Oksigen

HENRY’S LAW & GAS SOLUBILITY ‘At a constant T, the amount of gas absorbed by a given volume of liquid is proportional to the pressure in atmospheres that the gas exert’ CαP c=Kxp c: the concentration of gas that is absorbed (moles @ mg/L @ ml/L) p: the partial pressure that the gas exerts K: solubility factor Factors affecting gas solubility: xv.Altitude altitude

Pressure (p)

solubility

• each 100m rise above sea level, atmospheric pressure decreases by 8 to 9 mm Hg

ii. Temperature With p held constant: P

Temperature

solubility

iii. Salinity P

salinity

solubility

Sources of gas in aquatic system:

Gas

Process

CO2

Respiration, decomposition

O2

Photosynthesis

CH4 (methane)

Anaerobic decomposition

H2S

Chemical & bacterial mineralization

NH3 (ammonia)

Heterotrophic bacterial decomposition

NH4+, NH4OH

Excretion

OXYGEN The importance of dissolved oxygen (DO): • essential to the metabolism of aquatic organisms that possess aerobic respiratory biochemistry • the dynamics of O2 distribution are basic to the understanding of the distribution, behaviour & physiological growth of aquatic organisms • the distribution of O2 affects the solubility of

many inorganic nutrients • the variations in DO of lakes & rivers are a good measure of their trophic states

Source of oxygen: 2. Atmosphere The addition of atmospheric O2 to a lake involves 2 processes: •

Suitable gradient of partial pressure differences of O2 b/ween the atmosphere & the water

•

Turbulence (wind) carries the absorbed O2 to lower levels

2. Photosynthesis

1600 1200 800 400 0 0800

1200

1600

Time of the day

2000

O2 produced (mgO2/m2/h)

C6H12O6 + 6O2

6CO2 + 6H2O

Loss of O2: • respiration • bacterial aerobic decomposition • chemical oxidation • erosion & gas bubbles from the sediments remove O2 • the warming of a summer epilimnion 1º Production measured by the oxygen method The light-dark bottle technique 3 bottles: IB: Initial bottle LB: Light bottle DB: Dark bottle LB – IB = net gain in O2 (net 1º production) DB: purely respiratory (O2 remaining in the dark bottle after a period of total respiration)

IB – DB = R (respiration in both bottles) Net production + respiration = gross production (GP) (LB – IB) + (IB – DB) = GP GP = LB – DB GP: the total of O2 produced by photosynthesis

In polluted waters, long incubation will lead to: DB:

respiration

anaerobic

The disadvantages of light-dark bottles: • misconception of net production; respiration in DB • photorespiration in LB - favored by high light intensities & low CO2 - oxidation of glycolate (photosynthate) • the assumption that the R derived from DB is the same as that in LB Diel variations in O2 • eutrophic lake: - below-oxygen saturation in early morning - supersaturation in late afternoon/midday -drop to zero during the night • the more productive the environment – the greater the fluctuation in concentration of O2 • large variation of O2 concentration also occur near shore (aquatic plants) • O2 deficit by decaying organic material

Aerator used to increase the dissolved oxygen (DO) level in aquaculture pond

DISTRIBUTION OF DISSOLVED OXYGEN IN LAKES •

4 general types of O2 distribution in thermally stratified lakes:

iii. Orthograde O2 profile •

Oligotrophic lakes

•

O2 in the hypolimnion remain saturated from the period of spring turnover Summer stratification

Depth

0

4

8

12

O2(mg/l)

ø O2

0

10

20

30

T(°C)

Winter stratification 4

Depth

0

12

ø

0

10

O2(mg/L)

O2

20

30

T(°C)

• O2 saturation in the hipolimnion • in ultra-oligotrophic lakes with minimal biotic influence • in the Arctic & Antarctic • rare occurrence in dimictic lakes

ii. Clinograde O2 profile Summer stratification 0

4

8

12

O2(mg/L) Depth

O2

ø 0

10

20

30

T(°C)

• eutrophic lakes • oxidative processes at the hypolimnion • O2 at the hypolimnion becomes undersaturated • hypolimnion is anaerobic • bacterial respiration in decomposition of sedimenting organic matter • large, deep lakes: bacterial respiration of organic matter of phytoplanktonic origin dominate • shallow lake: benthic decomposition dominate • lakes high in humic organic compounds – chemical oxidation or photochemical oxidation by ultraviolet light

Winter stratification 4

Depth

0

12

O2

ø 0

O2(mg/L)

10

T(°C)

30

20

• respiratory utilization & chemical oxidation increase with depth at slower rate Spring/fall turnover 4

Depth

0

12

ø 0

10

O2(mg/L)

O2

20

30

T(°C)

Variations in oxygen distribution iii. Positive heterograde O2 profile 0

4

8

12

O2(mg/L)

depth

ø

Epilimnion

O2

Hipolimnion

0

10

20

30

T(°C)

• O2 in metalimnion increase during stratification – positively correlated with water transparency • blue-green algae (Oscillatoria); major contributors • lake with stable stratification – high relative depth • O2 in the littoral dissipate (disperse)into the metalimnion

iv. Negative heterograde O2 profile 0

4

8

12

Depth

ø

O2(mg/L) Epilimnion Metalimnion

Hipolimnion

O2 0

10

20

30

T(°C)

• a metalimnetic O2 minimum • the sinking rate of organic matter is slow at metalimnion - decompose • decomposition rate higher at metalimnion • respiratory consumption of O2 by zooplankton in the metalimnion

OXYGEN DEFICIT • the difference in amount of O2 present at the beginning & at the end of stratification below a given depth • the amount of O2 needed to reach saturation minus the amount of O2 present • indicates the relationship of the metabolism in the trophogenic zone and that in the tropholytic zone Trophogenic zone Organic matter tropholytic zone

Utilization of hypolimnetic O2

• provides an indirect estimate of the productivity of the lake