Interannual and seasonal surface water pCO2 in a marsh-dominated estuary

Climate change, local anthropogenic pressures, and naturally occurring processes have impacted biogeochemical cycling in coastal systems. Here, a coastal estuarine ecosystem in North Carolina was studied in order to determine spatial relations, interannual and seasonal changes, and overall fluxes of carbon. pCO₂, percent dissolved oxygen (DO), particulate organic carbon (POC), total dissolved inorganic carbon (DIC), total alkalinity (TA), δ¹³C_DIC and δ¹³C_POC —amongst additional data—were collected from numerous study locations in the Cape Lookout region of North Carolina over several seasons (mostly spring and fall) between 2017 and 2019. Carbon isotopes of POC ranging between ‑30 and -17.8‰ coupled with a decreasing trend in C/N values moving down-estuary indicate that the organic carbon in the system is mainly sourced from upland vascular plant and agricultural inputs, with a small influence from in-estuary Spartina marsh grasses. The majority of the estuary was over-saturated with CO₂ compared to the atmosphere during all seasons, with the marsh-creek Smyrna Creek consistently exhibiting the most extreme pCO₂ values, peaking at 14606 µatm in the head of the creek in June 2018. Some estuarine sites were occasionally undersaturated in CO₂, likely from local phytoplankton blooms occurring during spring and summer. TA values throughout the system range from 1872–2342 µmol kg^-1, excluding Smyrna and Williston marsh-creeks which exhibited anomalous TA in several different seasons. Omitting these two creeks, the remainder of the system shows an increasing spatial TA trend moving down-estuary over the salinity gradient with the lowest values in Jarrett Bay and the highest values in the ocean. Due to seasonal mixing trends, DIC concentration increased down-estuary in the Summer and Spring and decreased over the salinity gradient in the Fall; however, the head of Smyrna Creek typically exhibited notably high DIC compared to the rest of the system (due to high CO₂). DIC and TA relationships indicate that inorganic carbon likely originates from a combination of sulfate reduction, denitrification, CO₂ invasion, and aerobic respiration. Calculations of air–sea CO₂ flux indicate that the estuarine waters as a whole are a source of CO₂ to the atmosphere with an average air–sea CO₂ flux of 13.4 mmol m^-2 day^-1.