Coastal wetlands, including seagrass meadows, emergent marshes, mangroves, and temperate tidal swamps, can efficiently sequester and store large quantities of sediment organic carbon (SOC). However, SOC stocks may vary by ecosystem type and along environmental or climate gradients at different scales. Quantifying such variability is needed to improve blue carbon accounting, conservation effectiveness, and restoration planning. We analyzed SOC stocks in 1,284 sediment cores along>6,500 km of the Pacific coast of North America that included large environmental gradients and multiple ecosystem types. Tidal wetlands with woody vegetation (mangroves and swamps) had the highest mean stocks to 1 m depth (357 and 355 Mg ha-1, respectively), 45% higher than marshes (245 Mg ha-1), and more than 500% higher than seagrass (68 Mg ha-1). Unvegetated tideflats, though not often considered a blue carbon ecosystem, had noteworthy stocks (148 Mg ha-1). Stocks increased with tidal elevation and with fine (<63 μm) sediment content in several ecosystems. Stocks also varied by dominant plant species within individual ecosystem types. At larger scales, marsh stocks were lowest in the Sonoran Desert region of Mexico, and swamp stocks differed among climate zones; otherwise stocks showed little correlation with ecoregion or latitude. More variability in SOC occurred among ecosystem types, and at smaller spatial scales (such as individual estuaries), than across regional climate gradients. These patterns can inform coastal conservation and restoration priorities across scales where preserving stored carbon and enhancing sequestration helps avert greenhouse gas emissions and maintains other vital ecosystem services.

During his long career, Howard Odum focused on both conceptual and theoretical issues such as maximum power and the development of energy circuit language as well as ecosystem-level studies of natural systems and human interactions with these systems. He and colleagues carried out large macrocosm studies on a variety of ecosystems. We review here studies initiated by Howard Odum as well as whole system studies carried out by a number of post docs, his students and their students. We focus on studies involving John Day, Katherine Ewel, and William Mitsch. Ewel and Mitsch worked on fertilized cypress systems in Florida. While at Ohio State University, Mitsch initiated a wetland research park on the university campus. He and Day carried out a series of holistic studies of the Mississippi basin to address water quality issues caused by agricultural runoff and developed a basin-wide plan for wetland reclamation to reduce nutrient inputs to streams and rivers. More recently, studies have been carried out on the long-range and large-scale effects of hydrology and macrophyte planting on phosphorus removal by wetlands adjacent to Lake Erie. Major ecosystem-scale studies in coastal Louisiana have investigated the value of coastal wetland and aquatic systems in improving water quality and restoring lost landscapes. These studies in the Midwest and Mississippi delta form the basis of determining design standards for creating and restoring wetlands in the Mississippi Basin to reduce Gulf of Mexico hypoxia and regain many lost ecosystem functions over a large part of North America. Studies have also been conducted of the use of mangroves to treat effluent from shrimp ponds. These studies were intended to increase our understanding of water quality improvement and ecosystem budgets, demonstrating the importance of pulsing, self-design, and emergy analysis.

Estuarine systems that provide valuable ecosystem services to society and important foraging and rearing habitat for fish and wildlife species continue to undergo degradation. In Puget Sound, WA, as much as 70–80

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha −1 in the top 30 cm and 231 ± 134 Mg SOC ha −1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

To assess the effect of sea level rise (SLR) on a 3,800-ha eelgrass meadow in Padilla Bay National Estuarine Research Reserve in Puget Sound, Washington, USA, we coupled the Marsh Equilibrium Model (MEM) with the Relative Elevation Model (REM), combining their respective strengths in simulating aboveground and belowground processes. We then modified the hybrid model to reflect an empirical relationship between eelgrass stem density and sediment accretion, making it the first model of its kind to do so. We used field data to initialize and calibrate the model, then simulated surface elevation change under various SLR and suspended sediment scenarios and tested it against a 12-year surface elevation table dataset from the site. 100-year simulations projected relative elevation loss along at least half of the elevation gradient for all SLR scenarios, and along the entire gradient for three SLR scenarios, with greater loss at higher elevations. The current suspended sediment concentration is thus insufficient for the entire eelgrass meadow to keep pace with SLR, with up to a four-fold increase required, however this presents a management conundrum in that the required sediment load may prevent eelgrass from meeting its light requirements. The main contributions of this study thus include: the novel combination of MEM and REM models, the inclusion of stem density as a factor controlling accretion, the use of a long-term data record for model initialization, calibration, and validation, and the finding that increasing sediment inputs to maintain the elevation of the habitat in the long term may be detrimental to eelgrass health in the short term.

