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At least 145 records · Page 8Linked to original sources
USGS2021

Emerging dominance of Paratrochammina simplissima (Cushman and McCulloch) in the northern Gulf of Mexico following hydrologic and geomorphic changes

Grand Bay estuary in coastal Mississippi and Alabama (USA) has undergone significant geomorphic changes over the last few centuries as a result of anthropogenic (bridge, road, and hardened shoreline construction) and climatic (extreme storm events) processes, which reduce freshwater input, sediment supply, and degrade barrier islands. To investigate how geomorphic changes may have altered the Grand Bay estuary, sediment push cores were collected for foraminiferal, sedimentological (organic matter content, grain-size distribution), and radiochemical ( 210 Pb, 137 Cs, and 7 Be) analyses. Clay normalized geochronologies were determined with a constant rate of supply model. Based on downcore age-depth relationships, select intervals were analyzed for foraminifera in order to assess alterations in the microfossil assemblage in Grand Bay estuary over the 20th Century. All estuarine samples were low diversity (species richness: 1–10; Fisher's alpha diversity: 0.14–1.75); two species, Ammotium salsum and Paratrochammina simplissima, dominated all downcore assemblages. Paratrochammina simplissima increased in abundance up-core from a minor subsidiary species (median = 4.7% at 19–20 cm) to dominant or co-dominant with A. salsum over the 20th and early 21st Centuries in six cores, comprising up to 60.7% of a single sample. The emerging dominance of P. simplissima since ~1950 along with the reduction of brackish-estuarine taxa and introduction of calcareous species signifies increased salinity and less marsh organic matter preserved in the sediments. While seasonal dissolution limits our ability to chronologically constrain the introduction of calcareous species, P. simplissima , a species not referenced in taxonomic data from the northern Gulf of Mexico until 2012, is well constrained, following its first occurrence in the 1930s.

Mississippi↗
USGS2021

Wetlands in intermittently closed estuaries can build elevations to keep pace with sea-level rise

Sea-level rise is a threat to coastal ecosystems, which have important conservation and economic value. While marsh response to sea-level rise has been well characterized for perennially open estuaries, bar-built intermittently-closed estuaries and their sea-level rise response are seldom addressed in the literature – despite being common globally. We seek to advance the conceptual understanding of sea-level rise response of marshes by incorporating the unique nature of intermittently-closed estuaries in a marsh model. We hypothesize that intermittently-closed-estuary marshes may be more resilient to sea-level rise than open-estuary marshes due to greater initial elevation capital and higher accretion rates due to closure events. Using California, USA as a case study, spatial analysis shows that marshes in intermittently-closed-estuaries had significantly greater elevations (x̄ = 1.93 m ± 0.2 standard error, n = 14) than marshes in permanently open estuaries (x̄ = 0.94 m ± 0.1 standard error, n = 8; P = 0.003). We then used a process-based model to determine marsh elevation change under 840 simulated responses to sea-level rise to 2100. Our modeling shows that regular annual mouth closure can promote accretion rates and increase marsh elevations fast enough to match even high rates of sea-level rise, as fluvial sediment pulses can be captured in the estuary. Modeled suspended sediment concentration had the strongest effect on accretion, followed by probability of annual mouth closure. Intermittently closed estuaries are critical environments where marshes may be sustained under high rates of sea-level rise, thus reducing the anticipated global loss of these important ecosystems. Our results begin to fill an important gap in the knowledge about marsh accretion and identify research needs to inform coastal management.

Estuarine, Coastal and Shelf Science↗
USGS2021

Land-based sediment sources and transport to southwest Puerto Rico coral reefs after Hurricane Maria, May 2017 to June 2018

