Recycled glass sand (cullet) is a potential alternative to soil fill in salt marsh restoration projects, given its similarity to natural silica sands. However, the ability for common salt marsh plants to grow in glass sand has not yet been tested. An outdoor mesocosm experiment assessed the ability of the dominant mid‐marsh plant, Juncus roemerianus, and the high marsh plant, Spartina patens, to grow in various mixtures of glass sand versus fill soil. Results showed that J. roemerianus can survive in glass sand, but growth declined as the proportion of sand increased. However, S. patens grew comparably, if not better, in treatments with up to 75% glass sand. In the face of increased coastal land loss and a global sediment deficit, the use of glass sand in mid‐to‐high elevation salt marsh restoration projects can simultaneously divert glass from landfills and promote more sustainable coastal restoration strategies.

One of the world’s largest “blue carbon” ecosystems, Louisiana’s tidal wetlands on the US Gulf of Mexico coast, is rapidly being lost. Louisiana’s strong legal, regulatory, and monitoring framework, developed for one of the world’s largest tidal wetland systems, provides an opportunity for a programmatic approach to blue carbon accreditation to support restoration of these ecologically and economically important tidal wetlands. Louisiana’s coastal wetlands span ∼1.4 million ha and accumulate 5.5–7.3 Tg yr−1 of blue carbon (organic carbon), ∼6%–8% of tidal marsh blue carbon accumulation globally. Louisiana has a favorable governance framework to advance blue carbon accreditation, due to centralized restoration planning, long term coastal monitoring, and strong legal and regulatory frameworks around carbon. Additional restoration efforts, planned through Louisiana’s Coastal Master Plan, over 50 years are projected to create, or avoid loss of, up to 81,000 ha of wetland. Current restoration funding, primarily from Deepwater Horizon oil spill settlements, will be fully committed by the early 2030s and additional funding sources are required. Existing accreditation methodologies have not been successfully applied to coastal Louisiana’s ecosystem restoration approaches or herbaceous tidal wetland types. Achieving financial viability for accreditation of these restoration and wetland types will require expanded application of existing blue carbon crediting methodologies. It will also require expanded approaches for predicting the future landscape without restoration, such as numerical modeling, to be validated. Additional methodologies (and/or standards) would have many common elements with those currently available but may be beneficial, depending on the goals and needs of both the state of Louisiana and potential purchasers of Louisiana tidal wetland carbon credits. This study identified twenty targeted needs that will address data and knowledge gaps to maximize financial viability of blue carbon accreditation for Louisiana’s tidal wetlands. Knowledge needs were identified in five categories: legislative and policy, accreditation methodologies and standards, soil carbon flux, methane flux, and lateral carbon flux. Due to the large spatial scale and diversity of tidal wetlands, it is expected that progress in coastal Louisiana has high potential to be generalized to similar wetland ecosystems across the northern Gulf of Mexico and globally.

Seagrasses and other submerged aquatic vegetation (SAV) provide critical nearshore habitats. Information on the location, extent, and condition of SAV resources in Mississippi (MS) and Alabama (AL) is often crucial in coastal management decision-making, like permitting for dock or pier construction, sediment dredging for navigation, living shoreline construction, and others. We are evaluating the benefit and challenges of using drones and sonar for detecting and mapping SAV in nearshore, shallow, and optically complex waters to support coastal management and permitting needs in the region. We are researching these technologies to determine the conditions under which each method is most cost-effective, maximizes efficiency, and yields high-accuracy data that can improve management strategies and outcomes. Results indicate that drone platforms can detect most SAV in shallow waters at low tide. Sonar data is more difficult to extract and process, and further sampling is required to provide a sufficient database with which to compare against aerial imagery. Regression and ordination techniques will be needed to cross-validate aerial imagery with sonar maps. Having recommendations under which conditions each method is most suited can provide easier decision-making for natural resource managers.

Beneficial Use (BU) of dredge sediments has been used in coastal salt marsh restoration to renourish areas with excessive edge erosion and marsh platform subsidence. Restoration of these degraded salt marshes can include the construction of new marsh habitats at appropriate elevations for vegetation establishment. However, little is known about how these newly constructed vegetative communities develop and function over time. Two such projects were constructed with BU sediments at Deer Island, Mississippi, USA, in 2004 and 2015. They were planted with native vegetation in anticipation that they would recover in _Juncus roemerianus_ (Black needlerush) dominated salt marshes. The two constructed sites were compared to an adjacent reference salt marsh using metrics that included vascular plant diversity, standing stock biomass, and sediment composition. Sampling over six seasons from spring 2017 to fall 2019 demonstrated the establishment of new vegetation resulting in a diverse floristic community. The two constructed sites were found to have higher species richness and plant diversity than the natural reference marsh. However, the two constructed sites had significantly lower below-ground biomass and sediment organic content (SOC) compared to the natural reference site. Soil bulk density, SOC, and grain size of the sediments at the two BU sites were also dissimilar to the natural marsh reference. All metrics evaluated indicated the two BU restorations were not yet identical to the reference salt marsh, even after more than a decade of ecosystem recovery, and that _J. roemerianus_ had failed to establish as expected.

