current, occupying the upper ocean, flows equatorward carrying fresh, cold Sub-Antarctic surface water northward, along the outskirts of the subtropical gyre. The main flow of the current veers offshore in southern Peru, as a weaker limb continues to flow equatorward. Around 18th parallel south the fresh, cold waters begin to mix with the warm, high salinity Subtropical Surface waters. This collision causes partial subductions. Within this region, the equatorial undercurrent (EUC) flows eastward along the equator, feeding the Peru-Chile undercurrent (PCU) that moves poleward. Off the coast of central Chile, there is a coastal transition zone (CTZ), which is characterized by high eddy kinetic energy. This energy forms mesoscale eddies which extend offshore. The CTZ has three distinct regions within its boundaries: high chlorophyll-a concentrations in wide regions off the coast of Peru (10–15°S), high chlorophyll-a concentrations in wide regions off the coast of Chile (30°S), and high chlorophyll-a concentrations in narrow regions off the coast of northern Chile (Montecino and Lange 2008). High chlorophyll-a concentrations are generally found within 50 km of the coast. The limb of the Humboldt Current System that veers off the coast of Peru creates a decrease in ventilation within the system. This lack of ventilation is the primary driver of an intense oxygen minimum zone (OMZ) which is formed in the sub-surface to intermediate depths. In the north, the EUC ventilates the OMZ, and in the south the PCU advects low oxygen waters southward towards northern Chile. This OMZ is the fourth largest permeant hypoxic zone in the world's oceans. It occupies an area about 2.18 ± 0.66 × 106 km3. The core of this zone is centered off Peru, creating a shallow upper boundary that reaches from about down to . Another factor contributing to the OMZ is sinking and decay of primary productive resources. Consequently, the OMZ forces many organisms to stay near the surface where nutrients and oxygen are obtainable. The presence of a shallow OMZ restricts the migration of zooplankton within the water column. Between , many species of zooplankton occupy this space within the OMZ. This allows for a substantial exchange of carbon between the euphotic layer and the OMZ. 75% of the total zooplankton biomass move in and out of the OMZ. The OMZ also serves as a refuge for organisms that can live in hypoxic conditions. Coastal upwelling is the main factor contributing to the high biological