Factsheet on Water Quality Parameters Dissolved Oxygen

Dissolved oxygen (DO) is the amount of oxygen in water that is available to aquatic organisms. DO is necessary to
support fish spawning, growth, and activity.
Dissolved Oxygen
Why do we measure dissolved oxygen?
DO is an important indicator of the overall biological
health of a waterbody and is required for a waterbody to
support aquatic life. It is generally measured in the field
along with water temperature, turbidity (clarity), specific
conductance, and pH. This information is then assessed
against water quality standards to determine whether
the water is fit for aquatic life.
Figure 1 is a generalized illustration of how DO affects
fish health - sensitivities vary by species, in the range
labeled as "too low", DO is too low to support fish. In the
"stressful" range, DO conditions impede spawning and
reproduction, and limit growth and activity. A higher DO
is needed to be "supportive" offish spawning, growth,
and activity. Different levels of DO are required to
support aquatic life depending on the species present
and their stages of life (spawning, larvae, etc.). Trout,
for example, require higher DO, while carp can survive
in lower DO conditions. Among the macro nver.ebrates,
many immature insects require a high DO content,
while other species such as aquatic worms and snails
can tolerate lower DO concentrations. Hypoxic (low DO
concentration) or anoxic (virtually no DO) conditions do
not support fish or macroinvertebrate populations.
Figure 7. General freshwater fish tolerance for dissolved
oxygen concentrations - tolerances vary by species.
RANGE OF TOLERANCE FOR
DISSOLVED OXYGEN IN FISH
mg/L Dissolved Oxygen
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Too Low Stressful Supportive
What affects dissolved oxygen?
The primary sources of oxygen in surface waters are
transfer of oxygen from the air and by plants and algae
in the water due to photosynthesis. When the water is in
equilibrium with the atmosphere and is holding as much
DO as expected for the temperature, barometric pressure,
and salinity conditions, it is said to be saturated. Aeration
or photosynthesis can cause DO concentrations to
become even higher and exceed saturation (the water
becomes supersaturated).
For factsheets on other water quality parameters, visit:
epa.qov/awma/factsheets-water-quality-parameters.
For more information about the CWA Section 106 Grants Program, visit:
epa.qov/water-pollution-control-section-106-q rants.

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Dissolved Oxygen
Seasonal cycles
Seasonal changes in water temperature (T) of lakes
affect DO concentrations. Figure 2 shows these
seasonal changes in a eutrophic (nutrient rich) lake.
photosynthesis and more organisms consume DO
deeper in the water.
These seasonal factors combine to amplify the daily DO
cycles described next. Warmer water and high nutrient
concentrations (eutrophication) can result in excess algal
growth. The eventual die-off of algae in an algal bloom
and the increased rate of decomposition of organic
matter in warmer temperatures ultimately reduce DO.
Other factors that can affect DO
concentrations include:
Salinity - Increased salinity reduces the ability for
water to absorb DO.
Altitude - Lower barometric pressure at higher
elevations reduces the ability for water to absorb DO.
Daily cycles
DO levels in surface waters usually follow a daily cycle
(Figure 3). During the day oxygen is added to the water
through photosynthesis by aquatic plants and algae and
can create saturated, or even supersaturated surface
waters. At night, photosynthesis stops, and DO levels
drop as oxygen is consumed through respiration by
aquatic plants and animals. Therefore, DO levels in
surface water will likely be lowest early in the morning,
and aquatic organisms are most vulnerable at that time
(assuming all other conditions are the same). DO levels
rise again during the day as photosynthesis resumes.
SPRING TURNOVER
0 10 20 30 T (°C)
DO
	
0 4 8 12
DO
mg/L)
SUMMER STRATIFICATION
0 10 20 30
thermoclineJ
T(°C)
0 4 8 12
DO
mg/L)
FALL TURNOVER
0 10 20 30
I ii i i | i i
T(°C)
DO
0 4 8 12
DO
mg/L)
WINTER STRATIFICATION
^ ice cover	o 10 20 30
K'		

