With the passage of time, organic matter begins to accumulate and the biological productivity of such lakes also tends to increase. The lakes first change to a mesotrophic condition and finally become eutrophic. (A third term, dystrophic, is sometimes used: dystrophic lakes are those in which nutrients occur but various organic acids and other substances also present inhibit the growth of plants and animals.

Dystrophic lakes are usually parts of certain bogs. “Eutrophication” thus signifies the change (increase) in biological productivity and nutrient content which all lakes and reservoirs undergo during their life. The problems that arise following the discharge of wastes containing nutrient salts into water bodies come not from the nutrients themselves but from the changes they induce in the aquatic productivity and the phytoplankton.

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The term eutrophication denotes the lowering or deteriora­tion of water quality for domestic, recreational and other uses. The continual deposition of organic debris and its degradation by benthic bacteria, etc., in oligotrophy reservoirs is generally so slow that no significant algal and higher plant populations can be supported, and the water remains “clean”. On the other hand, in a eutrophic pond, the deposition of materials is so great that a variety of nutrients becomes released into the water which can thus support large populations of algae and higher plants. Oligotrophic waters tend to have a N:P ratio greater than 10.

This ratio approaches 10 as some sewage is discharged into an oligotrophic lake to eutrophic is a very slow process, occurring in thousands or millions of years. But the great advances in technology, industry and agriculture, have led to a remarkable acceleration of this rate with the result that a completely oligotrophic lake often becomes eutrophic within the course of a few decades. Eutrophication is, in fact, just another form of pollution. The trend in normal succession seems to be from less to more productivity and it has generally been assumed that the underly­ing basis for the increased productivity is increased nutrient supply. However, sometimes towards the end of a lake’s life, plant production or abundance may increase in the absence of any significant increase in nutrient input. The eutrophication concept has been confused since the early 1920s because it was based on such instantaneous parameters as oxygen tension, nutrient concentration and phytoplankton density. This kind of empirical and static concept neither supported any functional relationship between the different state variables nor the prediction of the dynamics of the system’s response to pertuibations.

Later, the concept of rates, i.e., the velocity of exchange between various mass and energy compartments, was introduced and it then became possible to relate concentrations to supply and consump­tion as well as phytoplankton density to primary production. The classical trophic state indexes are connected to external parameters by the rate of primary production. Qualitatively, eutrophication can be simply defined as an increase in the primary productivity of a lake, and recent workers have used primary production per unit of lake surface and time as a quantitative measure of the lake’s trophic states (see Imboden and Gachter, 1979). These workers have chosen the annual primary production as a trophic state index and have shown that this index is greatly influenced by such physical processes as the intensity of vertical mixing.

Of the nutrients entering the lake, a major proportion becomes incorpo­rated into algae which release the nutrients back into water either when still alive or after their death and decay. In fact, certain blue-green algae can serve to indicate eutrophic conditions. Oscillatoria rubescens, Aphanizomenon flosaquae and Microcystis aeruginosa have often been regarded as indicators of lake eutrophication. Biologically, oligotrophic lakes are poor in plankton, support many diverse species of such algae as Chlorophyta, Chrysophyta and Bacillariophyta (Tabellaria, Cyclotella) and have these algae distributed to great depths. Algal blooms are formed very rarely, In contrast, eutrophic waters are rich in plankton, have larger numbers of individuals but these belong to very few species, and water blooms occur frequently.

The common algae of such waters belong to Cyanophyta and Bacillariophyta (Melosira, Fragilaria, Asterionella). Chemically, oligotrophic lakes are rich in dissolved oxygen and poor in nitrogen and phosphorus whereas eutrophic lakes are poor in dissolved oxygen and rich in nitrogen and phosphorus. Oligotrophic waters are clear and transparent whereas eutrophic generally turbid and muddy and much less transparent. Eutrophication is commonly accompanied by a concomitant change in the species composition of the aquatic biota. These changes may be caused either directly due to the nutrients or, more commonly, may be caused by some underlying homoeostatic factors, e.g.

, change in grazing behaviour of animals. Such changes in homoeostatic relations can in certain cases delay or prevent the required desirable changes or improvements in a lake even after the nutrient load has been diminished significantly. Another similar factor that can cause delayed effects is the deposition of nutrients in bottom sedi­ments. Such nutrients may continue to diffuse into and enrich the water long after further inputs of nutrients into the lake have been checked.