The effects of runoff from land on nearshore ecosystems, including coral reef communities, are influenced by both sediment supply and removal by coastal processes. Integrated studies across the land-sea interface describing sources and transport of terrestrial sediment and its nearshore fate allow reef protection initiatives to target key onshore and offshore areas. Geochemical signatures in the fine fraction of terrestrial sediment from watersheds in southwest Puerto Rico were determined by multivariate principal component analysis and used to identify terrestrial sources of sediment runoff to nearshore coral reefs. Sediment settling out of suspension at reefs was collected at approximately 2 month-long intervals in bottom-mounted sediment traps from May 2017 to June 2018, a period that included Hurricanes Irma and Maria. Bulk sediment accumulation rates in traps exceeded a 10 mg/cm 2 /d threshold found to stress corals at 5 of 7 reef sites throughout the 13 month-long study. Geochemical signatures showed that watersheds 10s km to the east were a predominant, year-round source of fine sediment to reefs offshore of Guánica Bay and could have introduced sediment-bound contaminants due to a higher degree of industrialization and urbanization than the local watershed. Sediment runoff from the local watershed appeared to be constrained to a narrow band close to shore. During the 2.5 months after Hurricanes Irma and Maria, bulk sediment accumulation rates increased substantially and fine sediment geochemical signatures were indicative of predominantly distal sources, except outside of the mouth of Guánica Bay, which was strongly impacted by local runoff. Mass wasting, sediment runoff, and coastal turbidity persisted for months after Hurricane Maria and could account for the appearance of a small fraction of geochemical variance from a distal sediment source that appeared in reef traps 4 months post-hurricane and persisted through the end of the study 9 months post-hurricane. Sediment geochemical sourcing in temporally resolved records from sediment traps showed how landscape-scale changes after a major hurricane affected both near-term and long-term sediment delivery to reef communities. In addition, the importance of fine sediment advection from distal sources indicates that successful reduction of land-based pressures on nearshore ecosystems will require cross-jurisdictional strategies.

Puerto Rico↗
USGS2022

Ocean connectivity drives trophic support for consumers in an intermittently closed coastal lagoon

Estuarine food webs are complex, as marine, freshwater, and terrestrial inputs combine and contribute variable amounts of organic material. Seasonal fluctuations in precipitation amplify the dynamism inherent to estuarine food webs, particularly in lagoonal estuaries, which can be seasonally closed and disconnected from the ocean in low-runoff periods (bar-built lagoons). Despite their abundance along coastlines in Mediterranean climates, the organic matter sources fueling bar-built lagoon food webs are poorly understood, particularly with respect to seasonal hydrologic variability, episodic marine connections, and internal nutrient cycling. In this study, we evaluate the food web of a bar-built lagoon with respect to seasonal differences in lagoon water quality, the sources of organic matter which support consumers, and the trophic ecology of resident fishes. Observed water quality conditions reflected biogeochemical processes associated with salinity-driven stratification and high lagoon residence times and were associated with strong seasonal differences in the contribution of different organic matter sources to lagoon consumers. A variety of organic matter sources supported consumers; marine inputs were important to lagoon food webs in spring when the lagoon was open, while summer food webs were largely driven by phytoplankton which was likely fueled by internal nutrient cycling. Fish diets were largely comprised of crustaceans and fish eggs, with clearly defined trophic niches in spring but high overlap in summer. This study demonstrates that the seasonal changes in bar-built lagoon food webs are largely dependent on ocean connectivity and internal cycling within the lagoon, rather than watershed processes as is typical for many estuaries.

California↗
USGS2022

Elevations of mangrove forests of Pohnpei, Micronesia

Mangrove surface elevation is the crux of mangrove vulnerability to sea level rise. Local topography influences critical periods of tidal inundation that govern distributions of mangrove species and dictates future distributions. This study surveyed ground surface elevations of the extensive mangroves of Pohnpei, Federated States of Micronesia, integrating four survey technologies to solve issues of canopy blocking satellite reception, dense aerial roots limiting line-of-sight, and remoteness from surveyed datums. The island-wide average elevation of the mangrove seaward edge was −0.57 ± 0.13 m relative to MSL, while the landward average elevation was 0.33 ± 0.12 m relative to MSL. The overall mangrove elevation range was thus estimated to be 0.90 m. Mangrove species Bruguiera gymnorrhiza , Rhizophora apiculata and Sonneratia alba had large, overlapping elevation ranges, while Rhizophora stylosa occurred low in the tide frame. These species are likely to be less vulnerable to rising sea level given their greater range of elevation occurrence and presumably flooding tolerance, and hence have the highest adaptive capacity to rising sea level. Some landward edge species had very narrow elevation ranges, increasing their vulnerability to sea-level rise, with adjacent potential upland migration areas limited due to steep topography and human development. Pohnpei mangroves occupied 74% of the mean tidal range, similar to surveys elsewhere in the Pacific. This study demonstrates how more extensive understanding of the elevation distributions of intertidal species can contribute to sea-level rise vulnerability assessments, to allow prioritised climate change adaptation. However, more work is needed in standardizing approaches for global comparisons.

