In March 1976 and 1977, nitrate reductase, nitrate, nitrite, and oxygen were measured between depths of 30 and 250m in the subsurface waters overlying the Peruvian continental shelf and shelf edge at 15S latitude. Oxygen concentrations of less than 1 ml/1 began between 20 and 30m in both years. The oxygen deficient waters extended to the bottom on the shelf and to 440m at the shelf edge....
Packard, T. T., R. C. Dugdale, J. J. Goering, and R. T. Barber. 1978. "Nitrate reductase activity in the subsurface waters of the Peru Current." Journal of Marine Research 36, (1). https://elischolar.library.yale.edu/journal_of_marine_research/1423