Limnology (2+1)

Unit 2- Lakes

2.1.2 Thermal Classification of lakes
According to Hutchinson (1957), following are the classification of lakes based on changes in temperature of surface water.
a. Amictic: No mixing of bottom and top water; lakes insulated or protected by ice-corer, there is no effect of weather or external factors.
b. Monomictic: One mixing of the two waters during the year (most deep lakes of the world).
c. Cold monomictic: Water here at any depth never exceeds 4°C; they are ice-bound or ice-covered only in winter; there are inverse thermal stratification top waters 0°C and bottom waters 4°C (since water at 4°C is heaviest); only one mixing at temperatures not more than 4°C in spring / summer eg, Polar lakes.
d. Warm monomictic: Temperature of water never falls below 4°C at any depth. Direct thermal stratification top waters 10 - 20°C and bottom waters 8 - 4°C; only one mixing in a year in a winter eg, Most subtropical deep lakes.
e. Ploymictic : Mixing is continuous, but occurs only at low temperatures.

Size of lake
Lakes differ in area from those ranges from a pond to those of great size. Lake Superior, the largest body of freshwater flow has an area of more than 49,600 km². The Caspian sea with an area of 2,72,000 km² is sometimes considered as having the quality of lake.
Lake Chad in Africa has 64,000 km² during wet season, but is reduced to 9,600 km² in the dry season. Ten of the large lakes in America including Great lakes have an combined area of about 2,03,200 km². However the number of lakes whose area exceeds more than 8,000 km² is insignificant when compared to many thousands of lakes of lesser magnitude of 11,000 or more lakes or ponds in Michigan.
Depth of lake
Lakes vary in depth but even the deepest lake will never approaches the depth of ocean. It is important to note that the lake Baikal has a greatest depth contains about 20% of the total volume of freshwater and it is also the deepest known lake with a maximum depth of 1620 m. In North America, Crater lake in Oregon is about 608m. Lake Tahoe is 487m, Lake Chelan (Washington) 457m. Seneka lake 188m, Lake Superior 393m, Lake Michigan 281m, Lake Huron 228m, Lake Ontario 273m, Lake Erie 64m, and the last 5 lakes constitute Great lakes of America.
Lake margin
Nature of margin
The line of demarcation between land and water is the margin of the lake which depends upon a number of circumstances.
Shore dynamics
Water is in some form of motion ranging from gentle to violent. It has a great potentiality to cause changes on the shore against which it beats. In lake, the wave action is the principle form of water movement that cause shore changes. Modification of the original shore line has been accomplished by two main phenomenon such as shore cutting and shore building.
Lake bottom
The term lake bottom includes all part of bottom of lake basin from the water edge to the deepest region. All lakes of particular region may have the same origin, may have begun the history with same material and exist at the same climatic condition, yet the bottom material may often be different in different lakes. The kind of bottom deposits and the rate of deposition may depend upon the local circumstances. The nature of bottom deposits determines the biological productivity. The principle sources of bottom materials are
i. Bodies of plankton organisms which die and sink.
ii. Plant and animal remains.
iii. Organic and inorganic materials.
iv. Silt, clay and similar materials.
v. Marl or CaCO₃ precipitated produced by plants and animals.
vi. Remains of floating blanket algae.
Diversity of lake
Though all the lakes appear to be similar, there may be differences in colour, taste, hardness, turbidity and aquatic animals and plants. With the knowledge and the modern methods of environmental analyses, the lakes posses physical, chemical and biological diversity. According to a great diversity, lake may be stated into different forms as under :
a. Large, medium or small.
b. Deep or shallow.
c. Protected or unprotected.
d. With or without tributaries and outlets.
e. Fresh, brackish or salt.
f. Turbid or clear.
g. Acid, natural or alkaline
h. Hard, medium or soft.
i. Surrounded by bog, swamp, forest or open shores.
j. High or low in dissolved content.
k. With or without stagnation zones.
l. With mud, muck or mucky sand or false bottom.
m. With high, medium / low biological productivity.
n. With / without vegetation beds.
o. Young, mature and senescent.