Seasonal Carbon Dynamics in a Temperate Lagoonal Estuary: New River, NC, USA

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.