Preprints
https://doi.org/10.5194/bg-2021-111
https://doi.org/10.5194/bg-2021-111

  05 May 2021

05 May 2021

Review status: this preprint is currently under review for the journal BG.

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

Shao-Min Chen1,2, Ulf Riebesell1, Kai G. Schulz3, Elisabeth von der Esch1, Eric P. Achterberg1, and Lennart T. Bach4 Shao-Min Chen et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Department of Earth and Environmental Sciences, Dalhousie University, Halifax, Canada
  • 3Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, Australia
  • 4Institute for Marine and Antarctic Studies, University of Tasmania, Tasmania, Australia

Abstract. Oxygen minimum zones (OMZs) are characterized by enhanced carbon dioxide (CO2) levels and low pH and are being further acidified by uptake of anthropogenic atmospheric CO2. With ongoing intensification and expansion of OMZs due to global warming, carbonate chemistry conditions may become more variable and extreme, particularly in the Eastern Boundary Upwelling Systems. In austral summer (Feb–Apr) 2017, a large-scale mesocosm experiment was conducted in the coastal upwelling area off Callao (Peru) to investigate the impacts of on-going ocean deoxygenation on biogeochemical processes, coinciding with a rare coastal El Niño event. Here we report on the temporal dynamics of carbonate chemistry in the mesocosms and surrounding Pacific waters over a continuous period of 50 days with high temporal resolution observations (every 2nd day). The mesocosm experiment simulated an upwelling event in the mesocosms by addition of nitrogen (N)-deficient and CO2-enriched OMZ water. Surface water in the mesocosms was acidified by the OMZ water addition, with pHT lowered by 0.1–0.2 and pCO2 elevated to above 900 μatm. Thereafter, surface pCO2 quickly dropped to near or below the atmospheric level (405.22 μatm in 2017, NOAA/GML) mainly due to enhanced phytoplankton production with rapid CO2 consumption. Further observations revealed that the dominance of dinoflagellate Akashiwo sanguinea and contamination of bird excrements played important roles in the dynamics of carbonate chemistry in the mesocosms. Compared to the simulated upwelling, natural upwelling events in the surrounding Pacific waters occurred more frequently 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 local CO2 source during our study, which may have been impacted by the coastal El Niño. However, our observations of DIC drawdown in the mesocosms suggests that CO2 fluxes to the atmosphere can be largely dampened by biological processes. Overall, our study characterized carbonate chemistry in near-shore Pacific waters that are rarely sampled in such temporal resolution and hence provided unique insights into the CO2 dynamics during a rare coastal El Niño event.

Shao-Min Chen et al.

Status: open (until 16 Jun 2021)

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Shao-Min Chen et al.

Shao-Min Chen et al.

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Short summary
Oxygen minimum zones in the ocean are characterized by enhanced carbon dioxide (CO2) levels and are being further acidified by increasing anthropogenic atmospheric CO2. Here we report CO2 system measurements in a mesocosm study offshore Peru during a rare coastal El Niño event, to investigate how CO2 dynamics may respond to on-going ocean deoxygenation. Our observations show that nitrogen limitation, productivity and plankton community shift play an important role in driving the CO2 dynamics.
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