The effects of vegetation on island geomorphology in the Wax Lake Delta, Louisiana

Understanding how deltas build and maintain themselves is critical to predicting how they will respond to perturbations such as sea level rise. This is especially an issue of interest in coastal Louisiana, where land loss is exacerbated due to subsidence and decreased sediment supply. Feedbacks between ecology and geomorphology have been well documented in tidal environments, but the role of vegetation in delta morphodynamics is not well understood. This study investigates spatial and temporal correlations between vegetation succession and sediment accumulation at the Wax Lake Delta in Louisiana. I established a 2500 m long transect along the western levee of Pintail Island, capturing the full range of island elevations and the transition from bare sediment to herbaceous plants and trees. Shallow (50-100 cm deep) sediment cores taken along this transect were analyzed for particle size, organic matter content, and bulk density, and dated using ²¹⁰Pb. The resulting sedimentation rates and composition trends over time were compared to remote sensing-based analyses of temporal changes in island topography and flooding frequency derived from historical Landsat images. We found that the topography of Pintail Island has developed from a non-systematic arrangement of elevations to a discrete set of levees and intra-island platforms with distinct vegetation types, designated as high marsh, low marsh, and mudflat habitat. This elevation zonation is consistent with alternative stable state theory as so far applied to tidal salt marsh systems. At all but the youngest sampling site, sediment cores showed a significant decrease in organic matter content and a significant increase in grain size with depth. The total organic matter contribution to vertical growth was not sufficient to account for all the elevation change required to achieve the differentiation from low marsh to high marsh deduced from the time-lapse Landsat imagery analysis. Mineral sediment accumulation rates suggested that elevation growth was accelerating or holding steady over time, in contrast to theory suggesting rates should slow as elevation increases. These results provide an empirical foundation for future mechanistic models linking mineral sedimentation, organic sedimentation, vegetation succession, elevation change, and flood frequency in the delta.