Oceanographers and limnologists use pCO to measure the amount of carbon dioxide dissolved in water, as well as to parameterize its flux into (influx) and out of (efflux) the atmosphere.  Carbon dioxide reacts with water to form bicarbonate and carbonate ions, such that the relative solubility of carbon dioxide in water is greater than that of other unreactive gasses (e.g. Helium).  As more carbon dioxide dissolves in water, its pCO rises until it equals the pCO of the overlying atmosphere.  Conversely, a body of water with a pCO greater than that of the atmosphere effluxes carbon dioxide.
pCO is additionally affected by water temperature and salinity.  Carbon dioxide is less soluble in warmer water than cooler water, so hot water will exhibit a larger pCO than cold water with the same concentration of carbon dioxide.  pCO can be used to describe the inorganic carbon system of a body of water, together with other parameters such as pH, dissolved inorganic carbon, and alkalinity.   Together, these parameters describe the concentration and speciation of inorganic carbon species (CO, HCO, CO) in water.
Biological processes such as respiration and photosynthesis affect and can be affected by aquatic pCO.  Respiration degrades organic matter, releasing CO into the water column and increasing pCO.  Photosynthesis assimilates inorganic carbon, thereby decreasing aquatic pCO.