The insatiable desire of mankind to alter the environment to suit its own need has often done untold damage to the environment. This is partly due to negligence. The Dal Lake recently would have almost ceased to be as it was shrinking fast while its aquatic life was left gasping for oxygen. The lake was saved through a massive effort and at a great cost. Many of our present fresh-water lakes are in a great danger. The Nainital Bhutal is a case in point. The
problem is not only in India. Many European and North American lakes and rivers are dying out in much the same way. The reasons behind this disaster are explained by the process of entrophication.
The growth of plant biomass in aquatic environment is most commonly limited by nutrient concentrations and/or by the grazing of herbivores. In most lakes a bloom (a period of rapid growth in plant biomass) of phytoplankton occurs in spring. This bloom usually coincides with an input of nutrients in spring runoff and an increase in temperature. During the spring bloom, plant biomass grows until the nutrients are exhausted; then the plants die and the living biomass declines.
The size of the spring bloom characterises the trophic status of the lake. In some lakes bloom size is controlled by nutrient concentrations. For example the productivity of plants in many lakes can be predicted simply by the quantity of phosphorous in the lake water at the end of winter. In other aquatic environments, however, grazing by herbivores may have a very great influence on phytoplankton population.
Excessive nutrient input results in plants escaping the control of the grazing trophic level. This is the basic cause of some of the symptoms that define culturally eutrophic environment. If plants grow more rapidly than the ability of grazers to remove them, the excess plant biomass get deposited as sediments in a stagnant water reservoir. This excess biomass is decomposed.
The bacteria responsible for decomposition require oxygen. If enough decomposing biomass is present, the bacteria can consume all the oxygen in the water. The bacteria that require oxygen may then be replaced by bacteria that thrive on sulfate or carbon dioxide to derive oxygen. These bacteria then generate toxic compounds: hydrogen sulfide and methane which have a foul smell.
The escape of plants from the control of grazers may also be enhanced in extremely nutrient-rich environment by changes in the types of plants that grow. Blue green algae and dinoglagellates, which secrete toxins, are examples of plants that do not compete well with other types of phytoplankton when nutrients such as phosphorous are in short supply. When phosphorous becomes abundant things however, change; plant biomass grows untill some other nutrients, such as nitrogen or iron, become ‘limiting’. Blue green algae have a special advantage over other plants under such circumstances. First, they fix nitrogen from the air. This means that unlike most other plants, they can escape the limitation of nitrogen in water by converting nitrogen from the air into a useful nutrient. Second, they secrete compounds that bind iron into a form that renders it useless for other plants. In addition to controlling all the iron another potentially limiting nutrient-they are often inedible by many grazing organisms.
Thus the layers, which are mixed by wind and wave action, form the epilimnion. The deeper, colder layer that lies below it is the hypoliminion. The zone between the two is the thermocline.
Plant growth occurs only in the epilimnion to the depth that light penetrates. Because oxygen diffuses into water form the surface, the thermocline is the last layer to contain oxygen. Further, dying plant bodies usually fall below the thermocline into the hypolimnion to decay. This means the input of oxygen to the hypolimnion is almost negligible, but at the same time the consumption of oxygen by bacteria in the hypolimnion may be fast.
Cultural eutrophication is a widespread problem because so many human activities accelerate the rate of release nutrients inot the aquatic environment. Thousands of kilograms of nutrients per year may be discharged from a city’s sewage outlet. Clearing vegetation from land increases erosion which accelerates the leaching of both phosphorous and nitrogen from soil. The runoff streams from urban streets and surfaces also are highly enriched in nutrients as are many industrial discharges. Furthermore, sewage may contain a high concentration of waste organic materials that is decomposed by bacteria (leading to the removal of oxygen from water) and broken down into phosphorous and nitrogen.
Eutrophication is preventable and in some cased its effects may be reversible. Methods of advanced sewage treatment can be highly effective in reducing the nutrient content in sewage.
Although eutrophication is obviously not an insurmountable problem serious eutrophication continues to characterise a large number of small rivers, lakes and ponds worldwide. Oceans, in general, have not been affected by eutrophication because of their immense capacity to dilute pollutants and nutrients. Cultural enrichment has, however, affected several large marine water bodies including the Baltic Sea, the Northern Adriatic Sea, ocean basins off Sweden and Poland, Tokyo Bay and Mobile Bay in the United States.
How can eutrophication be controlled?
One common approach can be to divert waste water to fast moving streams or to the ocian. But this will transfer the problem as has happened with the Dal Lake. The lake sediments can be dredged to remove excess nutrient built up but i can be effective only in deep lakes. The underwater plant growth can be controlled with herbicides and algicides but there is every likelihood that addiction to such toxic materials may upset the ecosystem by magnification of food chain.
Any effort should, however, try to eliminate the problem at the source. It includes treatment of the sewage before it reaches the lake or river as is being done in Delhi for Yamuna.