Sea-level rise (SLR) and obstructions to sediment delivery pose challenges to the persistence of estuarine habitats and the ecosystem services they provide. Restoration actions and sediment management strategies may help mitigate such challenges by encouraging the vertical accretion of sediment in and horizontal migration of tidal forests and marshes. We used a process-based soil accretion model (Coastal Wetland Equilibrium Model) combined with a habitat classification model (MOSAICS) to estimate the effects of SLR, suspended sediment, and inland habitat migration on estuarine habitats, soil carbon accumulation, and economic value of climate change mitigation of carbon accumulation (social cost of carbon dioxide) in a macrotidal estuary in the northwest USA over 100 years (2011 to 2110). Under present-day sediment levels, we projected that after 100 years, most high salt marsh would remain with < 100 cm SLR, but substantial area converted to transitional (low) salt marsh and mudflat with ≥ 100 cm SLR. Increasing sediment availability increased the projected resilience of transitional salt marsh to SLR but did not prevent declines in high marsh area. Projected total carbon accumulation plateaued or declined with ≥ 100 cm SLR, yet the economic value of carbon accumulation continued to rise over time, suggesting that the value of this ecosystem service was resilient to SLR. Doubling or tripling sediment availability increased projected carbon accumulation up to 7.69 and 14.2 kg m −2 and increased total economic value up to $373,000 and $710,000, respectively. Allowing marsh migration supported conversion of upland to freshwater marsh, with slight increases in carbon accumulation. These results inform climate adaptation planning for wetland managers seeking to understand the resilience of estuarine habitats and ecosystem services to SLR under multiple management strategies.

The use of surface elevation table (SET) instruments to monitor elevation changes at low elevation coastal locations has steadily increased in recent years. A primary focus in the analysis of SET data is the estimation of the overall rate of elevation change, and numerous approaches have been used for this purpose. In this work, we compare and contrast several methods used for estimating the true rate of elevation change at SET station locations, including a novel approach proposed in this work that incorporates spatial dependence. We also discuss theoretical properties of one class of estimators, and undertake a comprehensive simulation study. Additionally, we present two case studies where we illustrate these differences using real SET data. All methods considered here tend to produce similar point estimates, but some confidence interval procedures can generate intervals with empirical coverage rates lower than specified. However, the best analysis approach is likely dependent upon selecting the method that best coincides with the true underlying process. Thus, we do not uniformly recommend one approach for all situations. Instead, we suggest carefully weighing potential advantages and disadvantages of each method before conducting analysis, while keeping in mind the ways in which modeling assumptions may impact this decision.

The salt marsh response to a changing climate may be more complex than that of either terrestrial or marine ecosystems because salt marshes exist at the interface of land and sea and both bring changes to the marsh. Climate change may exacerbate anthropogenic-related stresses that salt marsh plants are already experiencing, limiting their resilience (Keddy 2011). In this chapter we discuss major climate change impacts likely to affect salt marshes including temperature, sea level rise (SLR), salinity, CO2, freshwater flow, sediment, and nutrients, and consider how salt marsh plants respond to these impacts and potential interactions of these impacts. Specifically, we explore changes in plant productivity and decomposition rates, aboveground and belowground biomass, and stem density as they are central to understanding marsh responses on a larger scale, with implications for species composition, elevation change, nutrient cycling, carbon sequestration, food webs, and ultimately marsh survival. Although this chapter is focused on salt marshes, examples from tidal fresh and brackish marshes are also included to a limited extent where relevant.

Tidal marshes have been recognized globally for their ability to sequester “blue carbon” but there is still a need for studies investigating the marsh response to restoration, particularly in the Pacific Northwest United States. Here we report carbon stocks and accumulation rates for restored and natural tidal marshes in the Stillaguamish River estuary in Puget Sound, Washington, where a 60-hectare marsh was reintroduced to the tidal regime from its previous use as diked and drained farmland. We found that the restoration not only maximized carbon accumulation but also enhanced resilience to rising sea levels. Four years after restoration, mean sediment carbon stocks in the upper 30 cm within the restored marsh (4.43 kg C m -2 ) were slightly lower than those measured in the adjacent natural marshes (5.95 kg C m -2 ). Mean carbon accumulation rates, however, were nearly twice as high in the restored marsh (230.49 g C m -2 yr -1 ) compared to the natural marshes (123.00 g C m -2 yr -1 ) due to high rates of accretion in the restored marsh (1.57 cm yr -1 ). Mean elevation change rates were nearly twice that of corresponding 210 Pb accretion rates, but all were greater than the current rate of sea level rise.

