Modeling net ecosystem carbon balance and loss in coastal wetlands exposed to sea‐level rise and saltwater intrusion

Abstract

Coastal wetlands are globally important stores of carbon (C). However, accelerated sea‐level rise (SLR), increased saltwater intrusion, and modified freshwater discharge can contribute to the collapse of peat marshes, converting coastal peatlands into open water. Applying results from multiple experiments from sawgrass (Cladium jamaicense)‐dominated freshwater and brackish water marshes in the Florida Coastal Everglades, we developed a system‐level mechanistic peat elevation model (EvPEM). We applied the model to simulate net ecosystem C balance (NECB) and peat elevation in response to elevated salinity under inundation and drought exposure. Using a mass C balance approach, we estimated net gain in C and corresponding export of aquatic fluxes (FAQ$$ {F}_{\mathrm{AQ}} $$) in the freshwater marsh under ambient conditions (NECB = 1119 ± 229 gC m−2 year−1; FAQ = 317 ± 186 gC m−2 year−1). In contrast, the brackish water marsh exhibited substantial peat loss and aquatic C export with ambient (NECB = −366 ± 15 gC m−2 year−1; FAQ = 311 ± 30 gC m−2 year−1) and elevated salinity (NECB = −594 ± 94 gC m−2 year−1; FAQ = 729 ± 142 gC m−2 year−1) under extended exposed conditions. Further, mass balance suggests a considerable decline in soil C and corresponding elevation loss with elevated salinity and seasonal dry‐down. Applying EvPEM, we developed critical marsh net primary productivity (NPP) thresholds as a function of salinity to simulate accumulating, steady‐state, and collapsing peat elevations. The optimization showed that ~150–1070 gC m−2 year−1 NPP could support a stable peat elevation (elevation change ≈ SLR), with the corresponding salinity ranging from 1 to 20 ppt under increasing inundation levels. The C budgeting and modeling illustrate the impacts of saltwater intrusion, inundation, and seasonal dry‐down and reduce uncertainties in understanding the fate of coastal peat wetlands with SLR and freshwater restoration. The modeling results provide management targets for hydrologic restoration based on the ecological conditions needed to reduce the vulnerability of the Everglades' peat marshes to collapse. The approach can be extended to other coastal peatlands to quantify C loss and improve understanding of the influence of the biological controls on wetland C storage changes for coastal management.

Modeling strategies and data needs for representing coastal wetland vegetation in land surface models

“…In some cases, saltwater intrusion to fresh or brackish wetlands has decreased plant productivity aboveground and belowground, leading to elevation loss and peat collapse (Wilson et al ., 2018; Charles et al ., 2019). More work is needed to understand the conditions under which fresh and brackish mashes are converted to salt marsh or unvegetated ponds (Ishtiaq et al ., 2022). Salt marshes and mangroves that do not accrete sediment fast enough to keep pace with sea level rise or that face inland barriers to lateral migration may become permanently flooded, leading to a loss in vegetation (Wasson et al ., 2019).…”

Biological Flora of Coastal Freshwater and Brackish Marshes: Cladium jamaicense Crantz

“…Rodriguez et al (2021) stated that subsidence in sawgrass peatlands is related to drainage. Ishtiaq et al (2022) stated that saltwater intrusion and altered hydrological conditions lead to a collapse of peat and elevation loss. Wilson et al (2018b) found that salinity pulses in brackish sawgrass marshes followed by seasonal drought resulted in a loss of root-rhizome biomass.…”

“…(2021) stated that subsidence in sawgrass peatlands is related to drainage. Ishtiaq et al . (2022) stated that saltwater intrusion and altered hydrological conditions lead to a collapse of peat and elevation loss.…”

Shifting Sources and Fates of Carbon With Increasing Hydrologic Presses and Pulses in Coastal Wetlands

“…Increasing hydrologic connectivity in coastal ecosystems—which occurs with sea‐level rise as well as restoration—can increase and synchronize nutrients and microbial activities (Kominoski et al., 2020). The press of sea‐level rise increases marine water intrusion that alters soil and water chemistries (Herbert et al., 2015; Tully et al., 2019) and influences wetland plant productivity and net ecosystem carbon storage (Ishtiaq et al., 2022; Wilson et al., 2019). Restoration of degraded ecosystems is increasing water levels and reducing light attenuation and oxygen availability to the benthos, all of which change relative contributions and production of autochthonous and allochthonous carbon (Cory et al., 2014; Howard‐Parker et al., 2020).…”