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Dissolved Oxygen
Dissolved Oxygen (DO) is the amount
of oxygen dissolved in lake water.
Oxygen is necessary for all living organisms to survive except for some bacteria. Living organisms breathe in oxygen that
is dissolved in the water. The amount
of oxygen lake water can hold is directly related to temperature.
The colder the water, the more dissolved oxygen it can hold.
Sources of DO
Dissolved oxygen is supplied to
a lake from two main sources: plant and algae photosynthesis and diffusion from
the atmosphere. In photosynthesis,
plants use the sun's energy to convert carbon dioxide and water into oxygen and
cellular material (growth). Because
there is a higher concentration of oxygen in the atmosphere (air) than water, oxygen
diffuses into the surface of the lake from the atmosphere.
Uses of DO
Dissolved oxygen is used by two main processes: respiration
and decomposition. Respiration is when
animals breathe in oxygen and use it to produce energy, releasing carbon dioxide
and water as by-products. In simpler
terms, it is the act of breathing.
Decomposition is when invertebrates, bacteria and fungi break down dead organic
material. Most decomposition uses oxygen
in the process.
Distribution of Oxygen
Oxygen is only added to the lake
near the surface because that's where the plants are (euphotic
zone) and where diffusion
from the atmosphere occurs. In the
summer and winter in a eutrophic or
mesotrophic lake, the lake is usually separated into a top layer and a bottom layer (stratification).
During
stratification, the bottom of
the lake becomes anoxic, void of oxygen.
Anoxia occurs because respiration and decomposition takes place at the bottom of
the lake and use up oxygen. The oxygen
can't be replenished at the bottom of the lake because it is cut off from the top
of the lake. In the spring and fall
when the lake mixes again, oxygen gets replenished at the bottom of the lake.
Biological Implications
All organisms, except some bacteria,
need oxygen to survive. If the bottom
of the lake becomes anoxic, the organisms either die or move up from the bottom
to where there is oxygen. For example,
in late summer fish usually move closer to the surface because there is no oxygen
available at the bottom of the lake.
In shallow lakes in the summer and winter, the entire lake can become anoxic, causing
a fish kill.
Chemical Implications
When the bottom of a lake is
anoxic (usually in late summer and late winter), chemical processes at the
sediment/water interface cause phosphorus to be released from the sediments (see below). When the lake mixes again, this
increased phosphorus fuels algae growth. This phenomenon is called internal loading because phosphorus is entering the lake from within the lake (from the sediment) (more
information on lake mixing).
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