Dissolved Oxygen

The Good GasThe Good Gas

Photosynthesis: Your one-stop shop for all of your oxygen needs!

Carbon Dioxide

(from air)

Water (from ground)

Oxygen

(to air)Carbohydrate (plant material)

Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2

Happy Rays of Sunshine

DO: A Soluble Gas

H2O

H2O

H2O

H2O

H2O

O2 H2O

H2O

H2OO2

O2

O2

O2

O2

H2OO2

H2O

                             

Hydrilla

Coontail

Parrotfeather

Submerged Aquatic Vegetation (SAV’s)

Phytoplankton (single cell plants)

0

0.5

1

1.5

2

2.5

3

DO

Co

nc

en

tra

tio

n (

mg

/L)

Hypoxic

Anoxic

Normoxic

Habitat Classification Based on DO Concentration

0 – 0.2 mg/L

0.2 – 2 mg/L

> 2 mg/L

Respiration: How we, the animals, use oxygen (and plants too!)

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (in form of ATP)

Carbohydrate + Oxygen Carbon Dioxide + Water + Energy

Oxygen in

Carbon Dioxide out

Previous Meal

Carbohydrate in

Energy out

Water out (later)

CO2: A Soluble Gas Too!

H2O

H2O

H2O

H2O

O2 H2O

H2O

H2OO2

O2

O2

O2

O2

H2OO2

H2O

CO2

CO2CO2

CO2

CO2

CO2

CO2

C6H12O6

Solar Energy

Heat Energy

Biomass (g/m2/yr)

O2

Chemical Energy (ATP)

Photosynthesis Respiration

Biological Production and Consumption of Oxygen

Daily Aquatic Oxygen Cycle

0

1

2

3

4

Sunrise Noon SunsetMidnight Midnight

Dis

solv

ed O

xyg

en

Sunshine MoonshineMoonshine

∞

Day∞

Night

Photosynthesis

Respiration

∞

Photosynthesis

Respiration

∞Low

RespirationHigh

Respiration

∞ ∞High

PhotosynthesisLow

Photosynthesis

0

1

2

3

4

Sunrise Noon SunsetMidnight Midnight

CO2+H2OH2CO3H++HCO3-2H++CO3

-

An increase in CO2 causes an increase in H+ pH

Sunshine MoonshineMoonshine

pH

Lev

el

5

10

6

9

7

8

7

7

7

7

Water Flow

Blood Flow

Counter Current Flow

5

6

6

7

7

8

8

9

9

10

Blood Flow

Water Flow

5

6

6

7

7

8

8

9

9

10

Blood Flow

Water Flow

Hi DO

1

2

2

2

2

2

2

2

2

2

Blood Flow

Water Flow

Low DO

5

6

6

6

6

6

6

6

6

6

Blood Flow

Water Flow

Med DO

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!!

Abiotic Factors That Affect DO Concentration

• Temperature

• Water Clarity

• Current Velocity (Flow)

• Wind

• Cloud Cover

Temperature• The warmer water is, the less DO it can

hold– Think about opening a coke bottle after it sat a

few hours on the dash of your car in August.

•Excess DO evaporates into the atmosphere!

100% DO Saturation

02468

101214161820

0 5 10 15 20 25 30 35

Temperature (C)

100

% S

atu

rati

on

Lav

el

Oxygen > 100% Saturation

Oxygen < 100% Saturation

Oxygen diffuses out of water column

Oxygen diffuses into water column

Water Column

Atmosphere

Oxygen Can Diffuse Out of or Into the Water Column

Wind

Stirs in atmospheric oxygen

Current Velocity• The faster water flows, the more atmospheric

oxygen is mixed into the water.

Water Clarity

Pure water

Secchi Disk (cm)

Oceanic seawater

Cloud Cover• Clouds decrease the amount of sunlight reaching

aquatic plants, thus oxygen production is reduced.

Why Measure DO?

• Indicator of primary production (growth of plant material)– More plants = more animals that eat plants =

more animals that eat animals that eat plants = happy Cajuns!

• Most aquatic animals can not live in water without sufficient oxygen

Measuring DO• Winkler method – a titration method

– Not very practical in the field– Great to calibrate probes

• Electronic Probes – easy to use, but needs to be calibrated on a regular basis– Usually accurate to 0.2 mg/L

YSI 550A DO Meter w/12' cable

Putting It All Together

• Most aquatic DO is biologically produced through photosynthesis

• Respiration / Decomposition rates can affect DO concentration

• DO concentration typically cycles with the photoperiod

• Temperature, Wind, Current Velocity, Water Clarity, and Cloud Cover can indirectly affect oxygen concentration

Finally!!• A single DO measurement provides little information

• Regular monitoring provides a better picture of the health of a waterway – based on oxygen.