Carbon dioxide is more soluble in water than oxygen. Nitrogen gas is only half the soluble as oxygen gas. The higher the tempratures the lower the solubility is for oxygen gas. As pressure increases the amount of oxygen that will dissolve in one liter of water increases. Also, if the water contains to much dissolved oxygen, it will become toxic to the oragnisms living there. Three differnt things that will affect the level of dissolved oxygen in the water: 1. Temperature, 2. The pressure of water on the oxygen, 3. The level of dissolved salt that mixes into the water.