Author ORCID Identifier

https://orcid.org/0000-0001-8061-4560

https://orcid.org/0000-0002-5894-6240

https://orcid.org/0000-0003-2896-3063

https://orcid.org/0000-0003-0726-0131

Abstract

The capacity of aquatic systems to buffer acidification depends on the sum contributions of various chemical species to total alkalinity (TA). Major TA contributors are inorganic, with carbonate and bicarbonate considered the most important. However, growing evidence shows that many rivers, estuaries, and coastal waters contain dissolved organic molecules with charge sites that create organic alkalinity (OrgAlk). This study describes the first comparison of (1) OrgAlk distributions and (2) acid–base properties in contrasting estuary-plume systems: the Pleasant (Maine, USA) and the St. John (New Brunswick, CA). The substantial concentrations of OrgAlk in each estuary were sometimes not conservative with salinity and typically associated with very low pH. Two approaches to OrgAlk measurement showed consistent differences, indicating acid–base characteristics inconsistent with the TA definition. The OrgAlk fraction of TA ranged from 78% at low salinity to less than 0.4% in the coastal ocean endmember. Modeling of titration data identified three groups of organic charge sites, with mean acid–base dissociation constants (pKa) of 4.2 (+/- 0.5), 5.9 (+/- 0.7) and 8.5 (+/- 0.2). These represented 21% (+/- 9%), 8% (+/- 5%), and 71% (+/- 11%) of titrated organic charge groups. Including OrgAlk, pKa, and titrated organic charge groups in carbonate system calculations improved estimates of pH. However, low and medium salinity, organic-rich samples demonstrated persistent offsets in calculated pH, even using dissolved inorganic carbon and CO2 partial pressure as inputs. These offsets show the ongoing challenge of carbonate system intercomparisons in organic rich systems whereby new techniques and further investigations are needed to fully account for OrgAlk in TA titrations.

Date Created

December 16, 2024

Department

Ocean Process Analysis Laboratory

Publication Date

12-16-2024

Subject

organic acid-base biogeochemistry

Grant/Award Number and Agency

NSF Awards 1658377 and 1658321

Journal Title

Limnology and Oceanography

Language

English

Publisher

Wiley

Digital Object Identifier (DOI)

https://doi.org/10.1002/lno.12761

Document Type

Article

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