30
oxygen-rich circulating water layer which is
called epilimnion . Further, the littoral zone
includes rooted vegetation.
(b) Sublittoral zone , which extends from rooted
vegetation to the noncirculating cold water
with poor oxygen called the hypolimnion .
(c) Limnetic zone , which is the open water zone
away from the shore. It is said to be zone of
effective light penetration where rate of photosynthesis is said to be equal to the rate of
respiration.
(d) Profundal zone , which is the deep water area
beneath the limnetic zone and beyond the
depth of effective light penetration.
(e) Abyssal zone , which is found only in very
deep lakes. It begins from a depth of about
2,000 m from the surface.
It may be noted here that the wetlands usually
have only littoral and limnetic zones.
2.5
Depth and Shoreline
Development
‘Depth’ is a further factor related to the stratifi cation and nutrient status. It is diffi cult to
defi ne ‘shallow’ with any degree of precision,
e.g. for some purposes, 100 m is shallow; for
others, 30 m is deep. However, the extent to
which the benthos are illuminated and the
degree to which the benthos are separated from
the surface waters are perhaps the actual variables involved.
Thus, it could be said that all ecological systems, however aqueous or arid, conform to and
refl ect the same general principles and processes.
The patterns of expression of these principles and
the processes may, however, be different.
2.6
Movements and Water
Currents
The distinction between plankton and nekton is
almost entirely based on size. The adults are
generally larger in size and strong enough to
move independently on the currents. On the
contrary, the younger ones are generally smaller
in size and are often physically unable to move
against the current. Nevertheless, there is a
wide range to the distances moved by members
of the nekton. Some species may have a
restricted distribution throughout their adult
life, e.g. fi sh may stay in a particular section of
a stream or on one coral reef, while other
species make seasonal inshore and offshore
movements of up to a few kilometres. However,
some animals make much longer journeys
covering distances of several thousands of
kilometres, e.g . diadromous species like the
salmons and eels which move between the FW
and the high seas; tuna and whales, which
make transoceanic migrations.
Concomitant to above, it is sometimes found
that aquatic animals collect close to the temperature and salinity gradients which occur at fronts
between water masses. Further, currents must
have considerable infl uence on the distribution
and life histories of the nekton.
2.7
Dissolved Salts and Nutrient
Status
Most practical classifi cations of aquatic habitats
are based on the quantities of dissolved inorganic salts or other solutes. Salinity is of more
importance as a variable within the non-marine
aquatic habitats, because there it is generally
correlated with levels of primary production. In
hot dry regions, lakes may be found in which the
volume of FW input is almost balanced by evaporation from the lake surface. Further, nutrients
like NO 3 and PO 4 are also concentrated by evaporation. Algal productivity is usually higher
than in most of the productive FW lakes.
Moreover, the lakes may change their nutrient
status relatively rapidly as the rocks and soils in
their watersheds undergo erosion and become
leached and as particulate materials accumulate
in their basins. Excess input of nutrients, mainly
due to human intervention, may lead to ‘eutrophication’ of the water bodies. Natural eutrophication may be considered as a glacial
2 The Limnology of Lentic System
oxygen-rich circulating water layer which is
called epilimnion . Further, the littoral zone
includes rooted vegetation.
(b) Sublittoral zone , which extends from rooted
vegetation to the noncirculating cold water
with poor oxygen called the hypolimnion .
(c) Limnetic zone , which is the open water zone
away from the shore. It is said to be zone of
effective light penetration where rate of photosynthesis is said to be equal to the rate of
respiration.
(d) Profundal zone , which is the deep water area
beneath the limnetic zone and beyond the
depth of effective light penetration.
(e) Abyssal zone , which is found only in very
deep lakes. It begins from a depth of about
2,000 m from the surface.
It may be noted here that the wetlands usually
have only littoral and limnetic zones.
2.5
Depth and Shoreline
Development
‘Depth’ is a further factor related to the stratifi cation and nutrient status. It is diffi cult to
defi ne ‘shallow’ with any degree of precision,
e.g. for some purposes, 100 m is shallow; for
others, 30 m is deep. However, the extent to
which the benthos are illuminated and the
degree to which the benthos are separated from
the surface waters are perhaps the actual variables involved.
Thus, it could be said that all ecological systems, however aqueous or arid, conform to and
refl ect the same general principles and processes.
The patterns of expression of these principles and
the processes may, however, be different.
2.6
Movements and Water
Currents
The distinction between plankton and nekton is
almost entirely based on size. The adults are
generally larger in size and strong enough to
move independently on the currents. On the
contrary, the younger ones are generally smaller
in size and are often physically unable to move
against the current. Nevertheless, there is a
wide range to the distances moved by members
of the nekton. Some species may have a
restricted distribution throughout their adult
life, e.g. fi sh may stay in a particular section of
a stream or on one coral reef, while other
species make seasonal inshore and offshore
movements of up to a few kilometres. However,
some animals make much longer journeys
covering distances of several thousands of
kilometres, e.g . diadromous species like the
salmons and eels which move between the FW
and the high seas; tuna and whales, which
make transoceanic migrations.
Concomitant to above, it is sometimes found
that aquatic animals collect close to the temperature and salinity gradients which occur at fronts
between water masses. Further, currents must
have considerable infl uence on the distribution
and life histories of the nekton.
2.7
Dissolved Salts and Nutrient
Status
Most practical classifi cations of aquatic habitats
are based on the quantities of dissolved inorganic salts or other solutes. Salinity is of more
importance as a variable within the non-marine
aquatic habitats, because there it is generally
correlated with levels of primary production. In
hot dry regions, lakes may be found in which the
volume of FW input is almost balanced by evaporation from the lake surface. Further, nutrients
like NO 3 and PO 4 are also concentrated by evaporation. Algal productivity is usually higher
than in most of the productive FW lakes.
Moreover, the lakes may change their nutrient
status relatively rapidly as the rocks and soils in
their watersheds undergo erosion and become
leached and as particulate materials accumulate
in their basins. Excess input of nutrients, mainly
due to human intervention, may lead to ‘eutrophication’ of the water bodies. Natural eutrophication may be considered as a glacial
2 The Limnology of Lentic System