Limnology (2+1)

Unit 4- Chemical characteristics

4.1. Dissolved gases – Oxygen, Carbon dioxide and other dissolved gases
Dissolved gases
No naturally occurring body of water is free of dissolved gases. Their spatial and temporal distribution is dependent on factors such as precipitation, inflow and outflow, physical factors like temperature, movement of water and chemical factors such as solution processes, combination and precipitation of reactions, complex formation etc.
Among the dissolved gases present in water, oxygen and carbon dioxide are direct indicators of biological activity of water bodies. Gaseous nitrogen only enters the metabolic cycle of a few specific microorganisms. Hydrogen sulphide and methane occur in small localized amounts due to bacterial activity under conditions of low redox potential and are incorporated into the material budget of water bodies by certain bacteria.
The Liebig’s law of minimum states that the yield is dependent on whatever growth factor is at a minimum in proportion to all the other similar factors.
Solubility of Gases in water
The solubility of gases in water decreases with increasing temperature and decrease of pressure. When a gas comes in contact with water, it dissolves in it until a state of equilibrium is reached in which the solution and the emission of the gas are balanced. Total solubility of gas is expressed by Henry’s law. The concentration of a saturated solution of gas is proportional to the pressure at which the gas is supplied.
Condition affecting the solubility of gases in water
Solubility of gases differs widely even when their pressures are equal. It is therefore necessary to find out the solubility constants.
Henry’s law is stated as :
C= K p
Where, C = Concentration of gas in solution
p = Partial pressure of gas
K= Constant of solubility
The following general conditions affect the solubility of a gas:
i. Rise in temperature reduces solubility
ii. Increasing concentration of dissolved salts diminishes solubility
iii. Rate of solubility is greater when the gases are dry than when they contain water vapour
iv. Rate of solubility is increased by wave action and other forms of surface water agitation

A. Oxygen
The main sources of dissolved oxygen in water are:
i) The atmosphere and
ii) By photosynthetic activity of aquatic plants
Atmospheric oxygen enters the aquatic system:
a) By direct diffusion at the surface and
b) Through various forms of surface water agitations such as wave action, waterfalls, and turbulences due to obstructions.
Aquatic chlorophyll bearing plants release oxygen as a byproduct of photosynthesis, which gets distributed into the different layers of lake water by movements. In most lakes the phytoplankton contributes the bulk of the oxygen supply because of the huge amounts of chlorophyll of algae in the epilimnion zone. In shallow waters like ponds and swamps the limnetic photoautotroph may be overshadowed by littoral macrophytes, attached algae, and the benthic algal mats. In small rivulets and brooks the periphyton account for most of the production of oxygen.
The main causes of decrease of oxygen in water are:
i. Respiration of animals and plants throughout the day and night and
ii. Decomposition of organic matter – Aerobic bacteria use up of the oxygen of water while decomposing organic matter. Chemical oxidation of sediments also takes place. Purely chemical oxidation may also occur, but most of the oxidative processes in aquatic habitats are probably mediated through bacterial action.
iii. Reduction due to other gases – A gas may be entirely removed from solution by bubbling another gas through the water in which it is dissolved. In nature, gases like CO₂, methane and hydrogen sulphide often accumulate in large amounts and the excess amounts rise in the form of bubbles removing the dissolved oxygen.
iv. By physical process – In summer days the heat warms up the epilimnion zone of the lake, which could account for oxygen depletion of water. The combined effects of all or some of the above mentioned processes may completely deplete oxygen content of the system.
Diel oxygen changes in freshwaters
The concentration of oxygen in an aquatic environment is a function of biological processes such as photosynthesis and respiration and physical processes such as water movement and temperature. Diel variations occur in both day and night hours. Estimates of diel production can be made in natural waters by considering night as the dark bottle and day as the clear bottle. The increase in oxygen from dawn to dusk reflects net primary productivity. The decrease from dusk until dawn represents half the diel respiration. Adding the oxygen that disappeared at night to the day time gain gives a sum that is daily gross primary productivity.

