Hurricanes along the Southeast Atlantic Coast build marsh resiliency through mineral sedimentation

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Salt marsh stability and patterns of sedimentation across a backbarrier platform

Abstract. The persistence of tidal wetland ecosystems like salt marshes is threatened by human interventions and climate change. In particular, the threat of accelerated sea level rise (SLR) has increasingly gained the attention of the scientific community recently. However, studies investigating the effect of SLR on plants and vertical marsh accretion are usually restricted to the species or community level and do not consider phenotypic plasticity or genetic diversity. To investigate the response of genotypes within the same salt-marsh species to SLR, we used two known genotypes of Elymus athericus (Link) Kerguélen (low-marsh and high-marsh genotypes). In a factorial marsh organ experiment we exposed both genotypes to different flooding frequencies and quantified plant growth parameters. With increasing flooding frequency, the low-marsh genotype showed higher aboveground biomass production compared to the high-marsh genotype. Additionally, the low-marsh genotype generally formed longer rhizomes, shoots and leaves, regardless of flooding frequency. Belowground biomass of both genotypes decreased with increasing flooding frequency. We conclude that the low-marsh genotype is better adapted to higher flooding frequencies through its ability to allocate resources from below- to aboveground biomass. Given the strong control of plant biomass production on salt-marsh accretion, we argue that these findings yield important implications for our understanding of ecosystem resilience to SLR as well as plant species distribution in salt marshes.

Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO(2) concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO(2)] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO(2) (ambient + 340 ppm) accelerated soil elevation gain by 3.9 mm yr(-1) in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO(2) effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO(2), may paradoxically aid some coastal wetlands in counterbalancing rising seas.

Increased sea level rise accelerates carbon sequestration in a macro-tidal salt marsh.

Wetland soil formation in the rapidly subsiding Mississippi River Deltaic Plain: Mineral and organic matter relationships

Salt marsh elevation and geomorphic stability depends on mineral sedimentation. Many Mediterranean-climate salt marshes along southern California, USA coast import sediment during El Nino storm events, but sediment fluxes and mechanisms during dry weather are potentially important for marsh stability. We calculated tidal creek sediment fluxes within a highly modified, sediment-starved, 1.5-km2 salt marsh (Seal Beach) and a less modified 1-km2 marsh (Mugu) with fluvial sediment supply. We measured salt marsh plain suspended sediment concentration and vertical accretion using single stage samplers and marker horizons. At Seal Beach, a 2014 storm yielded 39 and 28 g/s mean sediment fluxes and imported 12,000 and 8800 kg in a western and eastern channel. Western channel storm imports offset 8700 kg exported during 2 months of dry weather, while eastern channel storm imports augmented 9200 kg imported during dry weather. During the storm at Mugu, suspended sediment concentrations on the marsh plain increased by a factor of four; accretion was 1–2 mm near creek levees. An exceptionally high tide sequence yielded 4.4 g/s mean sediment flux, importing 1700 kg: 20 % of Mugu’s dry weather fluxes. Overall, low sediment fluxes were observed, suggesting that these salt marshes are geomorphically stable during dry weather conditions. Results suggest storms and high lunar tides may play large roles, importing sediment and maintaining dry weather sediment flux balances for southern California salt marshes. However, under future climate change and sea level rise scenarios, results suggest that balanced sediment fluxes lead to marsh elevational instability based on estimated mineral sediment deficits.

