Dissolved Oxygen
The Good Gas
Dissolved oxygen (DO)
Dissolved oxygen (DO) is the amount of oxygen that is present in the water.
It is measured in milligrams per liter (mg/L), or the number of milligrams
of oxygen dissolved in a liter of water.
How does oxygen get into the water?
Oxygen gets into the water when:
➢Oxygen from the atmosphere dissolves and mixes into the water’s surface
➢Algae and bay grasses release oxygen during photosynthesis
➢Water flows into the Bay from stream, river and the ocean.Ocean waters
generally have more oxygen. River waters are fast-moving, which helps oxygen
from the air mix in.
Photosynthesis: Your one-stop
shop for all of your oxygen needs!
Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2
Happy Rays
of Sunshine
Carbon
Dioxide
(from air)
Oxygen
Carbohydrate
(plant material)
(to air)
Water (from ground)
Aquatic plants and phytoplankton
(single cell floating plants) release
oxygen into the water as a product
of photosynthesis
Happy Rays
of Sunshine
CO2
O2
Oxygen: A Soluble Gas
H 2O
O2
H2O
H
O
2
O
O2
2
O2
H 2O
O2 H O
H
O
2
2
O2 H2O
O2 H O
H
O
2
2
H2O
Habitat Classification Based on DO
Concentration
DO Concentration (mg/L)
3
2.5
> 2 mg/L
Normoxic
2
1.5
1
0.2 – 2 mg/L
Hypoxic
0 – 0.2 mg/L
Anoxic
0.5
0
Most fish need oxygen levels > 2.0 mg/L
0-2 mg/L: not enough oxygen to support life.
2-4 mg/L: only a few fish and aquatic insects can survive.
4-7 mg/L: good for many aquatic animals, low for cold water fish
7-11 mg/L: very good for most stream fish
Daily Aquatic Oxygen Cycle
Dissolved oxygen reaches its peak during the day. At night, it
decreases as photosynthesis has stopped while oxygen
consuming processes such as respiration, oxidation, and
respiration continue, until shortly before dawn.
Daily Aquatic Oxygen Cycle
Moonshine
Sunshine
Moonshine
Dissolved Oxygen
4
3
2
1
0
Midnight
Sunrise
Noon
Sunset
Midnight
Dissolved oxygen reaches its peak during the day. At night, it
decreases as photosynthesis has stopped while oxygen
consuming processes such as respiration, oxidation, and
respiration continue, until shortly before dawn.
Decomposition – Not good for DO
• Decomposer organisms (mainly bacteria)
consume oxygen
– Sometimes consume oxygen faster than plants
can produce it, even during the middle of the
day!
• A sudden increase in organic matter (think
leaf litter) can create a spike in
decomposition activity – especially if it is
hot
– Hurricanes not only add organic matter to our
waterways, but also stir up the sediment.
– Can cause fish kills!!
Factors Affecting DO
➢Volume and velocity of water flowing in the water body
➢Climate/Season
➢The type and number of organisms in the water body
➢Altitude
➢Dissolved or suspended solids
➢Amount of nutrients in the water
➢Organic Wastes
➢Riparian Vegetation
➢Groundwater Inflow
Wind
Stirs in atmospheric oxygen
Current Velocity
• The faster water flows, the more atmospheric
oxygen is mixed into the water.
Water Clarity
Amount of Sunlight Reaching Plants
The muddier the water is, the less light reaches the plants!
Cloud Cover
• Clouds decrease the amount of sunlight reaching
aquatic plants, thus oxygen production is reduced.
Human factors
that affect dissolved oxygen in streams include :
➢ addition of oxygen consuming organic wastes such as sewage,
➢ addition of nutrients,
➢ changing the flow of water,
➢ raising the water temperature, and
➢ the addition of chemicals.
Sample Collection
➢ Remember that the water sample must be collected in such a
way that you can cap the bottle while it is still
submerged.
➢ Carefully wade into the stream. Collect the sample so that you
are not standing upstream of the bottle.
➢ Remove the cap of the BOD bottle. Slowly lower the bottle into
the water, pointing it downstream, until the lower lip of
the opening is just submerged.
➢ Cap the bottle while it is still submerged. Lift it out of the
water and look around the "collar" of the bottle just
below the bottom of the stopper. If you see an air bubble,
pour out the sample and try again.
➢ "Fix" the sample immediately following the directions in your
kit.
SAMPLE HANDLING AND PRESERVATION
Preservation of sample is not practical. Because biological activity will
continue after a sample has been taken, changes may occur during handling
and storage. If analysis is to be carried out within two hours of collection, cool
storage is not necessary. If analysis cannot be started with in the two hours of
sample collection to reduce the change in sample keep all samples at 4°C. Do
not allow samples to freeze. Analysis should begin as shown as possible. Do
not open sample bottle before analysis. Begin analysis within six hours of
sample collection.