The amount of oxygen that dissolves in water can vary daily and seasonally, and decreases with higher temperature, salinity, and elevation. The maximum solubility of oxygen in water at one atmosphere (standard air pressure at sea level or 101,325 kPa) ranges from about 15 mg/L at 0 ºC to 8 mg/L at 30 ºC—that is, ice-cold water can hold twice as much dissolved oxygen as warm water (Wetzel 2001). Dissolved oxygen comes from the atmosphere and from photosynthesis by aquatic plants, and is depleted through both chemical oxidation and respiration by aquatic animals and microorganisms, especially during the decomposition of plant biomass and other organic material.
Surface water, near the water-atmosphere interface and with sufficient light for photosynthesis, is generally saturated or even supersaturated with oxygen. Deeper water receives oxygen through mixing by wind, currents, and inflows. Mixing and aeration also occur at waterfalls and rapids. Dissolved oxygen can be greatly depleted during summer months, in stagnant pools, or where algal blooms occur.
Dissolved oxygen is essential for a healthy aquatic ecosystem. Fish and aquatic organisms need oxygen dissolved in the water to survive. The need depends on the species and life stage; some are adapted to lower oxygen conditions, others require higher concentrations. Dissolved oxygen can affect the solubility and availability of nutrients, which can be released from sediments under conditions of low dissolved oxygen (B.C. MELP et al. 1998).
Mixing and aeration of water occurs at waterfalls and rapids.
Source: Vogel 2003
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