The seagrass Halodule wrightii (shoal grass) is found throughout the Caribbean and Gulf of Mexico where it can be an important or dominant component of the flora. It is generally more tolerant to disturbance than other seagrass species, including tolerance to reduced salinity. This study investigated the low salinity tolerance of a population of H. wrightii that has a long history of exposure to river flooding located at Cat Island, Mississippi (MS). Plants were collected after record prolonged flooding of the Mississippi River in 2019 and subsequent reduced salinities at Cat Island, MS, then allowed to recover at a salinity of 25 before being subject to three low salinity exposures of 5, 10, and 15. Plants were still alive after more than 12 months even at the lowest salinity of 5. Morphological and biomass metrics were recorded at the end of this prolonged low-salinity exposure. Plants exhibited low biomass and shoot density. This population of H. wrightii was able to survive very low salinities for longer than previously reported. Other seagrass populations with enhanced tolerance to abiotic stressors may become important for survival as climate change alters coastal habitat conditions.

Coastal wetlands are one of the most valuable ecosystems; however, they have an uncertain future when faced with increasing sea-level rise (SLR) and both human-induced and environmental stressors. To better understand future marsh resiliency, we examined above- and belowground biomass, carbon (C), nitrogen (N), and phosphorus (P) tissue content and sediment characteristics along a coastal elevation gradient at the Grand Bay National Estuarine Research Reserve (GNDNERR). Vegetation and sediment characteristics were compared across several marsh types, and data was used to make updated predictions using the marsh equilibrium model (MEM). Standing and total biomass increased from 869 g/m2 and 4250 g/m2 respectively at low marsh sites to 2197 g/m2 and 6789 g/m2 at high marsh sites. N/P ratios increased from 10.25 to 20.43 from the low to high marsh, largely driven by decreasing P content in tissues. Spartina alterniflora and Juncus roemerianus biomass and tissue nutrient content were lower than other marshes in the northern Gulf of Mexico. Juncus roemerianus total biomass was around 50% of other coastal Mississippi locations. A meteorologically driven tidal regime and no major freshwater inflow could cause a lack of nutrient and sediment delivery to the marsh surface stunting total biomass and reducing accretion. Under various SLR rate scenarios, the MEM estimates that coastal wetlands at GNDNERR will undergo substantial changes within the next 50–100 years. The lack of nutrient and sediment delivery means that preserving coastal wetlands at GNDNERR will depend on aiding landward marsh migration.

This study was conducted to compare the sediment parameters of 2 project sites located near Dauphin Island, AL, which each contained a natural shoreline (NS), a living shoreline (LS), and a hardened shoreline (HS) treatment. We collected sediment core samples from the shallow intertidal zone along 3 replicate transects and sectioned them into shallow (0-15 cm) and deep (16-30 cm) fractions. We analyzed sediment subsamples for bulk density, sediment grain size, total carbon, total nitrogen, and extractable phosphorus. At both project sites, HS sediments contained significantly more sand (>90% percent) and higher bulk density than NS and LS sediments. The HS sediments also contained significantly less C and N than the LS and NS. These data suggest that LS treatments over time facilitate conditions that favor organic-matter rich and fine-grained sediments and approach the conditions found in the NS, in contrast to HS sediments that become less suitable for marsh development. These sediment changes in LS treatmnts promote habitat conditions that help facilitate marsh expansion and thereby provide important habitat for estuarine dependent wildlife.

Restoration of lost marsh platforms can include de-novo construction using dredged sediments. Two restoration projects constructed with beneficial use material in 2004 and 2015 were planted with native vegetation in the anticipation they would function similarly to adjacent Juncus roemerianus (Black needlerush) dominated salt marshes. Planted J. roemerianus , however, failed to establish and exists sparsely in the restored marsh. To better understand potential sources of failure, this study assessed vascular plant diversity in Spring and Fall of 2017 through 2019 and demonstrated establishment of both planted and naturally recruited vegetation. The two constructed sites were found to have higher species richness and plant diversity (Shannon-Wiener H’, and Simpson’s D) than the natural reference marsh, in part due to the higher elevations of the two constructed sites. The plant diversity metrics indicated the two beneficial use restorations did not meet the species composition of the reference marsh. Further monitoring should be conducted to observe the species development over the long term with considerations given to the potential future marsh change.

