Using the Provenance of Sediment and Bioavailable Phosphorus to Help Mitigate Water Quality Impact in an Agricultural Catchment

The water quality and health of surface waters can be impaired by sediment and sediment-bound phosphorus (P). The Waituna Lagoon catchment in southern New Zealand has undergone agricultural intensification that has been linked to increases in sediment and sediment bound P in the lagoon. Time-integrated samplers trapped suspended sediment from the water column and their geochemical signature was compared to likely sources (stream banks, stream beds, topsoil and subsoil) in each of the lagoon’s contributing streams and rivers. The proportion of bioavailable P (BAP) within trapped sediment was much greater in samples from the Moffat and Carran Creeks, than from the Waituna Creek, probably due to the erosion of organic-rich soils that had little capacity to retain P compared to the more mineral soils of the Waituna Creek. The majority of trapped sediment was estimated to come from stream banks in the Waituna Creek catchment, with the remainder attributed to topsoil. Topsoil, subsoil, and bank sediments were all potential sources of trapped sediment at the other sites, but not bed sediment. To mitigate BAP and sediment loss, strategies should prevent stream bank collapse and erosion through processes such as better fencing out of stock, reconstructing collapsed banks or modification of current drain clearing methods. Additional strategies should also minimise contributions from topsoil in the Waituna Creek from, including landuses such as the grazing winter forage crops. This study highlighted the need to identify sources of sediment and BAP to help target mitigation practices and their likelihood of success.