System-scale dynamics of long-term sediment and phosphorus accumulation in contrasting valley-bottom wetlands in dryland landscapes of South Africa

Abstract

Anthropogenic-induced acceleration of sediment and associated nutrients presents a serious challenge to water resource management across the globe. While wetlands are potential natural solutions, little is known of how well key wetland types attenuate downstream sediment fluxes and how these relate to local- and catchment-scale factors. This research aimed to develop an understanding of patterns and rates of sediment and phosphorus retention in valley-bottom wetlands in South Africa. The geomorphology, sedimentology and historical accretion rates of three wetlands which offered diversity in size, catchment position and degree of organic accumulation were compared to ascertain the impact of local- and catchment-scale factors. Estimates of sediment and associated phosphorus accumulation suggest higher rates in clastic sediment-dominated wetlands (1089–7655 g sediment m−2 yr−1, 0.4–1.6 g-P m−2 yr−1) compared to an organic sediment-dominated system (601–1308 g-sediment m−2 yr−1, 0.2–0.3 g-P m−2 yr−1). Sediment accretion rates generally increased longitudinally, indicating that distal wetland reaches act as sediment and phosphorus retention hotspots. Phosphorus distribution patterns were primarily attributed to variations in organic matter content and associated aluminium and iron complexing agents in fine-grained sediment deposits, while particle size distributions were less important. However, the relevance of individual complexing agents in phosphorus dynamics varied among wetlands with differing landscape conditions. Contrasts between these systems reflect variations in catchment lithology and hydroclimatic regime, which influence the relative magnitude of sediment supply and vegetation productivity. We postulate that intermediate-sized wetlands characterised by catchments around 900 ha in size, gentle longitudinal slopes (∼2 %), a high proportion of fine sediment deposition, limited organic matter breakdown, and a long vegetation growing season provide optimal conditions for sediment and phosphorus accumulation. This system-scale information is essential to guide management decisions to identify and prioritize within-catchment ‘hotspots’ of sediment and phosphorus retention.