Dissolved oxygen What is dissolved

Dissolved oxygen


What is dissolved oxygen?

Measures of dissolved oxygen (DO) refer to the amount of oxygen contained in water, and define the living conditions for oxygen-requiring (aerobic) aquatic organisms. Oxygen has limited solubility in water, usually ranging from 6 to 14 mg L -1 [1]. DO concentrations reflect an equilibrium between oxygen-producing processes (e.g. photosynthesis) and oxygen-consuming processes (e.g. aerobic respiration, nitrification, chemical oxidation), and the rates at which DO is added to and removed from the system by atmospheric exchange (aeration and degassing) and hydrodynamic processes (e.g. accrual/addition from rivers and tides vs. export to ocean) [1].



Figure 1. Oxygen dynamics in coastal waters. Processes that increase dissolved oxygen concentrations are shown with green boxes. Processes that decrease dissolved oxygen concentrations are shown with orange boxes. (modified after Connell and Miller, 1984 [1]).




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What causes dissolved oxygen concentrations to change?
•Oxygen solubility varies inversely with salinity, water temperature and atmospheric and hydrostatic pressure.
•Dissolved oxygen consumption and production are influenced by plant and algal biomass, light intensity and water temperature (because they influence photosynthesis), and are subject to diurnal and seasonal variation [1].
•DO concentrations naturally vary over a twenty-four hour period due to tidal exchange, and because there is net production of oxygen by plants & algae during the daytime when photosynthesis occurs. By comparison, plants and algae only respire at night time, and this process consumes oxygen. Highly productive systems are expected to have large diurnal DO ranges (see Figure 2).
•Nutrient enrichment stimulates plant and algal growth (and algal blooms) and often results in a mass influx of particulate organic matter to the sediments (eutrophication). The decomposition of this labile organic matter by aerobic microorganisms leads to a rapid acceleration of oxygen consumption, and potential depletion of oxygen in bottom waters.
•Stratification can isolate bottom waters from oxygen enriching processes and can give rise to anoxic & hypoxic events. This problem is most acute in wave-dominated coastal systems (e.g. deltas, estuaries and strandplains and lagoons) because these systems typically have low internal tide penetration. Tides mix the water column and can replenish coastal waterways with oxygen. The baffling effect of seagrass meadows can also impede the mixing process, and maintain bottom water anoxia [3].
•Coastal discharges of wastes rich in organic carbon (e.g. from sewage treatment plants, paper manufacturing, food processing and other industries) are produced in large quantities in urban population centres, and can substantially reduce dissolved oxygen concentrations [1].
•The oxidation of pyrite found in acid sulfate soils can rapidly strip oxygen from the water, and gives rise to acid drainage. Acid drainage may result from natural processes but in many cases the draining of coastal wetlands (e.g. mangroves and salt marshes) is the cause.