The area of salt marshes does no longer increase. The recent erosion coincides with a rise in MHT-level in the last 25 years. Despite the decrease in area, sedimentation continues, especially in the lower salt marsh, which acts as a sink of nitrogen. Assimilation and mineralization of nitrogen are in balance in most plant communities along the gradient from lower to higher salt marshes. Mineralization of nitrogen increases towards the higher salt marsh, whereas the above-ground production and the mean nitrogen content of plants decrease. There is a positive correlation between quality of food plants in salt marshes and breeding success of Brent geese in the arctic tundra. Sedimentation on mainland salt marshes can compensate for the expected sea level rise. This is not the case for island salt marshes, if the relative sea level rise is more than 0.5–1.0 cm yr−1. The natural succession on salt marshes results in an accumulation of organic material, which is related to the dominance of single plant species. It is not clear to which extent this process is enhanced by eutrophication from acid deposition and seawater. Human exploitation of unprotected salt marshes is old and heavy in the system of mound settlements. Reclamation rates by dikes in the last centuries were higher than the rate of area increase. Grazing by cattle as a management practice results in both a higher plant species-richness and community diversity than abandoning; hay-making is intermediate, but shows less structural diversity than grazing with low stocking density. The invertebrate fauna is favoured by a short period of abandoning, but eventually characteristic salt marsh invertebrates are replaced by inland species. Many bird species prefer grazed salt marshes. The final section gives some perspectives. Provided that no further embankments take place the optimal nature management option for plants and animals is a vegetation pattern, which includes areas with a low canopy (grazed) and areas with a tall canopy.

Salt marsh peat deposits at many locations around the world, including the U. S. Atlantic and Gulf coasts, provide proxy data for reconstructing the history of changes in relative sea level (RSL). A geological database of index points has been established through sampling and analysis of salt marsh peat collected at many of those locations. However, spatial gaps in this database still exist in the available data for southern New England. A notable gap is the southern coast of Rhode Island, primarily due to the lack of precalculated tidal datums and the complexity of producing records in an active back barrier salt marsh setting. The primary goal of this research was to provide additional data to fill those gaps by establishing rates of sea-level rise in the salt marshes of the coastal ponds behind the barrier beach of southern Rhode Island. This project used basal-peat deposits, radiocarbon dating, and foraminifera that are indicators of local tide level. Exploratory/reconnaissance coring of the salt marshes of three ponds: Ninigret Pond, Quonochontaug Pond and Winnapaug Pond generally revealed very shallow peat deposits compared to those found around Narragansett Bay. Three locations were found that provided data sufficient to calculate usable rates of sea-level rise. The deepest peat deposit and oldest date was recovered at a salt marsh adjacent to Wildflower Road (WFN), Charlestown, RI. At 121 cm below the surface and a date of 1230 yrs. BP, the corresponding sedimentation rate and estimated rate of sea-level rise from the age/depth relationship was ~ 1 mm/yr. using the surface. This depth/age relationship and estimated rate of sea-level rise provides a frame of reference for sea level dynamics consistent with data of the same age and older in Narragansett Bay. Climate change, rising sea-level and storm waves have had an impact on the coastal salt marshes of Rhode Island and coastal communities of all the states along the U. S. Atlantic coast. The potential impacts from sea-level rise to salt marsh ecosystem services, coastal infrastructure, and coastal cultural resources, especially older indigenous cultural sites are likely to increase for decades if not centuries to come.

Salt marshes in the northeastern United States support several specialized breeding bird species that are threatened by sea level rise (SLR) and coastal development, processes that drive habitat change and fragmentation. There have been rapid, widespread declines in some species, but mechanisms driving population change and whether declines continue remain unclear. We examined the influence of phenomena expected to modify salt marshes, including SLR, sediment delivery rates, and land use, on the population trajectories of saltmarsh breeding birds. We modeled population trajectories of 5 species with spatially extensive point count surveys conducted from Maine to Virginia from 2011 to 2022. We used Bayesian hierarchical abundance models and model selection to identify phenomena that had the strongest effect on population change. Clapper rails (Rallus crepitans) continued their long-term decline (-4.1%/year). Willets (Tringa semipalmata semipalmata) (2.6%/year) and saltmarsh sparrows (Ammospiza caudacuta) (4.1%/year) increased, and seaside (Ammospiza maritima) and Nelson's (Ammospiza nelsoni subvirgatus) sparrows exhibited no clear change in abundance. The estimated increase for saltmarsh sparrow was not consistent with trends over the previous 25years but aligned with prior demographic modeling, which predicted a short-term stabilization during the study years before an expected return to a long-term decline. Road density and other tidal restrictions near marshes were generally good predictors of abundance over the study period, as was marsh habitat composition. Local rates of SLR and sediment delivery were not as good predictors. During periods of relatively low rates of realized SLR, local-scale drivers of population trends had relatively stronger effects than global drivers on the persistence of several saltmarsh breeding birds. Conservation practitioners, however, should be attentive to global drivers, especially as rates of SLR accelerate in the future.