T(°C)
DOJ
			I	 DO
0 4 8 12 (mg/L)
12
10
>
X
O
T3

O
6»
0|	1	1	1
Midnight Sunrise	Noon	Sunset Midnight
Figure 2. DO and temperature trends in eutrophic (nutrient
rich) lakes by season. Adapted from Wetzel (1975)
After spring turnover occurs, the lake water is evenly
mixed, so the temperature and DO are generally the
same throughout the lake. During summer stratification,
the top layer of the lake is warm and DO is high from
the transfer of oxygen in the air and from algae due
to photosynthesis. As depth increases, temperature
decreases and DO also decreases as there is less
photosynthesis and more organisms are consuming
DO. After fall turnover occurs, the lake water is evenly
mixed, so the temperature and DO are generally the
same throughout the lake. During winter stratification,
when the top of the lake freezes, the cold water just
beneath the ice can hold the most DO, and the sunlight
shining through the ice allows algae to photosynthesize.
DO then decreases with depth as there is less
Figure 3. Generalized depiction of daily variation in dissolved
oxygen concentrations.
2

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Dissolved Oxygen
Other water quality parameters
Changes in DO levels may be associated with changes
in other water quality parameters. For example:
Chlorophyll a - Chlorophyll a can be a useful
indicator of an emerging algal bloom. During a
bloom, DO can increase due to photosynthesis by
the algae. Later, DO concentrations may decline
because of oxygen consumption by consuming
dead algae.
pH - pH may change throughout the day along
with DO fluctuations. As algae and aquatic plants
draw carbon dioxide (C02) out of the water during
photosynthesis, pH may increase throughout
the day. At night, when aquatic plants, algae, and
decomposers respire, they return C02 to the water,
and pH decreases again.
Minerals - Under anoxic conditions, minerals (such
as iron oxide) in the sediment can dissolve, largely
due to microbial activity. Any phosphorus associated
with these minerals will also be released into the
water further exacerbating nutrient rich (eutrophic)
conditions.
What are EPA's recommended criteria for dissolved oxygen?
EPA's Quality Criteria for Water (1986) establishes
recommended criteria for DO concentrations to protect
aquatic life. Water quality criteria provide guidance for
setting region-specific standards and can be adopted or
adjusted as appropriate.
Criteria are based on the lowest DO concentrations
needed by freshwater fish. Freshwater criteria are
broken down into warmwater fish and coldwater fish
because certain species such as trout and salmon
require higher DO levels than others (such as pike).
Table 1 shows the freshwater criteria.
Table 1. Recommended criteria for the protection of aquatic life in freshwaters (mg/L of DO).
Coldwater Criteria
Warmwater Criteria

Early Life Stages1'2
Other Life Stages
Early Life Stages
Other Life Stages
30 Day Mean
NA3
6.5
NA
5.5
7 Day Mean
9.5 (6.5)
NA
6.0
NA
7 Day Mean
Minimum
NA
5.0
NA
4.0
1 Day Minimum4,5
8.0 (5.0)
4.0
5.0
3.0
1 These are water column concentrations recommended to achieve the required intergravel DO concentrations shown in
parentheses. The 3 mg/L differential is discussed in the criteria document. For species that have early life stages exposed
directly to the water column, the figures in parentheses apply.
2 Includes all embryonic and larval stages and all juvenile forms to 30-days following hatching.
3 NA (not applicable).
4 For highly manipulatable discharges, further restrictions apply.
5 All minima should be considered as instantaneous concentrations to be achieved at all times.
Source: USEPA (1986)
3

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Dissolved Oxygen
How do we measure dissolved oxygen?
DO can be measured at discrete points in time
or continuously. Monitoring sensors (continuous
monitoring) aiiow assessment of DO changes
throughout the day and are a cost-effective option for
collecting DO data. DO can be measured using water
quality probes, which often report DO measurements
in both mg/L (milligrams of oxygen in a liter of water)
and percent saturation. Concentrations can vary
greatiy, ranging from 0 mg/L to as high as 12 mg/L or
more. Low DO concentrations are considered hypoxic.
Concentrations below 0.2 mg/L are often considered
anoxic (virtually no oxygen).
DO can vary both horizontally and vertically in a
waterbody. Water samples should, therefore, be taken at
regular increments across a waterbody and at various
depths (or depth integrated, which is a sample that
represents the entire water column). Also, because DO
levels vary throughout the day, an hourly profile can be
informative, if a single sample is to be taken, consider
sampling as early in the morning as possible, when the
DO levels are likely to be at a minimum.
What are the challenges of using dissolved oxygen as a water quality parameter?
Because DO varies naturally throughout the day and
throughout the waterbody, it: can be difficult to determine
if low DO measurements are a true reflection of the
overall DO levels of the waterbody. This can make it
challenging to assess whether the waterbody is attaining
water quality criteria.
EPA 841F21007B | July 2021
SERA

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