1.3.2 Lakes

Unit 1- Limnology- An introduction

1.3.2. Lakes
Forel (1982) defined lake as a body of standing water and occupying basin and lacking continuity with the sea. He also defined pond as a lake of small depth, and a swamp has been defined as a pond of small depth and occupied by rooted vegetation. Carpenter (1928) formulated that the true difference between lake and pond is depth and not area accordingly a pond is a quiet body of water where floating vegetation extends to the middle of basin in which the biota is very similar to littoral zone of lake.

Lake Morphology
The shape of a lake basin is largely determined by its mode of origin. The depth and contour of lake bottom can be determined by lowering a weighted line or much more quickly with an echo sounder. Physical structural components of lakes include their shape, distribution of light, distribution of heat, and movement of water. The hydraulic retention time (time required for all the water in the lake to pass through its outflow) is an important measure for lake pollution studies and calculations of nutrient dynamics. The hydraulic retention time is mainly determined by the interplay between inflow of water into the lake and the basin shape.

Lake Zonation
The following depth zones are recognized in lakes:
a. littoral zone extends from the shore just above the influence of waves and spray to a depth where light is barely sufficient for rooted plants to grow.
b. photic (euphotic) zone is the lighted and usually well-mixed portion that extends from the lake surface down to where the light level is 1% of that at the surface.
c.aphotic zone is positioned below the littoral and photic zones to bottom of the lake, where light levels are too low for photosynthesis. Respiration occurs at all depths so the aphotic zone is a region of oxygen consumption. This deep, unlit region is also known as the profundal zone.
d.compensation depth is the depth at which rates of photosynthesis and respiration are equal.
e.sublittoral zone, which is the deepest area of plant growth, is a transition between the littoral and profundal zones.
f.pelagic zone (limnetic zone) is the surface water layer in offshore areas beyond the influence of the shoreline.
Boundaries between these zones vary daily and seasonally with changing solar intensity and transparency of the water. There is a decrease in water transparency with algal blooms, sediment inflows from rivers or shore erosion, and surface waves.


sd


Last modified: Tuesday, 24 April 2012, 5:35 AM