We studied the aquatic macrophytes of Muriyad wetlands to assess the ecological effects and role of macrophytes in relation to rice cultivation and fishing. Muriyad wetland is a seasonal freshwater floodplain wetland of 63 km 2. The whole area is inundated by water for about six months.  

Ipomoea carnea , Salvinia sp ., and Limnocharis flava are some of the major weeds seen in this wetland. Ipomoea carnea is a plant with high regenerative capacity and is very difficult to eradicate by physical methods. Salvinia spp . and Cyperus cephalotes together form heavy floating mats in the wetlands. Considerable amount of labour is needed to remove these weeds manually. A few years back Limnocharis flava was found only in some abandoned paddy fields but now have spread into larger areas.   Removal of the weed manually   is not effective since only petioles will get detached, allowing it to regenerate later.   Nymphaea, sp., Nelumbo sp . and Ipomoea aquatica are some of the other major weeds seen in this wetland.

Freshwater ecosystems are an integral part of a geographical region. These systems provide habitats for many plant species, fishes, birds, insects, and other animals. Their interaction with physical factors results in a balanced ecosystem, with plants providing food and shelter for other organisms that live in and close to the water. Thus, the plants in a water body may influence not only the aquatic system but also the surrounding areas. 12% of all animal species live in freshwater ecosystems and many more species are closely associated with these ecosystems (World Resources, 2000-2001)                       

Aquatic plants respond strongly to the environmental conditions within a wetland. The chemical environment of many aquatic systems has changed at an alarming rate during the last century due to the intensified exploitation of the land for farming, urban expansion, and due to atmospheric and water pollution.

Macrophytes, plants observable by the naked eye, are an important component of aquatic ecosystems. The tolerance limit of aquatic macrophytes is linked with the chemical and physical quality of water (Heegaard et al, 2001). It is therefore, possible to quantify environmental changes of a wetland by analysing these plants.   Changes in the community composition, or   the abundance of individual species, provide valuable information on how and why an ecosystem might be changing (Scott et al, 2002). Macrophytes are also becoming increasingly valued as a means of indirectly monitoring water quality. Eutrophication, for example, can produce a progressive change in species composition resulting in the eventual loss of species diversity (Kelly and Whitton, 1998).

Macrophytes provide a refuge for small animals against predation, change in the nutrient dynamics of the system, and prevent re-suspension of the sediments. Vegetated lakes usually harbour a richer community of invertebrates and fishes and attract large number of birds than lakes without vegetation (Scheffer, 1998). Eutrophication due to anthropogenic intervention results in the decline of submerged vegetation and the domination of phytoplankton in many lakes (Korner, 2001).   

The increase in plankton abundance may be mediated by the presence of dense beds of aquatic macrophytes. Macrophyte beds are favourable for zooplankton development, owing to the refuge they provide against fish predation. Many studies have shown that in the presence of planktivorous fish, zooplankton abundance is much higher within the beds of macrophytes than in open water (Lauridsen et al, 1996). In fluvial lakes with a strong central current, macrophyte beds may provide zooplankton, a refuge from flushing. These beds in fluvial lakes also favour the development of phytoplankton and zooplankton by increasing the water residence time (WRT), allowing more time for populations of planktonic organisms to develop (Basu, et al., 2000). Macrophytes contribute a bulk of biomass production in floodplain wetlands. They act as substratum for periphytic growth, provide shelter to various aquatic fauna and serve as a breeding ground for associated fauna (Mitra, 1997).

The Muriyad wetland is situated 8 km northeast of Irinjalakuda town, Trissur district of Kerala state. The area for an intensive study was determined based on the watershed boundaries of the locality.   The study area falls between 10 o 25' 32'', 10 o 18' 47'' N latitude and 76 o 17' 19”, 76 o 12' 48” E longitude. The micro watersheds together cover an area of 57.75 km 2 . The narrow central strip of about 16 km 2 running north to south from Karuvannur River is permanently or seasonally water logged. Most part of this wetland is situated below mean sea level. Direct observation was employed as the major study method. The taxonomic position of the plants was determined based on the classification of Bentham and Hooker. The influence of these macrophytes on rice cultivation and fishery was recorded through discussion with farmers and fishermen of the locality.

Fifty major plants were identified from the study area. Drosera burmanii, Linn. is a threatened species seen in this area. Limnocharis   flava, Buch   rapidly   spreading in this area is one the major weeds in paddy fields. Ipomoea   carnea, Jack., Salvinia molesta and Eichhornia crassipes, Solms are other weeds seen in this area.

