Limnology (2+1)

Unit 1- Limnology- An introduction

1.2.2. Distribution of inland waters
Inland waters cover less than 2% of the earth’s surface, approximately 2.5 x 106 km2. About 20 lakes are extremely deep (in excess of 400 m). A significant portion of the world’s freshwater is contained in lake. Some regions are very generously supplied with lakes and streams particularly those regions once subjected to ancient glaciation. Canada and northern United States possess an immense supply of lakes, among them the Great lakes, which constitute the greatest body of freshwater on the globe. Portions of Europe are also noted for their generous supply of lakes and streams. In certain regions, disappearance of inland waters during the dry season forms the basis for special biological phenomena resulting from the intermittent character of the environments.
In India, most of the wetlands including flood plain wetlands are situated in the eastern parts of the country whereas, reservoirs and tanks have been created mainly for irrigation are distributed throughout the country. Large rivers and streams are well distributed in the northern and eastern region of the country. However, southern regions of the country also have a good number of inland waters. Many of them are seasonal in nature.

Dynamics of Lotic and Lentic environment
In the lotic series, the tiny rivulet gradually deepens, widens its bed, and cuts back at its head, thus in time extending its length and increasing its cross section to that size which justifies the designation of brook. This process continues by the same general type of action, ultimately producing a creek and then finally a river, with all of the integrating conditions produced in such a gradual transformation.
Faunas occupying each of the different environments must accompany these migrations or become adapted to the gradually altering conditions or they will become extinct. These environmental migrations are very slow, in point of time, and give ample opportunity for the characteristic organisms of particular environments to make the necessary responses. The ultimate fate of any lotic series is the degradation of the land is the reduction of its bed to base level.
In the lentic series, natural processes work toward extinction, mainly by the gradual filling of basins.
Lake –> Pond –> Swamp
In larger lakes, natural filling takes much longer time, even many centuries also hence the filling is primarily due to :
• Wind blown materials such as dust, sand and debris of various sorts.
• Sediments brought into a lake by inflowing streams and by incoming run-off water as it flows down adjacent land slopes.
• Wave action, cutting away exposed shores and depositing eroded material in lake basin.
• Plants, particularly the higher aquatic plants which grow in shallow water, produce deposits of organic matter.
• Accumulating remains of animal life especially shells.
Not all lakes become extinct by filling alone. Other process also contributes to this for example an outlet may cut down its level at the point of exit from a basin, thus gradually draining the lake.
These stages in the extinction of standing waters result in a more or less definite, predictable ‘evolution of environment’ in the long run has a profound influence on the history and fate of lake organisms.

Running waters (Lotic series)
There are many different kinds of running waters, several of them occurring, inter-connected, within a single drainage system. The range covered within the series includes small trickles and seepages, ditches, larger fast flowing streams and rivers, large slow flowing rivers and canals.
The flow characteristic of any running water system are also closely connected with the geology, notably in the control exerted by the nature and structure of rock and soil formations, and also in the relationship between the amount of ground water and surface water flowing through the system. The basic flow pattern depends largely on the nature of this relationship.
Most of the water on the earth is in constant circulation within what is known as ‘hydrologic cycle’. The energy utilized within the cycle comes mainly from the sun. Water evaporates from both land and sea to be re-precipitated, usually somewhere else. On most part of the land, precipitation exceeds evaporation, and run off towards the sea occurs.

Difference between running water and standing water
• Current : Unidirectional main current is found in running water but not in standing water.
• Depth: It is small in running water, more in standing water.
• Condition of gradient from source to mouth : In running water, physical, chemical conditions usually change from the source to the mouth and the difference in many factors may be great between those extremes, but it is more homogeneous in standing water.
• Water of the basin: Running water systems are very shallow and have long, complex narrow channels, but standing water reach great depth, have broad basins.
• Permanent removal of eroded and transported materials: Constant erosion is common in running water and materials so removed are transported to distance. Erosion occurs in standing water, but it is rarely severe, eroded materials and not carried far away but remains within the same basins.
• Absence of prolonged stagnation: Consequence of erosion and deposition, most of the running waters tend to increase the length of their channels with age. In standing water materials constantly being deposited tend to fill in the basin.
• Physical factor: It is more important in running water and standing water.
• Basic food materials: Most running water manufacture themselves little basic food, but depend on the contribution from the surrounding land than the standing water.