Limnology (2+1)

Unit 7- Aquatic plants

7.1 Aquatic plants- Character, classification, zonation, seasonal relations
Character of larger aquatic plants
Larger aquatic plants constitute a heterogeneous group composed of a few Bryophytes and Pteridophytes and many of the families of Spermatophytes. They are restricted in distribution to the general vicinity of the shores and to the shallow water areas. The larger aquatic plants of United States are cosmopolitan within the continent although they are highly specific in habitat requirements.
In general, the aquatic plants can be grouped in to 3 assemblages
1. Emergent – those rooted at the bottom and projecting out of the water for part of their length eg, common species of bulrush, Scirpus
2. Floating - these which wholly or in part float in the surface of water and often do not project above it eg. Duck weed Lemna and
3. Submerged - those which are continuously submerged eg, Vallisneria.
More or less elaborate grouping or classifications of larger aquatic plants have been proposed from time to time. Hence, the following classification taken with some modification from Akber’s work on Water plants in 1920, has many point of usefulness.
Biological classification of the larger aquatic plants
A.Plants roots in the bottom
1) Terrestrial plants capable of living at least temporarily as submerged water plants without any marked adaptation of leaves to aquatic life eg: Achillea ptarmica, Nepeta hederacea
2) Plants sometimes terrestrial, sometimes with submerged leaves but different from aerial type and associated with flowering stage
3) Plants which produce 3 types of leaf (a) submerged (b) floating and (c) aerial
4) Plants which in certain instances may occur as land forms but normally submerged and characterised by a creeping axis bearing long, branching leafy shoots with no floating laves
a. Leafy aerial shoots produced at flowering period eg, Myriophyllum
b. Inflorescence raised out of water but not aerial foliage leaves eg. Potamogeton, Myriophyllum
c. Inflorescence submerged but organs raised to the surface eg. Anacharis
d. Inflorescence entirely submerged eg. Najas
5) Plants which may occur as land forms but commonly submerged characterized by an axis forms with linear leaves arise
6) Plants which are entirely submerged having vegetative either root or shoot naturally attached to the substratum
B. a) Plants which are not rooted to the bottom but live unattached in the water
1) Plants with floating leaves, flowers raised into the air and roots not penetrating the bottom eg. Spirodella, Lemna
2) Rootless eg. Wolffia
b) Plants entirely or practically submerged; Rooted but not penetrating bottom eg. Lemna and Rootless eg. Utricularia, Ceratophyllum
Zonation
It is the most noticeable features of the larger aquatic plants is the distinct tendency in more or less parallel zones along the margins of lakes ponds and similar body and water.
Smaller lakes or the protected area of the water body and marginal regions of larger lakes the zonation may exhibit great regularity. A lake margin possessing this typical zonation shows the following sequences :
1) Zone of emergent hydrophytes – Here plants are rooted to the bottom, submerged at the basal portions and elevated into the air at the tops. It is a shoreward zone extending from near the edge of water lakeward up to a depth of 2 m. Most commonly found plants include Scirpus (bulrush), Typha (cat tail), Sagittaria (arrow heads) etc.
2) Zone of floating hydrophytes – This zone occurs beyond (lakeward) the emergent zone comprising plants which are rooted to the bottom but the foliage float on the surface. The depths occupied somewhat but usually about 10 cm to 2.5 m. The characteristic plants are water lilies Nelumbo, Nymphaea, Nuphar, pond weed Potamogeton.
3) Zone of submerged hydrophytes – Typically, this zone occupies deeper water beyond the zone of floating plants extending downward to a depth not exceeding 6 m. The plants of certain species of pond weed (Potamogeton), water milfoil (Myriophyllum), Vallisnaria, Najas and others are commonly found in this zone.
Seasonal relations
In regions of well defined winter season accompanied by the development of ice water and plants of emergent and floating types die down and disintegrate with the onset of winter. This disintegration occurs among the wholly submerged plants and still uncertain and evidences have shown that atleast few of them were in active condition throughout the year even under the ice.