Food Web Linkages

Coastal marshes in the northern Gulf of Mexico provide essential habitat for various consumer species, however, land loss has severely degraded marsh habitat in this region. Few studies have examined restored black needlerush (Juncus roemerianus) marshes, such as those found in Mississippi (MS), and how they affect faunal inhabitants.

Restoration of Juncus-dominated marshes on Deer Island, MS sought to reestablish ecological functions with the intention of supporting natural consumer assemblages. To test this, quadrat and minnow trap sampling were used to compare invertebrate and nekton abundance, species richness, and diversity of two restored marshes (5+ yrs and 15+ yrs) with a natural reference marsh (100+ yrs old). Stable isotope analysis was also used to compare basal carbon sources and trophic support between sites. Quadrat sampling showed invertebrate abundance did not reach natural levels in either restored site, but minnow trap sampling showed abundance and species richness at the younger 5+ yr site surpassed that of the natural marsh. A comparison of community assemblage and stable isotope analysis showed similarity between the 5+ yr restored site and the natural site. The 5+ yr site better resembled a natural marsh than the 15+ yr site in many ways, suggesting that certain ecological processes are recovering faster in the younger site.

Our assessment of consumer community structure, combined with previous studies evaluating environmental and vegetative characteristics provide a thorough assessment of restoration efforts on Deer Island, MS. It also gives insight into future Beneficial Use restoration projects on Juncus-dominated marshes in this area.

The goal of this study was to evaluate consumer trophic support at restored sites on Deer Island, MS. Based on structural (diversity and abundance) and isotopic metrics of invertebrates and nekton species, it appears that the constructed sites may not be progressing along a similar succession trajectory toward conditions at the natural site. If created marsh age was the driving factor in restoring the consumer community, we would expect to see abundances approaching natural levels over time, with the older restored site being more similar to the natural marsh than the younger restored site (Fig 5.1). However, we observed the younger 5+ yr restored site exceeding its older counterpart in many ways, such as nekton abundance and species richness, suggesting that other environmental factors are likely influencing consumer recolonization.

In our study, slow succession of foundational characteristics, such as elevation and vegetation assemblage, may have prevented the constructed sites from progressing along this expected successional trajectory. For instance, J. roemerianus largely failed to reestablish in either of our constructed sites, which resulted in a system that does not yet reflect a natural Mississippi marsh. There is potential for the younger restored site to increase in elevation over time and achieve more natural vegetation and tidal inundation, as accretion rates are higher in newly created marshes (Nyman et al. 2006). In addition, projected sea level rise may lower the relative elevation at the 15+ yr site and alter ecosystem processes to support more natural vegetation and marsh inhabitants in the future.

After a restored marsh achieves reference levels of landscape, soil, and vegetation characteristics over time, then we may also expect the stable isotopes of consumers to change in response. For instance, if a created site were dominated by S. alterniflora (C4) initially but recolonized with J. roemerianus (C3) over time, then we would expect C isotopes of consumers to become more depleted as the system shifts from a C4- dominated system to a mixed system of C3 and C4 primary producer signals (Fig. 5.2). We would also expect C and N levels in the soil to change. A site filled with dredged material may reflect a wide range of N isotope values initially because of heterogeneity in sediment origins, but over time N values could level off to a more consistent range with establishment of natural microbial processes in the sediments and plant rhizosphere.

Although the two constructed sites are still lagging in many ways, the 5+ yr site appeared to reflect more similar consumer characteristics to the natural marsh compared to the 15+ yr site, suggesting that it may be approaching functional equivalence faster. If the goal of marsh creation includes natural levels of consumer trophic support, it is imperative to consider construction methods that create natural landscape, hydrology, soil, and vegetation.