulators of periphyton growth, which plays an integral role in nutrient cycling in stream ecosystems. Freshwater snails are also intermediate hosts for a variety of parasites of medical and veterinary significance. Biofilm assemblages are patch scale communities which dominate the metabolism and biogeochemical cycles in stream ecosystems. To determine the effects of eutrophication, one of the most common disturbances to stream ecosystems, on the structure and function of heterotrophic stream biofilms, we created an enrichment gradient by amending darkened stream channel mesocosms with a stochiometrically balanced solution of sucrose, NH4, and PO4. A total of ~2000 high quality bacterial partial 16S rRNA gene sequences yielded 381 unique phylotypes (