Documents - Publications - Journal Articles

Abstract - We present our methodology and data for science ready vertical carbon fluxes from a Spartina alterniflora tidal salt marsh as part of the Georgia Coastal Ecosystems Long Term Ecological Research (GCE-LTER) site on Sapelo Island, Georgia, USA. Vertical carbon fluxes were measured using the eddy covariance (EC) method from 2014 to 2024. The EC flux tower was located on the western side of Sapelo Island bounded by the Duplin River and Barn Creek. The proportional influence of marsh habitats on the flux measurements were 4% tall, 38% short, and 58% medium height form Spartina alterniflora. We provide the net ecosystem exchange (NEE), ecosystem respiration (ER), and gross primary production (GPP) at 30-minute fluxes (µmol CO2 m−2s−1), daily averages (µmol CO2 m−2s−1) and totals (g C m−2 day−1), and annual (g C m−2 year−1) quantities. We estimated uncertainty for each flux at each integrated timescale as 95% confidence intervals. Providing open access to 10-year carbon flux datasets can facilitate collaboration for advancing regional and global blue carbon synthesis and scale-up studies.

Abstract - The Altamaha River estuary off the southeastern U.S. is an important source of dissolved organic carbon (DOC) to the coastal ocean. The estuary is formed by three interconnected sounds, and it is characterized by a complex network of narrow channels and creeks with high spatial heterogeneity, making it difficult to study with in situ observations alone. Here, use used satellite data from Landsat available in high resolution (30 m) to investigate DOC distribution and variability in the system. Our analyses show that DOC variability in the estuary is characterized by two seasonal peaks, one in spring and one in fall, while over the shelf maximum DOC content is observed during summer. A multiple regression analysis was used to quantify physical mechanisms controlling DOC variability in the estuary. In addition to seasonal variations, anomalies in river discharge are the dominant factor controlling DOC variability throughout much of the estuarine system. Tides play a key role near the mouth of each sound and in some upstream regions, likely associated with inputs from salt marshes. The influence of winds is smaller and is restricted to the area near the mouth of the Altamaha Sound. Landsat data also captured an input of ~ 2 Gg of DOC to the estuary associated with the passage of Hurricane Irma in 2017. Our results demonstrate that Landsat can provide useful information about scales of variability in narrow estuaries, including capturing the occurrence of sharp fronts that would be difficult to observe with traditional in situ measurements alone.

Abstract - Environmental gradients can affect organic matter decay within and across wetlands and contribute to spatial heterogeneity in soil carbon stocks. We tested the sensitivity of decay rates to tidal flooding and soil depth in a minerogenic salt marsh using the Tea Bag Index (TBI). Tea bags were buried at 10 and 50 cm depths across an elevation gradient in a subtropical Spartina alterniflora marsh in Georgia (USA). Plant and animal communities and soil properties were characterized once, while replicate tea bags and porewaters were collected several times over 1 year. TBI decay rates were faster than prior litterbag studies in the same marsh, largely due to rapid green tea loss. Rooibos tea decay rates were more comparable to natural marsh litter, potentially suggesting that is more useful as a standardized organic matter proxy than green tea. Decay was slowest at higher marsh elevations and not consistently related to other biotic (e.g., plants, crab burrows) or abiotic factors (e.g., porewater chemistry), indicating that local hydrology strongly affected organic matter loss rates. TBI rates were 32 %–118 % faster in the 10 cm horizon than at 50 cm. Rates were fastest in the first 3 months and slowed 54 %–60 % at both depths between 3 and 6 months. Rates slowed further between 6 and 12 months, but this was more muted at 10 cm (17 %) compared to 50 cm (50 %). Slower rates at depth and with time were unlikely due to the TBI stabilization factor, which was similar across depths and decreased from 6 to 12 months. Slower decay at 50 cm demonstrates that rates were constrained by environmental conditions in the deeper horizon rather than the composition of this highly standardized litter. Overall, these patterns suggest that hydrological setting, which affects oxidant introduction and reactant removal and is often overlooked in marsh decomposition studies, may be a particularly important control on organic matter loss in the short term (3–12 months).

