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Importance of Dissolved Oxygen in the Ocean
The concentration of dissolved oxygen in the ocean is important because aquatic animals need oxygen just as land animals do. The gills on aquatic animals extract the dissolved oxygen from the water and it enters their bloodstream, oxygenating their organs. The higher the concentration of dissolved oxygen, the less hard aquatic animals have to work to "breathe."
Photosynthesis in Aquatic Plants
The amount of dissolved oxygen in the ocean depends partly on the type of organisms in the ocean. Aquatic plants and animals need oxygen to sustain their cells. Oxygen, therefore, is continually used up. Photosynthesis by aquatic plants during sunlight hours replenishes dissolved oxygen. The amount of oxygen replenishment depends on the amount of plants present in an area.
Surface Diffusion
Saltwater is primarily oxygenated at the surface. Replenishment of dissolved oxygen in the ocean also occurs through surface diffusion. This process is much slower than photosynthesis, since the diffusion of gases from the atmosphere to the ocean water is slow. In addition, replenishment of dissolved oxygen is dependent on water circulation, whether from winds moving surface water or destratification of ocean layers. Destratification causes upwelling to occur where cold, dense, more oxygenated waters push up to the coastal waters through bottom currents and replenish the oxygen supply.
Temperature
The maximum amount of oxygen replenishment in the ocean depends also on temperature. Dissolved oxygen concentrations increase with a decrease in temperature. For instance, cold water can hold more dissolved oxygen than warm water. However, it should be noted that not all cold water carries more dissolved oxygen since oxygenation depends on the amount of plant and animal life. Therefore, photosynthesis, surface diffusion and upwelling are all necessary for maximum oxygen replenishment.