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Dissolved Oxygen at the Preserve
May 12, 2004
What is Dissolved Oxygen?
Dissolved oxygen (DO) is molecular oxygen (a gas) that is dissolved
in water. Sources of dissolved oxygen are:
- Diffusion from the surrounding air
- Aeration of water from rapids in a stream or river
- A by-product of photosynthesis by aquatic plants
Dissolved oxygen concentrations are usually reported in units of
milligrams of gas per liter of water - mg/L.The graphs on this page show dissolved oxygen concentrations at
the “Gene Pool”, a restored wetland within the Preserve.
The Gene Pool was constructed in the 1998 to provide open water
habitat for ducks and other water birds. It is not connected to
the river during the warm summer months and does not support a fish
population.
Dissolved Oxygen in Early Summer 2003
Why does this graph have so many maximums and minimums?
Oxygen is produced during photosynthesis and consumed during respiration
and decomposition. Increased temperature or excess nutrients may
result in higher algal and plant growth, causing DO levels to increase.
The Gene Pool has aquatic plants and algae during the warmer months
of the year. Photosynthesis requires sunlight and occurs during
the daylight hours, but not at night. So at night the dissolved
oxygen levels decrease. This graph shows 14 days of data, so there
are 14 maximums and minimums. This daily pattern is called a diurnal
cycle.
Dissolved Oxygen in the Middle of the Summer 2003
Why is the Dissolved Oxygen in this graph so low?
The dissolved oxygen in this graph is low because
algae and other aquatic plants are decomposing after thriving during
the spring and early summer. Microbes use oxygen when they aerobically
decompose organic matter.
How much Dissolved Oxygen do fish need? Fish generally require 3-5 mg/l of dissolved oxygen, although some
very tolerant species like carp can survive in concentrations less
than 3 mg/l. Cold-water fish like trout or salmon generally require
higher concentrations than warm-water fish like bass or bluegill.
Background Information
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