Estuarine, Coastal and Shelf Science↗
USGS2022

Biomass allocation of tidal freshwater marsh species in response to natural and manipulated hydroperiod in coastal deltaic floodplains

Deltaic floodplains are highly vulnerable to relative sea level rise (RSLR) depending on the sediment supply from river channels that provides elevation capital as adaptation mechanism. In river channels where levees have restricted sediment supply to coastal deltaic floodplains, river sediment diversions have been proposed as a restoration strategy to increase elevation allowing for marshes to establish and cope with RSLR. The response of coastal wetlands to surface elevation has been well-defined for estuarine marshes, but models for coastal deltaic floodplain marshes have not been resolved. Here we coupled field observations from biomass plots and a mesocosm experiment (‘marsh organ’) with remote sensing techniques to assess biomass allocation of tidal freshwater marsh species in response to gradients in hydroperiod in Wax Lake Delta (WLD), coastal Louisiana, U.S.A.. We found that, contrary to salt-tolerant species, Colocasia esculenta aboveground biomass (AGB) is strongly positively correlated with percent inundated time (R2 = 0.79, P < 0.001), increasing from (mean ± 1SE) 186 ± 69 g/m2 in the supratidal zone to 1422 ± 148 g/m2 beyond its natural occurrence range in the lower intertidal zone. Belowground biomass consistently exceeded AGB at 2363 ± 294 g/m2 on average across elevation treatments. We also found that C. esculenta expanded its surface coverage area by 31% in five years consistent with the growth and emergence of WLD's subaqueous platforms, reflecting this species ability to cope with higher inundation time. In contrast to earlier studies conducted in brackish and saline settings, where longer hydroperiods had negative effects on biomass accumulation, our data suggest that tidal freshwater marshes can cope with longer hydroperiods caused by river sediment diversions.

Estuarine, Coastal and Shelf Science↗
USGS2022

Evidence of a dietary shift by the Florida manatee (Trichechus manatus latirostris) in the Indian River Lagoon inferred from stomach content analyses

Investigating the long-term fluctuations of the feeding ecology of megaherbivores such as sirenians is important, as any changes could be indicative of shifts in resource availability. The Indian River Lagoon (IRL), eastern Florida, USA, is a critical habitat for the Florida manatee ( Trichechus manatus latirostris ). However, the IRL has experienced a substantial decline in seagrass due to the persistence of several harmful algal blooms. Using microhistological analysis, we examined the diet of manatees over a discontinuous sampling period spanning over 38 years using stomach contents collected from carcasses recovered in the IRL. Samples collected between 2013–2015 (post-seagrass die-off, n = 90) were compared to archived stomach samples collected between 1977–1989 (pre-seagrass die-off, n = 103). Samples analyzed from 1977–1989 contained primarily seagrasses (61.7%), followed by algae (28.4%) and vascular plants (1.7%). In contrast, stomach samples from the post-seagrass die-off primarily contained algae (49.5%), followed by seagrasses (34%) and vascular plants (2.7%). Between 1977–1989 and 2013–2015, manatees in the IRL experienced a 44.9% decline in seagrass consumption, and a 74.3% increase in algal consumption. This dietary shift was not influenced by body length, a proxy of age, or sex. Our results indicate that the dietary shift experienced by manatees is due to the decline of available seagrass forage in the IRL, and highlight the dietary plasticity of manatees in the face of changes in resource availability. However, the individual health and population-level consequences of this dietary shift are unknown. An increase in mortality due to undetermined causes in this region since 2012 can be associated with deteriorating body conditions of manatees in the IRL, possibly resulting from a lack of seagrass diet. Future research should further investigate behavioral changes affecting manatees in relation to seagrass decline in the IRL, including the energetic costs of this dietary change.

