203
In addition to the above, in the freshwater sediments, morphologically recognisable parts of a
number of algae, e.g. diatoms, certain desmids
and green algae and cysts of certain dinofl agellates and chrysophytes, are well preserved. Parts
of other algae might have been less preserved or
not preserved at all. Thus, pigments, which were
relatively well preserved, might be restricted to
specifi c taxonomic groups which might be characteristic of relatively distinct ecological conditions. Their stratigraphy could be most useful in
interpretation of past conditions in the lake’s
ontogeny. The cosmopolitan distribution of chlorophylls among plants limits their specifi city to
protistan groups. In this regard, it could be
pointed out that certain bacterial carotenoids and
bacteriophyllous degradation products of purple
photosynthetic and green sulphur bacteria have
been found in sediment stratigraphy. These were
believed to be associated with events which led to
eutrophication and meromixis (Brown 1968 ).
The probable time of invasion and development
of BGA and other algae were ascertained in this
manner by using pigments like myxoxanthin and
myxoxanthophyll (Zullig 1981 ; Griffi ths 1978 ).
Recent workers had introduced a ‘pigment diversity index’. It is based on the enumeration of pigment
spots on two-dimensional thin-layer chromatograms
of acetone extracts, as a means of assessing the origins of sedimentary organic matter.
It is believed that during oligotrophic conditions, much of the organic matter preserved in
sediments is allochthonous in origin. Conversely,
under eutrophic conditions, pigments of autochthonous origins greatly exceed those sedimented
from external sources. However, differential degradation of pigments, during and after sedimentation, is a more serious problem which is quite
diffi cult to resolve. Likewise, destruction of pigments and derivatives formation were found to be
quite unrelated processes. Pheophytins and
pheophorbides accumulated only in lysed cells.
Also, the concentrations of these two derivatives
were found to increase in presence of dilute acids
and oxygen. Further, relatively low temperatures
in hypolimnetic waters may favour the preservation of pigments. As such, one could assume that
the best conditions of preservation occur in the
anoxic, cold and relatively dark monimolimnia of
meromictic lakes.
Therefore, it must be clearly appreciated that
the use of pigment concentrations and constituents as palaeolimnological indices requires critical evaluation of different parameters (Daley
et al. 1977 ).
12.5 Morphological Remains
In the course of its long history, palaeolimnology
began with analyses of morphological remains of
aquatic and terrestrial organisms. It could be well
known that preservation of dead organisms in the
sediments is generally incomplete. Also, the extent
of preservation usually depends both on the type
of organism and the environmental conditions that
had prevailed at the time of sedimentation. In the
sediments, pollen and spores of terrestrial plants
are generally most abundant, so are the frustules of
diatoms and cysts of chrysomonads. Many groups
of organisms may not be represented in the sediments. Among the animals, the remains of the cladocerans and the chironomid midges are the most
abundant and diversifi ed. However, almost all the
groups of animals are, to some extent, represented
in the sediments. Many of these remains could be
traced to the species level. This is true, to some
extent, in the case of diatoms, desmids, cladocerans, ostracods and beetles.
12.5.1 Pollen
Plant pollen and plant spores are produced plentifully. However, only a few of those produced
fulfi l their reproductive function. After the pollen
types are dispersed, they are well mixed by atmospheric turbulence in proportion to the quantity
and composition of the parent vegetation in and
around the lake(s). Most pollen grains are resistant to decay, if deposited in nonoxidising sites.
It is understood that the taxonomy of the pollen is
quite well known in many parts of the globe. The
pollen assemblage in sediments of known age
12.5 Morphological Remains
In addition to the above, in the freshwater sediments, morphologically recognisable parts of a
number of algae, e.g. diatoms, certain desmids
and green algae and cysts of certain dinofl agellates and chrysophytes, are well preserved. Parts
of other algae might have been less preserved or
not preserved at all. Thus, pigments, which were
relatively well preserved, might be restricted to
specifi c taxonomic groups which might be characteristic of relatively distinct ecological conditions. Their stratigraphy could be most useful in
interpretation of past conditions in the lake’s
ontogeny. The cosmopolitan distribution of chlorophylls among plants limits their specifi city to
protistan groups. In this regard, it could be
pointed out that certain bacterial carotenoids and
bacteriophyllous degradation products of purple
photosynthetic and green sulphur bacteria have
been found in sediment stratigraphy. These were
believed to be associated with events which led to
eutrophication and meromixis (Brown 1968 ).
The probable time of invasion and development
of BGA and other algae were ascertained in this
manner by using pigments like myxoxanthin and
myxoxanthophyll (Zullig 1981 ; Griffi ths 1978 ).
Recent workers had introduced a ‘pigment diversity index’. It is based on the enumeration of pigment
spots on two-dimensional thin-layer chromatograms
of acetone extracts, as a means of assessing the origins of sedimentary organic matter.
It is believed that during oligotrophic conditions, much of the organic matter preserved in
sediments is allochthonous in origin. Conversely,
under eutrophic conditions, pigments of autochthonous origins greatly exceed those sedimented
from external sources. However, differential degradation of pigments, during and after sedimentation, is a more serious problem which is quite
diffi cult to resolve. Likewise, destruction of pigments and derivatives formation were found to be
quite unrelated processes. Pheophytins and
pheophorbides accumulated only in lysed cells.
Also, the concentrations of these two derivatives
were found to increase in presence of dilute acids
and oxygen. Further, relatively low temperatures
in hypolimnetic waters may favour the preservation of pigments. As such, one could assume that
the best conditions of preservation occur in the
anoxic, cold and relatively dark monimolimnia of
meromictic lakes.
Therefore, it must be clearly appreciated that
the use of pigment concentrations and constituents as palaeolimnological indices requires critical evaluation of different parameters (Daley
et al. 1977 ).
12.5 Morphological Remains
In the course of its long history, palaeolimnology
began with analyses of morphological remains of
aquatic and terrestrial organisms. It could be well
known that preservation of dead organisms in the
sediments is generally incomplete. Also, the extent
of preservation usually depends both on the type
of organism and the environmental conditions that
had prevailed at the time of sedimentation. In the
sediments, pollen and spores of terrestrial plants
are generally most abundant, so are the frustules of
diatoms and cysts of chrysomonads. Many groups
of organisms may not be represented in the sediments. Among the animals, the remains of the cladocerans and the chironomid midges are the most
abundant and diversifi ed. However, almost all the
groups of animals are, to some extent, represented
in the sediments. Many of these remains could be
traced to the species level. This is true, to some
extent, in the case of diatoms, desmids, cladocerans, ostracods and beetles.
12.5.1 Pollen
Plant pollen and plant spores are produced plentifully. However, only a few of those produced
fulfi l their reproductive function. After the pollen
types are dispersed, they are well mixed by atmospheric turbulence in proportion to the quantity
and composition of the parent vegetation in and
around the lake(s). Most pollen grains are resistant to decay, if deposited in nonoxidising sites.
It is understood that the taxonomy of the pollen is
quite well known in many parts of the globe. The
pollen assemblage in sediments of known age
12.5 Morphological Remains