Limnology (2+1)

Unit 10- Biological productivity

10.3. Laws of minimum, biotic potential and environmental resistance, quantitative relations in a standing crop
Trophic dynamic
In the trophical dynamics of an ecological system, basic processes are in the nature of transfers of energy. The ultimate source of such energy is solar radiation. Earlier work have raised the biological conclusions which seem to have a certain validity like :
a. food cycles rarely have more than five trophic levels
b. separation of an organism from basic source of energy (solar radiation) lead to less chance of dependence upon trophic level for energy
c. the consumers seem to be progressively more efficient in the utilization of food supply
d. productivity and photosynthesis increase from oligotrophy to eutrophy and then decline in lake.
Successional phenomena
All environments are dynamic and undergo changes is the fundamental principle of ecology. These fundamental changes more or less predictable alternations involving expanses of time and these changes may be due to :
a) the action of predominating inorganic factors in the environment eg, erosion
b) the action of organisms in modifying the environment or
c) the combination of (a) and (b)
One fact common to all situations exists that the various components of the biota must meet the changing conditions in one of the following ways, such as :
a) adaptation
b) migration and
c) extinction
Ecological successions of various kinds go on in lakes and other inland waters as certainly as they do on land. The movement of the units of lentic series is in the direction of extinction by filling of basins and in the lotic series it is in the direction extension of stream length and a cutting of stream bed to base level.
Eutrophication
In general and within limits, the productivity increases with the age of a lake. Storm (1928) has stated the process as follows:
The natural process of the maturing of a lake is that of eutrophication. The original state of all lakes must be assumed to be oligotrophic but later due to surplus organic sediment occurring from life process of a lake is changed to eutrophic condition. The quantities of plankton, oxygen curves and average depths are the first features to be changed and later the bottom fauna.
It must be clearly understood that the maturing process takes place at very different rates in different lakes. For example, in northern United States, majority of lakes basin formed during glacial period but they have matured at different rates and many small basins have long ago passed the succession stages into old age and are become dry land. If the lake is smaller, a rapid eutrophication and further extinction take place but certain other lakes fail to go through the usual evolution from oligotrophy to eutrophy even though natural filling may render the hypolimnion smaller than the epilimnion.
Dystrophication
The dystrophic lake basins during their initial stages are low productive (essentially oligotrophic). These primitive basins varied greatly in size and depth covering considerable areas having hypolimnion exceeding the epilimnion. During certain circumstances an incomplete decay of plants and accumulation of humic materials appear the beginning of dystrophication. After the initiation of dystrophy, the succession progressed by marginal plant encroachment or by bottom accumulations in incompletely decayed plant materials or by both and passing through the stages into a peat bog.
Indices of productivity in lakes
Limnologists have looked for indices of general biological productivity in lakes. There are two considerations involved and must be kept clearly distinguished, they are (a) the inherent capacity of a lake to support life (biotic potential) and (b) the actual productivity at a given time. Obviously one or two indices of productivity would give a dependable evaluation.
1.Average depth : The average depth of a lake is the determining factor for productivity. The dissolved oxygen content of various layers of a lake is the indicator of richness in nutritive substances especially at a depth up to 10 m stratum. The dissolved oxygen content is greater in oligotrophic lakes than that of epilimnion. Certain other features mainly the degree of development of littoral regions constitutes important influences in determining the production of the lake.
2.Rooted submerged vegetation : Kluge (1926) claimed that the amount of rooted, submerged vegetation may be an index of lake productivity. It is a well known fact that the amount of rooted submerged vegetation is governed by a number of factors such as degree of exposure and slope of the submerged shelf. Large lakes for example, lake Nipigon may maintain a great fish production which could be predicted from the scanty vegetation, whereas the small lakes having submerged vegetation does not show productivity.
3.Plankton : Plankton is an index of general production. Eutropic lakes are characterized by quantitatively rich in plankton, while oligotrophic lakes have a plankton poor in quantity. It has been claimed that abundance of plankton is associated with rich bottom fauna and paucity of plankton accompanies a poor bottom fauna.
4.Bottom fauna : European workers have stressed the quality of the bottom fauna at deeper water as an indication of the productive character of a lake. Form of basin, character of bottom deposits, water movements etc. would be a true index of the general richness of a lake. When a rich benthic fauna is present a high total productivity is common.
5.Organic content of water: The standing crop of dissolved organic matter is much greater than the total organic matter in the plankton supported by the same water. The dissolved organic matter is said to be constant in quantity and composition that the character of a lake may be judged. It has been shown that a constant relation exist between the plankton and the total organic matter in the water. The presence of organic content has become a new subject to predict the general index of productivity.
6.Chlorophyll content : Chlorophyll content is used as an index of the photosynthetic capacity. This measurement can be used as a convenient method for evaluating biological productivity.
7.Other indices : The organic content of bottom deposits is important as food for benthic organisms. In general hard water lakes are not highly productive. The total alkalinity and total phosphorous appear to be the most valuable indices of productivity.
Artificial enrichment
The waste products of human beings and industries often find their way into natural waters and produce contaminations. Removal of forests and tilling of land bring changes through the medium of drainage but are not the nature of contaminations. The enrichment of water is due to the addition of substances and subsequent changes increase the amounts of essential nutritive materials. Among the contaminations, most likely are the domestic sewages downwash from manured fields and other organic matter.
The enrichment effects due to contamination are more to be expected in the smaller lakes than larger ones. Evidences have shown that the sewage from large cites brought into the larger lakes enters farther into the open lakes and dilutions becomes greater leading to enrichment having distinctly increased biota.
Sometimes and also at present use of fertilizers have favoured the fish production in the water bodies. Repeated application of artificial fertilizer to natural waters are dangerous leading to winter kill of fish in the northern United States lakes. Increasing production in enclosed waters such as ponds, small lakes and reservoirs is more prominent compared to large natural bodies and water.