## 4 Discussion
### Hydrological Impacts on Carbon Fluxes  
A higher variance in flooded conditions than in non-flooded conditions reflects the nonlinear and threshold-like responses of wetland ecosystems to saturation stress. Inparticular, NEE during flooding often fluctuated between net carbon uptake and release, suggesting that minor hydrological changes near critical thresholds can trigger large shifts in net carbon balance. This phenomenon is consistent with the concept of hysteresis in wetland carbon responses--where the same water level may result in different carbon flux outcomes depending on antecedent conditions and ecosystem memory [49]. Furthermore, saturated soils often reduce oxygen levels, leading to changes in microbial communities and enzyme functions that affect both autotrophic and heterotrophic respiration [50].  
Impeded drainage due to sea level rise may be contributing to higher groundwater tables along coastal areas. The relationship between increasing GWT (e.g., closer to the soil surface) and increasing SLR from 2009 to 2019 (R\({}^{2}\) = 0.11) suggests that, despite the \(\sim\)20 km distance of the flux tower from Croatan Sound and \(\sim\)40 km from the Atlantic Ocean, we could still detect a signal between rising sea levels and localized groundwater level changes. This rise in GWT likely exacerbates physiological stress in trees by creating hypoxic or anoxic soil conditions, which impair root respiration, reduce water and nutrient uptake, and ultimately lead to root death and crown dieback [3; 4]. Although not observed in the current study, saltwater intrusion in some coastal locations also introduces osmotic stress and ion toxicity that further disrupt cellular function and exacerbate hydraulic failure, predisposing trees to mortality under prolonged inundation [51; 52]. These combined stressors, hydrologic and salinity, contribute to the formation of ghost forests in many low-lying coastal ecosystems globally, and disentangling the importance of each is the subject of ongoing research.  
Meanwhile, greater RE relative to GPP during inundation reflects the activation of anaerobic decomposition pathways in saturated soils. Waterlogging may reduce CO\({}_{2}\) diffusion barriers while simultaneously enhancing fermentation and methanogenic processes in organic-rich substrates [53]. Notably, our observations suggest that RE is more consistently slightly elevated than GPP under prolonged flooding, indicating a potential shift toward net carbon loss. These results highlight the asymmetrical effects of hydrological stress on carbon fluxes. While both GPP and RE may initially increase under non-flooding, the prolonged imbalance between the two may lead to ecosystem destabilization. This aligns with previous studies in tidal wetlands and peatlands, showing that respiration tends to dominate under sustained inundation [54].  
Together, these findings support the conclusion that hydrological regimes are among the most important environmental drivers of carbon exchange in coastal wetlands. Importantly, they suggest that future increases in sea level and storm-driven flooding could intensify carbon loss through increased respiration, thereby weakening the long-term carbon sink function of these wetland ecosystems [54].