With increasing eutrophication, the diversity of the phytoplankton com­munity of a lake decreases and the lake finally becomes dominated by blue- green algae. Some striking examples of blue-green algal lakes are the hyper­trophic Wolderwijd and Veluwemeer lakes in the Netherlands. In these lakes, Oscillatoria agardhii is the dominant alga. The natural populations of this species are successively limited by phosphorus, light, and combined nitrogen. One interesting observation is that even at times when the growth of O.

agardhii is N-limited, this species is not succeeded by any nitrogen-fixing algae. These lakes are much less favourable to nitrogen fixers even when N- limitation prevails, and Zavenboom has concluded that the critical factor determining as to which algal species will dominate is the trophic state of the lake, especially with reference to organic matter and phosphorus. The phenomenon of hypertrophy involves such a high enrichment of a freshwater system with nutrients that a pronounced increase of biomass and a strong decrease in the number of species results. In those lakes which are only slightly eutrophic the maximum biomass of phytoplankton is controlled by amount of nutrients available for growth.

Under these conditions, typical oligotrophic species having a high affinity for the limiting factor and show in a marked light tolerance tend to be selected. At somewhat greater eutrophication level, the selection pressure of the limiting factor is somewhat lower, and a more diverse phytoplankton community develops whose biomass is higher. With continuing increase in eutrophication level, the light availability in the epilimnion is adversely affected and as a consequence that algal specie having a low light requirement (i.e., blue-green) is preferred. Most planktonic blue-greens have gas vacuoles which enable them to regulate their buoyancy in response to the available light conditions. These vacuoles enable the cyanobacteria to maintain themselves in water layers with optimum light conditions.

Deep lakes are generally more sensitive to hypereutrophication as compared to shallow lakes (Mur, 1980). The biomass concentration in deep lakes is low and the blue-green algal species are all gas vacuolate forms that grow in distinct strata. In contrast, in shallow lakes stratification does not occur and the light intensity becomes low only when the numbers of algae becomes very large. Non-gas vacuolate species tend to dominate here, and good examples are species of Oscillatoria; (Microcystis, Aphanizomenon and Anabaena exemplify gas vacuolate forms of deep lakes).

The crops of zooplankton in most hypertrophic lakes are also quite high. These microanimals feed on the phytoplankton. The populations of zooplank­ton are regulated by such fishes as roach and bream. Any marked increases in the populations of planktivorous fishes lead to steep declines in zooplankton and corresponding rises in the phytoplankton biomass.

According to Mur (1980), a selective removal of planktivorous fishes exerts a positive effect on the trophic state of a water body. This may be caused not only by predation of fish on zooplankton but also by the predation on zoobenthos that leads to a mobilization of phosphorus from the bottom sediments. The control of these fish population that feed on plankton and benthos does seem to constitute a promising method of lowering the trophic level of a lake. Although planktonic algae constitute a very significant factor in lake eutrophication, the effects of nutrient-enrichment on higher aquatic plants are by no means insignificant, especially in shallow waters. It has long been realized that primary production in aquatic habitats is under the influence of both solar energy and nutrients.

Recently, the latitude has also been found to influence productivity (Brylinsky and Mann, 1973). A regression of plankton production on latitude brought forth the interesting conclusion that whereas lakes at high latitudes had low productivity, those situated at low latitudes exhibited a wide range of levels of productivity. This work was done on Canadian lakes. This conclusion is, however, not accepted by some other Canadian workers, e.g.

, Schindler and Fee (1974). These latter workers are of the opinion that the observed correlation between geographical latitude and plankton production is due only in part to the effect of solar radiation. The major factor determining phytoplankton productivity seems to be phosphorus content rather than latitude. Village waters are generally not affected by point sources of pollution but are mainly subject to non-point sources.

Eutrophication of rural ponds is a good example of non-point pollution. Management of the effects of nutrient increases depends chiefly on the Vollenweider phosphorus-loading concept. Schindler (1980) has discussed the long-term background and a number of eutrophication and lake research programmes in North America and Canada.

