Results

Research Results

Sediment Bulk density

A total of 129 sediment cores were collected from both constructed marshes and the natural reference marsh (Table 3). The sediment cores from the 10+ y constructed site were the densest and ranged from 1.12 g/cm³ in Spring 2018 to 1.21 g/cm³ in Fall 2017. Sediment bulk density at the 2+ y constructed site was similar to the 10+ y constructed site and ranged from 1.02 g/cm³ in Spring 2018 to 1.21 g/cm³in Spring 2019. Sediment bulk density at the 100+ y reference marsh varied significantly from the two constructed sites and had the least dense sediment cores, which ranged from 0.44 g/cm³ in Fall 2018 to 1.09 g/cm³ in Spring 2019.

Grain size

Differences in grain size were mostly seen in the very-fine sand and silt/clay portions. The two-way ANOVA for coarse sand showed no significant site or season effect. Fine sand showed no significant variation among sites. The constructed sites were similar in both very-fine sand and silt/clay portions, with both having significantly higher very-fine sand than the reference site. The 100+ y reference site had significantly higher silt/clay than the 10+ y constructed site but was similar to the 2+ y constructed site.

Soil organic carbon

Soil organic carbon (SOC) followed a similar trend to what was observed in the bulk density values, where there was a significant difference between sites and the reference site had consistently more soil organic carbon than both constructed sites. SOC ranged at the 100+ y reference site from 10.00% in Fall 2017 to 14.83% in Fall 2018. The 10+ y constructed site had the lowest overall SOC with 1.5 % in Fall 2017. The 2+ y constructed site seemed to show development between sampling seasons with 2.14 % in Fall 2017 to 5.29 % in Fall 2018.

Vegetation Diversity

There were 37 total species found across the survey period from fall 2017 to spring 2019. The 10+ y constructed site had the highest species richness at n = 32. The 2+ y constructed site had half of the amount of species as the 10+ y constructed site at n = 16. The 100+ y reference marsh had the least amount of species at n = 5.

The 2+ y constructed site showed a steady development of total vegetative coverage when measured by both quadrat and point-intercept sampling. Quadrat sampling showed the 2+ y constructed site increased from 54% total vegetative coverage in Fall 2017 to 86% total vegetative coverage in Fall 2018 (Figure 7). Point-intercept sampling similarly showed an increase from 66% total vegetative coverage in Fall 2017 to 90% in Fall 2018. Total vegetative coverage at both the 10+ y constructed site and 100+ y reference marsh was relatively consistent over time, regardless of sampling method.

All sites shared commonly found salt marsh plants such as D. spicata (L.) Greene (saltgrass), J. roemerianus Scheele (black needlerush), S. alterniflora Loisel. (smooth cordgrass), and S. patens (Aiton) Muhl. (saltmarsh hay). The two constructed sites uniquely shared species such as the grasses Panicum amarum Elliott (bitter panicgrass) and Schizachyrium maritimum (Chapm.) Nash (gulf bluestem). The vine Vigna luteola (Jacq.) Benth(hairypod cowpea) was also found in the dry, sandy areas of both constructed sites. Ruppia maritima L. (widgeon grass) was found in a submerged portion of the 2+ y constructed site and an unsampled canal at the 10+ y constructed site. Species unique to the 2+ y constructed site were Panicum repens L., Sesuvium portulacastrum (L.) L. (shoreline seapurslane), and Uniola paniculata L. (seaoats). Notable species unique to the 10+ y constructed site are Baccharis halimifolia L. (eastern baccharis), Hydrocotyle bonariensis Comm. Ex Lam. (largeleaf pennywort), and Solidago sempervirens L. (seaside goldenrod). Invasive species were absent at 2+ y constructed site and the 100+ y natural reference marsh, but the invasive Imperata cylindrica (L.) P. Beauv. (cogongrass) was found at the 10+ y constructed site, albeit in small amounts.

Biomass development

Alive-, dead-, and below-ground biomass was measured from a total of 160 biomass cores from all sites and seasons. Alive plant biomass varied significantly among sites but was comparable across sampling seasons. Site contrasts for alive biomass showed that the 2+ y constructed site was comparable to both the 10+ y constructed site and the 100+ y reference marsh . Significant differences in alive plant biomass were found between the 10+ y constructed site and 100+ y reference site. There were no significant site differences for dead biomass. Belowground biomass showed significant site and season effects. The 2+ y and 10+ y constructed sites were similar to each other but different from the 100+ y reference marsh, suggesting development in the rhizosphere has yet to sufficiently take place.