Importance of quantifying the full-depth carbon reservoir of Jamaica Bay salt Marshes, New York

1 The relationships between soil development, vertical vegetation zonation, vegetation succession and herbivory by Brent geese, Branta bernicla, were studied in a coastal salt marsh. We were able to analyse up to 100 years of salt marsh development by comparing sites where vegetation succession had progressed for-various periods of time. These data were related to a continuous daily record of high water levels measured since 1824. 2 Most elevational variation in edaphic conditions (and therefore vertical vegetation zonation) could be attributed to variation in height of the sandy subsoil, as rapid dune formation occurs on the beaches early in succession. In the intermediate part of this elevational gradient, the maximum annual increase of 1.2 mm of silt corresponded to an annual increase of 5.6 g N m(-2) in the topsoil (0-50 cm). The average sea level rise in this area over the last 170 years was 0.63 mm year(-1). A sedimentation model suggests that this has had strong effects on sedimentation and the annual inundation frequency in the mid-part of the elevational gradient, thus affecting vegetation zonation on the salt marsh. For the major part of the investigated transects, sea level rise has probably speeded up succession due to an increased rate of sedimentation. 3 The occurrence and dominance of all plant species were recorded in 3927 plots, and and for the 11 most common species response surfaces were calculated for their dependence on elevation and transect age. Most plant species were clearly separated along these axes. Most halophytic species, which were preferred by the geese, occurred early in succession and low on the gradient, where we observed the highest densities ge quality of Festuca I rubra increased towards the lower salt marsh. Other preferred forage species (Puccinellia maritima and Plantago maritima) were gradually displaced during succession by the tall grass Elymus athericus, especially in the mid-and upper salt marsh. Few geese grazed in areas where Elymus was dominant. 4 Herbivores first increased in numbers but then declined along a gradient of primary productivity. We propose that declining forage quality (due to changing vegetation composition during succession) is a better explanation for this pattern than the classic explanation of predator control of herbivores at high levels of primary productivity. This quality threshold hypothesis, as an alternative explanation of the exploitation ecosystem hypothesis, is expected to hold especially where smaller (quality-sensitive) herbivores such as geese are present. 5 Grazing by cattle in a 200-year-old part of the salt marsh led to the disappearance of Elymus athericus, to establishment of early successional halophytes and a return of Brent geese. Crazing by a larger herbivore therefore facilitated conditions for smaller herbivores by preventing the dominance of plant species that were good light competitors, and thus improved forage quality. Populations of these small herbivores could then become regulated by predators, although none was present at our site.

Evolution of a salt marsh in the southeastern North Sea region – Anthropogenic and natural forcing

Sea level rise is a major stressor on many salt marshes, and its impacts include creek widening, ponding, vegetation dieback, and drowning. Marsh vegetation changes have been associated with sea level rise across southern New England, but most of these studies pre-date the current period of rapidly accelerating sea level rise coupled with episodic events of extreme increases in water levels. Here, we combine data from two salt marsh monitoring and assessment programs in Rhode Island that were designed to assess marsh responses to sea level rise and use these data to document temporal and spatial patterns in marsh vegetation during the current period of extreme water level increases. Vegetation monitoring at two Narragansett Bay salt marshes confirms the ongoing decline of the salt meadow species Spartina patens during this period as it becomes replaced by Spartina alterniflora. Bare ground resulting from vegetation dieback was significantly related to mean high water levels and led to the rapid conversion of mixed Spartina assemblages to S. alterniflora monocultures. A broader spatial assessment of RI marshes shows that S. alterniflora dominance increases at lower elevation marshes toward the mouth of Narraganset Bay. Our data provide additional evidence that S. patens continues to decline in southern New England marshes and show that losses can accelerate during periods of extreme high water levels. Unless adaptive management actions are taken, we predict that marshes throughout RI will continue to lose salt meadow habitat and eventually resemble lower elevation marshes that are already dominated by S. alterniflora monocultures.

Seaward expansion of salt marshes on the largest uninhabited island in Changjiang Estuary over the past two decades