Dredge-material created coastal marshes are more effective at improving water quality than natural marshes in early-stage development

Abstract

Coastal regions around the globe are experiencing land loss due to rising sea level. Some of the largest rates can be found in deltaic Louisiana, USA. Beneficial reuse of dredged sediments to create coastal marshes is one major effort in helping to reverse this land loss. In 2014, the Lake Hermitage Marsh Creation Project created 29 ha of marsh by dredging sand from the bed of the lower Mississippi River. The goal of this study was to determine the ability of these newly created mineral soil marshes to improve water quality. The natural marsh had 8.2 times greater organic matter content than the created marsh, on average. Under aerobic water column conditions, the mean denitrification rate of the natural marsh was 24.5 ± 3.53 mg NO3-N m2 d−1 while the mean denitrification rate of the two created marshes was significantly greater at 63.19 ± 3.44 mg NO3-N m2 d−1. This difference in rates is a result of internal N loading from the highly organic natural marsh soil, which had 3.7 times more extractable ammonium than the created marsh. As organic matter increases in the created marsh soil, the N removal efficiency may decline over time. This difference in efficiency has important implications not only for coastal restoration projects but also for created wetlands, which accrete significant organic matter over time and may lead to reduced efficiency of N removal, driven by internal N loading from soil.