29
For the sake of description, the pelagic
compartment could be further subdivided into
the ‘planktonic’ community (which is generally suspended in water) and ‘nektonic’ community of assemblage (which are larger and
more motile organisms which could swim
through water). Hutchinson ( 1967 ) pointed out
that in place of ‘planktonic’ and ‘nektonic’, the
words ‘planktic’ and ‘nektic’ are etymologically more correct. However, ‘planktonic’ and
‘nektonic’ are hallowed by decades of use.
Most of the aquatic systems support all the
four communities (namely, plankton, nekton,
benthos and fringing). However, their relative
importance varies widely. Some habitats may
lack one, or more rarely two, compartments. A
true planktonic community may be absent in
fast-fl owing rivers.
2.4
Stratifi cation and Zones
2.4.1 Stratifi cation
Stratifi cation in a water body takes place when
stable density differences are generated. This
could be due to surface heating with the establishment of a thermocline. It could sometimes be
also due to differences in salinity of the participating water masses with the formation of a halocline. However, such stratifi cation provides a
barrier to nutrient circulation. Wind is considered
as one of the major agencies promoting mixing
within the water body. However, wind-induced
turbulence may have insuffi cient power to penetrate a deep and marked thermocline. Thus, there
may be a stagnant and deoxygenated lower water
mass. On the other hand, the constantly high
energy input from the sun, in tropical and subtropical areas, creates a warm surface layer which
permanently fl oats upon colder water at greater
depth. However, in temperate regions, this stratifi cation is a seasonal phenomenon and occurs
only during the warmer months of the year.
Further, a thermocline greatly reduces mixing
between the productive surface layers (which is
the region of nutrient fi xation) and the zones
below, in which most of the decomposition
occurs (which is the region of re-mineralisation).
Thus, oxygen in the bottom waters may not be
replenished once a thermocline is established.
Oxygen is mostly used up in the microbial
decomposition of organic matter, and it may be
totally exhausted, if the thermocline persists for
long enough. During deoxygenation, the oxygen
donors are the nitrates at fi rst, and then the sulphates. However, there is liberation of toxic H 2 S
when the latter is used. Anoxia may, thus, be a
permanent feature of the bottom waters of deep
lakes. In such situations, life below the thermocline is, thus, generally restricted to anaerobes
equivalent to those living in the reduced layers of
all benthic sediments.
When the epilimnion of a lentic body begins
to warm up, heat takes quite a long time to diffuse
down through the water. This is so, particularly, if
the weather is calm and the water is not starred by
strong winds. When a lentic body is divided into
an upper warmer epilimnion and a lower colder
hypolimnion, it is said to be ‘stratifi ed’.
2.4.2 Chemical Characteristics
(Water Chemistry)
It is, perhaps, a fact that none of the wetlands and
lakes contain pure water. It is mainly because all
the water in a lentic body generally accumulate
after running overall through the rocks and soil of
its catchment area, apart from rain or snow falling on its surface. The water takes up, en route,
chemicals into solution. These chemicals are
often called salts which are ionisable. Anions and
cations carry electric charges. Water containing
them can conduct electricity. Ability to conduct
gives a measure of total quantity of charged particles (ions) dissolved in it. This is often referred
to as conductivity of the water.
Generally, the lakes (particularly the deep
ones) remain vertically stratifi ed in relation to
light intensity, wavelength absorption, hydrostatic pressure, temperature, etc. For example, in
a lake, there are three to fi ve well-recognised
horizontal strata, namely,
(a) Littoral zone formed by the shallow water
near the shore. It contains upper warm and
2.4 Stratifi cation and Zones
For the sake of description, the pelagic
compartment could be further subdivided into
the ‘planktonic’ community (which is generally suspended in water) and ‘nektonic’ community of assemblage (which are larger and
more motile organisms which could swim
through water). Hutchinson ( 1967 ) pointed out
that in place of ‘planktonic’ and ‘nektonic’, the
words ‘planktic’ and ‘nektic’ are etymologically more correct. However, ‘planktonic’ and
‘nektonic’ are hallowed by decades of use.
Most of the aquatic systems support all the
four communities (namely, plankton, nekton,
benthos and fringing). However, their relative
importance varies widely. Some habitats may
lack one, or more rarely two, compartments. A
true planktonic community may be absent in
fast-fl owing rivers.
2.4
Stratifi cation and Zones
2.4.1 Stratifi cation
Stratifi cation in a water body takes place when
stable density differences are generated. This
could be due to surface heating with the establishment of a thermocline. It could sometimes be
also due to differences in salinity of the participating water masses with the formation of a halocline. However, such stratifi cation provides a
barrier to nutrient circulation. Wind is considered
as one of the major agencies promoting mixing
within the water body. However, wind-induced
turbulence may have insuffi cient power to penetrate a deep and marked thermocline. Thus, there
may be a stagnant and deoxygenated lower water
mass. On the other hand, the constantly high
energy input from the sun, in tropical and subtropical areas, creates a warm surface layer which
permanently fl oats upon colder water at greater
depth. However, in temperate regions, this stratifi cation is a seasonal phenomenon and occurs
only during the warmer months of the year.
Further, a thermocline greatly reduces mixing
between the productive surface layers (which is
the region of nutrient fi xation) and the zones
below, in which most of the decomposition
occurs (which is the region of re-mineralisation).
Thus, oxygen in the bottom waters may not be
replenished once a thermocline is established.
Oxygen is mostly used up in the microbial
decomposition of organic matter, and it may be
totally exhausted, if the thermocline persists for
long enough. During deoxygenation, the oxygen
donors are the nitrates at fi rst, and then the sulphates. However, there is liberation of toxic H 2 S
when the latter is used. Anoxia may, thus, be a
permanent feature of the bottom waters of deep
lakes. In such situations, life below the thermocline is, thus, generally restricted to anaerobes
equivalent to those living in the reduced layers of
all benthic sediments.
When the epilimnion of a lentic body begins
to warm up, heat takes quite a long time to diffuse
down through the water. This is so, particularly, if
the weather is calm and the water is not starred by
strong winds. When a lentic body is divided into
an upper warmer epilimnion and a lower colder
hypolimnion, it is said to be ‘stratifi ed’.
2.4.2 Chemical Characteristics
(Water Chemistry)
It is, perhaps, a fact that none of the wetlands and
lakes contain pure water. It is mainly because all
the water in a lentic body generally accumulate
after running overall through the rocks and soil of
its catchment area, apart from rain or snow falling on its surface. The water takes up, en route,
chemicals into solution. These chemicals are
often called salts which are ionisable. Anions and
cations carry electric charges. Water containing
them can conduct electricity. Ability to conduct
gives a measure of total quantity of charged particles (ions) dissolved in it. This is often referred
to as conductivity of the water.
Generally, the lakes (particularly the deep
ones) remain vertically stratifi ed in relation to
light intensity, wavelength absorption, hydrostatic pressure, temperature, etc. For example, in
a lake, there are three to fi ve well-recognised
horizontal strata, namely,
(a) Littoral zone formed by the shallow water
near the shore. It contains upper warm and
2.4 Stratifi cation and Zones