Golterman (1980) has pointed out that although much is known about nutrient loading and lake productivity, there is a danger of oversimplification due to limitations of available models in describing the complexities of the phosphonis cycle. It has been felt that such key processes as recycling of nutrients and sediment formation or release should be included to represent the dynamic nature of ecosystems. Kajak (1980) has related the roles of phosphorus loading and dynamics in lake water to phosphorus nonpoint loads, agricultural impacts, loading rela­tionships, epilimnetic recirculation, the role of biota in the phosphorus cycle, and phytoplankton impacts. Skulberg (1980), working on Norwegian lakes, related the observed changes in plarktonic and benthic algae to eutrophication and to algal inva­sion of water courses. In the case of the American lake Mendota, Fallon and Brock (1980) studied growth, primary production and sedimentation over two annual phytoplankton cycles to observe the succession of blue-green algal following stratification of the lake.

He interpreted the observed declines in standing crop in terms of epilimnetic decomposition and sedimentation. Considerable attention is now being given to stop or reverse the eutroph­ication process with a view to ensuring an adequate supply of clean water for drinking and other purposes, and also with a view to preventing the lakes from too premature a death. The following procedures have been recommended by various limnologists to slow down the eutrophication process. (1) Limiting the amount of nutrients entering the lake.

(2) Reduction in the amounts of nutrients solubilized in water through microbial decomposition of bottom sediments; this can often be achieved by the bottom sealing technique of Sylvester and Seabloom (1965), i.e., artifi­cially planting an inert layer which covers bottom sediments. (3) Harvesting and removal of algal blooms and mechanical removal of higher plants; this can reduce the amount of nutrients recycled into the water upon death of algae and higher plants. (4) Removal of dissolved nutrients from water chemically or physically. (5) By encouraging the setting up of natural food-webs (e.g.

, daphnids and fishes), which can remove the algae, and subsequently harvesting the fish, etc. (6) By controlling the growth and multiplication of algae and higher plants through application of appropriate doses of copper sulphate and sodium arsenite, respectively. Of all the above methods of controlling eutrophication, that involving limitation of nutrient input is certainly the best and the most reliable. Al­though various kinds of major and minor nutrients are essential for plant growth, in natural habitats it is mostly nitrogen and phosphorus which are critical. However, certain blue-green algae can even fix the atmospheric nitrogen gas and hence their growth does not become limited by the deficien­cy of nitrogenous compounds in lake water, and in such cases phosphorus becomes the most critical element limiting algal growth. Most instances of man-made eutrophication have been traced directly to a superabundance of nitrogenous and phosphatic compounds in water. In many tropical habitats, nitrogen may become limiting for growth of non-nitrogen fixing algae.

Iron has also been shown to promote eutrophication in certain waters (see Russell and Gilson, 1972). Tertiary treatment of wastewater effluents is now being increasingly employed to decrease the phosphate-content of the effluent before being discharged into lakes and rivers. Oglesby and Edmondson (1966) have proposed three methods of nutrient limitation (a) removal of nitrogen and phosphorus at their source; (b) the diversion of nutrient-rich effluents or wastewaters from receiving bodies, and (c) dilution of nutrients in lakes by the controlled addition of water low in nutrients.

Kumar et al., (1974) envisage the production, through mutagenic treat­ments, of strains of algae endowed with greatly increased nutrient-uptake capacity. Such pollution-tolerant strains could then be grown in a eutrophic reservoir where they would strip the nutrients. Subsequent removal of such algae by harvesting may be expected to control or reverse the process of eutrophication. The Water Resources Research Council of the University of Massachu­setts have exploited the technique of “microbial intervention” for controlling eutrophication. The rationale underlying this technique envisages the man­agement of the water body in such a manner as to selectively favour the decomposer microbes over the algae. Since the bacteria and algae compete with each other for the nutrients derived from decaying lake sediments, artificially stimulating bacterial multiplication will lead to a disruption of the algal food-web.

In other words, the “self-cleansing” ability of the reservoir water will be maximized through microbial activity (see Erickson and Reynolds, 1971). The Massachusetts scientists are currently engaged in study­ing the algal and bacterial flora and also their biological interactions and the factors underlying their ecological competition. Their objective is to find out how best to tilt the balance in favour of bacteria.

