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Estuaries in NSW

Estuarine Water Quality

Oxygen

Oxygen demand

Dissolved oxygen

Dissolved oxygen (DO) enters the estuarine water mass in the following ways (sources):

from the photosynthetic activity of aquatic plants, including phytoplankton, macroalgae and macrophytes
by direct diffusion from the atmosphere
by turbulent aeration of estuary waters (eg breaking waves on estuary shores), and
as an import in tidal and freshwater inflows.

Dissolved oxygen is lost from the estuarine water mass in the following ways (sinks):

through the respiration of aquatic plants, animals and aerobic bacteria
by diffusion back to the atmosphere
by chemical oxidation processes, and
as an export in outgoing flows.

Aquatic plants are net producers of oxygen during daylight hours, but are net consumers during the night. Because of this, dissolved oxygen levels vary diurnally, with lowest concentrations occurring around sunrise.

Apart from the above factors, a number of other physical and chemical factors also affect dissolved oxygen levels, of which water temperature and salinity, turbidity levels (high turbidity suppresses photosynthesis) and pH are of importance. Table 2 shows the effects of temperature and salinity on the solubility of oxygen in water. Oxygen solubility falls as temperature and salinity increases.

Thus, the solubility of oxygen in estuarine waters varies in response to several effects: seasonally in response to freshwater inflows and temperature changes; possibly daily in response to daily temperature changes; and semi-diurnally in response to the changing salinity associated with tidal flows.

Because of the significant changes that can occur in the solubility or "saturation level" of DO, measured values are often quoted in relative terms as a percentage of the saturated value (in a similar fashion to relative humidity).

Most aquatic life requires oxygen to survive. Depending on temperature and salinity conditions, the saturated level of DO in an estuary generally lies between 6.5 mg/L and 9.0 mg/L. As DO levels fall, the most sensitive species of fish are either killed or move to other areas. As DO levels are further reduced only the most hardy species survive (eg mullet and eels), until ultimately all fish species are killed or driven away. Intermediate levels of DO cause environmental stress to animals, which renders them susceptible to disease and predation and leaves them less able to forage for food.

Table 2 Effects of temperature and salinity on oxygen solubility in water
Temp.	Salinity	DO solubility	Comments
(^oC)	(ppt)	(mg/l)
10	0	11.3	Freshwater at Sea Level
25	0	8.4	Freshwater at Sea Level
25	35	6.9	Seawater at Sea Level

Oxygen demand

The "oxygen demand" of a waterbody is a measure of the level of organic matter present in the waterbody (both particulate and dissolved). Under favourable conditions, water borne bacteria can readily multiply to metabolise (consume) non-living organic matter present in the waterbody. Aerobic bacteria use dissolved oxygen in this process.

Oxygen demand should not be confused with dissolved oxygen levels. Oxygen demand represents a demand placed on the waterbody, irrespective of the ability of the waterbody to satisfy that demand from dissolved oxygen.

Biochemical Oxygen Demand (BOD) is a measure of the actual amount of oxygen that will be consumed from the water mass within a given period of time (usually 5 days) when the existing bacterial populations break down what organic matter they can. BOD is determined by incubating a water sample for 5 days at 20^oC and measuring the actual oxygen uptake (BOD₅ value).

Not all of the organic matter is immediately available for bacterial breakdown. Much of it may be within living organisms (eg phytoplankton and bacteria themselves) or it may be highly resistant to breakdown (such as particles of plant origin).

Chemical Oxygen Demand (COD) is a measure of the oxygen required for the breakdown of all organic matter present in the water. Although COD is a less realistic assessment of the likely oxygen depletion due to bacterial processes, it is much quicker to measure than BOD, and is often used for this reason.

If the receiving water contains adequate levels of DO, bacterial activity will be of the aerobic type (i.e. oxygen consuming). As the bacteria break down the organic matter into simple compounds, they consume oxygen from the water mass and DO levels fall. If the concentration of readily metabolisable organic matter is sufficiently high, the bacteria may completely de-oxygenate the waterbody (sometimes leading to a "fish kill").

When DO levels fall below about 5% of saturation (0.2 - 0.4 mg/l), anaerobic bacteria, which obtain metabolic energy by the reduction of nitrates ("denitrification") and sulphates, then take over the further breakdown of any residual organic matter. However, as part of this process, hydrogen sulphide and methane gases, which are malodorous and aesthetically offensive, are produced. If anaerobic conditions persist for an extended period, the waterway may become "septic", i.e. black coloured, and foul smelling.

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