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Coastal marshes provide valuable ecosystem services yet are increasingly vulnerable to sea level rise SLR).To facilitate a better understanding of how fluvial marshes along the Gulf of Mexico coast are responding to regional SLR of around 3.7 mm per year, this study used aerial imagery to map land cover at the mouth of the Pascagoula River at 20-year intervals, beginning in 1955 and ending in 2014. High-resolution land cover maps were created for each image date based on a maximum likelihood classification scheme using spectral and textural image features. This marsh ecosystem, at the mouth of the largest free-flowing river by volume in the contiguous United States, should be more resilient to sea level rise than other Gulf Coast marshes, with little restriction to sediment supply and relatively low subsidence rates measured nearby. However, the results of this study show that marsh area declined by 1073 ha (17.5%) and rates of marsh conversion to open water increased over the studied time period. Although modeling studies indicate that coastal marshes worldwide may persist under accelerated SLR, these observations suggest that marsh extent in the sediment-rich Pascagoula River Estuary will continue to decline, signifying vulnerability among other marsh ecosystems along the northern Gulf of Mexico coast.

We studied the ecological resilience of salt marshes by deriving sea level rise (SLR) thresholds in two estuaries with contrasting upland hydrological inputs in the north-central Gulf of Mexico: Grand Bay National Estuarine Research Reserve (NERR) with limited upland input, and the Pascagoula River delta drained by the Pascagoula River, the largest unclaimed river in the continental United States. We applied a mechanistic model to account for vegetation responses and hydrodynamics to predict salt marsh distributions under future SLR scenarios. We further investigated the potential mechanisms that contribute to salt marsh resilience to SLR. The modeling results show that salt marshes in the riverine dominated estuary are more resilient to SLR than in the marine dominated estuary with SLR thresholds of 10.3 mm/yr and 7.2 mm/yr respectively. This difference of >3 m m/yr is mainly contributed by larger quantities of riverine-borne mineral sediments in the Pascagoula River. In both systems, sediment trapping by the above-ground vegetation appears to contribute more to marsh platform accretion than organic matter from below-ground biomass based on the medians of the accretion rates. However, below-ground biomass could contribute up to 90% of accretion in the marine dominated estuary compared to only 60% of accretion in the riverine dominated estuary. SLR thresholds of salt marshes are more sensitive to vegetation biomass in the marine dominated estuary while biomass and sediment similarly affect SLR thresholds of salt marshes in the riverine dominated estuary. This research will likely help facilitate more informed decisions on conservation/restoration policies for these two types of systems in the near-term needed to minimize future catastrophic loss of these coastal marsh habitats once SLR thresholds are exceeded. (C) 2020 Elsevier B.V. All rights reserved.

Persistent development, population pressures, and increasing natural hazards are unequivocally changing socio-ecological systems in the coastal zone. This essay provides direction and initiates scientific dialog on the potential role of mobility in adapting to natural and social changes in coastal environments. The essay identifies four key research areas on information needed to develop coastal management actions and policies that support and recognize socio-ecological coupling in coastal areas. The proposed research includes: (1) modeling localized scenarios that illustrate the tradeoffs associated with various sea level rise adaptation, (2) assessing and consolidating mobility terminology for different applications and contexts, (3) developing solutions to synchronize the co-migration of natural environments and built infrastructure, and (4) evaluating existing or creating new transparent, equitable, and sustainable policies and incentives to support socio-ecological mobility by using case studies and social science methods to understand how people make mobility decisions in different contexts.

The U. S. Gulf of Mexico is experiencing a dramatic increase in tidal marsh restoration actions, which involves planting coastal areas with smooth cordgrass (Spartina alterniflora) and black needlerush (Juncus roemerianus) for erosion control and to provide habitat for fish and wildlife. It can take decades for sedimentary cycles in restored marshes to approach reference conditions, and the contribution of the sediment microbial communities to these processes is poorly elucidated. In this study, we addressed this gap by comparing rhizosphere microbiomes of S. alterniflora and J. roemerianus from two restored marshes and a natural reference marsh located at Deer Island, MS. Our results revealed that plants from the restored and reference areas supported similar microbial diversity indicating the rapid colonization of planted grasses with indigenous soil microbiota. Although close in composition, the microbial communities from the three studied sites differed significantly in the relative abundance of specific taxa. The observed differences are likely driven by the host plant identity and properties of sediment material used for the creation of restored marshes. Some of the differentially distributed groups of bacteria include taxa involved in the cycling of carbon, nitrogen, and sulfur, and may influence the succession of vegetation at the restored sites to climax condition. We also demonstrated that plants from the restored and reference sites vary in the frequency of culturable rhizobacteria that exhibit traits commonly associated with the promotion of plant growth and suppression of phytopathogenic fungi. Our findings will contribute to the establishment of benchmarks for the assessment of the outcome of coastal restoration projects in the Gulf of Mexico and better define factors that affect the long-term resiliency of tidal marshes and their vulnerability to climate change.

