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

The 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 bioavailable P (BAP) in the lagoon. Time‐integrated samplers trapped suspended sediment from the water column, and their geochemical signature was compared with likely sources (stream banks, stream beds, topsoil, and subsoil) in each of the lagoon's contributing streams and rivers. The proportion of BAP, but not necessarily total P, 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 with the more mineral soils of the Waituna Creek. Annually, most BAP and sediment came from bank erosion, and strategies such as fencing out stock should focus on minimizing this throughout the catchment. However, when considering losses in space and time relative to the impact on the Waituna Lagoon, strategies the Waituna Creek catchment should also minimize contributions from topsoil in winter‐spring, whereas in the Carran and Moffat Creek catchments strategies need to decrease P inputs (e.g., effluent) to Organic soils likely to lose much BAP in summer‐autumn when the impact on the Lagoon is quickest. This study highlighted the need to identify sources and timings of BAP and sediment loss before recommending mitigation practices, which without this information may be slow or not succeed. Core Ideas The water quality of coastal lagoons is affected by sediment and P inputs. Knowing the provenance of sediment and BAP can help target strategies to mitigate loss. Organic‐rich soils lost more BAP but less sediment than mineral soils. Mitigating erosion and P form mineral soils should focus on stream banks and topsoil. Losses from Organic soils were from many sources; overall P inputs should be lowered.