Nutrients in the Swan-Canning Estuary  ||  Stream restoration of Wilson Creek
Ohio River ecosystem study  ||  James River ecosystem study

Nutrients in the Swan-Canning Estuary, Western Australia

I was extremely fortunate to be able to spend a sabbatical with the Centre for Water Research at the University of Western Australia. The sabbatical was facilitated by a Gledden Fellowship from UWA and my sponsor, David Hamilton. In addition to working on some of my own projects I had the opportunity to work with David and his post-doc, Barbara Robson to study nutrient fluxes in the Swan-Canning estuary.

Project overview
Despite decades of restoration and monitoring, ecosystems are more variable and unpredictable now than in the past. Aquatic ecosystems are subject to human and natural perturbations, which determine the timing and magnitude of water and nutrient delivery from their catchments. These processes give rise to complex ecosystem behavior and result in continual adjustment of management policies in response to unexpected changes in ecosystem state. Adaptive management is hampered by a lack of ecological and economic data needed to assess policy alternatives and by limitations in ecosystem modelling approaches for capturing the dynamic behavior of these ecosystems. An approach to address these deficiencies is the development of ecosystem models that couple natural and human processes relevant to management issues. Our research on the Swan-Canning Estuary demonstrates the utility of linking mass balance and model approaches to understanding ecosystem function and guiding management strategies.

Research approach
The Swan-Canning Estuary in Western Australia has been important to the establishment and growth of the city of Perth and is an integral part of regional tourism and fisheries. During the past century, extensive land clearing in the Swan Coastal Plain has resulted in the replacement of natural vegetation with agricultural crops. Runoff from agricultural and urban areas contributes salt, nutrients and sediments to the estuary. To better understand the sources and fate of nutrients in the estuary, we combined mass balance and modeling approaches to quantify nutrient loading from tributaries, nutrient export to the ocean and retention within the estuary. Traditional mass-balance budgets allow estimation of ecosystem nutrient retention, but generally provide little information about nutrient transformations and their sensitivity to environmental factors. On the other hand, process-based numerical models are capable of depicting nutrient transformations and their sensitivity to environmental influences but often are not validated with respect to system-scale nutrient balances. In this study, we adopt a combined approach, using a mass-balance to delineate net fluxes of nutrients within an estuary, and a time-dependent model with rate equations to resolve environmental forcing on specific processes affecting nutrient fluxes.

Project results
Annual nitrogen and phosphorus loads from tributaries to the Swan River Estuary were linearly correlated with annual total freshwater discharge, but urban drains and groundwater contributed a higher proportion of nutrients in high-flow years than in low-flow years. The nutrient budget indicates that the estuary acts as a significant sink of N and P, with water column losses from sedimentation and denitrification exceeding regeneration of nutrients from the bottom sediments. Sediment nutrient stores increased in most years, but occasionally decreased, depending on residence time and nutrient loads. Hence, changes in sediment stores in the estuary are a function of timing as well as magnitude of freshwater inputs. We are completing a manuscript (Robson et al.) for submission in early 2005.