Estuaries act as conduits for and modifiers of carbon delivery from watersheds to the coastal ocean. Here we hypothesize that a long residence time estuary receiving high nutrient and humic-rich dissolved organic carbon (DOC) loads from the watershed will efficiently produce and trap autochthonous carbon while conservatively transporting watershed-derived DOC to the coastal ocean. We assessed temporal changes in estuarine carbon dynamics in the New River Estuary, NC, USA (NRE) by measuring concentrations and ¹³C isotopic composition of particulate organic carbon (POC), DOC, and dissolved inorganic carbon (DIC) monthly for 16 months. Data were evaluated with respect to seasonal changes in each carbon pool inventory and non-conservative mixing along the estuarine axis. The estuary was predominantly a sink for POC trapping 65–95% of watershed-derived POC under low/moderate flow conditions and up to half of the autotrophic POC produced in the upper estuary seasonally. DIC inventories were dominantly driven by salinity. Monthly DIC and δ¹³C-DIC mixing curves rarely showed large deviations from non-conservative behavior but did reflect an estuarine gradient consistent with higher autotrophy upstream and increased heterotrophy downstream. Mixing curves for DOC primarily reflected conservative mixing of riverine DOC with seawater. However, there was a 50% increase summertime estuarine DOC inventory concurrent with a 2.5‰ decrease in δ¹³C-DOC suggesting diffuse lateral sources of isotopically light DOC with low lability. For POC, the NRE traps a large fraction of imported and autochthonous carbon and exports POC derived primarily from estuarine autotrophy. For DOC, we suggest that most of the DOC produced internally is mineralized within the estuary and that the NRE conservatively transports DOC derived from riverine and lateral wetland sources to the coastal ocean.

河口是从流域到沿海海洋碳输送的管道和调节器。在这里，我们假设从流域接收高营养和富含腐殖质的溶解有机碳 (DOC) 负荷的长停留时间河口将有效地产生和捕获本地碳，同时将流域衍生的 DOC 保守地运输到沿海海洋。我们通过测量浓度和¹³颗粒有机碳 (POC)、DOC 和溶解无机碳 (DIC) 的 C 同位素组成每月一次，持续 16 个月。评估了每个碳库清单的季节性变化和沿河口轴的非保守混合的数据。河口主要是一个汇，在低/中等流量条件下捕获 65-95% 的流域衍生的 POC，并且季节性地在河口上游产生的自养 POC 的一半。DIC 库存主要受盐度驱动。每月 DIC 和 δ ¹³C-DIC 混合曲线很少表现出与非保守行为的大偏差，但确实反映了河口梯度，与上游较高的自养和下游异养增加一致。DOC 的混合曲线主要反映了河流 DOC 与海水的保守混合。然而，夏季河口 DOC 存量增加了 50%，同时 δ ¹³减少了 2.5‰C-DOC 表明具有低不稳定性的同位素轻 DOC 的扩散横向源。对于 POC，NRE 捕获了大部分进口和本土碳，出口 POC 主要来自河口自养。对于 DOC，我们建议内部生产的大部分 DOC 在河口内矿化，并且 NRE 将来自河流和侧向湿地来源的 DOC 保守地运输到沿海海洋。