Florida↗
USGS2022

Porewater chemistry of Louisiana marshes with contrasting salinities and its implications for coastal acidification

Dissolved inorganic carbon (DIC) and total alkalinity (TA) are fundamental components of carbonate systems that control pH and buffering capacity of the receiving water body. Three coastal marshes with contrasting salinities in Barataria Basin, Louisiana, USA, were sampled to understand seasonal changes in porewater carbonate chemistry and its impact on surrounding water bodies. Each marsh was sampled five times between December 2018 and October 2019. Porewater DIC and TA increased with depth irrespective of marsh type and ranged from 4.47 to 31.61 mmol/kg and from 1.78 to 28.56 mmol/kg, respectively. The salt marsh had higher porewater DIC and TA compared to the lower salinity intermediate and brackish marshes, probably due to sulfate reduction in the salt marsh. However, it is likely that denitrification is the dominant anaerobic process in these marshes because of low porewater TA/DIC ratios in all three marshes. Porewater TA and DIC concentrations were generally higher during warmer months than colder months. However, the marsh flooding regime had a profound influence on TA and DIC concentrations by changing the redox potential of the marsh soil. Porewater TA/DIC ratios in all three marshes were generally less than 1, while surface water TA/DIC ratios were around 1, suggesting that export of DIC and TA from coastal marshes have the potential to contribute to coastal acidification.

Louisiana↗
USGS2023

A numerical investigation of the mechanisms controlling salt intrusion in the Delaware Bay Estuary

Salinity intrusion in coastal systems is mainly controlled by freshwater inflows. However, extreme events like drought, low-pressure storms, and longer-term sea level rise can exacerbate the landward salt migration and threaten economic infrastructure and ecological health. Along the eastern seaboard of the United States, approximately 13 million people rely on the water resources of the Delaware River basin. Salinity intrusion is actively managed through river discharge targets to suppress the propagation of the salt front (∼0.52 daily averaged psu line). The purpose of this study is to examine the mechanisms controlling the location of the salt front in the Delaware Bay estuary using a calibrated three-dimensional hydrodynamic model, the Coupled Ocean Atmosphere Wave and Sediment Transport modeling system. This study explored how river discharge, tidal motions, interactions with bathymetric and topographic features, and meteorological events affected the location of the salt front. The model was forced with tides, subtidal water levels, bulk atmospheric conditions, and waves. Compared with the observationally derived location of the salt front line, the model captured the major dynamics throughout the year and performed particularly well during times of low discharge, when salinity intruded up estuary at a constant rate of 0.4 km /day . The daily average salt front moved almost 16 km (10 mi) within a neap-spring tidal cycle, and low-pressure storm systems were found to move the daily averaged salt front by 13–16 km in one event.

Delaware, New Jersey, Pennsylvania↗
USGS2023

Flushing time variability in a short, low-inflow estuary

Flushing time, the time scale for exchange and mixing between embayed and oceanic waters in an estuary, plays an integral role in determining water quality and aquatic ecosystem health. Here, we investigated the spatiotemporal variability of flushing times throughout Morro Bay, a short, low-inflow estuary (LIE) on the California coast, using a calibrated and validated hydrodynamic model (Delft3D). Morro Bay has historically supported an extensive eelgrass ( Zostera marina ) habitat, which declined substantially from 139 to 5.4 ha during 2007–2017. Eelgrass decline motivated the current research into the role of changing bed roughness and oceanic drivers (i.e., tide and sea-level rise) on estuarine hydrodynamics and flushing times. We found that tidal variability exerts the strongest control on flushing times compared to other effects, i.e., bed roughness or sea-level rise. Additionally, we found that increasing sea level and decreasing bed roughness (associated with declining seagrass coverage) yielded higher rates of mixing (lower flushing times). We detected a strong correspondence between areas having shorter flushing times (e.g., near the estuary mouth) and areas occupied by resilient eelgrass populations in Morro Bay. Our findings further indicated that flushing times in short LIEs are particularly sensitive to several factors (e.g., bed roughness, sea level) that are susceptible to anthropogenic disturbance and future climate change.

