Evaluation of hydro-geomorphic controls on the spatial distribution of sediment-associated phosphorus concentrations in a semi-arid floodplain

Abstract

Floodplain wetlands are valued for providing ecosystem services, yet these services are threatened due to land-use pressures and climate change. Further, regulating ecosystem services such as phosphorus (P) assimilation and storage are influenced by floodplain morphology and water and sediment dispersal processes, which vary over time. To investigate the influence of hydro-geomorphic processes on total sediment-associated P in surface sediment, this study evaluated the surface inundation and sedimentary characteristics of different regions of a spatially complex semi-arid floodplain in South Africa. Data collection involved stratified floodplain surface sediment sampling to characterise surface variations in geochemistry, particle size and organic matter content. This was paired with a Sentinel-2 imagery-derived frequency of inundation and a LiDAR DEM to incorporate both the influence of hydro-geomorphology and sedimentology on sediment dispersal dynamics and P sequestration on the floodplain. P in surface sediment was strongly affiliated with organic content, Fe, Al and mean particle size. PCA revealed that sample variability was associated with sediment geochemistry and particle size (component one, 63.48% of variance explained) and frequency of inundation and distance to continuous channel (component two, 15.19% of variance explained). An analysis of geomorphic units within the floodplain found that the incomplete avulsion and resultant crevasse splay, as well as abandoned channels, were hotspots of P in surface sediment. However, the spatial extent of other features, with moderate concentrations of P in surface sediment, highlights the importance of geomorphic diversity in overall ecosystem service provision.