Aquatic Pollution and Coastal Management

Unit 3 : Agricultural wastes and eutrophication

3.7.What causes dissolved oxygen concentrations to change?

• Solubility of oxygen 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.
• DO concentrations naturally vary over a twenty-four hour period due to tidal exchange, and because there is net production of oxygen by plants and 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.
• 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 and 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.
• 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.
• 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.

Oxygen dynamics in coastal waters.