In other words, all of the organic matter produced by photosynthetic algae is respired by consumer and decomposing organisms. But photosynthesis and respiration do not balance at any given depth in the oceans. Because of that imbalance, the concentrations of TDC and O2 also vary with depth. Photosynthesis only occurs in the upper 150 m. Respiration reaches a maximum at 200-800 m, and thus O2 concentrations are low.
Respiration continues at depth.
But there is often an increase in O2 in deep and bottom water. This is due to the sinking and spreading of cold, O2-saturated water from high latitudes. These waters transport O2 faster than it is utilized by respiration. Dissolved CO 2 system in sea water. Reactions involving dissolved CO 2 species buffers oceans against sudden changes in acidity.