Limnology

1. Chemical characteristics of lake
Dissolved gases – Oxygen, Carbon
dioxide and other dissolved gases

2. 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.

3. • 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.

4. 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.

5. 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

6. 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

7. 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

8. 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.

9. 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

10. • 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.

11. 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.

12. 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.

13. • 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.

14. 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

15. • 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.

16. • v) 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.

17. Dissolved Solids and Dissolved Organic Matter
• All waters in nature contain dissolved solids .Water is
the universal solvent dissolving more different materials
than any other liquid.
• Natural waters come in contact with soluble substances
in many ways such as mere contact with its own basin,
erosion at shore line, wind blown materials, inflow of
surface waters, inflow of seepage and other forms of
subterranean waters and decay of aquatic organisms.
Rain water contains 30 to 40 ppm of dissolved solids.

18. Dissolved inorganic solids
• i) Nitrogen compounds
• ii) Phosphorus compounds
• iii) Sulfur compounds
• iv) Silicon
• Other Elements
• i) Calcium
• ii) Magnesium
• iii) Sodium
• iv) Potassium

19. Dissolved Organic Matter
• Freshwater contains 0.1 to 50 mg dissolved
organic compounds (DOC) per litre. Various free
sugars, amino acids, organic acids, polypeptides
and other substances have been reported.
• There are probably four sources of these
dissolved materials
• 1. Organic compounds of allochthonous origin
• 2. Soluble organic material from the decay of
aquatic organisms
• 3. Extra cellular metabolites excreted by littoral
macrophytes

20. Chemical characteristics of lake
Dissolved gases – Oxygen, Carbon
dioxide and other dissolved gases

21. 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.

22. • 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.

23. 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.

24. 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

25. 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

26. 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

27. 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.

28. 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

29. • 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.

30. 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.

31. 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.

32. • 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.

33. 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

34. • 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.

35. • v) 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.

36. Dissolved Solids and Dissolved Organic Matter
• All waters in nature contain dissolved solids .Water is
the universal solvent dissolving more different materials
than any other liquid.
• Natural waters come in contact with soluble substances
in many ways such as mere contact with its own basin,
erosion at shore line, wind blown materials, inflow of
surface waters, inflow of seepage and other forms of
subterranean waters and decay of aquatic organisms.
Rain water contains 30 to 40 ppm of dissolved solids.

37. Dissolved inorganic solids
• i) Nitrogen compounds
• ii) Phosphorus compounds
• iii) Sulfur compounds
• iv) Silicon
• Other Elements
• i) Calcium
• ii) Magnesium
• iii) Sodium
• iv) Potassium

38. Dissolved Organic Matter
• Freshwater contains 0.1 to 50 mg dissolved
organic compounds (DOC) per litre. Various free
sugars, amino acids, organic acids, polypeptides
and other substances have been reported.
• There are probably four sources of these
dissolved materials
• 1. Organic compounds of allochthonous origin
• 2. Soluble organic material from the decay of
aquatic organisms
• 3. Extra cellular metabolites excreted by littoral
macrophytes

39. Lakes

40. Lakes
• Forel (1982) defined lake as a body of standing
water and occupying basin and lacking continuity
with the sea. He also defined pond as a lake of
small depth, and a swamp has been defined as a
pond of small depth and occupied by rooted
vegetation.
• Carpenter (1928) formulated that the true
difference between lake and pond is depth and
not area accordingly a pond is a quiet body of
water where floating vegetation extends to the
middle of basin in which the biota is very similar

41. Lake Zonation
• The following depth zones are recognized in lakes:
a. littoral zone extends from the shore just above the
influence of waves and spray to a depth where light is barely
sufficient for rooted plants to grow.
b. photic (euphotic) zone is the lighted and usually well-mixed
portion that extends from the lake surface down to where the
light level is 1% of that at the surface.
c. aphotic zone is positioned below the littoral and photic
zones to bottom of the lake, where light levels are too low for
photosynthesis. Respiration occurs at all depths so the aphotic
zone is a region of oxygen consumption. This deep, unlit
region is also known as the profundal zone.

