Limnology

Unit 7- Aquatic plants

7.2 Quantities produced, chemical composition, distribution in different waters, limnological role
Quantities produced
Production of aquatic plants varies greatly with the nature of the water. It has been studied in lake Mendota, the Vallisneria constituted about 1/3rd of the total quantity; while in Green lake it composed < 10% where Chara comprised roughly one-half of the total quantity whereas in lake Mendota it amounted < 5%. Therefore it has been predicted that the production of larger plants differ widely both qualitatively and quantitatively.
Chemical composition
During the growth period of plants, temporarily certain essential substances from water and bottom deposits the chemical composition of plants which may give some information on amount of substances used. Plants make demand upon the supply of essential materials in the water. The amount of different substances removed or returned to the water have been studies. The most important ones are Ash, crude protein, ether-extract, fiber and carbohydrates have been determined.
Distribution in different waters
It is well known fact that the chemistry of water influence in determining general distribution. The qualitative composition of aquatic flora differs in different types of water bodies in inland areas. The aquatic flora has been divided into 3 major groups namely
1. soft-water flora
2. hard-water flora and
3. alkali or sulfate-water flora
Limnological role
It is necessary to consider the functions which the larger aquatic plants play in aquatic complex. The relations, direct and indirect are numerous but the following ones are probably the most important.
a. Utilization of non-living matter
The utilization of mineral salts and CO₂ in the building up of green plant tissue needs more emphasis here. It has been reported that the roots of larger aquatic plants serve primarily as provisions for holdfast having very little physiological function in absorbing nitrates from bottom and the absorption of nutrient materials is performed mostly by the body of the securing substances from water only. Some of the plants roots are very much reduced and the function of absorption is insignificant. However, the function of anchorage has an outstanding role in these plants and like wise the reduction of absorption by the roots compared with land plants is also established.
b. Food for animals
The role of the phytoplankton in basic food supply has been known but different opinion existed as for as larger aquatic plants concerned as food materials for aquatic plants. Recent studies have shown that large quantities of these plants are often consumed by great variety of animals. Berg (1950) studied that about 17 species of Potamogeton have been consumed by 2 dozen different species of insects. Fragments of larger aquatic plants occur in stomachs of fishes. Analysis of stomach contents have been reported in certain fishes the plant materials composing up to 50% of the total food content and hence it is mentioned that some fishes are largely plant feeders. Aquatic birds (ducks, geese), mammals (musk rat, deer, moose and beaver) secure food from aquatic plants. Nelsom et al. (1930) found that some of the plants like Anacharis, Myriophyllum and Vallisneria have high protein and carbohydrate content including several vitamins.