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. Oxygen minimum zones (OMZs) are characterized by enhancedcarbon dioxide (CO2) levels and low pH and are being further acidifiedby uptake of anthropogenic atmospheric CO2. With ongoingintensification and expansion of OMZs due to global warming, carbonatechemistry conditions may become more variable and extreme, particularly inthe eastern boundary upwelling systems. In austral summer (February–April) 2017, alarge-scale mesocosm experiment was conducted in the coastal upwelling areaoff Callao (Peru) to investigate the impacts of ongoing ocean deoxygenationon biogeochemical processes, coinciding with a rare coastal El Niñoevent. Here we report on the temporal dynamics of carbonate chemistry in themesocosms and surrounding Pacific waters over a continuous period of 50 dwith high-temporal-resolution observations (every second day). Themesocosm experiment simulated an upwelling event in the mesocosms byaddition of nitrogen (N)-deficient and CO2-enriched OMZ water. Surfacewater in the mesocosms was acidified by the OMZ water addition, withpHT lowered by 0.1–0.2 and pCO2 elevated to above 900 µatm.Thereafter, surface pCO2 quickly dropped to near or below theatmospheric level (405.22 µatm in 2017; Dlugokencky and Tans, 2021; NOAA/Global Monitoring Laboratory (GML)) mainly due toenhanced phytoplankton production with rapid CO2 consumption. Furtherobservations revealed that the dominance of the dinoflagellate Akashiwo sanguinea andcontamination of bird excrements played important roles in the dynamics ofcarbonate chemistry in the mesocosms. Compared to the simulated upwelling,natural upwelling events in the surrounding Pacific waters occurred morefrequently with sea-to-air CO2 fluxes of 4.2–14.0 mmol C m−2 d−1. The positive CO2 fluxes indicated our site was a localCO2 source during our study, which may have been impacted by thecoastal El Niño. However, our observations of dissolved inorganic carbon (DIC) drawdown in themesocosms suggest that CO2 fluxes to the atmosphere can be largelydampened by biological processes. Overall, our study characterized carbonatechemistry in nearshore Pacific waters that are rarely sampled in sucha temporal resolution and hence provided unique insights into the CO2dynamics during a rare coastal El Niño event.

biogeosciences

Temporal dynamics of surface ocean carbonate chemistry in response to natural and simulated upwelling events during the 2017 coastal El Niño near Callao, Peru