## 1 Introduction  
Coastal wetlands serve important functions, such as providing food and bio-materials as direct resources, acting as wildlife sanctuaries, storing a large fraction of the terrestrial carbon (C) pool, protecting against storm surges, and facilitating sediment accumulation for land accretion [1; 2; 3]. They also provide water purification, tourism resorts, and other functionalities [1], and are considered among the most productive and economically valuable ecosystems in the world [4]. They occupy 4%-6% of Earth's land area and store approximately 202-535 Gt of C [5], with 0.031 to 0.034 Gt of wetland C stored in the USA [6]. Along the coastal plain of the southeastern USA, wetlands play a significant role in regulating regional ecohydrology [4; 7] and in the productivity of economically important crops, timber, and fishery resources [8]. The importance of coastal forested wetlands in providing a range of ecosystem services and their role in aboveground-and-behaviour C storage is well-recognized. However, deeper understanding of the net C balance of coastal forested wetlands has not been well-explored [2; 3; 9; 10; 11; 12; 13].  
Coastal wetlands face numerous natural and anthropogenic disturbances that threaten their ecological integrity and C storage potential. These threats include land-use change [7; 9], climate change [12; 13; 14; 15; 16; 17; 18], and sea-level rise (SLR) [19; 20; 21; 22; 23; 24; 25; 26; 27], leading to the problems of saltwater intrusion [7; 28; 29] and prolonged inundation [30; 31; 32]. With climate change, SLR is expected to attain between +0.4 m and +1.2 m relative to today by the year 2100 [15; 33; 34], accompanied by a higher frequency and intensity of coastal storms [35; 36]. These coastal phenomena are already resulting in 'ghost forest' formation due to the prolonged inundation and submergence of low-lying land [21; 28; 37]. Thus, studies to understand the short-term and long-term impact of these disturbances are needed to assess the forest C budget temporal patterns and future trajectories.  
Carbon fluxes, such as gross primary productivity (GPP) and ecosystem respiration (RE), and the corresponding net ecosystem exchange (NEE), are strongly influenced by climate [38; 39; 40; 41; 42; 43]. Light, atmospheric temperature, and water availability mainly control C fluxes [39; 40; 44; 45; 46; 47]. Years with high rainfall amounts result in less sunlight, which may reduce GPP [41] while dryer years are associated with more sunlight, which may increase GPP; however, it may also result in greater deficits in soil moisture, causing stomatal closure, hence reducing GPP [48; 49]. On the other hand, RE increases exponentially with temperature, given sufficient soil moisture, yet declines if soils are too dry or wet [50; 51]. Understanding the influence of climatic drivers on the forest C balance is important because the climate-C cycle feedbacks control landscape C balances in response to climate warming [45; 52; 53], yet studies that examine these effects remain limited [40].  
Therefore, to monitor the long-term net C balance of a coastal forested wetland, we established an eddy covariance flux tower within a natural undrained bottomland hardwood forest in eastern North Carolina, USA. We examined the long-term trends (2009-2017) in gross primary production (GPP), ecosystem respiration (RE), and net ecosystem C exchange (NEE), seasonally and inter-annually. We analyzed the response of C fluxes and balance to climatic and hydrologic forcings, and examined the possible effect of rising sea levels on the inland groundwater dynamics. This study contributes to carbon accounting using a more accurate approach to improve ecosystem models relating C dynamics to temporal trends in wetland ecosystems.