What is Dissolved Oxygen in Water and how relevant it is for Urban Water Quality?
Do you know you can estimate the living conditions or the health of a water body just by measuring the content of Dissolved Oxygen in Water? Absolutely. Check it out and the impact it has on humans.
What is Dissolved Oxygen in water or DO?
Dissolved oxygen in water is the amount of free Oxygen available dissolved in water. It is one of the most critical parameters of water. This is so because free oxygen (O2) as we state is a non-compounded form of oxygen. This is different from the molecular state of that compounded oxygen atom which is bonded with 2 molecules of hydrogen (H+) to form water (H2O).
The free non-compounded oxygen is used by all organisms living inside water to breathe. Critical functions like cleaning of a water body by decomposition caused by the bacteria use dissolved oxygen. In fact it has such a significant effect onto their life that this dissolved oxygen in water parameter is used to determine the 'Health' of the water body. Low DO level in water makes life unsustainable for aquatic animals. Too low DO level in lakes are deemed as hypoxic or dead and sustains no life.
Availability of Dissolved Oxygen level decreases with the increase in ocean depth level. The amount of Dissolved Oxygen needed by aquatic organisms depend on the depth of the water body they are living in. Eg. The deep sea creatures that have less sunlight penetration and low metabolism uses less DO while shallow fishes need much higher DO to survive.
Relation between DO and other water parameters
The water oxygen content level depends upon:
Water Temperature
As we know the solubility of gases in water decrease with the temperature. Hence higher the water temperature, lower is the amount of DO.
DO and Water Pressure
As we know, atmospheric pressure decrease with altitude. Similarly DO level decreases with depth of the ocean. That is why deep sea ocean level have lesser amount of DO than that of the surface water.
Salinity in Water
It is observed that high level of water salinity often has low levels of DO.
Velocity of Water and DO
Rapidly moving water as in rivers have more DO level than stagnant water like ponds. This is because due to higher velocity of water there is greater turbulence. This exposes more surface area of water to eventually dissolve more of oxygen from the atmosphere.
Turbidity in Water
The high turbidity has amount of sediments in water block the sunlight. In the process they absorb a lot of heat thus raising water temperature. This eventually lower down the DO level in water.
Rate of Aquatic Photosynthesis
DO level in water also depends on the rate of photosynthesis by aquatic plants. It rise during the day as photosynthesis and falls during night.
Seasonality Trends
The four seasons of the year have different levels of DO. The temperature of water in the seasons are main factor responsible for variation in the DO level.
Supersaturation
Supersaturation of water with oxygen is the state of water having very high levels of dissolved oxygen. Many factors among above can contribute to it eg. Rapid moving water could get supersaturated.
Supersaturation of water with oxygen (or nitrogen) can cause disease such as gas bubble disease in fishes.
Why analyze Dissolved Oxygen for Water Quality?
Dissolved Oxygen parameter determines the health of a water body. Measuring it will give you an idea of :
The type of aquatic organisms found with the ocean depth
- Aquatic animals survival and their metabolism
- Aquatic plant health and the rate of photosynthesis
- Eutrophication if any causing depletion of dissolved oxygen
- Rate of decomposition by bacteria in water
DO level chart/Depth
DO level in water | Soft water | Types of Creatures | Sunlight penetration |
|---|---|---|---|
6 mg/L | Top most Ocean surface | Most fishes like tuna | Maximum Sunlight zone |
5 mg/L | Shallow depth | Most fishes like white perch | Optimum Sunlight zone |
| Mid deep | Crabs | Mid Sunlight zone |
3 mg/L | Deep water depth | Angler fish | Low Sunlight zone |
2 mg/L | Deep channel | Squids | Very less light penetration |
1 mg/L | Deep ocean floor | Worms | Approximately dark |
0 mg/L | Deepest ocean trenches | Little to no life | Absolutely no light |
Aquatic animals survival and their metabolism
Lower Dissolved Oxygen level tends to lower down the metabolism of aquatic animals. Beyond a certain level these organisms can even perish to extinction.
Aquatic plant health and the rate of photosynthesis
Oxygen is released by both aquatic and terrestrial plants as a by-product of photosynthesis during the day. During the night oxygen is used for the metabolic activities releasing carbon dioxide. This varies the DO level during the night and day but also gives an idea of the health of the aquatic plants and their state of photosynthesis.
Eutrophication
Eutrophication is the process of oxygen depletion in water bodies caused by the growth of too many phytoplankton like algae in it. The unnatural growth of algae is attributed to the artificial supply of nutrients such as nitrogen and phosphorus. These nutrients used in farms as pesticides are often carried by the runoffs during rainfall.
When an algae dies it is decomposed by the bacteria in water. These bacteria uses the dissolved oxygen in water for decomposition. Normally this works well. But due to high growth and death of algae, the rate of decomposition increases and so does the use of dissolved oxygen. This depletion of oxygen can completely kill the entire aquatic ecosystem. Eutrophication is a direct response of water pollution by humans and should be taken care of accordingly in due time.
Rate of decomposition by bacteria in water
Lower amount of DO is an indication of presence of high number of bacteria in that water body.
Origin
Sources
- Dissolved Oxygen or DO is the free oxygen available in the atmosphere which dissolves in water.
- By-product of photosynthesis released during the day by aquatic plants.
Photosynthesis can be summed up as:
CO2 + H2O = CH2O+ O2
Glucose (CH2O) formed is used by the aquatic plant. The oxygen(O2) is released and dissolved in water.
Urban Water Dissolved Oxygen Standards
Dissolved Oxygen level states the well being of a water body and the organisms living in it. It is usually expressed in milligrams per liter or mg/L.
DO level in water | Health of the water body |
|---|---|
Above 3 mg/l | Good state |
Below 3 mg/l | state of concern |
below 1 mg/l | Hypoxic or not life sustaining |
The dissolved oxygen level in water does not adversely affect the water quality for human consumption. But more oxygenated the water is, the better it tastes. Of course along with many other benefits it has.
Operations
Detection of Dissolved Oxygen in water
Color
No distinguish color is observed to any change in dissolved oxygen content in water.
Odor
No odor is detected on change in DO level.
Measuring the dissolved oxygen level in water
Multi parameter monitor is used to record water oxygen content.
Water oxygen meter can do the same.
Urban DO Water Treatment
Aeration
Is one of the best and convenient methods to increase the oxygen level in water.
Manually with a Stick - DO levels in lakes and ponds can be increased physically by using a stick to stir water just as you do to dissolve sugar cubes.
Artificial wind generators - to blow air causing movement through ripples in water, increasing the DO level.
Generally simple stick would do better for the risk of supersaturation by artificial heavy machinery.
Impact
Nature
Dissolved Oxygen level is the lifeline of all aquatic activities. Their survival depends on its optimum levels in water.
Human
Though low DO level is not known to prove fatal to humans but it still has a key role to play in water quality. The DO affects the taste of the water by improving it.
High DO level has high level of oxygen which can cause corrosion of water supply pipeline. This is so genuine a problem that industrial water supply cut down its DO level to prevent corrosion.
High level of Dissolved Oxygen in drinking water has certain health benefits.
Check out Oxygenated water.