Salt marsh restoration with sediment-slurry amendments following a drought-induced large-scale disturbance

Abstract

Approximately 40,500 ha of Spartina-dominated marshes died during a drought-induced disturbance in Louisiana. This caused concern because in the absence of recovery, dieback marshes can subside, increasing the present high rates of wetland loss in coastal Louisiana. We assessed the recovery of one such dieback area after hydraulically dredged sediment-slurries were applied to the site to compensate for post-dieback soil consolidation. Five treatment-levels resulted from the sediment-slurry addition: 1) high elevation 2) medium elevation, 3) low elevation, 4) pop-up, and 5) vegetated. Plant recruitment within the five sediment treatment-levels were compared to two types of reference marshes: 1) ambient marshes, which neither died-back nor received sediment-enrichment, and 2) dieback marshes, which did not receive sediment addition. High and medium elevations had minimal recovery two years following the slurry addition, similar to that in reference dieback marshes. The low elevation, pop-up (highly organic sections of the original substrate that detached during slurry application and settled on top of the sediment-slurry), and vegetated (dieback areas that recovered by the start of the study) treatment-levels, all of which received sediment-slurry application, had rapid plant recovery. Plant recolonization was governed by optimal inundation, high organic matter content at high elevation, and rhizome survivability following sediment burial. If applied appropriately, sediment-slurry amendments can restore salt marshes submerging due to subsidence or other events, like sea-level rise, that may result in excessive plant submergence.