Abstract - We developed a phenology-based growth model(PG model) for Spartina alterniflora that incorporates the effects of light, temperature, and salinity on plant production. The PG model is the first to quantify carbon translocation between both above- and below-ground biomass across three phenological periods: growth, senescence, and dormancy periods. This model, fitted to field data from short, medium, and tall S. alterniflora types, estimates physiological parameters such as mass-specific rates of carbon translocation. Once parameterized, the model is applied in forward mode to predict whole-plant production, growth, respiration, mortality, and translocation. Model results reveals that short forms allocate 82 % of photosynthate to below-ground biomass during the growing season, compared to tall (52 %) and medium (22 %) types. However, tall forms, with extensive above-ground biomass, show the highest absolute carbon translocation to below-ground tissues during growth(ave. 3940 g dry weight m−2) and senescence(ave. 265 g dry weight m−2) period. An average mortality rate of 52 % of net production in the tall form below-ground biomass throughout the year indicates a substantial contribution to organic carbon sequestration within the habitat sediment. Model results also reveal that the carbon translocation from below- to above-ground tissues may not be required for survival during winter in milder climate like Sapelo Island, Georgia.

Abstract - Tidal flooding can significantly impact vegetation pixel reflectance of coastal salt marshes, presenting a problem for remote sensing studies of these highly productive ecosystems. The current study aimed to spatially and temporally expand our previously developed Flooding in Landsat Across Tidal Systems (FLATS) model to detect and analyze the long-term changes in flooded marsh pixels in Landsat 5–9 imagery. As the FLATS index is only calibrated for Landsat 8, our goal was to expand the use of FLATS to a greater range of Landsat imagery and facilitate the masking of flooded pixels in long-term time series of vegetation indices. Using areas of salt marsh in the Georgia Coastal Ecosystems (GCE) Long Term Ecological Research (LTER) site, images from Landsat 5 through 9 were paired with near-coincident Landsat 7 images for a novel cross-calibration. Indices in the FLATS algorithm (Normalized Difference Water Index, NDWI and the Enhanced Vegetation Index, EVI) were calibrated for each image pair using linear regression models, and an adjusted FLATS index (FLATS+) was created to be used on a substantially expanded Landsat dataset from 1984 to 2023. The R2 scores for the vegetation index calibrations ranged from 0.78 to 0.87 for EVI, and 0.73–0.82 for NDWI. Additionally, this study sought to monitor changes in flooding patterns at the study site, utilizing the expanded temporal range of FLATS+. The trend in FLATS+ value exhibited significant spatial autocorrelation at three LTER sites, with areas of marsh experiencing significant changes in inundation over the 39-year period (Moran's I, p < 0.01 at all sites). The FLATS+ index is a tool that is able to identify flooded pixels in Landsat 5–9. The index can be used to study salt marsh productivity, carbon uptake, flooding, and resiliency in response to sea level rise.

Abstract - Coastal tidal wetlands and estuaries play important roles in the global carbon budget by contributing to the net withdrawal of CO2 from the atmosphere. We quantified the linkages between terrestrial and oceanic systems, marsh-to-bay carbon exchange, and the uptake of CO2 from the atmosphere in the wetland-dominated Plum Island Sound (MA, USA) and Duplin River (GA, USA) estuaries. The C budgets revealed that autotrophic marshes [primary production:ecosystem respiration (P:R) ~1.3:1] are tightly coupled to heterotrophic aquatic systems (P:R ~0.6:1). Levels of marsh gross primary production are similar in these systems (865 ± 39 and 768 ± 74 gC m−2 year−1 in Plum Island and the Duplin, respectively) even though they are in different biogeographic provinces. In contrast to inputs from rivers and coastal oceans, tidal marshes are the dominant source of allochthonous matter that supports heterotrophy in aquatic systems. Dissolved inorganic carbon (DIC) exported from marshes to the coastal ocean was a major flux pathway in the Duplin River; however, there was no evidence of DIC export from Plum Island marshes and only minor export to the ocean. Burial was a sink for 53% of marsh net ecosystem production (NEP) on Plum Island, but only 19% of marsh NEP in the Duplin. Burial was the dominant blue carbon sequestration pathway at Plum Island, whereas in the Duplin, DIC and organic carbon export to the ocean were equally important. Regional- and continental-scale C budgets should better reflect wetland-dominated systems to more accurately characterize their contribution to global CO2 sequestration.

