Seasonal Changes in Carbonate Saturation State and Air‐Sea CO 2 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 CO 2 partial pressure ( p CO 2 ) in a north Patagonian fjord with a focus on changes in freshwater inflows and biological processes. To do this, we monitored pH and p CO 2 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 p CO 2 , pH, and DO of the fjord's surface waters. During the summer, p CO 2 reached its annual minimum (range: 187–571 μatm) and pH its maximum (range: 7.98–8.24), coinciding with lower freshwater inflows (204–307 m 3 /s) and high DO (280–378 μmol/kg), as well as aragonite saturation states (Ω Arag ) higher than 1. In contrast, in winter, p CO 2 ranged from 461–1,008 μatm and pH from 7.57–8.03, coinciding with high freshwater inflows (1,049–1,402 m 3 /s), lower oxygen (216–348 μmol/kg), and constant undersaturation of Ω Arag . Reloncaví Fjord had an annual air‐water CO 2 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 CO 2 sink in late spring and summer, caused by high primary production rates, to a CO 2 source during the rest of the year caused by high community respiration due to allochthonous organic carbon inputs.