Limnology (2+1)

Unit 10- Biological productivity

10.1 Circulation of food material, classification of lakes based on productivity
Circulation of food materials
Essential food stuffs in a lake undergo continuous, more or less definite cycles. Natural water body is completely closed community since there are various direct and indirect influences exerted from outside. In considering the circulation of food materials in a typical lake, the following fundamental facts must be kept in mind.
1)The ultimate basic substances are
a) inorganic nutritive materials dissolved in water and
b) certain energies and gases from the atmosphere
2)Chlorophyll bearing plankton and flora and certain bacteria can utilize directly basic materials in constructing living mater
3)Other organisms, plants or animals are of a dependent superstructure
4)Every organisms of a lake population may
(a) by death and disintegration contributes directly to the dissolved materials and detritus or
(b) be consumed as food by other organisms.
Circulation of food materials
Two general set of processes are construction process and reduction process. Building upward from the simple food material into the higher is a construction process. Reduction through bacterial action into simple substances is a reduction process. Cycles are so interrelated that materials may be built up to the same level in different ways. One cycle through which animals are freed from dependence upon photosynthetic plants ie from basic dissolved nutritive materials through bacteria which change CO₂ to organic carbon in the absence of sunlight to certain animals which consuming them.

Figure : Circulation of Food material in a Lake
Classification of lakes on the basis of productivity
Many attempts have been made to classify lakes on limnological bases, related in some way or another to productivity during the past five decades. The combined contributions of a number of those attempts have been lead to the classification of lakes into three major types viz,
1. Oligotrophic lakes
2. Eutrophic lakes and
3. Dystrophic lakes

Following Table gives the details of difference between above listed lakes

Further, other difference between Oligotrophic and Eutrophic lakes is given below