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

• Published in: Earth surface processes and landforms Vol. 49; no. 3; pp. 1179 - 1198
• Main Authors: Wiener, Kenwinn D., Grenfell, Suzanne E.
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 15.03.2024
• Subjects: Annual rainfall; Arid lands; Arid zones; Catchment area; Catchments; drylands; Ecosystem services; Ecosystems; Environmental restoration; Forestry; Geomorphology; Inlets; Inlets (waterways); Morphometry; Outlets; Phosphorus; Phosphorus removal; Sediment; sediment (dis)connectivity; sediment dispersal; Sediment samplers; Sediment samples; Sediments; Slope stability; Suspended sediments; Trapping; Valleys; Water consumption; Water users; Wetland restoration; Wetlands
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.5761

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. Valley‐bottom wetlands in dryland landscapes are dynamic fluvial landforms and play an important role in delivering a myriad of ecosystem services and supporting human well‐being. Overall, results show that the efficiency of sediment trapping and phosphorus removal services in these wetlands are temporally variable and system‐specific, and likely depends on the interaction between annual rainfall regimes, catchment size and local‐scale wetland geomorphology. As these systems alternate between periods of net sediment trapping and short‐term episodic recycling of sediment, we suggest that this could serve as a mechanism to locally regulate morphological adjustment. This study therefore highlights the importance of considering the temporary nature and spatial variability of sediment recycling processes in catchment management and wetland restoration planning strategies.