Elevation and accretion dynamics at historical plots in the Biloxi Marshes, Mississippi Delta

Abstract

The objectives of this study were to examine changes in accretion and elevation change over periods of up to 15 years for the Biloxi marsh complex (BMC) in southeastern Louisiana, part of the Mississippi Deltaic Plain, identify factors affecting accretionary dynamics, and put these findings in the context of ongoing restoration. We present elevation and accretion data from Surface Elevation Table (SET) and feldspar marker horizon sites first established in 2003. The sites were clustered in two areas (East and West) in the central BMC on the eastern edge of the Mississippi delta. Accretion markers were used in conjunction with elevation measurements to calculate shallow subsidence. These data were analyzed along with similar data from nearby Coastwide Reference Monitoring System (CRMS) sites located around the periphery of the BMC. Elevation decreased at the Western sites by −0.35 ± 0.13 cm/yr, and increased at the Eastern and CRMS sites by 0.40 ± 0.03 cm/yr and 0.72 ± 0.09 cm/yr, respectively. The rate of accretion was similar at the Western (0.49 ± 0.14 cm/yr) and Eastern (0.64 ± 0.07 cm/yr) sites, and over twice as much (1.30 ± 0.11 cm/yr) at the CRMS sites. Shallow subsidence, calculated as the difference between vertical accretion and surface elevation change, was 0.76 ± 0.49 cm/yr at the Western sites, 0.23 ± 0.06 cm/yr at the Eastern sites, and 0.58 ± 0.11 cm/yr at the CRMS sites. These trends are consistent with the observation that sediment is brought in from Chandeleur Sound to the east and is attenuated as deposition occurs across the landscape from east to west, and that levee flank depressions associated with Bayou La Loutre, an abandoned Mississippi River distributary ridge, are causing locally high subsidence in the Western region. Without intervention, these localized areas of the Western region will be submerged within the next several decades at current rates of elevation loss and eustatic sea-level rise, while the Eastern sites and the wetlands on the periphery of the BMC are likely to keep pace with sea level rise well into the second half of this century. These results demonstrate the importance of accurate knowledge of both subsidence and accretionary dynamics in determining coastal wetland sustainability and restoration approaches.