Aquatic weeds

AQUATIC VEGETATION
    Aquatic vegetation refers to plants which grow partially or completely in waterbodies. They may have their roots grow from the bottom of a waterbody (e.g Nymphaea or water lily) or may be floating plants (e.g Pistia or water lettuce) or may grow near the waterbody (e.g Ipomoea). Aquatic plants, at low levels, have beneficial effects on fish in that they serve as food for some species (e.g Oreochromis niloticus eats Lemna), vegetative cover against predators like birds etc.
        However, when aquatic weeds become excessive, they:
-Limit light penetration into the pond
-Compete with fish for pond nutrients
-Cause oxygen depletion in the pond
-Provide refuge for other animal competitors and predators of fish
-Hamper movement of fish
-Hinder the feeding and harvesting of fish
-Accelerate silting in ponds.
-It is easier and cheaper to weed out excessive vegetation regularly than to restore a pond in which vegetation and silting constitute a problem.

Classification of aquatic weeds/plants as discussed during industrial attachment

Algae – Algae can also be classified into phytoplankton and filamentous algae.
      Phytoplankton are algal plants which may multiply rapidly into dense masses (algal blooms) thereby imparting colours ( green, blue-green, reddish-brown etc.) to a waterbody. 
Filamentous algae (e.g. Spirogyra spp and Cladophora spp)

1) Marginal weeds – These grow around a waterbody but may spread all over the water surface e.g Typha

2) Floating weeds – The leaves and flowers float on the water surface while the roots are within the water column e.g Azolla, Pistia, Lemna and Eichhornia

3) Emergent weeds – The leaves, flowers and in some cases the stem, emerge above the water surface but the roots grow from the bottom of the water body e.g. Myriophyllum and Nymphaea

4) Submerged weeds – Rooted or rootless plants that grow below the surface of a waterbody e.g Hydrilla and Ceratyphyllum.

Control of aquatic weeds/vegetations
       The control of excessive vegetation will improve fish production from a waterbody. Excessive vegetation is controlled by manual, mechanical, chemical, biological and integrated methods.

1)   Manual method
This entails the manual removal of aquatic weeds either by pulling or picking with the hand or harvesting weeds with the nets and other hand held implements. This is a very tedious job in large waterbodies overgrown with weeds or fertile waterbodies with rapid weed growth.

2)  Mechanical method
This involves the use of machines (e.g floating weed harvesters and diesel winches) for controlling weed. Such machines may be difficult to operate and expensive for small scale farmers.

3)  Biological method
This method utilizes animals (herbivorous aquatic animals like fish and ducks, insects like weevil etc.) and plants (aquatic weeds like Lemna which prevent algal bloom) to control weed growth. Fish species such as grass carp, common carp, Tilapia zillii are known to be very effective in the control of aquatic weeds. Swans and ducks, which move around the edge of the pond in search of food, assist in the control of submerged vegetation when they dig into the soil. Lemnaor duckweed is a delicacy for ducks.

4)  Chemical method
This entails the use of chemicals at levels which are not lethal to the fish but are capable of controlling the target weed e.g. copper sulphate at 0.02-0.5mg/litre is used for the elimination of excessive plankton bloom in water. No single chemical can control all aquatic weeds and knowledge of the weed species, active ingredients in the chemical, controlling action, persistence of the chemical, method and time of application of the chemical etc. is crucial in order to apply it in an environmentally friendly manner.

5)  Integrated method
This involves the combination of two or more of the foregoing methods in aquatic weed control e.g chemical along with manual method,manual method and mechanical method etc.

Uses of aquatic weeds

a) Pollution Control – Some weeds absorb organic and inorganic chemical compounds from water and this helps in the removal of harmful substances like pesticides, heavy metals, phenols and purifies water for use by man and animals.

b) Production of biogas – Aquatic weeds have been successfully used in biodigesters for the production of biogas (mainly methane).

c) Animal feed –Aquatic weed can serve as food for fish either while it is growing in the pond or fed after being cut into smaller pieces. It can also be incorporated as a component of processed feed.

d) Fertilizer – Decayed aquatic weeds can be used as compost material for crop farming and pond fertilization.