DISSOLVED GASES

RABIYA NASIR

DISSOLVED GASES IN SEAWATER

H2O: Universal Polar Solvent

What is seawater?

* Seawater is a solution of about 96.5% water and

* 3.5% dissolved salts. • The most abundant salt in seawater is sodium

chloride (NaCl). • Most elements on Earth are present in seawater. • Because these substances are dissolved, they

are in the form of ions (positive or negative

atoms).

gases

Most gases in the air dissolve readily in seawater at the ocean’s surface

Major gases found in seawater (in order of relative abundance)NitrogenOxygenCarbon dioxide

Important Gases 6 important gases are

dissolved in lakes, streams, seas

Nitrogen Oxygen Carbon dioxide Methane Hydrogen sulfide Ammonia All have important functions,

but differ in behavior, origin

Air Provides Some Gases Atmosphere has enough

nitrogen (78%), oxygen (21%), and carbon dioxide (0.03%) to serve as primary source

Others present only in trace amounts in atmosphere

Other Gas Sources Methane - anaerobic

breakdown of plants/animals

Hydrogen sulfide - chemical/bacterial transformations

Ammonia - breakdown of nitrogenous materials by bacteria, some animals

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

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)

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

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

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

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

CO2 + H2O = H2CO3 Carbonic acid

H2CO3 = HCO3- + H+ bicarbonate

HCO3- = CO3

2- + H+ carbonate

Putting it all together

CO2 + H2O = H2CO3

Sensitive to changes in pH

Low pH - left side dominates

High pH - right side dominates

= HCO3- + H+ = CO3

2- + 2H+

Putting it all together

Addition of CO2 via respiration pushes equilibrium to right and lowers pH

Removal of CO2 via photosynthesis pulls

equilibrium to left and raises pH

Buffer System

CO2 + H2O = H2CO3 = HCO3- + H+ = CO3

2- + 2H+

CaCO3 + H2CO3 = Ca (HCO3)2

Ca (HCO3)2 = CaCO3 + H2O + CO2

Little change in pH despite additions of lots of acids orbase, as long as supply of carbonates & bicarbonatesholds out

Nitrogen Exists in many

different forms in natural freshwater systems

A major nutrient that affects the productivity of aquatic systems

Nitrogen

Dissolved gas - N2

Ammonia - NH3 NH4+

Nitrite - NO2-

Nitrate - NO3-

Dissolved organicsAmino acidsPolypeptidesProteins

Sources: atmosphere, rain, runoff, groundwater **

Losses: water outflow, adsorption to sediments, dinitrification by bacteria

Nitrogen Cycle

Ammonia Readily

assimilated by plants

Nitrification by bacteria

Present in low concentrations in oxygenated waters

Ammonia Accumulates in

hypolimnion No

photosynthesis or nitrification

Release from sediments during anoxia

Nitrate Nitrates high in presence of

oxygen Nitrification Nitrates not assimilated

easily by plants Molybdenum needed to

reduce nitrate Poor abundance in igneous

basins

Nitrate

Denitrification to N2 only by anaerobic bacteria in hypolimnion

Nitrate:ammoniaCalcareous runoff 25:1Igneous runoff 1:1Sewage or fertilizer

1:10

Phosphorus

Total concentrations in unpolluted waters 0.01-0.05 mg/L

Sources:Rainfall (unpolluted <0.03 mg/L) (polluted

>0.1 mg/L)Groundwater ~0.02 mg/LSurface runoff - variable - often major

contributor to lakes (especially with pollutants)

Phosphorus

>90% of P in water is in form of organic phosphates or related materials in living things or their secretions

Great scarcity - limiting factor Rapid turnover of organic P between

living organismsBacteria, phytoplankton, zooplankton, others5-100 minutes, more rapid under deficiency

Phosphorus

In presence of O2, various forms of phosphates form complexes, chelates, and insoluble salts with several metal ionsE.g., calcium and ironInduce precipitation of P in oxygenated

waters

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 boilingPrecipitation of CaCO3

Ca(HCO3)2 = CaCO3 + H2O + CO2

Permanent hardness - sulfates, chlorides, other anions

SALINITY is the total quantity of dissolved inorganic solids in water. You can see here that the most abundant dissolved solids are chloride, sodium, and sulfate.

Salinity

Concentrations of Ca2+ Mg2+ Na+ K+ and HCO3

- CO32- SO4

2- Cl-

Plus other ionized components of other elements