IMPACTS OF CONTAINMENT AND ELEVATION ON CREATED MARSH PLANT COMMUNITY AND ECOSYSTEM DYNAMICS

Coastal marsh restoration is increasing globally, as marshes are impacted by anthropogenic impacts. Billions of dollars are allocated to marsh restoration in the Mississippi River delta where wetland loss rates are extremely high. The recovery of structural and functional attributes of created marshes varies by attribute measured and project. Here, two marshes created in 2018 and one natural marsh were used to examine how variations in hydrologic connectivity and marsh elevation influence subsurface hydrology and salinity, vegetation community structure, soil properties, accretion, and denitrification. I established a comparative field study in a created marsh contained within a higher elevation containment dyke (Confined), an unconfined created marsh where the dredge sediment was allowed to settle naturally (Unconfined), and a nearby natural marsh on the northeast shore of Lake Pontchartrain in Big Branch National Wildlife Refuge, Louisiana. Most marsh creation projects in Louisiana have a surrounding higher-elevation containment dyke to limit the loss of dredge sediment. The long-term effects of containment dykes on tidal exchange, plant dispersal, sediment delivery, and accretion are unknown. Marshes were surveyed for elevation and vegetation by species (n = 12). Surface accretion rates were measured in the Natural marsh and created marshes using feldspar marker horizons (n = 5). Vegetation communities differed among marshes with Spartina patens dominating in the Natural marsh, Typha domingensis dominating low to moderate elevations and Baccharis halimifolia dominating high elevations in the Confined marsh, and Schoenoplectus americanus, which is a desirable species for wildlife, dominating in the Unconfined marsh. The Unconfined marsh accreted more mineral matter than the Natural marsh. Denitrification rates were lower in the Confined marsh than in the Unconfined and Natural marshes. Overall, this study illustrates large differences among marshes related largely to elevation, though containment dykes may also be fostering the colonization of wind-dispersed species, such as Typha domingensis. The Unconfined marsh has higher accretion rates, lower maximum salinities during low water events, and higher denitrification rates than the Confined marsh. Ultimately, these findings show the importance of marsh creation design in facilitating the development of desired ecosystem structure and functions.