Evaluation of Plot-Scale Methods for Assessing and Monitoring Salt Marsh Vegetation Composition and Cover.

Vegetation is a key component of salt marsh monitoring programs, but different methods can make comparing datasets difficult. We compared data on vegetation composition and cover collected with 3 methods (point-intercept, Braun-Blanquet visual, and floristic quality assessment [FQA]) in 3 Rhode Island salt marshes. No significant differences in plant community composition were found among the methods, and differences in individual species cover in a marsh never exceeded 6% between methods. All methods were highly repeatable, with no differences in data collected by different people. However, FQA was less effective at identifying temporal changes at the plot scale. If data are collected from many plots in a marsh, any of the methods are appropriate, but if plot-scale patterns are of interest, we recommend point-intercept.

Runnels mitigate marsh drowning in microtidal salt marshes.

As a symptom of accelerated sea level rise and historic impacts to tidal hydrology from agricultural and mosquito control activities, coastal marshes in the Northeastern U.S. are experiencing conversion to open water through edge loss, widening and headward erosion of tidal channels, and the formation and expansion of interior ponds. These interior ponds often form in high elevation marsh, confounding the notion applied in predictive modeling that salt marshes convert to open water when elevation falls below a critical surface inundation threshold. The installation of tidal channel extension features, or runnels, is a technique that has been implemented to reduce water levels and permit vegetation reestablishment in drowning coastal marshes, although there are limited data available to recommend its advisability. We report on 5 years of vegetation and hydrologic monitoring of two locations where a total of 600-m of shallow (0.15-0.30-m in diameter and depth) runnels were installed in 2015 and 2016 to enhance drainage, in the Pettaquamscutt River Estuary, in southern Rhode Island, United States. Results from this Before-After Control-Impact (BACI) designed study found that runnel installation successfully promoted plant recolonization, although runnels did not consistently promote increases in high marsh species presence or diversity. Runnels reduced the groundwater table (by 0.07-0.12 m), and at one location, the groundwater table experienced a 2-fold increase in the fraction of the in-channel tidal range that was observed in the marsh water table. We suggest that restoration of tidal hydrology through runnel installation holds promise as a tool to encourage revegetation and extend the lifespan of drowning coastal marshes where interior ponds are expanding. In addition, our study highlights the importance of considering the rising groundwater table as an important factor in marsh drowning due to expanding interior ponds found on the marsh platform.

Laying it on thick: Ecosystem effects of sediment placement on a microtidal Rhode Island salt marsh.

Heightened recognition of impacts to coastal salt marshes from sea-level rise has led to expanding interest in using thin-layer sediment placement (TLP) as an adaptation tool to enhance future marsh resilience. Building on successes and lessons learned from the Gulf and southeast U.S. coasts, projects are now underway in other regions, including New England where the effects of TLP on marsh ecosystems and processes are less clear. In this study, we report on early responses of a drowning, microtidal Rhode Island marsh (Ninigret Marsh, Charlestown, RI) to the application of a thick (10-48 cm) application of sandy dredged material and complimentary extensive adaptive management to quickly build elevation capital and enhance declining high marsh plant species. Physical changes occurred quickly. Elevation capital, rates of marsh elevation gain, and soil drainage all increased, while surface inundation, die-off areas, and surface ponding were greatly reduced. Much of the marsh revegetated within a few years, exhibiting aspects of classic successional processes leading to new expansive areas of high marsh species, although low marsh Spartina alterniflora recovered more slowly. Faunal communities, including nekton and birds, were largely unaffected by sediment placement. Overall, sediment placement provided Ninigret Marsh with an estimated 67-320 years of ambient elevation gain, increasing its resilience and likely long-term persistence. Project stakeholders intentionally aimed for the upper end of high marsh plant elevation growth ranges to build elevation capital and minimize maintenance costs, which also resulted in new migration corridors, providing pathways for future marsh expansion.

Drainage enhancement effects on a waterlogged Rhode Island (USA) salt marsh.

Drainage enhancement (e.g., ditch digging, open-marsh water management, runnelling) has long been used to reduce tidal marsh soil waterlogging and surface ponding to promote salt hay production and mosquito control. Now it is also being used as a tool to enhance marsh resilience to sea-level rise despite a lack of studies that evaluate its effectiveness as an intervention approach. We therefore conducted a controlled field experiment to evaluate short-term responses to drainage enhancement of a Rhode Island (USA) salt marsh. Drainage enhancement elicited rapid physical changes in portions of the marsh including declines in water levels and marsh elevation, but the biological components examined (e.g., vegetation and bird community composition) were largely unaffected. In two of the four areas monitored, marsh surface inundation duration declined from > 75% to 3-10% and low water levels dropped by 20 cm. Mean annual marsh surface elevation in monitoring plots increased 5 mm one year after drainage enhancement but dropped to 11 mm below initial conditions after three years. The decline in elevation varied among habitats, with the greatest decline (18 mm) found in areas dominated by Spartina alterniflora and/or bare ground. Vegetation community composition and % cover and heights of dominant species were unchanged, but areas that were initially bare had fully revegetated after three years. Drainage enhancement also had no effects on bird community composition or marsh sparrow (Ammodramus spp.) density. Our study provides evidence that drainage enhancement can relieve waterlogging and some of its impacts without any apparent adverse effects on the composition and abundance of existing vegetation and bird communities. At the same time, it can induce a loss of marsh platform elevation that has the potential to offset declining water levels and inhibit high marsh enhancement. Finally, unanticipated findings from our study provide evidence that the effects of larger-scale drivers such as sea-level rise may predominate over localized responses to drainage enhancement itself.

A climate change adaptation strategy for management of coastal marsh systems.

Sea level rise is causing shoreline erosion, increased coastal flooding, and marsh vulnerability to the impact of storms. Coastal marshes provide flood abatement, carbon and nutrient sequestration, water quality maintenance, and habitat for fish, shellfish, and wildlife, including species of concern, such as the saltmarsh sparrow (Ammodramus caudacutus). We present a climate change adaptation strategy (CCAS) adopted by scientific, management, and policy stakeholders for managing coastal marshes and enhancing system resiliency. A common adaptive management approach previously used for restoration projects was modified to identify climate-related vulnerabilities and plan climate change adaptive actions. As an example of implementation of the CCAS, we describe the stakeholder plans and management actions the US Fish and Wildlife Service and partners developed to build coastal resiliency in the Narrow River Estuary, RI in the aftermath of Superstorm Sandy. When possible an experimental BACI (Before-After, Control-Impact) design, described as pre- and post-sampling at the impact site and one or more control sites, was incorporated into the climate change adaptation and implementation plans. Specific climate change adaptive actions and monitoring plans are described, and include shoreline stabilization, restoring marsh drainage, increasing marsh elevation, and enabling upland marsh migration. The CCAS provides a framework and methodology for successfully managing coastal systems faced with deteriorating habitat, accelerated sea level rise, and changes in precipitation and storm patterns.