204
could be determined by various dating techniques.
This provides an index of vegetation in and
around the lake in the past. It also refl ects changes
which had occurred in the vegetation through
time. There had been detailed works done on the
principles of pollen analysis including pollen
identifi cation and interpretation of pollen data
(Birks and Birks 1980 ).
12.5.2 Algal Remains
In the lake sediments, different types of algae are
well preserved. The siliceous frustules of the diatoms are by far the most common algal microfossils. The remains of the diatoms generally occur
plentifully. They are often well preserved. Further,
most of them could be identifi ed to species level
from frustule morphology. Hence, diatoms are useful palaeontological indicators. Diatom remains
help in reconstruction of probable past lake conditions. Nevertheless, remains of other groups, notably certain green algae (e.g. Pediastrum ), scales
and cysts of chrysophytes, cysts of dinofl agellates
and heterocysts of BGA are also preserved in lake
sediments (Munch 1980 ). The past conditions of
the lakes could be well be interpreted with the help
of stratigraphy of these algae and the diatoms.
Also, study of the changes in the other parameters
recorded in the sediments could further assist in the
interpretation.
It is believed that the centric diatoms are generally associated with more oligotrophic waters,
while the planktonic pennate diatoms tend to be
more characteristic of eutrophic waters. The ratio
of these two indicator groups of diatoms in the
stratigraphic record could offer some help in
interpreting the chronology and in ascertaining
positive or negative changes in productivity.
Further, re-construction of past chemical conditions in lakes may be possible based on past diatom communities (Richardson et al. 1978 ).
12.6 Plant Macrofossils
In the lake sediments, remnants of large plants
are often found preserved as macrofossils. They
usually consist of fruits, seeds, megaspores and
fragments of leaves, rhizomes, fl owers and
woody tissues (Birks 1980 ). In addition, seeds
and other reproductive parts are often well preserved and identifi able to the species level. These
macrofossils supplement the pollen record.
Further, these plant macrofossils are usually
deposited close to their sites of origin. Also, the
limited transport of plant macrofossils could
result in heterogeneous distributions within sediments in different areas of a lake. In addition to
above, macrofossils may also provide insight into
the rates and types of plant succession which had
occurred in and around the lakes.
12.7 Animal Remains
Some identifi able morphological remains in lake
sediments are generally left by almost all groups
of animals. The most plentiful animal remains in
the sediments at the microscopic level are those of
ostracods, cladocerans and midges, spicules of
sponges, egg cocoons of turbellarians, resting eggs
of rotifers, oribatid mites and even cases of testaceous rhizopods, etc. (Frey 1976 ). Exoskeletons of
cladoceran zooplankton had generally been reasonably well preserved in lake sediments. Further,
species of chydorid cladocerans had largely been
littoral and were said to occur on a variety of substrata. In this regard, there had been considerable
evidences that the littoral faunal remains had, perhaps, been re-distributed by currents within the
lake basins. Here, they possibly had become integrated with the remains of planktonic cladocerans
(Goulden 1969a ). On the other hand, molluscs and
beetle remains dominate at the macroscopic level.
The FW molluscs are considered as relatively poor
indicators of past palaeoclimatic fl uctuations and
lake conditions. It could be because of their generally broad environmental tolerances (Ouellet
1975 ). However, remains of the fi sh scales, bones,
etc., are in less abundance.
With regard to enumeration, the numbers of
species or groups are often expressed in relative %
of the total assemblage. Concomitantly, varying
rates of sedimentation could be determined from
radiocarbon dating and certain other stratigraphic
criteria which permit expression as an absolute
rate of accumulation (Davis and Deevey 1964 ).
12 Historical Records (Palaeolimnology)
could be determined by various dating techniques.
This provides an index of vegetation in and
around the lake in the past. It also refl ects changes
which had occurred in the vegetation through
time. There had been detailed works done on the
principles of pollen analysis including pollen
identifi cation and interpretation of pollen data
(Birks and Birks 1980 ).
12.5.2 Algal Remains
In the lake sediments, different types of algae are
well preserved. The siliceous frustules of the diatoms are by far the most common algal microfossils. The remains of the diatoms generally occur
plentifully. They are often well preserved. Further,
most of them could be identifi ed to species level
from frustule morphology. Hence, diatoms are useful palaeontological indicators. Diatom remains
help in reconstruction of probable past lake conditions. Nevertheless, remains of other groups, notably certain green algae (e.g. Pediastrum ), scales
and cysts of chrysophytes, cysts of dinofl agellates
and heterocysts of BGA are also preserved in lake
sediments (Munch 1980 ). The past conditions of
the lakes could be well be interpreted with the help
of stratigraphy of these algae and the diatoms.
Also, study of the changes in the other parameters
recorded in the sediments could further assist in the
interpretation.
It is believed that the centric diatoms are generally associated with more oligotrophic waters,
while the planktonic pennate diatoms tend to be
more characteristic of eutrophic waters. The ratio
of these two indicator groups of diatoms in the
stratigraphic record could offer some help in
interpreting the chronology and in ascertaining
positive or negative changes in productivity.
Further, re-construction of past chemical conditions in lakes may be possible based on past diatom communities (Richardson et al. 1978 ).
12.6 Plant Macrofossils
In the lake sediments, remnants of large plants
are often found preserved as macrofossils. They
usually consist of fruits, seeds, megaspores and
fragments of leaves, rhizomes, fl owers and
woody tissues (Birks 1980 ). In addition, seeds
and other reproductive parts are often well preserved and identifi able to the species level. These
macrofossils supplement the pollen record.
Further, these plant macrofossils are usually
deposited close to their sites of origin. Also, the
limited transport of plant macrofossils could
result in heterogeneous distributions within sediments in different areas of a lake. In addition to
above, macrofossils may also provide insight into
the rates and types of plant succession which had
occurred in and around the lakes.
12.7 Animal Remains
Some identifi able morphological remains in lake
sediments are generally left by almost all groups
of animals. The most plentiful animal remains in
the sediments at the microscopic level are those of
ostracods, cladocerans and midges, spicules of
sponges, egg cocoons of turbellarians, resting eggs
of rotifers, oribatid mites and even cases of testaceous rhizopods, etc. (Frey 1976 ). Exoskeletons of
cladoceran zooplankton had generally been reasonably well preserved in lake sediments. Further,
species of chydorid cladocerans had largely been
littoral and were said to occur on a variety of substrata. In this regard, there had been considerable
evidences that the littoral faunal remains had, perhaps, been re-distributed by currents within the
lake basins. Here, they possibly had become integrated with the remains of planktonic cladocerans
(Goulden 1969a ). On the other hand, molluscs and
beetle remains dominate at the macroscopic level.
The FW molluscs are considered as relatively poor
indicators of past palaeoclimatic fl uctuations and
lake conditions. It could be because of their generally broad environmental tolerances (Ouellet
1975 ). However, remains of the fi sh scales, bones,
etc., are in less abundance.
With regard to enumeration, the numbers of
species or groups are often expressed in relative %
of the total assemblage. Concomitantly, varying
rates of sedimentation could be determined from
radiocarbon dating and certain other stratigraphic
criteria which permit expression as an absolute
rate of accumulation (Davis and Deevey 1964 ).
12 Historical Records (Palaeolimnology)