They exposed an oak-pine forest community to irradia­tion from Cs137 source for six months and observed that five clear zones of modification had resulted following the administration of different doses of radiation: (a) Following the highest dose of about 200 r/day, a central zone where all higher plants were destroyed and only some lichens and mosses could survive; (b) At somewhat lower doses (150 r/day), sedge zone remained: i.e., everything was destroyed except for Carex pennsylvanica; (c) At still lower doses, a shrubzone in which a few shrubs could survive but trees were eliminated; (d) After low doses of about 16 r/day, the pine was eliminated but the oak survived; and (e) At about 2 r/day, no significant changes in communi­ty structure resulted (Woodwell, 1970). Most of the observed changes were found to affect the structure of the community, i.e.

, different strata of the forest were eliminated, layer by layer, at progressively increasing radiation dosage, with the tallest plants being eradicated first and the smallest last. However, certain changes also took place in respect of the diversity, primary production, respiration and nutrient budget. These observed effects of irradia­tion were broadly similar to the previously well-studied effects of fire out­breaks in the same oak-pine type of community. The shift in the physiognomy of the community from a predominant tree- dominated one toward shrubby, herbaceous and cryptogamic communities, also involved a reduction of the total standing crop of organic matter and of nutrients trapped within the ecosystem. These changes and losses of nutrients can be significant in relation to the potential capacity of the land to support life. Serious losses of nutrients have been reported by other workers follow­ing the cutting of trees in a watershed and allowing them to decay naturally. Jordan (1976) has studied the effects of gamma radiation (Cs137 source) on a tropical rainforest in Puerto Rico. The forest was irradiated for 4 months.

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Apart from other effects, which were similar to those caused by mechanical stripping and herbicide’ treatment, one significant difference which was uniquely a radiation response was sprouting of trees near their bases where they were shielded from radiation exposure. Jordan found that the recovery of the irradiated forest closely resembled secondary succession following other types of disturbances in the same area. Judicious use of radioisotopes can furnish valuable information about the kinetics of chemical, physical, biological, or geological processes in aquatic ecosystems. Many radionuclide’s are suitable for systems with time constants ranging from a few hours to centuries. Any information on rates, routes and reservoirs, derived from studying the behaviour or radioisotopes in the envi­ronment is valuable for our proper understanding of the fate of pollutants and chemicals. Radioisotopes are also being increasingly used to study rate constants of rate-limiting processes in controlled experimental ecosystems (Grice and Reeve, 1981) and also to identify the transport agent and the reservoirs for the transport of a particular chemical in such ecosystems. The use of radiotracers also enables one to characterize primary production, respiration, accumula­tion or exchange of nutrients, trace metals or food particles of benthic or planktonic organisms (Grice and Reeve, 1981; Santschi, 1983). Pollution—the ‘skull and crossbones’ of modem civilization threatens not only terrestrial vegetation but also inland surface waters even though the general tendency among scientists concerned with assessing the effects of pollution on ecosystems has been to minimize or even ignore the importance of aquatic microbial communities and to emphasize higher taxa.

The func­tioning of the aquatic ecosystem is often significantly affected by its pollution and eutrophication. Much work is now being done on the functioning of major components within such ecosystems, supplemented by studies of circulation of materials within and amongst different aquatic ecosystems and also be­tween these and the adjoining terrestrial ecosystems. Special attention has been given under the MAB Programme to the effects of man-caused stresses on inland waterways.

Since standing (or slow flowing) waters are commonly more sensitive to the impacts of stresses, these are receiving greater attention (see UNESCO, 1972). A number of more or less similar kinds of projects concerning aquatic ecosystems being undertaken by various international organizations and agencies are listed in UNESCO (1972). The ecological impacts of long-range pollutant transport are highly significant but predicting environmental transport, transformation, and eco­logical effects of pollutants is often highly imprecise. Most of the work on the ecological effects so far has concentrated on human health effects and effects on individual organisms, with much little work on ecosystems. Ecosystem degradation is a difficult concept since ecosystems are composed of popula­tions of many diverse organisms in addition to abiotic components.

It is only recently that due recognition and appreciation is being given to the fact that microbial communities are not haphazard assemblages of species but rather structured and organized communities with diverse, interlocking cause-effect pathways. The ecological and biological requirements can often reach the same complexity as those of higher plants and animals and any perturbation of microbial communities by pollution can affect the entire aquatic food-web (see Cairns et al., 1972).

Caims et al., have studied gross pollutional effects on aquatic algal and protozoan communities. They have analyzed the responses of those microbial communities to different kinds of pollutional stress. Various types of waste discharges were found to decrease species diversity of aquatic microbial communities. Microbial species vary markedly both spatially and temporally and due appreciation of this variabil­ity is a basic prerequisite for assessing pollution-caused changes in such communities. The structure of freshwater algal and protozoan communities seems to be flexible since the number and type of species involved differ in different areas.

Sometimes pollutants may not directly change microbial communities; e.g., pollutants may eliminate organisms which graze or eat the algae or protozoa. Many predators are highly selective and the results of selective action on population densities of different species can erroneously be confused with and attributed to those caused by pollution stress. Out of the various kinds of aquatic microbes, diatoms are perhaps the most suitable and convenient for monitoring pollution effects.

Their numbers and specific or generic identities can be determined more easily as compared to other algae, e.g., cyanophytes. In fact, a simple (slide) device, called diatometer, has been evolved and extensively used for collection and estima­tion of diatoms and other unicellular algae from aquatic habitats (see Cairns etal., 1972). Microbial communities of aquatic habitats are known to constitute the best possible material for biological indicators. Bisson and Cabelli (1980) report that Clostridium perfringens is not a good indicator for recreational water but is a fairly good indicator for chlorinated drinking water.

It is always advisable to include an invertebrate and a fish (in addition to a primary producer, a decomposer, and/or a detritus feeder) so as to conduct a reliable bioassay of the aquatic habitat. Sometimes even this assay package may prove inadequate in view of our ignorance of the niches and microhabitats of these organisms.