Oxygen Deficient Conditions and Nitrate Reduction in the Eastern Tropical North Pacific Ocean1

Oxygen concentrations in the oxygen‐minimum zone of the eastern tropical North Pacific Ocean are at least an order of magnitude lower than previously reported. Concentrations of less than 1 µ g‐atom/liter are common throughout the region from below the pycnocline to depths of several hundred meters. In these nearly anoxic waters, nitrate appears to be reduced to nitrite and free nitrogen. Nitrate deficits, derived from material balance calculations, suggest that up to 13–14 µ g‐atoms/liter of nitrate‐nitrogen have been so reduced. This is 40–50% of the nitrate normally present in this depth interval. Secondary nitrite concentrations rarely exceed 1.5 µ g‐atoms/liter and are generally associated with oxygen concentrations of less than 2 µ g‐atoms/liter. Ammonia concentrations appear to be slightly lower in the region of denitrification, probably because of bacterial assimilation. If the nitrate reduction rate observed in Darwin Bay in the Galapagos Islands were to apply as a rate of denitrification in the entire oxygen‐minimum zone in the eastern tropical North Pacific, the annual loss of 920 × 10 12 g of combined nitrogen could be expected. However, considerations of environmental differences suggest that a figure somewhat less than a quarter of this amount is more probable.