Eutrophication can also be reversed or controlled by removal of nutrients such as phosphorus and nitrogenous compounds from the water. Phosphorus can be removed by pre-precipitation, simultaneous precipitation, and postprecipitation methods (see Baalsrud and Balmer, 1973); in these methods biological steps are not involved. Removals as high as 90 per cent or more can be achieved through physiochemical methods. Kumar and Rai (1978) have found that zirconium oxychloride effectively precipitates phosphates in pol­luted water or culture medium, thereby relegating it to the status of a limiting nutrient. This in turn can prevent algal blooming. Most Indian village ponds are heavily polluted and infested with obnoxious algal blooms. Their water is rich in phosphates, nitrates and other plant nutrients which arise from sewage, cattle and dairy wastes, and other organic matter, etc. If the phosphates are precipitated, algal blooming would be checked and this can at least partially control eutrophication in these ponds.

Zirconium oxychloride (100 ppm) has been found to precipitate as much as about 95 per cent of the phosphate content in algal culture medium. These experiments have also been done in small outdoor cemented tanks but they have not yet been tried at the pond level. However, aquarium scale experiments have indicated that zirconi­um oxy chloride (100 ppm) does not exert any adverse or toxic effects on fish. This method seems potentially promising for eutrophication control in village ponds, but further work is necessary at the pond level before a definite conclusion is drawn. Ammonia seems to be the main oxygen consuming component in un- nitrified, low-BOD water or effluents. Nitrification can be a very useful process to achieve nitrogen removal from the effluent. Nitrogen can be removed by (a) biological nitrification and denitrification; (b) air stripping of ammonia from an alkalized wastewater, (c) ion exchange; (d) electrodialysis; and (e) reverse-osmosis.

Out of these various methods the first two are more commonly employed. Ammonium nitrogen can be biologically oxidized to nitrate which is in turn biologically reduced to nitrogen gas. Both nitrifying and denitrifying bacteria have much lower growth rates than those of carbon oxidizing bacteria. It is therefore customary first to reduce the organic content of the sample by means of carbon oxidizing organisms and then to decrease its nitrogen content through the mediation of nitrifying and denitrifying bacteria. Nitrogen removal by air stripping involves raising the pH of water to make it sufficiently alkaline; this shifts the ammonium-ammonia equilibrium to the ammonia side and the NH3 gas so produced is trapped in water in air stripping towers.

Some general features and attributes of eutrophication are summarized below: (1) It is a natural process, but can be greatly speeded up by man’s activities. (2) Many different factors in the aquatic habitat can accelerate eutrophi­cation; such chemical factors as nitrogen and phosphorus enrichment are only a few of the factors involved. (3) The rate of eutrophication is very difficult to measure and no suitable quantitative scale of trophic level has so far been proposed.

Temporal and spatial variations in nutrients, primary productivity, species diversity and other parameters which signify different trophic levels minimizes the utility of short-term investigations of eutrophication rates; and (4) It is perhaps not necessarily true that eutrophication is an entirely irreversible process (see Cairns eta/., 1972). Although it is well known that nitrogen and phosphorus are perhaps two of the main factors determining eutrophication, there is some indication that in some cases carbon, rather than phosphorus and nitrogen, may be the chief nutrient limiting the production of algal blooms (King, 1970), and that free CO2 concentrations may regulate the structure of algal communities. In cyanophyte-bacteria associations, bacteria degrade organic matter, producing CO2 which is utilized by the blue-greens in their photosynthesis.

This photo­synthesis generates oxygen which is, in turn, used by bacteria to oxidize or decompose organic matter. During this efficient symbiotic system, nitrogen, phosphorus and other nutrients are cycled between the algae and bacteria, and amongst the two and the environment. It is also known by means of CO2 enrichment studies that carbon may stimulate primary productivity when other factors are not limiting. Jackson (1969) has studied primary productivity of a highly polluted lake, the Onondaga Lake, in relation to its organic and inorganic pollutants. The northern area of this lake was mostly inorganically polluted whereas the southern zone had largely organic pollutants. The organically polluted part was found to be richer in total nitrates, phosphates and algal individuals as compared to the inorganically polluted water.

This organic zone also con­tained very high concentrations of dissolved silica (exceeding 1 ppm) and Jackson’s work has indicated that this is an important factor influencing algal communities, especially diatoms. It is already known that some correlation exists between dissolved solids and cell dimensions of certain algae, e.g., algal cells tend to be larger in freshwaters rich in dissolved solids than those found in waters low in these (see Cairns et al., 1972).