Linking Net Community Production and Hydrography Under La Nina and El Nino conditions in the Eastern Tropical South Pacific (ETSP)

Presence of the Subantarctic Mode Water (SAMW) in the upper thermocline of the ETSP results in steep vertical gradients in the major nutrient stoichiometry, specifically the silicate to nitrate ratio. Thus, biological export production within the High Nutrient Low Chlorophyll (HNLC) and low-silicate waters of the northern ETSP, the region of high CO2 outgassing, is likely to be particularly sensitive to the variability in the hydrographic conditions that determine the depth of origin for the waters upwelled into the euphotic zone. During two cruises, in spring 2010 (mild El Nino), and in spring 2011 (moderately strong La Nina), we quantified Net Community Production rates (NCP, as Net biological O2 production), based on O₂/Ar supersaturation ratios measured along 10S between the coast of Peru and 100W. Biases in the estimates of the net biological O2 production within the mixed layer arising from non-equilibrium O2 fluxes from the Oxygen Minimum Zone below, were quantified with a regional box model of coupled oxygen-nitrate mass balances. Potential export efficiencies along the 10S transect were derived from the comparison between satellite-based Net Primary Production and our field-based NCP estimates. Somewhat predictably, regional NCP rates and potential export efficiency, as well as the degree of the biological nitrate uptake were higher under the La Nina than under the El Nino conditions, likely due to deeper origin of the upwelled waters characterized by the higher silicate to nitrate ratios. A less intuitive implication is that while reducing the CO2 outgassing by the increased biological carbon uptake locally, the La Nina-enhanced export production within the ETSP may transiently diminish basin-scale capacity for the oceanic biological CO2 uptake by enhancing the fixed nitrogen losses, This may occur via expanding the vertical extent of the regional OMZ, as was observed at several stations along the cruise tracks in 2011, which in turn may drive an increase in the magnitude of regional denitrification. Further work would test this hypothesis.