Hypoxia refers to low
oxygen conditions. Normally 20.9% of the gas in the atmosphere is oxygen. The
partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water however, oxygen levels are much lower, approximately 1%, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface (if there is more oxygen in the air, it will diffuse across the
partial pressure gradient).