Marine plants also sometimes consume, but they produce more oxygen than they consume. Indeed, they are the source of much of the oxygen in the atmosphere, since some of the oxygen they produce escapes to the air. All plants need light to grow and in the oceans the maximum depth of light penetration is typically about 100 meters. Well-mixed water in contact with the atmosphere generally has enough oxygen to meet the respiratory requirements of aquatic organisms. The metabolic processes of many aquatic organisms are seriously impaired at values below 3 ppm.
Several conditions lead to oxygen depletion in the marine environment.
Hot summer temperatures and stagnant water may result in severe local oxygen depletion. Isolation of intermediate and deep waters from the atmosphere for long periods of time gives rise to a widespread oxygen minimum zone in the world's oceans. On a small scale, a poorly aerated fish tank with too many animals can result in oxygen depletion and death. Working in a group you will measure dissolved oxygen, temperature, and salinity at 5-meter depth intervals for two stations, one in Castine Harbor and the other in Smith Cove. If possible, work at the shallow end of Smith Cove where water depths are about 9 meters, and if time permits, take samples at 3-meter depth intervals.