Valley‐bottom wetlands as temporary buffers for source‐to‐sink dispersal of sediment and associated phosphorus in dryland landscapes

Abstract

Sediment trapping in wetlands is an essential ecosystem service, with implications for downstream ecosystems and water users. There is however limited empirical evidence of the contemporary rates and magnitude of sediment trapping in valley‐bottom wetlands. Time‐averaged suspended sediment samples from the inlets and outlets of forestry‐ and agriculturally‐impacted valley‐bottom wetlands with contrasting morphometric characteristics were compared in terms of suspended sediment and associated total phosphorous (total P) fluxes over annual scales, a dataset that was limited by Covid travel constraints. Although both wetlands were net depositional, contemporary suspended sediment mass balances for the agriculturally‐impacted wetland revealed a temporal change in the amount of sediment trapped over two water years (2019/2020 and 2020/2021), with trapping efficacies of 91% and 24%, respectively. The proportion of sediment trapped in the water year of 2020/2021 within the adjacent wetland, with a small commercially forested catchment, was up to 4 times higher than the agriculturally‐impacted wetland, which drained a larger catchment. Rates of total P retention showed that the agriculturally‐impacted wetland was a net sink for phosphorus in 2019/2020, but shifted to a source of phosphorus in 2020/2021 as the export of suspended sediment was enhanced. However, this contrasts with the forestry‐impacted wetland, which was a net sink of sediment and associated phosphorus during the one‐year study period of 2020/2021. Overall, despite data constraints, this study suggests that the efficacy of valley‐bottom wetlands in the delivery of sediment trapping and phosphorus removal ecosystem services varies temporally and spatially. This variability is potentially related to the interaction between annual rainfall regimes, catchment size and wetland geomorphic character. The temporary nature of sediment recycling processes could serve to balance wetland dynamics by regulating vertical growth of valley floors and longitudinal slope stability and should be considered in catchment management and wetland restoration planning strategies.