Discussion

Discussion and Conclusions

Constructed wetlands are becoming more prevalent as humans attempt to offset the loss of valuable coastal environments. Enhancing coastal wetlands by restoring environments to their natural footprint can lead to regaining ecosystem functions and services, such as water quality improvement, carbon sequestration, storm surge protection, and wildlife habitat. Wetland loss is rampant in the Gulf of Mexico, more so than the watersheds of the Atlantic coastline and the Great Lakes. Restoration in the United States has generally targeted Spartina-dominated marshes in Texas, Louisiana, and much of the Atlantic coast. The Juncus-dominated marshes of Mississippi and Alabama have gone relatively-unnoticed and thus assessments of marsh restoration in these areas are few and far between.

The success of marsh restoration should be measured by a project’s progress towards goals specifically stated by in the restoration management plan, however, the goals often have little specificity or a timeline for meeting them. This study measured progress of the two marshes constructed with beneficial-use material on Deer Island by comparing classic developmental indicators such as plant diversity and biomass, as well as SOC. Salt marsh development is only able to take place once vegetation is planted or grown in adequate densities at a proper elevation with soil that is nontoxic and unrestricting of root growth.

The elevation and physical sediment characteristics varied significantly between the constructed and reference sites. The 10+ y constructed site tended to be higher in elevation and have higher portions of larger sand grain sizes than both the 2+ y constructed site and the 100+ y reference marsh. Soil organic content from sediment cores taken from the 10+ y constructed site and the 2+ y constructed site was comparable between the two sites, despite them being more than a decade apart from each other. Grain size has been shown to play a role in the accretion of SOC within salt marsh sediments, as finer sediments tend to accumulate SOC more rapidly. SOC at constructed sites tend to be comparable to natural reference sites within a decade, meaning that the 10+ y constructed site is on the upper-limit of the age expectation for this indicator, while the 2+ y constructed site still has time. The differences in SOC at the 10+ y constructed site are likely a reflection of the coarser sediments used to fill the site, which can increase porosity and thereby oxygen exposure, resulting in more rapid decomposition of organic matter at the expense of building the SOC pool. Despite the relatively lower SOC in samples collected from the constructed sites, the bulk density was significantly higher than the 100+ y reference marsh, even though these variables tend to correlate positively. Higher sediment bulk density has been shown to correlate with S. alterniflora aboveground biomass. Any relationship between bulk density and belowground biomass is unexplored in S. alterniflora or J. roemerianus dominated marshes, however, the bulk density and root biomass relationship tends to vary by species.

Ideal soil conditions for future projects in Mississippi would more closely mimic reference soil conditions to promote development of marsh productivity and community composition. During periods of drought, S. alterniflora marshes with lower silt content, higher bulk density, and lower water retention are more likely to experience marsh dieback. Silt and clay particles have a greater ability to retain plant nutrients and organic matter due to high surface area and cation exchange capacity. Burial of soil organic carbon within J. roemerianus marshes is likely enhanced with more frequent tidal inundation, where mineralization is lessened. Based on the lower SOC at the 10+ y constructed site, it would likely be beneficial for future restoration efforts at Deer Island, MS to acquire more fine sediments as well as ensure that the site is constructed at an elevation that would be more frequently tidally inundated. These modifications to site structure would likely increase ecological functions such as carbon burial and promote a plant community structure more similar to the 100+ y reference site, at least in the portions of the constructed site where J. roemerianus and S. alterniflora were planted.

The future of Mississippi salt marsh restoration using beneficial-use sediments is going to rely on successful colonization of J. roemerianus, whether by planting or natural recruitment, as this species is indicative of a natural northern Gulf of Mexico salt marsh. It is unclear how the plant community and marsh platform will respond to rising sea level as there are many factors that determine the resilience of individual wetlands, but it is apparent that Deer Island is at risk of being submerged or heavily fragmented if CO₂ emissions and SLR are not reduced.