Limnology (2+1)

Unit 10- Biological productivity

10.2. Laws of minimum, biotic potential and environmental resistance, quantitative relations in a standing crop
Law of the minimum
Existence and production of animal and plant life depend upon the proper quantitative and qualitative composition of the environment for each component organisms. Liebig’s law of the minimum originally applied to plants and is stated as each organism required a certain number of food materials and each of these materials must be present in a certain quantity. If one of these food substances is absent or present in minimal quantity the growth will be minimal. The yield of a plant or animal according to this law is determined by the quantity of that particular substance present in minimal amount as per the demands of the organism.
The law of minimum has had a wide acceptance. Liebig’s law of the minimum is the foundation of law of limiting factors which is dependent on one factor upon another. Various environmental factors acted independently wherein if one factor in present in limiting quantity and increase of other factors would effect no change.
Biotic potential and environmental resistance
The biological productivity of any body of water or any portion of that body of water is the end result of the interaction of organism present with the surrounding environment. Biotic potential is the characteristics and abilities inherent within an organism which enable it to exist and reproduce. It is the sum total of all of those capacities of an organism which determine its relative success in solving all problems of maintenance. It is a sum of the number of young produced at each production.
Every environment contains active features which work toward the control of production in various organisms involved. Thus the environment resists to a greater or less extent for the fulfillment of biotic potential. In the long run, nature acts toward a balance between these two tendencies in which each organism maintains itself in a suitable environment without overpopulation.
The principles of biotic potential and the environmental resistance together with all of their associated features are just a pertinent in aquatic biology as in terrestrial situations. Biological productivity of any aquatic community is a general measure of all of adjustments between biotic potential and environmental resistance existing within it.
Quantitative relationships in a standing crop
Any body of water maintains a certain standing crop of organisms composed primarily of 5 large groups viz, phytoplankton, bottom flora, zooplankton and fishes. This series composes a nutritional chain in which the first two constitute a producing class and the other three are the consumers. Therefore these organisms are an expression of the productivity of water concerned.

Figure : Diagram showing weight relationship of Biota and dissolved organic matter

The actual values expressed into this pyramid of aquatic life are different to some extent in different lakes but some form of pyramid is the rule. This figure illustrate the dissolved organic matter composes approximately 60% of total diagram, the fish only one-half of 1% and the other animals slightly more than 5%.