Limnology of tropical impoundments: a comparative study of the major reservoirs in Madras State (India)

Summary A limnological study of seventeen man-made lakes has been made. Thermal features showed absence of stable thermal stratification. Cognisable thermocline did not exist. The South-Western reservoirs with low nutrient and salt content were very poor in productivity, but the high alkaline reservoirs were also not very productive. Some reservoirs with lower alkalinity, hardness and total solids developed algal blooms and were highly productive. Bhavanisagar and Stanley reservoirs with optimal levels of dissolved salts and alkalinity-hardness were fairly productive. Reservoirs with organic soils were better in productivity than those with higher silica content (Hope Lake, Poondi Reservoir, Vaigai etc.) Dissolved oxygen depletion in the bottom was characteristic of productive waters. Total oxygen depletion occurred in some of them but in a few depletion was rare and not of great severity. Klinograde curves were characteristic of productive reservoirs. Even when the bottom waters were low in or were devoid of dissolved oxygen the outflow from this level picked up oxygen within a short distance of 100 m. Effect of water replacement on plankton production is discussed. Primary production in the reservoirs has been compared and discussed. It is the silt-laden turbid waters that are poor in production. Water releases could be manipulated to assist spawning of fishes and for amelioration of pollution downstream.