Analysis: low oxygen concentrations and biogenic elements associated with Equatorial Subsurface water, and variability in Humboldt Current System off Chile

The Equatorial Subsurface Water (ESSW), which is characterized by low dissolved oxygen content (<2 ml l-1) and high nutrient concentrations, is one of the main sources of upwelling water in the area.

It is noticeable that the upper boundary of the oxygen minimum layer (OML) is found at shallower depths in the coastal zone (25-50m) than in the adjacent oceanic waters (50-100m), both off northern Chile and Peru. This is a special feature of the Humboldt Current Eastern Boundary System and is associated with an important de-nitrification regime in the water column. Before this work was completed, information on the sources of variability associated with the distribution of the ESSW OML, the impact of these changes upon pelagic productivity in the region and biogeochemical cycling was very limited.