There is increasing urgency to implement climate change mitigation strategies that enhance greenhouse gas removal from the atmosphere and reduce carbon dioxide (CO 2 ) emissions. Recently, coastal “blue carbon” habitats⁠—mangroves, salt marshes, and seagrass meadows—have received attention for their ability to capture CO 2 and store organic carbon (OC), primarily in their sediments. Across habitat types and regions, however, information about the sequestration rates and sources of carbon to local sediments remains sparse. Here we compiled recently obtained estimates of sediment OC stocks and sequestration rates from 139 cores collected from temperate seagrass ( Zostera marina ) meadows in Alaska, British Columbia, Washington, and Oregon. Across all cores sediment OC content averaged 0.75%. Organic carbon stocks in the top 25 cm and 1 m of the sediment averaged 1,846 and 7,168 g OC m −2 , respectively. Carbon sequestration rates ranged from 4.6 to 93.0 g OC m −2 yr −1 and averaged 24.8 g OC m −2 yr −1 . Isotopic data from this region suggest that OC in the sediments is largely from noneelgrass sources. In general, these values are comparable to those from other temperate Z. marina meadows, but significantly lower than previously reported values for seagrasses globally. These results further highlight the need for local and species‐level quantification of blue carbon parameters. While temperate eelgrass meadows may not sequester and store as much carbon as seagrass meadows elsewhere, climate policy incentives should still be implemented to protect existing sediment carbon stocks and the other critical ecosystem services associated with eelgrass habitats.

Global change will impact coastal systems in a number of ways. Climate change will result in higher temperatures, accelerated sea-level rise, changed freshwater discharge, more intense precipitation events and droughts, and changes in the frequency and intensity of tropical storms as well as winter storms. Increasing temperature will shift the tropical-temperate interface toward higher latitudes. Sea level will likely rise from 1.0 to 1.5 m by 2100. In general, precipitation will increase near the equator and at higher latitudes and decrease in the outer tropics and lower temperate zone. Accelerated sea-level rise will threaten coastal wetlands and other lowland environments. Fresh water discharge will increase to some coastal areas and decrease for others with potential impacts on severe floods, wetland vegetation, and coastal productivity. Climate change will interact with other global change forcings such as environmental degradation, population pressure, and energy costs to exacerbate climate impacts. Resource scarcity, especially energy, will make energy intensive restoration and adaptation projects more expensive. Some coastal systems such as arid systems, areas with large expanses near mean sea level like deltas, and those with high human impact will be less sustainable. Management of coastal systems should be based on system functioning that uses natural energies to the greatest extent possible. It is important to understand that there are biophysical limits that need to be understood and taken into consideration for sustainable management and restoration to take place.

An assimilation wetland is a natural (non-constructed) wetland into which secondarily-treated, disinfected, non-toxic municipal effluent is discharged. In the Mississippi River Delta, the wetland is typically either a freshwater forested wetland (e.g., baldcypress-water tupelo) or a freshwater emergent wetland. These wetlands have been hydrology altered, some extensively, with freshwater input reduced from historical norms. Discharge of freshwater effluent with nutrients and suspended sediments into an assimilation wetland increases vegetation productivity and accretion and combats subsidence. Effluent discharge rate into an assimilation wetland depends on wetland size and effluent nutrient concentrations. Design and construction of an assimilation wetland requires a Louisiana Department of Natural Resources (LDNR) Coastal Use Permit (CUP), a Louisiana Department of Environmental Quality (LDEQ) Louisiana Pollutant Discharge Elimination System (LPDES) permit, a US Army Corps of Engineers (USACE) 404 permit, and an LDEQ Water Quality Certification, along with potential levee board permit applications. Both a feasibility study and an ecological baseline study are conducted before discharge of treated effluent begins. Assimilation wetlands are designed with a minimum of four monitoring sites; three located along a transect from the discharge to the area where surface water leaves the wetland, and the fourth, a reference area, located in an ecologically similar wetland nearby. As part of the LDEQ LPDES permit, study sites within an assimilation wetland are monitored continually for the life of the project, including vegetation productivity and species composition, sediment accretion, hydrology, and surface water nutrient and metals concentrations. There are ten active assimilation wetlands in coastal Louisiana and another four with permit applications pending. Results of annual monitoring show nutrient concentrations of surface waters decrease with distance, reaching background levels before water leaves the wetland. While nutrient concentrations decrease, vegetative productivity is enhanced. In degraded forested wetlands being used as assimilation wetlands, baldcypress and water tupelo seedlings are often planted, which thrive in the nutrient rich environment. However, nutria are attracted to vegetation with increased nutrient concentrations, and herbivory severely damaged one emergent wetland receiving municipal effluent, killing both herbaceous vegetation and unprotected tree seedlings. After culling of nutria, the wetland recovered. This introduced species must be monitored and controlled in any assimilation wetland. Here we review the history of assimilation wetlands in the Mississippi River Delta to show how advances in scientific understanding, growing regulatory sophistication, and controversy have shaped this program.

