The downward flux of organic matter decreases sharply with depth, with 80–90% being consumed in the top . The deep ocean thus has higher oxygen because rates of oxygen consumption are low compared with the supply of cold, oxygen-rich deep waters from polar regions. In the surface layers, oxygen is supplied by exchange with the atmosphere. Depths in between, however, have higher rates of oxygen consumption and lower rates of advective supply of oxygen-rich waters. In much of the ocean, mixing processes enable the resupply of oxygen to these waters (i.e. waters that are part of the wind-driven subtropical gyre circulations are rapidly exchanged with the surface and never acquire a strong oxygen deficit).