Catchment export of carbon, nitrogen, and phosphorus across an agro-urban land use gradient, Swan-Canning River system, southwestern Australia

Coastal regions in many regions of the world are under increasing pressure from the expansion of agriculture and urbanization associated with elevated N and P loading and eutrophication of coastal estuaries. I compared how mixed land use catchments deliver dissolved and particulate forms of C, N, and P in streamflow to the Swan-Canning estuary that bisects Perth, Western Australia. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) composed the majority of the total C and N load, particulate C and N fluxes were minor, and P fluxes were evenly split between soluble reactive phosphorus and particulate/organic P. In contrast to current biogeochemical theory, DON export was dominant in urban and agricultural catchments in the low-gradient environment of the Swan Coastal Plain, whereas NO3 export was a greater factor in higher-gradient, forested catchments on the urban fringe. This trend suggests that hydrologic conditions that supported coastal wetlands prior to human development may still promote DON mobilization as well as dissolved inorganic nitrogen loss along hydrologic flow paths. Substantial variability in export of C, N, and P across catchments highlights the unique hydrologic properties of Australian catchments. Areal C, N, and P export was significantly related to catchment runoff which was lowest in a catchment with inland drainage, but greatest in urban catchments with impervious surfaces and shallow groundwater. The effective delivery of DOC and DON to aquatic ecosystems in urbanizing coastal catchments underscores the importance of restoration efforts that address hydrologic retention as well as the source and bioavailability of dissolved organic matter.