42. d. compensation depth is the depth at which rates of
photosynthesis and respiration are equal.
e. sublittoral zone, which is the deepest area of plant
growth, is a transition between the littoral and
profundal zones.
f. pelagic zone (limnetic zone) is the surface water
layer in offshore areas beyond the influence of the
shoreline.
Boundaries between these zones vary daily and
seasonally with changing solar intensity and
transparency of the water. There is a decrease in
water transparency with algal blooms, sediment

45. Streams and
Rivers
• Streams are zones where a rapid flow of shallow water
produces a shearing stress on the stream bed, resulting in a
rocky or gravel substratum covered by fully oxygenated water.
Streams may vary in size from tiny rivulet to rivers.
• As time goes the stream may develop into river or increase its
size, whereas the size of reservoirs decreases as time passes.

46. Physical conditions of Streams
• The annual change in stream temperature is 10 to 20°C.
• The velocity of stream water varies with the landforms. In
plains, streams are slow and sluggish throughout their length.
In mountain stretches the speed of water may be rapid.
• Extreme of turbidity occur in running water series and
streams with rock beds the turbidity is minimal.
• Stream systems increase their length, width and depth with
increasing age. This is in distinct contrast to the reduction
processes characteristic of all standing water units.

47. Chemical conditions
• The dissolved oxygen supply in uncontaminated stream is high
at all levels often near saturation. The polluted streams show
low dissolved oxygen due to accumulation of organic wastes.
• Current in streams tends to keep the pH in uniform over
considerable distances. It keeps any acidity due to
accumulating free CO2 reduced. Streams waters do not
develop the more intense acidities.

48. RIVER
• River is said to be a natural stream of water usually fresh water
flowing towards an ocean. In some cases river flows into the
ground or dries up completely before reaching another body of
water.
• Usually larger streams are called rivers while smaller streams are
called creeks, brooks, rivulets, rills, and many other terms.
• A river is a component of the hydrological cycle. The water
within a river is generally collected from precipitation, through
surface run off, ground water recharge and release of stored
water in natural reservoirs such as glacier.

49. Planktonic organisms in Inland
Water

50. Planktonic organisms
• The term plankton was first proposed by an
Oceanographer, Victor Hensen in 1887 to designate that
the heterogeneous assemblage of minute organism and
finely divided, non-living materials which are known to
occur in the waters and to float at the will of the waves
and other water movements.

51. Classifications
• Plankton of the various freshwaters differs widely
in quality. The following outline is an abbreviated
indication of the organism which may occur in
the plankton of fresh water.
• 1. Plants :
• a) Algae are represented in the plankton of
inland waters
• b) Fungi, which occur as bacteria abundantly in
the plankton, in fact it is likely that no water in
nature is free from them.

52. • 2. Animals :
• a) Protozoa – These are the representative of the plankton
by many genera and species
• b) Coelenterate - Mainly hydra is facultative plankter
occurring at times free in open water
• c) Rotatoria is the most important groups of zooplankton
• d) Gastrotricha - occur in limited numbers
• e) Bryozoa whose larvae are free swimming
• f) Arthropoda – Mainly i) Crustacea and ii) Insects

53. • 3.Occasional plankters :
• a) Flowering plants for eg. Wolffia (Lemnaceae) occur at
various depth and are recorded as plankton organisms
especially in rivers.
• b) Platyhelminthes eg, Turbellaria are of less importance
in freshwaters.
• c) Coelentrata eg, Medusa (Craspedacusta)
• d) Insecta eg, May-fly nymphs
• e) Arachnida eg, Water mites
• f) Vertebrata eg, Juvenile stages of fishes

54. Classification and Terminology of
Plankton
• Some of the important groups of plankton are :
• 1. Classification based on Quality
• A. Phytoplankton – plant plankton
• a. Phytoplankton proper – chlorophyll bearing
plankton
• b. Saproplankton – bacteria and fungi
• B. Zooplankton – animal plankton