California↗
USGS2023

Metabarcoding analysis of meiobenthic biodiversity along the Gulf of Mexico continental shelf

This study explores how diverse the meiobenthic (meiofauna and other benthic micro-eukaryotes) community is throughout the United States Gulf of Mexico (GOM) continental shelf. In late 2010 and 2011, 51 sediment samples were collected along GOM from Texas through Florida at a range of depths (40m–496m). An additional six deep-sea slope sediment cores were collected in December 2010 near the Deepwater Horizon platform (1370–1385m and 1865m). Metabarcoding of the 18S hypervariable V9 region was conducted to assess biodiversity. Within continental shelf samples, there was greater meiobenthic diversity off the Eastern GOM coast in comparison to both Central and Western GOM coast locations. The Eastern GOM coast has known sediment differences from Western GOM sites. These sediment differences along with influences from the Gulf of Mexico Loop Current may account for observed variations in GOM meiobenthic diversity.

Gulf of Mexico↗
USGS2023

Estuarine salinity extremes: Using the Coastal Salinity Index to quantify the role of droughts, floods, hurricanes, and freshwater flow alteration

In the face of accelerating climate change, advancing understanding of how extreme climatic events influence estuarine salinities can help to inform resource management. Extreme salinities driven by droughts, hurricanes, floods, and freshwater flow alterations can lead to ecological transformations in estuarine ecosystems. Here, we applied the Coastal Salinity Index (CSI; Conrads and Darby 2017) to 22 years (1998–2020) of salinity data in a Louisiana estuary (Barataria Estuary, USA) to elucidate the impacts of extreme events on estuarine salinities. The CSI is an index to quantify salinity patterns at a specific location through long-term averages and deviations from historical average conditions. We calculated and compared CSI values for four stations distributed along an estuarine salinity gradient. We identified 10 events between 1998 and 2020 that produced extreme salinities, including two droughts, four hurricanes, three floods, and one freshwater diversion. The droughts of 2000 and 2006 caused surface water salinities to increase substantially throughout the estuary. The effects of hurricanes were highly variable, with some storms leading to elevated salinities throughout the entire estuary (e.g., Hurricanes Katrina and Rita in 2005), whereas other storms led to elevated salinities for some but not all stations (e.g., Hurricanes Gustav and Ike in 2008 or Hurricane Isaac in 2012). The opening of a freshwater river diversion in 2010 contributed to fresher conditions throughout the estuary and appeared to reduce or eliminate the increases in salinity that normally occur during the summer, although these effects were short-lived. Mississippi River floods in 2008, 2011, and 2019 reduced salinities throughout the estuary, but the effects were most pronounced in the lower estuary compared to the upper estuary. Collectively, our results advance understanding of the influence of extreme events on estuarine salinity regimes. Our analyses also highlight the value of the CSI for identifying periods with extreme salinities (i.e., extreme high or low salinities) via calculations that place salinity levels within and across estuaries within a historical context.

Louisiana↗
USGS2023

ENSO and NAO linkages to interannual salinity variability in north central Gulf of Mexico estuaries through teleconnections with precipitation

Though the importance of Earth's internal climate modes such as the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) to regional-scale climate variability is well recognized, the degree to which these oscillations are reflected by spatio-temporal salinity variability over interannual timescales in estuaries is less understood. Here an 11-year continuous salinity monitoring dataset spanning 223 stations across Louisiana's coastal wetlands along the northern Gulf of Mexico is examined with empirical orthogonal function (EOF) analysis to identify dominant modes of interannual variability in the salinity field. The first EOF mode accounts for 72% of the variance in the salinity field and captures a domain-wide pattern where salinities vary in-phase through space in response to local precipitation anomalies occurring in the vicinity of the study area. This local precipitation anomaly is positively correlated with ENSO (Nino3.4 index), consistent with the El Niño – wet (La Niña – dry) precipitation teleconnection that is prevalent throughout the northern Gulf of Mexico coast. The second EOF mode, which accounts for 13% of the variance in the salinity field, is expressed primarily in the marshes across the lower reaches of the Mississippi River deltaic plain (MRDP). EOF2 is anticorrelated with annual Mississippi River discharge anomaly such that salinities in the lower MRDP decrease as discharge increases, pointing to enhanced advection of fresh river plume waters over the shelf into the estuary via estuary-ocean exchange during years of anomalously high river discharge. Mississippi River discharge anomaly is positively correlated with the NAO at a one-year time lag, through a teleconnection with precipitation throughout much of the central region of the Mississippi River drainage basin. Together, these findings indicate that most of the interannual salinity variability across Louisiana's coastal wetlands can be linked to climate variability through teleconnections with precipitation. Incorporating these dynamics into restoration planning, monitoring, and adaptive management efforts may help constrain background environmental variation and better isolate restoration effects.