Coastal wetlands are known to be efficient carbon sinks due to high rates of primary productivity, carbon burial by mineral sediments, and low rates of sediment organic matter decomposition. Of the three coastal wetland types: tidal marshes, tidal forests, and seagrass meadows, carbon burial by seagrasses is relatively under-studied, and reported rates range widely from 45 to 190 g C m(-2) yr(-1). Additionally, most of these seagrass rates are biased toward tropical and subtropical species, particularly Posidonia oceanica, with few focused on Zostera marina, the most widespread species in the northern hemisphere. We measured sediment organic content, carbon content, and long-term accretion rates to estimate organic carbon stocks and sequestration rates for a Z. marina meadow in Padilla Bay, a National Estuarine Research Reserve in Washington. We found rates of carbon sequestration to be quite low relative to commonly reported values, averaging 9 to 11 g C m(-2) yr(-1). We attribute this to both low sediment organic content and low rates of accretion. We postulate here that Padilla Bay's low carbon sequestration capacity may be representative of healthy Z. marina meadows rather than an anomaly, and that Z. marina meadows have an inherently low carbon sequestration capacity because of the species' low tolerance for suspended sediment (which limits light availability) and sediment organic content (which leads to toxic sulfide levels). Further research should focus on measuring carbon sequestration rates from other Z. marina meadows, particularly from sites that exhibit, a priori, the potential for higher rates of carbon sequestration.

Large Mississippi River (MR) diversions (peak water flow > 1416 m(3)/s and sediment loads > 165 kg/s) have been proposed as part of a suite of coastal restoration projects and are expected to rehabilitate and rebuild wetlands to alleviate the significant historic wetland loss in coastal Louisiana. These coastal wetlands are undergoing increasing eustatic sea-level rise, land subsidence, climate change, and anthropogenic disturbances. However, the effect of MR diversions on wetland soil organic carbon (SOC) sequestration in receiving basins remains unknown. The rate of SOC sequestration or carbon burial in wetlands is one of the variables used to assess the role of wetland soils in carbon cycling and also to construct wetland carbon budgets. In this study, we examined the effects of MR water and sediment diversions on landscape-scale SOC sequestration rates that were estimated from vertical accretion for the next 50 yr (2010-2060) under two environmental (moderate and less optimistic) scenarios. Our analyses were based on model simulations taken from the Wetland Morphology model developed for Louisiana's 2012 Coastal Master Plan. The master plan modeled a "future-without-action" scenario as well as eight individual MR diversion projects in two of the hydrologic basins (Barataria and Breton Sound). We examined the effects that discharge rates (peak flow) and locations of these individual diversion projects had on SOC sequestration rates. Modeling results indicate that large river diversions are capable of improving basin-wide SOC sequestration capacity (162-222 g C.m(-2).yr(-1)) by up to 14% (30 g C.m(-2).yr(-1)) in Louisiana deltaic wetlands compared to the future-without-action scenario, especially under the less optimistic scenario. When large river diversions are placed in the upper receiving basin, SOC sequestration rates are 3.7-10.5% higher (6-24 g C.m(-2).yr(-1)) than when these structures are placed in the lower receiving basin. Modeling results also indicate that both diversion discharge and location have large effects on SOC sequestration in low-salinity (freshwater and intermediate marshes) as compared to high-salinity marshes (brackish and saline marshes).