Table 1. Ferns    

Class: Dicotyledons

Class:Monocotyledons

Marsilea is a weak herb usually seen in paddy fields and irrigation ditches. It is reported to have some adverse impacts in rice fields. However, it cannot be considered as a weed as far as the paddy fields of Muriyad wetland is concerned. Seedlings remain submerged and adults float as emergent plants. Azolla was another fern represented in this wetland. It harbours symbiotic nitrogen fixing Cyanobacteria, Anabaena azollae , and hence luxuriant growth of this fern can increase the fertility of paddy fields. It is calculated that Azolla can substitute about 25-30 kg of nitrogen/ha as applied through ammonium sulphate (Cook, 1996).

Salvinia molesta , considered as the ‘worst weed',   was introduced to India from South America. This free-floating plant has become a major threat in almost all the water bodies of Kerala. Due to its rapid growth rate it can spread over far and wide areas. During flood season, Salvinia covers almost the major part of this wetland. The usual agricultural practices of Muriyad wetlands commences with dewatering the fields and salvinia settles down as a thick covering over the paddy field. It is not mulchable and manual removal of this weed is labour intensive causing increase in the expenditure of   paddy cultivation. Attempts to eradicate this weed using biological control measures are not fully successful in this area.

Nelumbo nucifera , or ‘lotus' is a phanerogam usually seen in the deeper regions of this wetland. Although the flowers and buds of lotus is of great demand in the market, it is a weed which obstructs the fishing activity. The periphytonic algal growth on this plant can support several fresh water fishes as their food. Nymphaea stellata , is a rooted floating macrophyte. Usually it grows both in deeper and shallow waters.   The ovules of this plant is a favourite food material for several aquatic birds. Over growth of lotus can obsruct the smooth passage of country boats operated by the fishermen.

Ludwigia hyssopifolia is a semi aquatic erect under-shrub usually seen on the fringes of the paddy fields situated in the wetland. In Muriyad area it is slowly assuming the status of a weed to paddy.

Centella asiatica is a medicinal plant commonly   seen in this wetland. The extract from the whole plant is an effective paediatric medicine and the juice of the plant is used for controlling jaundice. Hygrophylla spinosa and Polygonum sp. are associated plants. The former has medicinal value and used against the diseases of the uro-genital tract and   for dropsy. These two plants usually invade   the abandoned paddy fields and as it is difficult to remove these weeds manually with in a short time,   the paddy fields are converted into swamps.

One of the major weeds invading   the Muriyad wetland is Ipomoea carnea . It is an exotic plant brought to India from South America as a hedge plant. Infestation of   Ipomoea carnea is common   in the northern portion of the wetland where the large scale sand and clay mining is going on. It is a versatile plant which can grow both in   aquatic and terrestrial habitats. In several localities of Muriyad wetland Ipomoea provides suitable habitats for birds. Luxuriant growth of Ipomoea in the water-logged area provides shelter and breeding ground for several species of fishes including Channa striatus and Channa marulius .

Utricularia is a submerged plant, represented by three species,   equipped with a bladder like structure to trap inseccts. Abundant growth of this can be seen during the post monsoon season. Nymphoides indica and Nymphoides cristata, are two quick growing species seen in this wetalnd. Both of them are rooted floating emergents. Due to the fast growth rate large scale biomass synthesis takes place during the post monsoon period. As it is a weak plant it can be very easily mulched into the soil and provide a high quality organic matter to the soil.

Eichhornia crassipes or the ‘water hyacinth' is also an exotic species introduced from America. It has high growth rate and clogs waterways making fishing, boating, and almost all other water related activities impossible. The mats of this plant block the air-water interface and diminish oxygen level leading to the degradation of water quality. This in turn affects the species richness of the aquatic ecosystem. The mats also eliminate submerged plants by blocking sunlight.

Limnocharis flava , (L), Buch, is slowly getting established as another weed, in central Kerala. In the near future it would become one of the most serious weeds of the paddy fields. It is a handsome exotic erect herb of marshes introduced to India from America. Sown rice seedlings suffer more heavily from Limnocharis than the transplanted rice.

Pandanus   grows on the banks of the canal and is abundant in   north-eastern part of the wetland. It provides excellent habitats for several species of small mammals and reptiles. The prop roots extending in to the water body provide specific breeding ground for Arias sp and fresh water prawn Macrobachium . The leaves are used for making mats which is now attaining the status of an export item.

We gratefully acknowledge the financial assistance provided by Kerala Research Programme for Local level development, CDS, Thiruvananthapuram. We also express our gratitude to the Management and Principal of Christ College, Irinjalakuda, for providing necessary facilities to carry out the research programme. Our sincere thanks are due to Dr. Subhash, for his support and encouragement, Mr. Raju Varghese and V.V. Binoy who helped us in the field investigations.

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Dept. of Zoology,
Christ College Irinjalakkuda,
Kerala, India
Ph: 0484-478880
E mail: sanilmalayil@rediffmail.com