The distribution of oxygen in the oceans is dependent on four major processes: (1) gas exchange with the atmosphere at the ocean surface, (2) mixing of waters away from the surface, (3) production of oxygen from phytoplankton photosynthesis in subsurface waters, consumption of oxygen by plants and animals throughout the water column, and oxidation of plant material at intermediate depths by microbial processes, and (4) increased oxygen in deep waters due to the sinking of cold, oxygen-rich water masses. Dissolved oxygen concentrations have been decreasing in the north Pacific for a variety of reasons.
Several types of oxygen sensors are commercially available, including membrane Clark-type cells and optode sensors. The OOI program uses both types of oxygen sensors on different platforms, often coupled to CTD instruments.
Seabird SBE 43 instruments are on the Shallow Profiler Mooring Science Pods. Aanderaa Optode 4831 instruments are on the stationary 200 m platforms (Platform Interface Assemblies), on the Deep Profiler Moorings and near the seafloor associated seafloor junction boxes coupled to CTD, and are also a component of the Oregon Shelf and Offshore Benthic Experiment Packages. Thus, real-time oxygen measurements are being made throughout the water column from coastal to far offshore environments.