An important biogeochemical link between organic and inorganic carbon cycling: Effects of organic alkalinity on carbonate chemistry in coastal waters influenced by intertidal salt marshes

Dissolved organic carbon (DOC) contains organic acid charge groups that contribute organic alkalinity (OrgAlk) to total alkalinity (TA). These effects are often ignored or treated as a calculation uncertainty in many aquatic CO₂ studies. This study evaluated OrgAlk variability, sources, and characteristics in estuarine waters exchanged tidally with a groundwater-influenced salt marsh in the northeast USA. OrgAlk provided a biogeochemical link between organic and inorganic carbon cycling through its direct effects on pH, and thus CO₂ system speciation and buffer capacity. Two main charge groups were identified including carboxylic and phenolic or amine groups. Terrestrial groundwater and in-situ production within salt marsh peat contributed OrgAlk to the tidal creek, with the former being a more significant source. Groundwater entering the marsh complex contained exceptionally high OrgAlk (> 150 µmol kg⁻¹), and these compounds were preferentially preserved within the DOC pool during groundwater transport and mixing with coastal water. OrgAlk:DOC ratios in groundwater and marsh-influenced water varied across space and time. This highlights the insufficiency of using a fixed proportion of DOC to account for organic acid charge groups. Accounting for OrgAlk altered H⁺ concentrations by ∼1–41 nmol kg⁻¹ (equivalent to a pH change of ∼0.03–0.26), pCO₂ by ∼30–1600 μatm and buffer capacity by ∼0.00–0.14 mmol kg⁻¹ at the relative OrgAlk contributions of 0.9–4.3% of TA observed in the marsh-influenced tidal water. Thus, OrgAlk may have a significant influence on coastal inorganic carbon cycling. Further theoretical calculations confirm that these concentrations of OrgAlk would have sizable impacts on both carbonate speciation and, ultimately, air-sea CO₂ fluxes in different coastal environments, ranging from estuarine to shelf waters. A new conceptual model linking organic and inorganic carbon cycling for coastal waters is proposed to highlight the sources and sinks of organic acid charge groups, as well as their biogeochemical behaviors and mechanistic control on the CO₂ system.