Man-induced eutrophication has several manifestations of which the most prominent is the nuisance growth of planktonic algal blooms and the luxuriant growth of benthic algae and rooted macrophytes in shallow near- shore areas.
Although the growth of algae and plants is influenced by several environmental and physiological factors, the greatest single factor influencing productivity is the nutrients, especially phosphorus. Hence, the control of eutrophication is generally directed toward preventing the input of nutrients in the lake. A phosphorus control programme is the most effective way of combating Lake Eutrophication.
Such a programme involves the reduction or removal of phosphorus from detergents, a nutrient control programme at wastewater treatment plants and processing or storage of urban runoff and agricultural wastes.
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These are two broad approaches to Lake Recoveiy, viz:
(1) Methods to limit fertility and/or sedimentation in lakes, and
(2) Procedures to manage the consequences of lake aging (Ryding, 1981). The former approach tackles the root cause of the problem whereas the latter approach attempts to treat the superficial symptoms.
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Several physical, chemical and biological factors affect water quality in a lake. Some of these factors are light, wind, nitrogen, phosphorus, and chlorophyll content, the growth limiting effect of nutrients, and phosphorus release from sediments.
Different combinations and degrees of these and other related factors generate different results and experiences in attempts at rehabilitating different lakes. No one method is capable of remedying completely the problem of overgrowth associated with nutrient-enriched waters.
Even though the relationship between nutrient loading and chlorophyll standing crop has been recognized since long, it remains impossible to predict precisely as to how any given lake will respond to an artificial reduction in nutrient inputs. In some cases, sewage diversion and improved wastewater treatment operations have successfully reversed the eutrophication as, for instance, in Lake Zurich, Lake Washington, Onondaga Lake and Lake Boren (Forsberg et al., 1978).
In these lakes the major source of nutrient input was domestic wastewater and, when its input was reduced, the lakes began to recover. However, similar attempts in Lake Leman and Lake Sillen have failed to reverse cultural eutrophication till now. Many shallow, heavily polluted lakes have likewise not responded to advanced wastewater treatment and reduced phosphorus input efforts mainly because of recycling of phosphorus from bottom sediments.
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On the basis of this work on several Swedish lakes, Ryding (1981) has recommended the adoption of algal assay results into lake management schemes for assessing the effect of excess plant nutrients in polluted lakes; plotting the algal growth potential in terms of chlorophyll against lake water chlorophyll can give some indication about the extent of inter-relationship between these parameters in lake recovery studies (Ryding, 1981). In those lakes, where no signs of decrease in the algal standing crop are manifested, after the adoption of remedial measures, it may be worthwhile to determine the C: N: P ratio of the algal particulate matter.