Louisiana↗
USGS2023

Two centuries of southwest Iceland annually-resolved marine temperature reconstructed from Arctica islandica shells

Iceland's exposure to major ocean current pathways of the central North Atlantic makes it a useful location for developing long-term proxy records of past marine climate. Such records provide more detailed understanding of the full range of past variability which is necessary to improve predictions of future changes. We constructed a 225-year (1791–2015 CE) master shell growth chronology from 29 shells of Arctica islandica collected at 100 m water depth in southwest Iceland (Faxaflói). The growth chronology provides a robust age model for shell oxygen isotope (δ 18 O shell ) data produced at annual resolution for 251 years (1765–2015 CE). The temperature reconstruction derived from δ 18 O shell shows coherence with May–October local surface temperature records and sea surface temperatures in the North Atlantic region, suggesting it is a useful proxy indicator of water temperature variability at 100 m depth within Faxaflói. Field correlations between the shell-based records and gridded sea surface temperature data reveal strong positive correlations between the 1-year lagged shell growth and temperatures within the subpolar gyre post-1972, suggesting a delayed influence of subpolar gyre dynamics on ecological indicators in southwest Iceland in recent decades. However, the shell growth chronology and δ 18 O shell record generally show relatively weak and insignificant correlations with larger region climate indices including the Atlantic Multidecadal Variability, North Atlantic Oscillation, and East Atlantic pattern. Therefore the interannual variations in the newly produced shell-based records appear to reflect more local to regional dynamics around southwest Iceland than large-scale modes of climate variability.

Estuarine, Coastal and Shelf Science↗
USGS2024

Patterns of marsh surface accretion rates along salinity and hydroperiod gradients between active and inactive coastal deltaic floodplains

High subsidence rates are inherent to coastal deltas worldwide, contributing to rapid rates of relative sea-level rise and compromising the sustainability of coastal wetlands. Different parts of river deltas, however, experience accretion or erosion, depending on the coupling between ecological and morphological processes. Wetland expansion occurs in active deltaic coastal basins that are connected to riverine sedimentation. In contrast, wetland degradation occurs in inactive deltaic coastal basins where river engineering strategies associated with flood control restrict river connectivity. Here, we investigated the relative role of inorganic and organic loading to marsh accretion rates spanning fresh to brackish to saline zones between active and inactive coastal deltaic floodplains of the Mississippi River Delta. Marsh surface accretion rates monitored over 36 months using the feldspar marker horizon technique ranged from 1.24 ± 0.35 cm yr −1 in the freshwater marsh to 2.94 ± 0.51 cm yr −1 in the saline marsh in the inactive coastal basin compared to an opposite trend in the active coastal basin with a low vertical accretion rate in the saline site at 1.12 ± 0.17 cm yr −1 and higher accretion values at the freshwater site (2.14 ± 0.49 cm yr −1 ). Our results suggest that saline marshes have high resilience identified by high vertical accretion rates exceeding those of river-dominated freshwater marshes in active deltaic floodplains. Overall, the marsh surface accretionary patterns detected in this study underscores the relative contribution of organic and inorganic sediments to elevation capital across salinity gradients between active and inactive basins in coastal Louisiana with particular interest to river management and restoration strategies. These findings, however, are applicable to coastal deltaic floodplains elsewhere given the repetition geomorphic forcings (e.g., relative contribution of riverine, tidal and wave power) and coastal typologies worldwide.