This special issue compiles original research and reviews of previous research on an important geographic constitution at the water-land interface: the seagrass/ Submerged Aquatic Vegetation (SAV) beds in the Gulf of Mexico (GoM) coastal zone. SAV are a group of vascular plants that grow underwater; and seagrass is a specialized subgroup of SAV that have adapted to live in the high salinity coastal and marine waters. Presence of, types of, and seasonality of SAV beds have substantial influences on geography, ecology, landscape, culture, and economy in the coastal zone. Healthy SAV beds help shape, maintain, and modify coastal landscapes by buffering wave energy, modifying water currents, protecting shorelines from erosion, aiding sediment deposition, consolidating substrate, and changing littoral profiles and water depth. Coastal seagrass resources are among the most productive ecosystems on Earth. Seagrass beds perform numerous vital ecological functions and provide food and shelter for commercially and ecologically important organisms, including blue crabs, shrimp, turtles, manatees, and waterfowl. Therefore, assessment of SAV distribution, composition, and abundance has been of particular interest to coastal environmental managers, scientists, developers, and recreationists as this information serves as an excellent indicator of estuarine environmental quality. Coastal areas containing seagrass and SAV beds carry geographical and historical importance in the Southeast, especially throughout the GoM. The GoM and its resources have immense ecological, economical, and historic values to the U.S. but have been going through several major environmental disturbances and disasters including, but not limited to, major hurricanes (e.g., 2005 Hurricane Katrina), the 2010 Deepwater Horizon Oil Spill, red tides, rapid subsidence, marsh erosion, threats from sea level rise, and river water diversions. Man-made levees

Along the mainland coastline of eastern Mississippi Sound, the seagrass species Ruppia maritima (Wigeongrass) and Halodule wrightii (Shoalgrass) co-occur in persistent and sometimes extensive beds within Grand Bay. This paper presents seasonal and annual changes in those Ruppia-Halodule beds that are part of the Grand Bay National Estuarine Research Reserve in western Grand Bay using 2005-2011 transect survey data of percent cover and 2009-2013 biomass sampling data. The seagrass beds exhibited significant interannual variation in areal coverage and seasonal variations in biomass, with peak total aboveground biomass (up to 140 g dry wt/m(2)) in July and August. Intact R. maritima and H. wrightii seeds were found in the sediment, with densities as high as 4160 seeds/m(2). The Ruppia-Halodule beds did not show any short-term declines in their growth or abundance following 2005 Hurricanes Katrina and Rita nor the 2010 Deepwater Horizon oil spill.

AbstractFeedbacks among inundation, sediment trapping, and vegetation productivity help maintain coastal wetlands facing sea‐level rise (SLR). However, when the SLR rate exceeds a threshold, coastal wetlands can collapse. Understanding the threshold helps address key challenges in ecology—nonlinear response of ecosystems to environmental change, promotes communication between ecologists and resource managers, and facilitates decision‐making in climate change policies. We studied the threshold of SLR rate and developed a new threshold of SLR acceleration rate on sustainability of coastal wetlands as SLR is likely to accelerate due to enhanced anthropogenic forces. Deriving these two thresholds depends on the temporal scale, the interaction of SLR with other environmental factors, and landscape metrics, which have not been fully accounted for before this study. We chose a representative marine‐dominated estuary in the northern Gulf of Mexico, Grand Bay in Mississippi, to test the concept of SLR thresholds. We developed a mechanistic model to simulate wetland change and then derived the SLR thresholds for Grand Bay. The model results show that the threshold of SLR rate in Grand Bay is 11.9 mm/year for 2050, and it drops to 8.4 mm/year for 2100 using total wetland area as a landscape metric. The corresponding SLR acceleration rate thresholds are 3.02 × 10−4 m/year2 and 9.62 × 10−5 m/year2 for 2050 and 2100, respectively. The newly developed SLR acceleration rate threshold can help quantify the temporal lag before the rapid decline in wetland area becomes evident after the SLR rate threshold is exceeded, and cumulative SLR a wetland can adapt to under the SLR acceleration scenarios. Based on the thresholds, SLR that will adversely impact the coastal wetlands in Grand Bay by 2100 will fall within the likely range of SLR under a high warming scenario (RCP8.5), highlighting the need to avoid RCP8.5 to preserve these marshes.