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

Abstract

AbstractChanges 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 (ΩArag) 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 ΩArag. 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.