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Plant
Bioindicators of Aquatic Environment
By:
Prakash Chandra and Sarita Sinha
Plants are
increasingly being used as highly effective and sensitive tools for recognizing
and predicting environmental stresses. The problem of water pollution has
become acute now a days due to industrialization and urbanization. The
industrial effluents contain substances like heavy metals, PCB’s and many other
organic and inorganic toxic compounds. In order to identify the nature of
pollutants and their impact, quick and reliable monitoring systems are
essential. Aquatic plants provide useful information on the status of aquatic
environment as they do not migrate from one place to another and they quickly
attain equilibrium with their ambient environment. They provide cumulative
information of preceding and present environmental conditions as against
chemical analytical methods which reveal only the current status of the
environment. While lot of information is available on the bioindicators of air
pollution, the information on the pollution indicator plants in aquatic
environment is rather scant.
In aquatic
habitat, plant species of different groups serve as reliable indices for
biological monitoring of pollution load. The aquatic vascular plants are
potentially useful as indicators of water status. By their ability to
accumulate toxic substances, they indicate their presence in the environment
even if they are present in very low concentrations. In many sensitive species
metal induced morphological and structural changes may also be indicative of
changes which are specific to some metals. The nutrient enrichment effect is
indicated by the disappearance of susceptible species leading to the change of
species composition. These may be successfully used as ecological indicators (bioindicators)
for assessing and predicting environmental changes.
Lower group
of plants as indicators of pollution
Amongst the
lower group of plants, algae have been extensively used as indicators of water
pollution. Colonies of Scenedesmus obliguos were used for monitoring
cadmium and lead pollution in river basin in Thailand and Federal Republic of
Germany. Brown algae have also been used as biological indicators of heavy
metal pollution. A change in species diversity of phytoplankton is the clear
indication of polluted aquatic ecosystem.
Induced
morphological changes in many algae have also been reported. Cellular
malformation, chlorosis and significant increase in heterocyst frequency have
been caused in Anabaena cylindrica under cadmium stress. Similarly in
A. inequalis, induction of abnormally long filaments and loss of cellular
content have also been reported as a result of cadmium pollution. Cains and
Cains (1983, 1984) have reported remarkable decrease in zygospore germination
in Chlamydomonas in the presence of selected herbicides and insecticides.
Aquatic
vascular plants as indicators of metal pollution
Heavy metals
pose a serious threat to aquatic environment as they do not degrade but
accumulate in aquatic micro and macrophytes and enter into the food chain. Many
sensitive aquatic vascular plants have been identified as indicators of metals
because of their potential to accumulate them substantially.
Indicators
of copper, lead and zinc pollution
Potamogeton sp.
occurring
wild in lakes and rivers in U. K. and Canada are employed to monitor levels of
copper, lead and zinc pollution in aquatic systems. Similarly,
Pontedaria
cordata
has been
found ideal for detecting copper and lead at various distances from the site of
discharge.
Indicators
of mercury pollution
In Thailand
and Finland,
Ceratophyllum demersum is
indicator of
mercury contamination. Plants showed as high as 1.72 ppm mercury accumulation
while growing near a caustic soda factory.
Panda and
co-workers have reported formation of micronuclei in dividing root cells of
Eichhornia
plants
growing in mercury polluted sites (Science Reporter, April-May 1989). The
sensitivity of this plant can be used to monitor mercury in aquatic ecosystem
at distances far away from the source of discharge. Similarly, in
Elodea densa,
surface membrane and chloroplast structure in leaves were severely damaged due
to excessive accumulation of methyl mercury.
Bioindicators of chromium and cadmium pollution
Induced
structural changes viz. disappearance of root hairs and colouration of
hydathodes in the juvenile leaves of
Limnanthemum
cristatum
due to high
leaf tissue chromium concentration (226µg/g dry wt.) have also been reported.
Plants of
Wolffia
globosa
have also
shown sensitivity to metal concentrations and potential to be used as indicator
of cadmium contamination.
Ray and
White (1979) suggested that an indicator species should be a representative of
the locality, abundantly available and easy to harvest. The species should also
have high tolerance for metal and be capable of its enrichment. In view of the
tremendous potential of biological systems, intensive efforts are now needed to
identify " Bioindicators" for assessing the state of aquatic systems and
working out suitable remedial methods to save the humanity from the hazards of
environmental pollution.
Dr.
Prakash Chandra is the former head of Aquatic Botany Division and Dr. Sarita
Sinha is a Scientist at the National Botanical Research Institute, Lucknow,
India. |