Abstract - Coastal and estuarine ecosystems are particularly sensitive to climate change, placing them at the forefront of challenges to mediate impacts of a warming atmosphere, rising sea-levels, and increasingly frequent extreme weather events. To model potential loss, predict and prepare for future regime shifts, or to build effective conservation policies, it is important to understand the long-term socioecological processes that structure modern ecosystems. We highlight how modern ecological baselines along the Georgia coast of eastern North America are shaped by 5000 years of Indigenous and Euro-American land use. We demonstrate the extent and intensity of manifestations of past land use on modern landscapes, especially by way of quantifying the scale of shell deposition by Indigenous communities and the landscape infrastructure of Euro-American plantations. Through both intentional and unintentional impacts, modern estuarine ecosystems globally are products of these engagements, alterations, and creative transformations that we refer to as deep-time legacy drivers.

Abstract - Salt marshes provide valuable ecosystem services but are vulnerable to drowning with accelerated sea-level rise (SLR). Marsh belowground biomass (BGB) production helps avoid drowning by building marsh surface elevation. Reductions in BGB can serve as an early warning sign of marsh deterioration, as they often precede decreases in aboveground biomass (AGB). However, landscape-scale BGB assessments to predict broad trends in marsh deterioration have not been previously available. We applied the Belowground Ecosystem Resiliency Model (BERM) to assess standing stocks and trends in both BGB and AGB over the past decade (2014–2023) across US Georgia coast Spartina alterniflora marshes (691 km2). Over this time period, BGB and AGB averaged 841 ± 323 and 221 ± 14 g m−2, respectively, but showed opposite trends. BGB decreased on average by 0.94% per year and over most of the marsh area (72%), while AGB increased on average by 0.66% per year and showed a net increase across most of the marsh area (88%). This disconnect suggests that AGB is not a good indicator of marsh resilience, and we highlight two areas with similar AGB but different BGB. Inundation intensity, an important predictor of BGB, rose through time and was negatively related to BGB. SLR trends suggest continuing increases in inundation, which will result in further declines in BGB followed by widespread marsh drowning. Landscape BGB assessments are a valuable tool to identify ecosystem vulnerability and proactively manage salt marshes and the services they provide under rising sea levels.

Abstract - In coastal regions and marginal bodies of water, the increase in partial pressure of carbon dioxide (pCO2) in many instances is greater than that of the open ocean due to terrestrial (river, estuarine, and wetland) influences, decreasing buffering capacity and/or increasing water temperatures. Coastal oceans receive freshwater from rivers and groundwater as well as terrestrial-derived organic matter, both of which have a direct influence on coastal carbonate chemistry. The objective of this research is to determine if coastal marshes in Georgia, USA, may be “hot-spots” for acidification due to enhanced inorganic carbon sources and if there is terrestrial influence on offshore acidification in the South Atlantic Bight (SAB). The results of this study show that dissolved inorganic carbon (DIC) and total alkalinity (TA) are elevated in the marshes compared to predictions from conservative mixing of the freshwater and oceanic end-members, with accompanying pH around 7.2 to 7.6 within the marshes and aragonite saturation states (ΩAr) <1. In the marshes, there is a strong relationship between the terrestrial/estuarine-derived organic and inorganic carbon and acidification. Comparisons of pH, TA, and DIC to terrestrial organic material markers, however, show that there is little influence of terrestrial-derived organic matter on shelf acidification during this period in 2014. In addition, ΩAr increases rapidly offshore, especially in drier months (July). River stream flow during 2014 was anomalously low compared to climatological means; therefore, offshore influences from terrestrial carbon could also be decreased. The SAB shelf may not be strongly influenced by terrestrial inputs to acidification during drier than normal periods; conversely, shelf waters that are well-buffered against acidification may not play a significant role in mitigating acidification within the Georgia marshes.

