Marsh submergence vs. marsh accretion: Interpreting accretion deficit data in coastal Louisiana

The apparent imbalance between relative sea-level rise and vertical marsh accretion is frequently cited as a major factor in the problem of wetland loss in Louisiana. Rates of relative sea-level rise are high in Louisiana due to high rates of subsidence. Although marsh accretion rates are also high, they are usually insufficient to maintain the relative elevation of the marsh surface. This situation is commonly referred to as an accretion deficit. The interpretation of subsidence and accretion data, and therefrom accretion deficit data, is confounded by the numerous geologic, biologic, and sedimentologic processes influencing coastal marshes in Louisiana. Therefore, calculation of accretion deficits can be influenced by the techniques used to measure subsidence and accretion. The concept of accretion deficit is based on the assumption that accretion rates are equivalent to elevation change rates, but this assumption may not necessarily be correct. We suggest that direct measurements of elevation change in marsh surface can provide better indications of the status of the marsh surface with respect to subsidence and accretion.