B. Carbon dioxide
i) Sources of carbon dioxide in freshwater
The atmospheric carbon dioxide mixes with the water when it comes in contact with the water surface, as it possesses the highest solubility in water. As the partial pressure of carbon dioxide in air is low, the amount which remains in solution in water at a given temperature is also low.
1. Rainwater and inflowing ground water
Rainwater is charged with 0.55 to 0.60 mg/I CO₂ as it falls towards earth. Water trickling through organic soil may become further charged with CO₂.
2. Byproduct of Decomposing Organic Matter (DOM)
Carbon dioxide is added to the water as a byproduct of decomposing organic matter which is a common phenomenon in natural waters. Large quantities of the gas are produced in this way. It is found that carbon dioxide is the second largest decomposition product, constituting 3 to 30 per cent of the total gas evolved.
3. Respiration of Animals and Plants
Respiratory processes produce and release carbon dioxide into the water. The quantities so added are governed by the magnitude of aquatic flora and fauna, the relative size of the individual organism and those factors which determine the rate of respiration.
ii) Reduction of carbon dioxide in freshwaters
The principal processes which tend to reduce the carbon dioxide supply are;
1. Photosynthesis of aquatic plants
Consumption of free CO₂ in photosynthesis depends upon amount of green plants which the water supports, duration of effective day light, transparency of water and the time of year.
Marl forming organisms
The following groups of aquatic organisms are known to form marl (=Crumble : large deposits of calcium and magnesium carbonate) in water bodies; aquatic flowering plants like Potamogeton, Ceratophyllum, Nymphaea, Vallisneria; many blue-green algae like Rivularia, Lyngbya nana, Lyngbya martesiana, Colacacia. Centrosphaeria facciolaea; many species of diatoms; mollusks which form calcareous shells; insects like Diptera larvae; the cray fishes and lime-forming bacteria. All these organisms function in the production of the insoluble carbonates which involves carbon dioxide, calcium and magnesium. Thus the process of lime formation binds up carbon dioxide supplied from circulation and removes the available calcium and magnesium from the system.
Agitation of water
Agitation is a very effective method of releasing free carbon dioxide from water. It is evident from the fact that sometimes when deeper layers of water has large amount of it, the surface water shows very little carbon dioxide.
Evaporation
Evaporation of waters containing bicarbonates results in the loss of half-bound carbon dioxide and precipitation of mono carbonate. The form of loss is greatest in shallow water bodies where evaporation is most effective.
Rise of bubbles from depths
Free carbon dioxide often accumulates in decomposing bottom deposit in such quantities that at frequent intervals increasing internal pressure of gas exceeds the external pressure and the excess gas rises in the form of masses of bubbles to the surface and is lost into the air.

Other dissolved gases
i) Methane
Methane, sometimes called marsh gas, is one of the products of decomposing organic matter at the bottoms of marshes, ponds, rice field and lakes. The methane bacteria are obligate anaerobes. They decompose organic compounds with the production of methane (CH₄) through reduction of either organic or carbonate carbon. Conditions favorable for production of methane appear at about the time the dissolved oxygen content is exhausted. This is because methane (CH₄), a compound of carbon and hydrogen burns in oxygen forming oxides of carbon and hydrogen ie, carbon dioxide and water.

It has been found that large quantities of methane are produced in marshes and eutrophicated lakes during summer time.
ii) Hydrogen Sulphide
Hydrogen sulphide dissolves very rapidly in water and is thus not dissipated like methane. The bottom water of stratified eutrophic lakes may contain appreciable quantities of the very soluble gas H₂S. This is especially marked in lakes of regions of high edaphic sulfate. The reduction of sulfate to sulfide is a phenomenon largely associated with anaerobic sediments. H₂S is poisonous to aerobic organisms because it inactivates the enzyme cytochrome oxidase.
iii) Nitrogen
Nitrogen has a low solubility in water. It is such an inert gas that the quantities which occur in lake water are not changed by the chemical and biological processes. The atmosphere usually supplies the greater amounts of nitrogen found in water. The minimum amount occurs in winter, since it is more soluble in cool water.
iv) Ammonia
Ammonia occurs in small amounts in unmodified natural waters. It is exceedingly soluble, 1 volume of water dissolving 1,300 volume of ammonia at 0° C. In lakes, it is the result of the decomposition of organic matter at the bottom. In summer, free ammonia ordinarily increases with depth.
v) Sulphur dioxide
Traces of sulphur dioxide may occur in natural waters.
vi) Hydrogen
Liberation of hydrogen in the anaerobic decomposition of lake bottom deposits seems likely. But, the amount so formed is small.
vii) Carbon Monoxide
Carbon monoxide may occur in the bottom of the hypolimnion in small amount.