55. • 2. On the basis of Size:
• A. Meroplankton – the large units of plankton, visible to the
unaided eye
• B. Net plankton (Mesoplankton) – plankton secured by the
plankton net equipped with No. 25 silk bolting cloth (mesh
0.03 to 0.04 mm)
• C. Nanoplankton (Microplankton) – very minute plankton
not secured by the plankton net with No 25 bolting cloth
• a) Ultra plankton : <5 µm (<0.005 mm)
• b) Nano plankton : 5-60 µm (0.005 – 0.06 mm) (Dwarf /
runts)
• c) Microplankton : 60-500 µm (0.06-0.5 mm) (Net plankton)
• d) Meso plankton : 500-1000 µm (0.5-1.0 mm)
• e) Macroplankton : 1.0 mm – 1.0 cm
• f) Megaloplankton : > 1.0 cm

56. • 3.On the basis of local environmental distribution:
• A. Limnoplankton : lake plankton
• B.Rheoplankton (Potamoplankton) : running water
plankton
• C. Heleoplankton : pond plankton
• D. Haliplankton : salt water plankton
• E. Hypalmyroplankton : brackish water plankton

57. • 4.Based on Origin
• A. Autogenic plankton : plankton produced
locally
• B. Allogenic plankton : plankton introduced from
other localities
• 5.On the basis of content
• A. Euplankton / True plankton
• B. Pseudoplankton (Flase plankton) : debris
mingled in plankton

58. • 6.Based on the life history / plank tonic life (Length of
time):
• A. Holoplankton / Permanent plankton : organisms free
floating throughout their life
• B. Meroplankton / Temperory plankton : organisms free
floating only at certain times or stages of life cycle
• 7.Based on habitat in water body
• A. Hypoplankton : Benthic
• B. Epiplankton : Surface
• C. Bathyplankton : Aphotic zone
• D. Mesoplankton : Disphotic/lighted zone

59. Aquatic plants- Character, classification,
zonation, seasonal relations

60. Character of larger aquatic plants
• Larger aquatic plants constitute a heterogeneous
group composed of a few Bryophytes and
Pteridophytes and many of the families of
Spermatophytes.
• They are restricted in distribution to the general
vicinity of the shores and to the shallow water
areas.

62. • In general, the aquatic plants can be grouped in
to 3 assemblages:
• 1. Emergent – those rooted at the bottom and
projecting out of the water for part of their
length eg, common species of bulrush, Scirpus
• 2. Floating - these which wholly or in part float in
the surface of water and often do not project
above it eg. Duck weed Lemna and
• 3. Submerged - those which are continuously
submerged eg, Vallisneria.

64. Biological classification of the
larger aquatic plants
• A. Plants roots in the bottom
• 1) Terrestrial plants capable of living at least temporarily
as submerged water plants without any marked
adaptation of leaves to aquatic life eg: Achillea ptarmica,
Nepeta hederacea
• 2) Plants sometimes terrestrial, sometimes with
submerged leaves but different from aerial type and
associated with flowering stage
• 3) Plants which produce 3 types of leaf (a) submerged (b)
floating and (c) aerial

65. Achillea ptarmica Nepeta hederacea

66. • 4) Plants which in certain instances may occur as
land forms but normally submerged and
characterized by a creeping axis bearing long,
branching leafy shoots with no floating laves
• a. Leafy aerial shoots produced at flowering
period eg, Myriophyllum
• b. Inflorescence raised out of water but not
aerial foliage leaves eg. Potamogeton,
Myriophyllum
• c. Inflorescence submerged but organs raised to
the surface eg. Anacharis
• d. Inflorescence entirely submerged eg. Najas

67. Myriophyllum
Potamogeton
Anacharis
Najas

68. • 5) Plants which may occur as land forms but
commonly submerged characterized by an axis
forms with linear leaves arise
• 6) Plants which are entirely submerged having
vegetative either root or shoot naturally attached
to the substratum
• B. a) Plants which are not rooted to the bottom
but live unattached in the water
• 1) Plants with floating leaves, flowers raised into
the air and roots not penetrating the bottom eg.
Spirodella,
• 2) Rootless eg. Wolffia

69. Spirodella, Wolffia
Ceratophyllum

70. Nekton
• The term nekton has been coined by Ernst Heckal
(1890). Nekton is derived from Greek word means
swimming.
• The term nekton is used to designate those
organisms which swim freely in water and possess
locomotion enabling them to have independent
drifting movement along the water flowage system.
• In limnetic regions of inland waters, the nekton is
composed almost entirely of fishes.