Abstract - Quantifying ecosystem resilience to disturbance is important for understanding the effects of disturbances on ecosystems, especially in an era of rapid global change. However, there are few studies that have used standardized experimental disturbances to compare resilience patterns across abiotic gradients in real-world ecosystems. Theoretical studies have suggested that increased return times are associated with increasing variance during recovery from disturbance. However, this notion has rarely been explicitly tested in field, in part due to the challenges involved in obtaining long-term experimental data. In this study, we examined resilience to disturbance of 12 coastal marsh sites (five low-salinity and seven polyhaline (=salt) marshes) along a salinity gradient in Georgia, USA. We found that recovery times after experimental disturbance ranged from 7 to >127 months, and differed among response variables (vegetation height, cover and composition). Recovery rates decreased along the stress gradient of increasing salinity, presumably due to stress reducing plant vigor, but only when low-salinity and polyhaline sites were analyzed separately, indicating a strong role for traits of dominant plant species. The coefficient of variation of vegetation cover and height in control plots did not vary with salinity. In disturbed plots, however, the CV was consistently elevated during the recovery period and increased with salinity. Moreover, higher CV values during recovery were correlated with slower recovery rates. Our results deepen our understanding of resilience to disturbance in natural ecosystems, and point to novel ways that variance can be used either to infer recent disturbance, or, if measured in areas with a known disturbance history, to predict recovery patterns.

Abstract - Predators regulate communities through top-down control in many ecosys-tems. Because most studies of top-down control last less than a year and focuson only a subset of the community, they may miss predator effects that mani-fest at longer timescales or across whole food webs. In southeastern US saltmarshes, short-term and small-scale experiments indicate that nektonic preda-tors (e.g., blue crab, fish, terrapins) facilitate the foundational grass, Spartinaalterniflora, by consuming herbivorous snails and crabs. To test both hownekton affect marsh processes when the entire animal community is present,and how prior results scale over time, we conducted a 3-year nekton exclusionexperiment in a Georgia salt marsh using replicated 19.6 m 2 plots. Our nektonexclusions increased densities of plant-grazing snails and juveniledeposit-feeding fiddler crab and, in Year 2, reduced predation on tethered juve-nile snails, indicating that nektonic predators control these key macroinver-tebrates. However, in Year 3, densities of mesopredatory benthic mud crabsincreased threefold in nekton exclusions, erasing the tethered snails’ predationrefuge. Nekton exclusion had no effect on Spartina biomass, likely because theobserved mesopredator release suppressed grazing snail densities and elevateddensities of fiddler crabs, whose burrowing alleviates soil stresses. Structuralequation modeling supported the hypotheses that nektonic predators andmesopredators control invertebrate communities, with nektonic predators hav-ing stronger total effects on Spartina than mud crabs by controlling densitiesof species that both suppress (grazers) and facilitate (fiddler crabs) plantgrowth. These findings highlight that salt marshes can be resilient to multiyearreductions in nektonic predators if mesopredators are present and thatmultiple pathways of trophic control manifest in different ways over time tomediate community dynamics. These results highlight that larger scale andlonger-term experiments can illuminate community dynamics not previouslyunderstood, even in well-studied ecosystems such as salt marshes.

Abstract - 1. Large grazers modify vegetated ecosystems and are increasingly viewed as keystonespecies in trophic rewilding schemes. Yet, as their ecosystem influencesare context-dependent,a crucial challenge is identifying where grazers sustain,versus undermine, important ecosystem properties and their resilience.2. Previous work in diverse European saltmarshes found that, despite changingplant and invertebrate community structure, grazers do not suppress below-groundproperties, including soil organic carbon (SOC). We hypothesised that,in contrast, eastern US saltmarshes would be sensitive to large grazers as extensiveareas are dominated by a single grass, Spartina alterniflora. We predicted thatgrazers would reduce above-andbelow-groundSpartina biomass, suppress invertebratedensities, shift soil texture and ultimately reduce SOC concentration.3. We tested our hypotheses using a replicated 51-monthlarge grazer (horse) exclusionexperiment in Georgia, coupled with observations of 14 long-termgrazedsites, spanning ~1000 km of the eastern US coast.4. Grazer exclusion quickly led to increased Spartina height, cover and flowering,and increased snail density. Changes in vegetation structure were reflected inmodified soil texture (reduced sand, increased clay) and elevated root biomass,yet we found no response of SOC. Large grazer exclusion also reduced drought-associatedvegetation die-off.5. We also observed vegetation shifts in sites along the eastern US seaboard wheregrazing has occurred for hundreds of years. Unlike in the exclusion experiment,long-termgrazing was associated with reduced SOC. A structural equation modelimplicated grazing by revealing reduced stem height as a key driver of reduced soilorganic carbon.6. Synthesis: These results illustrate the context dependency of large grazer impactson ecosystem properties in coastal wetlands. In contrast to well-studiedEuropean marshes, eastern US marshes are dominated and structured by a singlefoundational grass species resulting in vegetation and soil properties being moresensitive to grazing. Coastal systems characterised by a single foundation speciesmight be inherently vulnerable to large grazers and lack resilience in the faceof other disturbances, underlining that frameworks to explain and predict largegrazer impacts must account for geographic variation in ecosystem structure.