Estuarine, Coastal, and Shelf Science↗
USGS2025

Shifts in marsh erosion, migration, and wave exposure over nearly two centuries of sea-level rise in the Gulf of Mexico

Coastal wetlands are economically important ecosystems, but are at risk to erosion from waves, storms, and sea level rise. However, marshes can persist under rising sea level through vertical accretion and migration into adjacent higher-elevation habitats. We measured rates of marsh shoreline change and migration and compared the results for historical and modern periods in a largely undeveloped and marine-dominated estuary within the Mississippi-Alabama coast of the Northern Gulf of Mexico. Mean shoreline change rate for the modern (post-1957) period was higher than historical rates (pre-1957) at −1.55 ± 0.11 m yr −1 and -0.84 ± 0.07 m yr −1 , respectively. Shoreline change rates were highest in regions where exposure to wind-driven waves increased due to barrier island migration and land loss. Marsh migration (modern 1.25 ± 0.37 m yr −1 and historical 1.01 ± 0.13 m yr −1 ) occurred at similar rates as shoreline erosion except for the highest erosive shorelines, leading to an overall 15% marsh loss. Upland-to-marsh conversion occurred in forested areas and may have been encouraged by changes in management practices. In the Grand Bay estuary, our data show that marsh migration is not occurring at sufficient rates to compensate for marsh loss, resulting in overall loss in marsh habitat. Overall, migration may allow marsh to persist under rising sea levels but can lead to an overall reduction in forested or freshwater habitats if steep slopes or other barriers prevent their migration.

Alabama, Mississippi↗
USGS2025

Elevated CO2 enables brackish marsh transgression into freshwater forested wetlands while stimulating CH4 emissions

Wetlands are significant carbon (C) sinks and are expected to promote greater C assimilation as atmospheric CO 2 concentrations rise. However, the fate of C with environmental change along fresh-to-oligohaline wetland transitions is not well understood. We established an ex-situ mesocosm experiment to mimic future elevated atmospheric CO 2 concentrations ( e CO 2 , 720 ppm) versus current (380 ppm), and we exposed four co-occurring coastal wetland communities that naturally transgress (i.e., freshwater forest, mixed forest and marsh, marsh, mudflat) to these concentrations for two years. Overall, wetland communities with marsh plants in monoculture and mixed culture maintained high ecosystem C uptake with e CO 2 versus freshwater forested wetlands or mudflats, likely from superior plant species photosynthetic adjustment versus leaf area increases. e CO 2 promoted greater CO 2 uptake by leaves in all communities except mudflats, while promoting CH 4 efflux from whole ecosystems only when marsh plants were present. e CO 2 is projected to stimulate C gain 2.2-fold for forested wetlands and oligohaline marsh and 2.9-fold for forest-marsh mixture through greater CO 2 uptake. However, this comes at a cost of stimulated CH 4 flux by 1.4-to-1.7-fold in mixed and marsh communities versus reduced CH 4 fluxes with e CO 2 by forest and mudflat communities, perhaps through different oxidation pathways. Freshwater forested wetlands limited greenhouse gas emissions compared with transitional habitats, oligohaline marshes, and mudflats as atmospheric CO 2 concentrations increased. Stimulated C uptake in marshes may not offset higher methane emissions from these systems, potentially facilitating greater warming in a future with elevated atmospheric CO 2 .

Estuarine, Coastal and Shelf Science↗
USGS2026

Vulnerability of mangrove resources to sea-level rise on Sanibel Island, Florida, USA

Accelerating sea-level rise (SLR) is poised to reshape coastal environments over the coming decades, providing a challenge to land managers who need actionable information. Mangroves have an innate ability to keep pace with some SLR but may drown under the higher rates projected by the end of the century. Understanding local mangrove resilience to SLR requires understanding of historic and contemporary accretion rates, productivity, and forest elevations relative to tidal inundation. We applied the WARMER3 modeling framework to assess future mangrove resilience to SLR and blue carbon storage across the “Ding” Darling National Wildlife Refuge on Sanibel Island in southwest Florida, USA. We found that Sanibel mangroves are sensitive to the SLR scenario, with projected losses ranging from 19 to 70% by 2100. Across SLR scenarios, projected areal extent was similar until about 2035 and then diverge, coincident with projected acceleration in the rate of SLR. Threshold analysis indicates found that Sanibel mangroves are likely to submerge when rates exceed 6.5 mm yr −1 . Currently, the mangrove forest of our study domain holds an estimated 214,000 Mg of carbon but is likely to decrease as mangroves convert to open water in the second half of this century. For “Ding” Darling, these site-specific projections identify when losses in mangrove resources are most likely, providing a basis to prioritize local management actions and conservation resources.

Florida↗