Seasonal Changes in Carbonate Saturation State and Air-Sea CO₂ Fluxes During an Annual Cycle in a Stratified-Temperate Fjord (Reloncaví Fjord, Chilean Patagonia)

Changes may be occurring in the carbonate chemistry of fjords due to natural and anthropogenic disturbance of major freshwater sources. We present a high-frequency time series study of seasonal pH and CO₂ partial pressure (pCO₂) in a north Patagonian fjord with a focus on changes in freshwater inflows and biological processes. To do this, we monitored pH and pCO₂ in situ, along with river streamflow, salinity, temperature, and dissolved oxygen (DO) in the Reloncaví Fjord (41.5°S) for a full year (January to December 2015). Strong seasonal variability was observed in the pCO₂, pH, and DO of the fjord's surface waters. During the summer, pCO₂ reached its annual minimum (range: 187–571 μatm) and pH its maximum (range: 7.98–8.24), coinciding with lower freshwater inflows (204–307 m³/s) and high DO (280–378 μmol/kg), as well as aragonite saturation states (Ω_Arag) higher than 1. In contrast, in winter, pCO₂ ranged from 461–1,008 μatm and pH from 7.57–8.03, coinciding with high freshwater inflows (1,049–1,402 m³/s), lower oxygen (216–348 μmol/kg), and constant undersaturation of Ω_Arag. Reloncaví Fjord had an annual air-water CO₂ flux of 0.716 ± 2.54 mol·m⁻²·year⁻¹ during 2015 and thus acted as a low emission system. The annual cycle was mainly governed by seasonal changes in biological processes that enhanced the shift from a CO₂ sink in late spring and summer, caused by high primary production rates, to a CO₂ source during the rest of the year caused by high community respiration due to allochthonous organic carbon inputs.