Do the limnetic and littoral zones of the same lake have the same trophic structure?
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Answer:
A typical lake has three distinct zones (limnetic, littoral and the benthic zone; Fig. 11) of biological communities linked to its physical structure. The littoral zone is the near shore area where sunlight penetrates all the way to the sediment and allows aquatic plants (macrophytes) to grow. The 1 % light level defines the euphotic zone of the lake, which is the layer from the surface to the depth where light levels become too low for photosynthesis. In most lakes, sunlit euphotic zone occurs within epilimnion. In transparent lakes, photosynthesis may occur well below the thermocline, into the perennially cold hypolimnion. The higher plants in littoral zone, in addition to being a food source and a substrate for algae and invertebrates, provide habitat for fish and other organisms different from the open waterzones.
The limnetic (pelagic) zone is the open water area where light does not penetrate to the bottom. The third component of the lake habitat is benthic zone (the bottom of the lake), covered by fine layers of mud in which animals live. In the littoral zone, there is enough light for rooted plants to grow, but beyond this zone, there are no rooted plants as the water is too deep for light to reach them. The deepest part of the open water forms profoundal zone, but this is relevant only in extremely deep lakes.
In each community, green plants produce food upon which all aquatic organisms depend. Hence they are called as autotrophs. Animals, which feed on these plants, are called as herbivores and those that eat other animals are called carnivores. Omnivores have a mixed diet and detrivores feed on dead and decayed plant and animal material. All the feeding habits link to form the food-web.
Fish are usually the largest animals in the lake, feeding on most invertebrates; some eat other fish and very few are herbivores. There are more than 20,000 species of fish out of which 7000 live in fresh waters. Fish feed on plankton, plants and invertebrates and are in turn eaten by carnivores such as birds, otters and man. At each step, food is eaten to gain energy and nutrients. Most of the energy generated is used to support daily activity and is dissipated into the environment as heat. Only surplus energy is available for growth and reproduction, such that the energy available for the next link in the food chain is always less than that which entered the present trophic level. Hence, there cannot be more carnivores than herbivores.