71. • The limnetic nekton may inhabit the whole of
open water of a lake and down to its greatest
depth. In contrast to the limnetic region, the
littoral area is the zone of greater nekton
population. In addition to the fishes (young and
mature), numerous free swimming
invertebrates occur mainly insects.
• Major groups of vertebrates including fishes,
Amphibia, Retillia, Aves and Mammalian are
more or less representative of water and some
time free swimming in nature.

72. Benthos
• The term benthos includes all bottom dwelling
organisms, comprising the great assemblages of plants
and animals. Benthos includes the organisms of the
bottom from uppermost water bearing portion of a
body of water right up to the greatest depths.

73. Classification of benthic regions
• 1. Lake zone:
• There are usually 3 zones and in exceptionally in
deep lakes there are 4 zones, they are Littoral,
Sublittoral, Profundal and Abyssal zones.
• i) Littoral Zone : Extends from water’s edge to the
lakeward limit of rooted vegetation.
• ii)Sublittoral Zone : From lakeward limit of vegetation
down to about the level of the upper limit of the
hypolimnion.
• iii) Profundal zone : The entire lake floor that bounds
the hypolimnion
• iv) Abyssal : Present only in lakes of depths greater
than 600 m.

75. Biological productivity of Lake

76. Classification of lakes on the basis of
productivity
• Many attempts have been made to classify lakes on
limnological bases, related in some way or another to
productivity during the past five decades. The combined
contributions of a number of those attempts have been
lead to the classification of lakes into three major types
viz,
• 1. Oligotrophic lakes
• 2. Eutrophic lakes and
• 3. Dystrophic lakes

77. Following Table gives the details of difference
between above listed lakes

78. Further, other difference between Oligotrophic
and Eutrophic lakes is given below

79. Biological productivity of Lake

80. Classification of lakes on the basis of
productivity
• Many attempts have been made to classify lakes on
limnological bases, related in some way or another to
productivity during the past five decades. The combined
contributions of a number of those attempts have been
lead to the classification of lakes into three major types
viz,
• 1. Oligotrophic lakes
• 2. Eutrophic lakes and
• 3. Dystrophic lakes

81. Following Table gives the details of difference
between above listed lakes

82. Further, other difference between Oligotrophic
and Eutrophic lakes is given below

83. Nature of Inland water
environment

84. Physical Characteristics
• Pressure
• Water is a heavy substance. Pure water weighs 62.4 lb (pounds) per cubic
feet at 4°C.
• This is a direct result of density. Since, density changes with differences
in temperature, compression, substances in solution and substances in
suspension; the weight of a cubic foot of natural water is not always the
same.
• The pressure at any subsurface position is the weight of the
superimposed column of water plus the atmospheric pressure at the
surface.
• As depth increases, the pressure in water is rapidly become great, so that
ultimately a crushing effect is imposed upon objects submerged to
considerable depths. This collapse under pressure is called implosion.
• The pressure change in lakes and reservoirs are very small than
compared to sea. In lake, having maximum depth of 100 ft., the pressure
in the deepest region is about 58 lb. per sq. in.

