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Vol. 6 No. 1 - Millennium Issue - January 2000

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.

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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