Surviving a High Nutrient-Low Chlorophyll (HNLC) region: insights to the internal cycling of nitrogen and iron in the eastern equatorial Pacific

The marine nutrient nitrate is completely utilized in much of the global surface ocean, but persists in so-called High-Nutrient, Low chlorophyll (HNLC) regions where primary production is limited by iron. Accordingly, the addition of iron to equatorial Pacific HNLC waters increases nitrate utilization and therefore decreases nitrate concentrations [NO₃^-]. However, seasonal variability in HNLC surface [NO₃^-] occurs alongside changes in upper ocean upwelling and stratification despite little seasonality in the flux of iron. Here we use nitrate isotopes (δ¹⁵N and δ¹⁸O) to show that seasonal variability of eastern equatorial Pacific HNLC surface [NO₃^-] is caused by changes in the degree of nitrate utilization that cannot be explained by the available iron flux. The available iron can account for less than 3 μmol/kg of nitrate utilization (19% of source water [NO₃^-]) even though the observed nitrate drawdown ranges from 7 μmol/kg (during boreal fall when upwelling is strongest) to >11 μmol/kg (during boreal spring when upwelling is weakest). Based on these observations, we propose that the photosynthetic picoplankton (e.g., Prochlorococcus and Synechococcus) with a low iron / carbon requirement preferentially consume recycled N compounds and that much of the recycled iron fuels nitrate assimilation by larger phytoplankton (e.g., diatoms). Slower upwelling rates during boreal spring and El Niño events therefore allow for more internal cycling of iron and increased nitrate utilization, explaining the seasonal variability in surface nitrate concentrations.