Dissolved Oxygen

Dissolved Gases

Dr. Susanna Nurdjaman

Program Studi Oseanografi

Fakultas Ilmu dan Teknologi Kebumian

Why are gases of interest?

• Highly mobile chemicals, move into and out of the
ocean via the atmosphere and through different
compartments within the ocean •
Air-Sea exchange of gases is important for climate and
atmospheric chemistry
• Participate in many important biological reactions: •
Photosynthesis/respiration
• Nitrogen fixation/denitrification
• Tracers of water mass movement and mixing If
thermohaline circulation were to stop, the deep ocean
waters would lose their connection to the atmosphere
and they would go anoxic on time scale of ~1000 years

Chemical Oceanography

I. Gas Atmosferik

Gas

N2
O2
CO2
Ar

Ne

He

Kr

Partial pressure

0.7808

0.2095

0.00033

0.00934

1.82x10-5

5.24x10-6

1.14x10-6

(atm)

Gas
Partial pressure

Xe
8.7x10-8

H2
5x10-7

CH4
2x10-6

N2O
5x10-7

(atm)

Berlimpah : nitrogen (78 %), oksigen (21 %) dan argon (1 %)

Non-variabel dan variabel (uap air, gas by human activities (NO2, CO dan NH3))

Proporsi gas non variabel di atmosfer (Glueckauf,1951)

Other Gas Sources

➢ Methane - anaerobic

breakdown of
plants/animals

➢ Hydrogen sulfide -

chemical/bacterial
transformations

➢ Ammonia -

breakdown of
nitrogenous materials
by bacteria, some
animals

Fugacity

-This term is analogous to that of activity
for dissolved solids.

-

A fugacity coefficient is similar to an activity
coefficient.

-

For most work in surface seawater fugacity is very
close (within a few percent) to partial pressure.

-At high pressures, gases do not behave ideally, and
thus fugacities must be used.

How much gas is dissolved in

water at any given time?

➢ Dependent on several

factors:

➢ Solubility factor

➢ Pressure

➢ Temperature

➢ Salinity

Solubility Factor

➢ Not all gases

dissolve in water to
same extent

➢ Some gases

dissolve very easily
in water, some
dissolve very little

Pressure (atmosphere)

➢ Amount of gas

absorbed by water is
proportional to its
partial pressure in the
atmosphere (conc. =
solubility factor X
partial pressure)

➢ Altitude decreases

saturation level by
~1.4% per 100 m

Temperature

➢ Solubility of gas in water decreases as temperature

rises

➢ Generalization -

cold water can hold more gas in

solution than warm water

➢ Nearly linear relationship within normal range of

natural water temperatures

Salinity

➢ Presence of various

minerals in solution
lowers the solubility of
gases

➢ Generally disregarded

in limnology because
freshwaters have
salinity near zero

Salinity

➢ Oceans (salinity of

3.5%) have reduced
gas saturation values
of ~18-20%

➢ Saline pools/lakes

can have much higher
salinities (5-6 X ocean
values)

➢ Important

consideration here for
gas solubilities

Relative Saturation

➢ Relation between existing solubility (amount of

gas present) and the equilibrium content
expected at same temperature and partial
pressure

➢ Can be less, or more (supersaturation)

Nitrogen

➢ Exists in many

different forms in
natural freshwater
systems

➢ A major nutrient that

affects the
productivity of aquatic
systems

Nitrogen

4

➢ Dissolved gas - N2

➢ Ammonia - NH3 NH+

2

➢ Nitrite - NO-

➢ Nitrate - NO-

3

➢ Dissolved organics

⚫ Amino acids

⚫ Polypeptides

⚫ Proteins

➢ Sources: atmosphere,

rain, runoff,
groundwater **

➢ Losses: water

outflow, adsorption to
sediments,
dinitrification by
bacteria

Oxygen

➢ Abundant and dissolves readily in water

➢ Needed for respiration by organisms and for

complete breakdown of organic matter

➢ Relatively easy to measure

Oxygen

➢ 1/4 as abundant as nitrogen in atmosphere, but

twice as soluble

➢ Solubility of oxygen increases as temp.

decreases, salinity decreases, and pressure
increases

Oxygen

➢ Two sources for

oxygen in lakes

➢ Atmosphere

➢ Photosynthesis

Atmosphere

➢ Diffusion across air-water

interface and down into
water column

➢ Years to reach depth of 5

m

➢ Wind-driven waves and

currents distribute oxygen
to lower levels

➢ Too much agitation can

prevent water from
becoming supersaturated

Photosynthesis

➢ Most oxygen in

standing waters is by-
product of
photosynthesis

➢ Phytoplankton

contribute most

➢ Rooted macrophytes,

attached algae,
benthic algae mats
are chief producers in
shallow lakes, lake
margins

Loss of Oxygen

➢Physical - change in temperature,

pressure

➢Biological - most important -

respiration by plants, animals,
bacteria (decay processes)

➢Other - methane bubbles rising from

sediments through water column

Dissolved Oxygen

Oxygen Distribution

➢Distribution changes as lake goes

through seasonal temperature cycle

➢Orthograde distribution during spring,

fall turnovers in dimictic lake

➢Clinograde distribution during thermal

stratification

Another distribution

➢ Extreme clinograde - permanently

meromictic lakes, anaerobic hypolimnion

Daily, seasonal variation in

oxygen concentrations

➢ The more plant material in a lake or pond,

the more prone that system is to both daily
and seasonal variations in dissolved
oxygen content

Daily variation in oxygen

concentrations

➢ O2 rises during day, declines at night

➢ The greater the plant biomass, the greater

the magnitude of the cycle

Daily variation in oxygen

concentrations

Seasonal variation in oxygen

concentrations

➢ O2 high during summer growing season,

low in late-summer when plants die

➢ May produce anoxia and die-offs of

animals (summerkill)

Seasonal variation in oxygen

concentrations

➢ O2 also may be low during winter in ice-

covered lakes

➢ Reduced light transmission, respiration

only - Winterkill of animals

Carbon Dioxide

➢ CO2 increasing in concentration in

atmosphere

➢ High solubility - 200 X > O2

➢ Follows solubility laws (pressure, temp.)

➢ Many sources other than atmosphere:

rainwater, runoff, groundwater, respiration,
decomposition in sediments

Carbon Dioxide

➢ CO2 behaves much differently than other

gases once it dissolves in water

➢ Exists in equilibrium with many additional

forms of carbon

Confusing, interrelated terms

➢ Alkalinity

➢ Hardness

➢ Salinity

Alkalinity

➢ Measure of buffering capacity of water

➢ Carbonates and bicarbonates of alkali

metals

Hardness

➢ Calcium and magnesium salt content

➢ Temporary hardness - carbonates and

bicarbonates, can be removed by boiling

⚫ Precipitation of CaCO3

⚫ Ca(HCO3)2 = CaCO3 + H2O + CO2

➢ Permanent hardness - sulfates, chlorides,

other anions