Abstract - Understanding how climate and local stressors interact is paramount for predicting future ecosystem structure. The effects of multiple stressors are often examined in small-scale and short-term field experiments, limiting understanding of the spatial and temporal generality of the findings. Using a 22-year observational dataset of plant and grazer abundance in a southeastern US salt marsh, we analyzed how changes in drought and grazer density combined to affect plant biomass. We found: (1) increased drought severity and higher snail density both correlated with lower plant biomass; (2) drought and snail effects interacted additively; and, (3) snail effects had a threshold, with additive top-down effects only occurring when snails were present at high densities. These results suggest that the emergence of multiple stressor effects can be density dependent, and they validate short-term experimental evidence that consumers can augment environmental stress. These findings have important implications for predicting future ecosystem structure and managing natural ecosystems.

Abstract - Situated in the transitional zone between non-tidal forests upstream and tidal freshwater marshes downstream, tidal freshwater forests (TFF) occupy a unique and increasingly precarious habitat due to the threat of saltwater intrusion and sea level rise. Salinization causes tree mortality and forest-to-marsh transition, which reduces biodiversity and carbon sequestration. The Altamaha River is the longest undammed river on the United States East Coast and has extensive TFF, but there have been only limited field studies examining TFF along the entire gradient of salinity and flooding. We surveyed thirty-eight forest plots on the Altamaha River along a gradient of tidal influence, and measured tree species composition, diameter, and height. Hierarchical clustering and indicator species analysis were used to identify TFF communities. The relationship of these communities to elevation and river distance was assessed using non-metric multidimensional scaling (NMDS). We identified six significantly different forest communities: Oak/Hornbeam, Water Tupelo, Bald Cypress/Tupelo, Pine, Swamp Tupelo, and Bald Cypress. Both elevation and river distance were significantly correlated with plot species composition (p = 0.001). Plots at the downstream extent of our study area had lower stem density, basal area, and species diversity than those further upstream, suggesting saltwater intrusion. This study demonstrates the importance of and need for thorough and robust analyses of tidal freshwater forest composition to improve prediction of TFF response to sea level rise.

Abstract - Ecosystems vary broadly in their responses to disturbance, ranging from highly impacted to resilient or resistant. We conducted a large-scale analysis of hurricane disturbance effects on coastal marshes by examining 20 years of data from 10 sites covering 100,000 ha at the Georgia Coastal Ecosystems Long-Term Ecological Research site distributed across gradients of salinity and proximity to the ocean. We analyzed the impacts of Hurricanes Matthew (in 2016) and Irma (in 2017) on marsh biota (plants, crabs, and snails) and physical attributes (erosion, wrack deposition, and sedimentation). We compared these variables prior to the storms (2000–2015) to years with storms (2016, 2017) to those after the storms (2018–2020). Hurricanes generated storm surges that increased water depth and salinity of oligotrophic areas for up to 48 h. Biological variables in the marsh showed few effects of the hurricanes. The only physical variable affected was creek bank slumping; however, slumping had already increased a year before the hurricanes, suggesting that slumping could have a different cause. Thus, our study uncovered only minor, ephemeral impacts on Georgia coastal marshes, highlighting their resistance to hurricane disturbance of the lower magnitude that typically confronts this region of coastline.