In Nepal, the study of the limnology was started in mid 19th century. Brehm in 1953 studied the aquatic fauna in kali pho Khari i.e east Nepal-William. Hirni and foster in 1965 published the paper about algae from Nepal. Miller 19961 studied the plankton of Ranipokhari. Loffer in 1969 studied the limnological characteristics of high mountain lakes of Khumbu Himal. Hickel in 1973 studied phytoplankton of Kathmandu valley and the Phewa, Rupa, and Begnas lake.Importance of limnology
Man are using the natural resources more and more. So it is being clear that the major effect to reduce their stress must come quickly. Otherwise, water will become the limiting factor for the man. Even though aquatic systems are constantly renewed because water is purified by the natural hydrological processes by the nutrient cycles and by the populations of the biological organisms which can replenish the water giving sufficient opportunity and the time. However, these life-sustaining process works only if they are not overloaded with the pollutants and excessive nutrient and are not overused.Freshwater bodies
Lakes are the lentic water bodies with the depth of more than 13 feet. Forel (1892) defined lake as the body of the standing water occupying a basin and lacking continuity with the sea.Multons (1918) refers to the lake as those bodies of the standing water which are of considerable expense and deep enough to the stratify thermally.
Distribution of lakes
Some estimation of the total number of lakes on the earth has been done based on a detailed account of the lake in the USSR, the total area of all the lakes on the earth surface is 12.7x106 km2 and their total volume is the 166000km3 size and the depth of the lake also differ from each other.
Lake Baikal in Siberia contains the 1/5th of the total earth fresh water having a volume of 23x103 km3,the maximum depth is about 1620m and the mean depth is about 720m i.e the deepest lake. Half of the lakes of the world are found in the Canada, largest lake is the superior lake. Some of the largest lakes are the Caspian sea(USSR), Victoria (Africa), Huron (Canada), Aval sea (USSR) etc. lake Phewa, Rara, Tilicho, Shey pho ks undo, Begnas are some of the natural lakes of Nepal.
The classic thermal stratification pattern is the temperate zone and pattern which is described as follows:
1: Summer stratification
During the summer, the top water becomes warmer than the bottom water. As a result, only warm top layer i.e Epilimnion circulated and does not mix with the colder bottom water i.e hypolimnion creating and zone with the steep temperature gradient in between called thermocline. This is called the summer stagnation in the hypolimnion. Summer stagnation is severe as the oxygen supply in hypolimnion is depleted as both the green plant and surface source is cut off. Also, the high rate of decomposition of dead organic matter in summer consumes the oxygen present. This causes summer killing of the fish.
2: Winter stratification
With the onset of the cooler season, the temperature of the epilimnion drops until it is same as that of the hypolimnion. Then, the water of the entire lake begins circulating and the oxygen is again turned to depths. This is called the fall overturn. As the surface water cools below 4-degree centigrade, it expands becomes lighter remains on the surface and freezes thus bringing stratification. The winter stagnation is not so severe because the bacterial decomposition and respiration of organism are not so great at the low temperature. However, exceptionally severe cases may occur when snow covers and the ice prevents photosynthesis resulting in the oxygen depletion of the entire lake and winter killing of fish.
ReferencesMiller, Jr. G.T. Living in the Environment. Wadsworth Publication, 2003.