85. Density
• Some of the most remarkable phenomena in
Limnology are dependent upon density relations in
water. The density of water depends on the quantity
of dissolved substances, the temperature and the
pressure. With increasing amounts of dissolved
solids the density increases in a roughly linear
fashion.
• The quantity of dissolved solids for inland waters is
usually below 1 g / l, except, for mineral waters
(springs) inland salt water bodies, and water bodies
subjected to marine influence.
• The density difference due to chemical factors is not
more than 0.85 g /l and the density differences

86. • i) Variations due to pressure
• Water at the surface, subject to a pressure of only 1
atmosphere, is considered as having a density of unity (1.0); at a
pressure of 10 atmospheres, the density is about 1.0005; at 20
atmospheres, the density is about 1.001; and at 30
atmospheres, it is about 1.0015.
• ii) Variations due to Temperature
• Pure water forms ice at 0°C, and steam at 100°C, but there is
change in the density of the liquid due to temperature. Water
possesses the unique quality of having its maximum density at
4°C and it becomes less dense when the temperature decreases
from 4°C to freezing point. Density of water will be less during
summer and it will be high during winter. Sea water becomes
heavier at 0°C. The temperature of maximum density of sea
water is 0°C, where as for fresh water it is 4°C.

87. Mobility (Viscosity)
• Water is an exceedingly mobile liquid. Nevertheless, it
has internal friction (viscosity). This viscosity varies with
the temperature. Water is distinctly more mobile at
ordinary summer temperatures than that are just before
it freezes.
• The viscosity changes with temperature. The response of
water to wind of fixed velocity would differ with different
temperature of the water. Pressure does not cause any
significant change in viscosity.

88. Temperature
• Temperature is one of the most important factors in an aquatic
environment. In fact, it is possible that no other single factor
has so many profound influences and so many direct and
indirect effects.
• Diurnal and seasonal variations are very much common in
freshwater environments than in marine environment. A
diurnal variation range of 4.8 to 5°C has been recorded in a
tropical pond with an average depth of 3.0 m. In shallow water
bodies within an average depth of 1.5 m, the lowest night
temperature was 26.6°C. The highest day time temperature
was 32°C with a variation of 5.4°C. In flowing water bodies like
streams and rivers there is no such wide fluctuations in
temperature.
• Lentic waters of lakes and ponds undergo thermal stratification
phenomenon according to seasons. Thermal stratification has
been reported most frequently in the lakes of tropical countries
such as Java, Sumatra and India.

89. • According to temperature relations lakes have been classified into three types:
• 1)Tropical lakes : In which surface temperature are always above 4°C.
• 2)Temperate lakes : In which surface temperature vary above and below 4°C.
• 3)Polar lakes : In which surface temperature never goes above 4°C.
• Decrease in temperature cause reduction in metabolism resulting in lower rate
of food consumption. Extreme higher or lower temperature has lethal effects
on the aquatic organisms.
• Fluctuation in temperature of water regulates the breeding periods, gonodal
activation and thermal induced migration. On the basis of their ability to
tolerate thermal variations, most fresh water organisms are classified into
stenotherm and eurytherm.
• Stenothermic are the organisms with a narrow range of temperature tolerance
while the eurythermic are those organisms with a wide range of temperature
tolerance.

90. Thermal stratification
• In tropical lake, heat intake at the surface leads to
the formation of a vertical temperature gradient,
within which the thermal resistance become too
great for the existing winds to continue mixing the
whole water masses. The upper warmer layer is
called epilimnion and the lower cooler layer is
called hypolimnion. In between the two distinct
portions, a layer called thermocline.
• Summer stratification
• In summer, there are three distinct layers are

91. • Epilimnion
• a) It is upper layer of water.
• b) It is warmer layer.
• c) The temperature of this layer fluctuates with the
temperature of the atmosphere. It will be about 27°C
to 21°C.
• Hypolimnion
• a) It is the bottom layer of water.
• b) At this layer, water will be cool.
• c) The temp is between 5°C and 7°C.
• d) It is a stagnant column of water.

92. • Thermocline (metalimnion):
• a) It is the middle layer.
• b) The temperature is in between the temp of the
upper layer and that of the lower layer.
• c) It is characterized by a gradation of temperature
from top to bottom.
• d) It is also called transition zone.
• In deeper lakes, a seasonal, thermal phenomenon
occur which is so profound and so far reaching in its
influence that it forms, directly and indirectly the
substructure upon which the whole biological
framework rests, particularly in the temperature zone.