Effects of invasive N2-fixing Acacia mearnsii on sediment nutrient concentrations in mountain streams: Implications of sediment geochemistry for ecosystem recovery

Abstract

Widespread invasive nitrogen-fixing plant species pose major threats to water resources, biodiversity and nutrient dynamics of river catchments around the world. However, the impacts of invasive N2-fixing plants on in-stream sediment nutrient dynamics remain poorly understood. Here, we quantified the impacts of invasive N2-fixing Acacia mearnsii and A. mearnsii clearing on two mountain streams in South Africa’s Cape Floristic Region. Nutrients were measured in fine sediment that had infiltrated into the gravel bed and surface water in three reaches associated with natural fynbos shrubs, A. mearnsii invaded and cleared riparian ecotones. Results showed that the impacts of A. mearnsii invasions and Acacia clearing in riparian areas on in-stream sediment-associated nutrient concentrations are context-dependent. The difference in channel morphometry and sediment geochemical attributes at reach-scale contributed to the contrasting spatial patterns of sediment-adsorbed nutrients within these rivers. There was also an indication of a long-lasting effect of invasion on total phosphorus that could persist in river sediments ≥10 years after the removal of dense A. mearnsii stands in close proximity to streams. This could be a result of the complexation between phosphorus and immobile iron in sediment, resulting in the retention of ‘legacy phosphorus’. This underlines the importance of fine sediments in capturing nutrients and ultimately regulating nutrient concentrations in the water column.