Coastal wetlands store more carbon than most ecosystems globally. As sea level rises, changes in flooding and salinity will potentially impact ecological functions, such as organic matter decomposition, that influence carbon storage. However, little is known about the mechanisms that control organic matter loss in coastal wetlands at the landscape scale. As sea level rises, how will the shift from fresh to salt‐tolerant plant communities impact organic matter decomposition? Do long‐term, plant‐mediated, effects of sea‐level rise differ from direct effects of elevated salinity and flooding? We identified internal and external factors that regulated indirect and direct pathways of sea‐level rise impacts, respectively, along a landscape‐scale salinity gradient that incorporated changes in wetland type (fresh, oligohaline, mesohaline and polyhaline marshes). We found that indirect and direct impacts of sea‐level rise had opposing effects on organic matter decomposition. Salinity had an indirect effect on litter decomposition that was mediated through litter quality. Despite significant variation in environmental conditions along the landscape gradient, the best predictors of above‐ and below‐ground litter decomposition were internal drivers, initial litter nitrogen content and initial litter lignin content respectively. Litter decay constants were greatest in the oligohaline marsh and declined with increasing salinity, and the fraction of litter remaining (asymptote) was greatest in the mesohaline marsh. In contrast, direct effects of salinity and flooding were positive. External drivers, salinity and flooding, stimulated cellulytic activity, which was highest in the polyhaline marsh. Synthesis. Our results indicate that as sea level rises, initial direct effects of salinity will stimulate decay of labile carbon, but over time as plant communities shift from fresh to polyhaline marsh, litter decay will decline, yielding greater potential for long‐term carbon storage. These findings highlight the importance of quantifying carbon loss at multiple temporal scales, not only in coastal wetlands but also in other ecosystems where plant‐mediated responses to climate change will have significant impacts on carbon cycling.

Salinity alterations will likely change the plant and environmental characteristics in coastal marshes thereby influencing soil carbon accumulation rates. Coastal Louisiana marshes have been historically classified as fresh, intermediate, brackish, or saline based on resident plant community and position along a salinity gradient. Short-term total carbon accumulation rates were assessed by collecting 10-cm deep soil cores at 24 sites located in marshes spanning the salinity gradient. Bulk density, total carbon content, and the short-term accretion rates obtained with feldspar horizon markers were measured to determine total carbon accumulation rates. Despite some significant differences in soil properties among marsh types, the mean total carbon accumulation rates among marsh types were not significantly different (mean ± std. err. of 190 ± 27 g TC m⁻² year⁻¹). However, regression analysis indicated that mean annual surface salinity had a significant negative relationship with total carbon accumulation rates. Based on both analyses, the coastal Louisiana total marsh area (1,433,700 ha) accumulates about 2.7 to 3.3 Tg C year⁻¹. Changing salinities due to increasing relative sea level or resulting from restoration activities may alter carbon accumulation rates in the short term and significantly influence the global carbon cycle.

Severe and persistent bottom-water hypoxia (≤2 mg O₂ L⁻¹) occurs on the Louisiana/Texas continental shelf from mid-May through mid-September over a large area (up to 23,000 km² in mid-summer). Benthic infauna are less mobile than demersal organisms and become stressed by the low dissolved oxygen; benthic community composition, abundance, diversity, and biomass become altered. From the 1950s to the early 1970s, when sediment core indicators identified the initiation and subsequent worsening of dissolved oxygen conditions, there were no hydrographic data or benthic infaunal studies within the current area of frequent bottom-water hypoxia. This study highlights the impacts of severe hypoxia on benthic macroinfaunal communities and how they may have changed from less-hypoxic periods. Polychaetes were and are the dominant taxa in the available studies, but polychaete species richness in summer is now curtailed severely beginning with our 1985–1986 data. Species richness of polychaetes in summer hypoxia (1985–1986 and 1990–1991) was about 60% less than comparable taxa in 1972–1973. Abundance of polychaetes was much less in summer than spring, and recent infaunal biomass in summer was only 15% of what was found in spring. The result is less prey for demersal penaeid shrimp and fishes. Over the period of our comparison, infaunal feeding modes shifted from subsurface deposit feeders and surface deposit feeders to primarily surface deposit feeders (i.e., 95.5% of all polychaetes). Most were opportunistic, hypoxia tolerant, and recruited in high numbers following hypoxia abatement, some in fall and winter but most in spring. As benthic communities succumb to the stress of severe and continued seasonal low oxygen, they occupy the few upper centimeters of the sediment profile above the redox discontinuity layer with negative feedbacks to the water column by way of altered biogeochemical processes.

