Implications of river reconnection on phosphorus cycling in coastal wetlands

Abstract

Louisiana's coastal wetlands are experiencing some of the world's largest land loss rates. This problem is partly due to levees along the Mississippi River, isolating the river from the coastal basins. This disconnect prevents delivering of sediment and nutrients to the wetland-dominated coastal basins, where sediments would increase marsh accretion. Louisiana's Coastal Master Plan aims to reconnect the river with riparian areas through construction of a diversion. Baseline phosphorus (P) dynamics were determined before river reconnection and compared to an area with an unmanaged connection to the river. In Barataria Basin, the equilibrium P concentration (EPC) was lower in both the marsh (0.039 ± 0.015 mg L⁻¹) and open water sediments (0.016 ± 0.008 mg L⁻¹) than the concentration of soluble reactive phosphorus (SRP) in the Mississippi River (∼0.075 mg L⁻¹). Additionally, total P was significantly higher in marsh soil (677 ± 183 mg P kg⁻¹) compared to the open water sediments (503 ± 90 mg P kg⁻¹). On average, the organic residual P fraction was the dominant individual P form, comprising 39 % (254 ± 78.2 mg P kg⁻¹) of total phosphorus (TP) in marsh soil and 45 % (208 ± 65.9 mg P kg ⁻¹) of TP in open water sediments. The primary form of total P in the river sediment is the Fe/Al mineral fraction at 43 % (469 mg P kg ⁻¹). Consequently, river reconnection, the dominant form of soil P will shift to inorganic Fe/Al-bound P. This shift will likely increase the internal loading of P over time due to iron reduction, releasing newly deposited mineral-bound phosphorus into the water column In vegetated wetland areas, this river-sourced P can be taken up by algae and macrophytes, while in open water areas, there could be an increase in algal blooms in these newly river-reconnected coastal basins, changing the P dynamics.