Highlights
- OrgAlk can be an important fraction of TA in coastal waters.
- OrgAlk acid-base properties are suggestive of carboxyl and phenolic-like signals.
- Complementary fluorescence analysis of DOM can elucidate origins and transformations of OrgAlk.
- OrgAlk can lead to the propagation of uncertainty on calculated carbonate parameters.
Abstract
Total alkalinity (TA) is a popularly measured carbonate system parameter and is widely used in calculations of key carbonate system descriptors such as the calcium carbonate saturation state, an important indicator of ocean acidification. Organic alkalinity (OrgAlk) is recognised as a contributor to TA in coastal waters, with this having implications on the use of TA to calculate key carbonate chemistry descriptors. As titratable charge groups of OrgAlk can act as unknown acid-base species, the inclusion of the total concentration and apparent dissociation constants of OrgAlk in carbonate calculations involving TA is required to minimise uncertainty in computed speciation. Here we present an investigation of the prevalence and properties of OrgAlk as well as the impact of OrgAlk on carbonate chemistry calculations in a transitional waterbody. Water samples were collected during low and high tide over a 5-week period in Rogerstown Estuary, Dublin Ireland. TA and OrgAlk were measured using modified Global Ocean Acidification Observing Network (GOA-ON) titration apparatus in conjunction with OrgAlkCalc, an open-source Python based computational programme. pH was measured on the total scale using meta-cresol purple (mCP) as the indicator dye. Dissolved inorganic carbon (DIC), the partial pressure of CO2 (pCO2), in situ pH on the total scale (pHT) and the saturation state of aragonite (∆ΩA) were calculated using pH and both OrgAlk adjusted TA and measured TA as the input parameters. Optical analysis of DOM was conducted to compliment OrgAlk characterisations and to further elucidate OrgAlk sources and dynamics. OrgAlk charge groups concentrations ranged from 35,198 μmol·kg−1, with the highest concentrations observed in more marine waters. Two apparent charge groups were associated with OrgAlk, with pK values of 4.38 ± 0.27 and 6.95 ± 0.43. Differences between calculated carbonate system parameters when using OrgAlk adjusted TA and non-OrgAlk adjusted TA ranged from 88 to 254 μmol·kg−1 DIC, −98–67 μatm pCO2, −0.02–0.12 pHT and 0.02–0.64 ∆ΩA. Variability in the differences in calculated carbonate systems was largely a factor of OrgAlk charge group concentration and pK. This work highlights the importance of considering OrgAlk if using TA as an input parameter in carbonate system investigations of coastal waters.
Kerr D. E., Grey A. & Kelleher B. P., 2023. Organic alkalinity dynamics in Irish coastal waters: case study Rogerstown Estuary. Marine Chemistry 254: 104272. doi: 10.1016/j.marchem.2023.104272. Article.