Coastal marshes are essential habitats for soil carbon accumulation and burial, which can influence the global carbon budget. Coastal Louisiana has extensive marsh habitats (fresh, intermediate, brackish, and saline) where soil cores were collected to a depth of 100 cm at 24 sites to assess long‐term carbon accumulation and burial rates. Select soil depth intervals were analyzed for bulk density, total carbon, and radionuclide (¹³⁷Cs and ²¹⁰Pb) dating. Marsh habitat maps (years 1949–2013) were also used to determine the most frequently occurring habitat at each field site. Over 5 decades, half of the sites transitioned between marsh habitats at least once. Saline marshes tended to have lower mean total carbon density (0.04 ± 0.002 g cm⁻³) and lower mean long‐term total carbon accumulation rates (211 ± 46 g TC m⁻² yr⁻¹, n = 5, based on ²¹⁰Pb) compared to the other marsh habitats. Using marsh habitat specific accumulation rates and area, the total carbon burial rate for coastal Louisiana in year 2013 was estimated at 4.3 Tg TC yr⁻¹ which accounts for about 5%–21% of the estimated tidal wetland burial rate globally. Historically, about 1.0 Tg TC yr⁻¹ was lost due to reduced marsh area from 1949 to 2013. With no coastal restoration activity, the predicted wetland loss over the next 50 years could reduce carbon burial in coastal Louisiana to 2.1 Tg TC yr⁻¹, a reduction of about 50% from the year 2013 rate, with potential to significantly alter the global carbon budget.

Coastal habitats can play an important role in climate change mitigation. As Louisiana implements its climate action plan and the restoration and risk‐reduction projects outlined in its 2017 Louisiana Coastal Master Plan, it is critical to consider potential greenhouse gas (GHG) fluxes in coastal habitats. This study estimated the potential climate mitigation role of existing, converted, and restored coastal habitats for years 2005, 2020, 2025, 2030, and 2050, which align with the Governor of Louisiana's GHG reduction targets. An analytical framework was developed that considered (1) available scientific data on net ecosystem carbon balance fluxes per habitat and (2) habitat areas projected from modeling efforts used for the 2017 Louisiana Coastal Master Plan to estimate the net GHG flux of coastal area. The coastal area was estimated as net GHG sinks of −38.4 ± 10.6 and −43.2 ± 12.0 Tg CO₂ equivalents (CO₂e) in 2005 and 2020, respectively. The coastal area was projected to remain a net GHG sink in 2025 and 2030, both with and without the implementation of Coastal Master Plan projects (means ranged from −25.3 to −34.2 Tg CO₂e). By 2050, with model‐projected wetland loss and conversion of coastal habitats to open water due to coastal erosion and relative sea level rise, Louisiana's coastal area was projected to become a net source of GHG emissions both with and without the Coastal Master Plan projects. However, in the year 2050, the Louisiana Coastal Master Plan project implementation was projected to avoid the release of +8.8 ± 1.3 Tg CO₂e compared with an alternative with no action. Reduction in current and future stressors to coastal habitats, including impacts from sea level rise, as well as the implementation of restoration projects could help to ensure coastal areas remain a natural climate solution.

Predictive tools are widely used to study coastal and deltaic systems in support of basic research, planning efforts, engineering design, and the implementation of restoration or protection strategies. They have been extensively used to evaluate the effectiveness of natural and nature-based solutions (NNBS) to support ecosystem functions and services of coastal ecosystems and human communities experiencing increased risk from sea-level rise and severe storms. The potential benefits of NNBS are being increasingly recognized, particularly in remote areas or areas that are either technically or financially infeasible to be protected with levees or other difficult engineering alternatives. Local communities, however, are often excluded from proposing, screening, or evaluating NNBS as restoration and protection strategies. Communities are also not sufficiently involved in the development or application of the predictive tools. This research effort outlines an approach to developing knowledge-based predictive tools and a community engagement process to evaluate NNBS strategies proposed predominantly by local communities. Incorporating knowledge from local communities benefits and potentially improves the performance of predictive tools and their ability to capture visible trends and observations. To illustrate this concept, the authors present landscape models for coastal Louisiana that successfully reproduced the frequency of flooding of local roads, rate of shoreline erosion, salinity pattern changes, and presence/absence of key species (e.g., brown shrimp, oysters, and so forth). While these qualitative measures are not a substitute for well-established rigorous and quantitative model performance assessment approaches, they offer an effective approach to engage local communities and incorporate their knowledge in the development of the predictive models and the proposed protection and restoration strategies to be examined.