Abstract

Little is known on the El Niño-related impacts affecting the CO2 dynamic and fluxes in productive Eastern Boundary Upwelling Systems (EBUS). Through 35 oceanographic campaigns conducted between January 2015 and April 2016 in the Humboldt EBUS (23°S), intraseasonal (i.e., daily-to-bimonthly) hydrographic variations and nearshore changes in surface carbonates system were assessed during the 2015 El Niño event. Collected hydrographic and carbonates system data (sea surface temperature (SST), subsurface temperature, salinity, oxygen, chlorophyll a, pH, total alkalinity (AT) and pCO2), were analyzed and interpreted considering local (i.e., satellite winds and tide gauge sea level) and remote (i.e., equatorial Kelvin waves) forcing. Permanent alongshore wind conditions prompted a relatively deep mixed layer favoring elevated acidity (7.8–7.9 pHT) and CO2 saturated (>450 μatm) conditions throughout most of the study period. Between austral summer-fall and fall-winter transitions, intermittent warm events increased water temperature (SST) above the long-term climatological state (2000–2010), and gave rise to high acidity-warm conditions. However, despite warm water particularly high in salinity (>35) and AT (>2,300 μmol kg−1) associated with downwelling Kelvin waves activity observed during the developing phase of the 2015 El Niño event, phytoplankton production and CO2 uptake were favored because of the maintenance of mean upwelling conditions. These results are discussed in terms of data limitations, coastal processes adding variability to carbonates system variability, and requirements for a better understanding of hydrographic forcing and responses in the Humboldt EBUS.