Abstract
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 (February–April) 2017, a large-scale mesocosm experiment was conducted in the coastal upwelling area off Callao (Peru) to investigate the impacts of ongoing 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 d with high-temporal-resolution observations (every second 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; Dlugokencky and Tans, 2021; NOAA/Global Monitoring Laboratory (GML)) mainly due to enhanced phytoplankton production with rapid CO2 consumption. Further observations revealed that the dominance of the 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 dissolved inorganic carbon (DIC) drawdown in the mesocosms suggest that CO2 fluxes to the atmosphere can be largely dampened by biological processes. Overall, our study characterized carbonate chemistry in nearshore Pacific waters that are rarely sampled in such a temporal resolution and hence provided unique insights into the CO2 dynamics during a rare coastal El Niño event.
Highlights
One of the most extensive oxygen minimum zones (OMZs) in the global ocean can be found off central/northern Peru (4– 16◦ S; Chavez and Messié, 2009)
Our observations in the mesocosms revealed that, following the addition of two OMZ water masses with different nutrient signatures, there was a higher drawdown of dissolved inorganic carbon (DIC) in response to slightly more dissolved inorganic nitrogen (DIN) input from the OMZ water addition but no difference in the buildup of particulate organic carbon (POC) and chl a (Figs. 2a and c, 3d)
The timing of the first phytoplankton bloom was consistent with a shift from a diatom-dominated community to A. sanguinea dominance in most mesocosms, indicating that A. sanguinea was a key factor driving the changes in carbonate chemistry under N-limited conditions
Summary
One of the most extensive oxygen minimum zones (OMZs) in the global ocean can be found off central/northern Peru (4– 16◦ S; Chavez and Messié, 2009). Denitrification and anammox processes that occur in O2-depleted waters remove biologically available N from the ocean and produce an N deficit and phosphorus (P) excess with respect to the Redfield ratio (C : N : P = 106 : 16 : 1) in the water column (Redfield, 1963; Deutsch et al, 2001, 2007; Hamersley et al, 2007; Galán et al, 2009; Lam et al, 2009) Upwelling of this N-deficient water has been found to control the surface water nutrient stoichiometry and influence phytoplankton growth and community compositions (Franz et al, 2012; Hauss et al, 2012)
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