Multiple stressors on water availability at global to catchment scales: understanding human impact on nutrient cycles to protect water quality and water availability in the long term

1. Freshwater systems are subject to multiple stressors that include changing climate, changing land use, changing demands on water resources and changing nutrient cycles. Global trends suggest these stressors that impact on water availability will increase over the coming decades, and - without action - will constrain opportunities to sustain ecosystem services to deliver the Millennium Development Goals. 2. Although a key 'service' freshwaters provide is buffering inputs from the land system, predicting ecosystem response through observation and modelling is complex because nonlinear and dynamic interactions amongst a large number of constituents operate to regulate biogeochemical transformations in freshwater systems. 3. Reductionist approaches have been successful at unravelling many of the processes and some of the interactions in freshwater systems. However, reductionist approaches cannot provide the concepts or methods to understand how system properties will emerge in response to a changing climate (particularly the changing spatial and temporal distribution of precipitation); to the consequent change in water availability and water quality in the context of social drivers on the demand for water; and to feedbacks arising from nutrient cycling across a range of scales. 4. This study presents a review of the evidence for landscape-scale filtering of nutrient delivery to receiving waters and examines the role of the internal processing of nutrients at critical interfaces such as the hyporheic in attenuating nutrient loads. Analysis of research on the spatial scales and time step appropriate for catchment and water quality observations indicates the importance of small scale and short time step data for unravelling nutrient cycling in freshwater ecosystems. 5. Thematic implications: leading work in the catchment and aquatic sciences on the importance of diffuse nutrient losses from land, and on nutrient cycling in freshwaters, in governing water quality and protecting water availability, is making an increasing contribution to mainstream environmental science. Critically, the evidence base is starting to grow to inform policy-related debates with respect to food security, to climate change adaptation and for sustaining ecosystem services in freshwater environments.