Contrasts in stream water chemistry impact aquatic ecosystem functioning in an agricultural landscape

Given the importance of freshwater ecosystems to human health and wellbeing, the resilience of aquatic communities to pollution from a variety of sources is of increasing concern. Current indices concentrate on structural measures to define stream health. These include community assemblages, in-stream water chemistry, and hydrological impairment through physical modifications. However, ecosystem services provided by freshwater communities rely on the underlying biogeochemical cycles that are a function of metabolic processes. At present, these are not routinely used in assessments of ecological status. A paired sub-catchment approach was used to study the effects of different land management practices on in-stream water chemistry, and their consequences to aquatic functional integrity in an agricultural landscape. The study provided an opportunity to assess the potential for ecosystem functional measures to complement the structural measures that are used to define impact on aquatic communities. High resolution analysis of the nutrient chemistry within two study reaches underpinned comparisons of community aerobic respiration, greenhouse gas transfer across the sediment-water interface and macro-invertebrate mediated processing of organic matter. This programme of measures identified clear differences between the study sub-catchments. It revealed that the management of animal waste, and control on the delivery of fine sediments to a watercourse, were key influences on in-stream functional integrity. The delivery of inorganic nutrients as a result of fertiliser application was also evident. However, the significance of this signal was masked by the overriding effect of high loads of organically loaded fine sediments and low flow in the Priors Farm reach