Oxygen consumption and dentrification below the peruvian upwelling

Abstract

Rates of oxygen consumption and denitrification were calculated from the respiratory electron transport activity of deep and intermediate water plankton of the Peru Current upwelling, and compared to rates calculated for the deep water under other oceanic regions. Oxygen consumption off Peru ranged from 1.13 ± 0.85 μℓ 02 h−1 ℓ−1 at the sea surface to 0.6 ±0.4 x 10 −3 μℓ 02 h −1 ℓ−1 at depths below 1000 m. The depth-dependence in oxygen consumption in the Peru up-welling at 15°S was described by the exponential function R= 1.5 Z−1.03 (r2 = 0.99) where R is oxygen consumption in μℓ 02 h−1 ℓ−1 and Z is depth in meters. In the intermediate water (100–500 m), oxygen consumption in oxygenated waters and denitrification in anoxic waters decreased seaward from the coastal zone along a gradient from high to low surface productivity. Denitrification in the southeastern Pacific removed 2.5 x 1013 g N yr −1 in 1976–1977. This process occurred in the oxygen deficient zone at 100–400 m between 10–25°S, and although the zone extends 1300 km westward into the Pacific, the denitrification rate is most intense in the inner 175 km. The organic carbon consumed by the 02 and NO 3 − based respiration in the water column is less than the organic carbon produced by the phytoplankton in the surface waters. The difference of 1.7 g C d−1 m−2 in the nearshore region and 0.6 g C d−1 m−2 in the offshore region could be lost to the sediments or exported offshore. Mechanisms by which this organic carbon is transported to the deep sea are discussed.