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Sapelo Island’s core consists of five separate barrier islands corresponding to five Heinrich events, H5–H1. During the present interglacial rise of the sea, Holocene barriers (H0) were attached to the most recent paleo-barrier, H1. All barriers have a recurved spit with an intervening backbay tidal marsh. Each barrier is composed of several dune ridges. Beach ridge editions were not formed at their present locations. Sand and other materials emerged from below sea level on the shelf in transgressive, elevated H seas and in the last post-glacial rise of the Holocene interglacial. Fourteen spurious increases of higher sea levels of warmer water in Dansgaard-Oeschger (D-O) events were followed by cooling H intervals with extreme waves during gales, hurricanes, and possibly tsunamis. Deltaic surplus materials on the shelf rolled shoreward. Storms drove deposits onshore in predominant wave episodes of overwash. Pliocene and Pleistocene sand surpluses on the continental shelf in relict river channels and deltas were and are source deposits. The preserved varved chronology is based on analyses of pollen and microfossils from Lake Tulane, Florida. Lake Tulane provides a continuous timeline of comparable radiocarbon ages to Sapelo. Both sites are considered coeval. Overwash deposits on the dry sand beach above average tides were forerunners of dune formation. After storms, backshore sand was trapped within strandlines of Spartina marsh wrack. Materials caught in the straw mulch on the dry sand produced aeolian dune ridges during windswept Nor’easters. High dunes grew from low and coalesced into a series of barriers to form Sapelo Island.

Predators regulate communities through top‐down control in many ecosystems. Because most studies of top‐down control last less than a year and focus on only a subset of the community, they may miss predator effects that manifest at longer timescales or across whole food webs. In southeastern US salt marshes, short‐term and small‐scale experiments indicate that nektonic predators (e.g., blue crab, fish, terrapins) facilitate the foundational grass, Spartina alterniflora, by consuming herbivorous snails and crabs. To test both how nekton affect marsh processes when the entire animal community is present, and how prior results scale over time, we conducted a 3‐year nekton exclusion experiment in a Georgia salt marsh using replicated 19.6 m² plots. Our nekton exclusions increased densities of plant‐grazing snails and juvenile deposit‐feeding fiddler crab and, in Year 2, reduced predation on tethered juvenile snails, indicating that nektonic predators control these key macroinvertebrates. However, in Year 3, densities of mesopredatory benthic mud crabs increased threefold in nekton exclusions, erasing the tethered snails’ predation refuge. Nekton exclusion had no effect on Spartina biomass, likely because the observed mesopredator release suppressed grazing snail densities and elevated densities of fiddler crabs, whose burrowing alleviates soil stresses. Structural equation modeling supported the hypotheses that nektonic predators and mesopredators control invertebrate communities, with nektonic predators having stronger total effects on Spartina than mud crabs by controlling densities of species that both suppress (grazers) and facilitate (fiddler crabs) plant growth. These findings highlight that salt marshes can be resilient to multiyear reductions in nektonic predators if mesopredators are present and that multiple pathways of trophic control manifest in different ways over time to mediate community dynamics. These results highlight that larger scale and longer‐term experiments can illuminate community dynamics not previously understood, even in well‐studied ecosystems such as salt marshes.

The ability to both resist and recover from disturbances like storm surge and saltwater intrusion plays a key role in shaping the structure and function of tidal marshes. In this study, porewater chemistry, vegetation, and soil elevation change were measured in field plots of a tidal freshwater marsh exposed to four years of experimental press (chronic) and pulse (acute) brackish water additions followed by five years of recovery to assess their resistance and resilience to saltwater intrusion. Press additions produced significant, widespread changes in marsh structure and function including increased porewater N and P, reduced macrophyte cover and species richness, and loss of soil surface elevation whereas pulse additions had little effect. Once dosing ceased, porewater chemistry, vegetation and soils in press plots recovered at differing rates, with porewater N and P declining to background levels after one year, plant cover and species richness increasing within two to four years, and soil surface elevation increasing to similar levels found in control plots after five years. The plant community in the press treatment converged with the other treatments after 3–4 years, though macrophyte species exhibited varying rates of recovery. Ground cover (Ludwigia repens) and soft stem species (Persicaria) that declined first, recovered faster than Zizaniopsis miliacea that was more resistant but less resilient to brackish water intrusion. While tidal freshwater marshes are resistant and resilient to pulses such as those that stem from hurricanes and storm surges, continued long-term intrusion events like sea level rise (SLR) will likely lead to conversion into brackish marsh. Understanding long-term responses and tradeoffs in resistance and recovery as shown in this experiment offers insight into the future trajectory of tidal freshwater marshes as well as broader ecosystem responses to disturbance and recovery crucial to management and restoration.

Tidal salt marshes are important ecosystems in the global carbon cycle. Understanding their net carbon exchange with the atmosphere is required to accurately estimate their net ecosystem carbon budget (NECB). In this study, we present the interannual net ecosystem exchange (NEE) of CO₂ derived from eddy covariance (EC) for a Spartina alterniflora salt marsh. We found interannual NEE could vary up to 3-fold and range from −58.5 ± 11.3 to −222.9 ± 12.4 g C m⁻² year⁻¹ in 2016 and 2020, respectively. Further, we found that atmospheric CO₂ fluxes were spatially dependent and varied across short distances. High biomass regions along tidal creek and estuary edges had up to 2-fold higher annual NEE than lower biomass marsh interiors. In addition to the spatial variation of NEE, regions of the marsh represented by distinct canopy zonation responded to environmental drivers differently. Low elevation edges (with taller canopies) had a higher correlation with river discharge (R² = 0.61), the main freshwater input into the system, while marsh interiors (with short canopies) were better correlated with in situ precipitation (R² = 0.53). Lastly, we extrapolated interannual NEE to the wider marsh system, demonstrating the potential underestimation of annual NEE when not considering spatially explicit rates of NEE. Our work provides a basis for further research to understand the temporal and spatial dynamics of productivity in coastal wetlands, ecosystems which are at the forefront of experiencing climate change induced variability in precipitation, temperature, and sea level rise that have the potential to alter ecosystem productivity.