Dissolved Oxygen
| http://www.ncsu.edu/sciencejunction/depot/experiments/water/lessons/do/ |Dissolved oxygen is one of the best indicators of the health of a water ecosystem.
Dissolved oxygen can range from 0-18 parts per million (ppm),
but most natural water systems require 5-6 parts per million to support a diverse population.Oxygen enters the water by direct absorption from the atmosphere or by plant photosynthesis.
The oxygen is used by plants and animals for respiration and
by the aerobic bacteria which consume oxygen during the process of decomposition.When organic matter
such as animal waste or
improperly treated wastewater
enters a body of water,
algae growth increases
and the dissolved oxygen levels decrease
as the plant material dies off and is decomposed through the action of the aerobic bacteria.Decreases in the dissolved oxygen levels can cause changes in the types and numbers of aquatic macroinvertebrates which live in a water ecosystem.
Species which cannot tolerate decreases in dissolved oxygen levels include
mayfly nymphs,
stonefly nymphs,
caddisfly larvae and
beetle larvae.As the dissolved oxygen levels decrease,
these pollution-intolerant organisms are replaced
by the pollution-tolerant
worms and
fly larvae.Dissolved oxygen levels change and vary according to the time of day, the weather and the temperature.
If yearly comparisons are made on dissolved oxygen levels, they should be done at the same time of day, during the same season and on a day with a temperature variation of only 10 degrees Celsius from the previous reading.
A decrease in the dissolved oxygen levels
is usually an indication of an influx of some type of organic pollutant.
