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The present paper is based on the results of taxonomic research work conducted in Dera Ismail Khan District of KPK, Pakistan, during 2005 – 2007. The area was extensively surveyed in order to collect floating aquatic weeds. From the study area 11 floating aquatic weed species belonging to 9 genera and 9 families were collected and identified in the light of available literature. These plants include Bryophytes: 1 species, Ricciocarpus natans (L.) Corda; Pteridophytes: 2 species, Azolla pinnata R.Br. and Marselia quadrifolia L., and Spermatophytes: 8 species, Lemna aequinoctialis Welw., L. gibba L., Marselia quadrifoliata L. Nelumbo nucifera Gaerth., Nymphoides cristata (Roxb.) O. Ketze. Nymphoides indica (L.) Kuntze:, Pistia stratiotes L. Potamogeton nodosus Poiret and Spirodela polyrrhiza (L.) Schleid. Floating weeds on one hand cause serious problems and on the other hand they are used for various purposes. Data inventory consists of botanical name, family, major group, habit and habitat, flowering period, availability, distribution in D.I.Khan, Pakistan and world, beneficial and harmful effects. Key to the floating aquatic species of the area was developed for easy and correct identification and differentiation.
Aquatic weeds are those unabated plants which grow and complete their life cycle in water and cause harm to aquatic environment directly and to related eco-environment relatively. Water is one of the most important natural resources and in fact basis of all life forms on earth (Lancar and Krake, 2002). Aquatic plants vary greatly in type, with some being quite similar to common land plants while others are quite different. They fall into one of four common class types: algae, floating plants, submerged plants and emerged plants. Grouping is based on the positioning of their roots and leaves (Lichtenstein, 2010). Floating aquatic weeds vary in size from single cell (algae) and may grow up to large vascular plants (Lancar and Krake, 2002). They can be found in fresh or salt water. The leaves of these plants are firm and remain flat in order to absorb more sunlight. (Lichtenstein, 2010)
A diversity of aquatic plants is necessary in water- ways for preventing excessive erosion and turbidity, and for maintaining the delicate nutrient balance in water, hydro-soil and plants. Pond side vegetation provides habitat for water fowl, cover for certain species of fish, and increases the density of planktons, the basis of fish production.
On the other hand, aquatic weeds present a world-wide problem. Such weeds invade aquatic habitats used by man for irrigation, transportation, recreation, drinking and other public purposes. They clog drainage ditches, obstruct navigation channels, limit fishing and boating, provide breeding places for insects and other pests, cause settlement of sediment and present many other problems (Ahmad and Younus, 1979).
Dera Ismail Khan (D.I.Khan) is the southern most district of Khyber Pukhtun Khwa (K.P.K.) lying between 31.15′ and 32.32′ north latitude and 70.11′ and 71.20′ east longitude with an elevation of 173 meters from the sea level. It has a total geographical land of 0.896 million hectares out of which 0.300 m.ha. is cultivated (Khan, 2003). The climate is continental with marked temperature fluctuations both seasonal and diurnal, with significant aridity. January is the coldest month of the year and July the hottest. The mean maximum and minimum temperatures during winter are 20.3°C and 4.2°C respectively, compared to 42°C and 27°C during summer. Average annual rainfall is 259 mm (Anonymous, 1998).
The research area (D.I.Khan District) was extensively surveyed from 2005 to 2007. Plants specimens were collected at least two times from 15 sites comprising Chashma lake, Badari Dam (at khisore range), Dara Zinda stream, Paharpur sewerage canal, Awaran canal, irrigation channels near Ara, Mandra Kalan, Paharpur and Rangpur Adda, ponds near Bilot Sharif, Dhakki Mor, Basti Dapanwala and Darya Khan Bridge and Baloch Nagar and water spur near Qureshi Mor. The specimens were carefully washed, preserved by using standard herbarium techniques. The following data were recorded in the field i.e. date of collection, collection number, habitat, flowering season and distribution. The collected material was identified by using voucher specimens of the herbarium (ISL), dissecting light microscope (Zeis-2000). In major identifications were made with the help of literature such as Jafri (1966); Stewart (1972); Beg and Samad (1974); Qaiser (1977, 1993); Ahmed and Younis (1979); Nazir and Younis (1979); Leghari et al. (1999); Leghari (2004). After correct identification the plants were given voucher numbers and deposited in the herbarium, Department of Plant sciences, Quaid-i-Azam University, Islamabad. Detailed morphological features of each species were compiled.
As a result of a detailed taxonomic research work, conducted during 2005–2007 in D.I.Khan District, 11 floating aquatic plant species belonging to 9 genera of 9 families were collected. These plants include 1 species [Ricciocarpus natans (L.) Corda] of Bryophyta; 2 species [Azolla pinnata R.Br. and Marselia quadrifolia L.] of Pteridophyta, and 8 species of Spermatophyta [Lemna aequinoctialis Welw., Lemna gibba L., Nelumbo nucifera Gaerth., Nymphoides cristata (Roxb.) O. Ketze. Nymphoides indica (L.) Kuntze:, Pistia stratiotes L. Potamogeton nodosus Poiret and Spirodela polyrrhiza (L.) Schleid]. Data inventory consists of botanical name, family, major group, habit and habitat, flowering period, availability, distribution in D.I.Khan, Pakistan and world, beneficial and harmful effects (Tables 1–4). Key to the aquatic floating species of the area was developed for easy and correct identification and differentiation. Photographs of 8 plants have been presented in fig. 1 and and22 (plates A–J).
Aquatic weeds create situations which are ideal for mosquito growth. The mosquitoes are sheltered and protected from their predators by aquatic weed roots and leafy growth and are responsible for the spread of Malaria, Yellow fever, river blindness and encephalitis. Snails are able to multiply, playing a crucial role in the life-cycle of blood and liver flukes (parasitic worms) as they shelter, and find sustenance among the root zones. Schistosomiasis and fuscioliasis diseases spread as the floating weed carry the snails to new locations. People living close to these areas complain of mosquito problems. Fish production is greatly affected by the presence of floating and submerged aquatic weeds. Isolated weed beds may be tolerated, providing shelter and shade for fish, but when the growth becomes thick and covers entire water body, it can be lethal for fish growth. Fish may suffocate from a lack of oxygen and it may cause death. When floating and submerged aquatic weeds become extremely dense, many fish species are unable to exist in such environments and vanish (Lancar and Krake, 2002).
Genus Azolla is a fern comprising 7 species which are almost cosmopolitan in distribution (Lumpkin, 2010). In different situations Azolla is considered to be an undesirable weed or a beneficial plant (Cook et al., 1974). 1 species Azolla pinnata is commonly found in D.I.Khan District. It is free floating, heterosporous. It has few fibrous roots. Stem with many crowded branches; the whole plant appears to be triangular in shape (Ahmad and younus, 1979). 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 (John et al., 2003).
Hg pollution is a matter of serious concern. A marked reduction in the content of chlorophyll a, protein, DNA and RNA occurs due to toxicity imposed by Hg. A. pinnata can remove Hg from industrial effluents by accumulating Hg more efficiently and is thus recommended for removal of Hg from contaminated waters (Rai ' Tripathi 2009).
A. pinnata is a food source for waterfowl, fish, shrimp, insects, worms, snails and crustaceans. Mats of Azolla can actually discourage blue-green algal blooms. They restrict the penetration of sunlight into the water, which is essential for algal growth, and take up nutrients from the water column, limiting the availability of this food source for the algae. The mats of Azolla can be a form of biological mosquito control, preventing mosquito larvae surfacing for air. On the other hand, it is possible that thick, complete coverings of Azolla can cause de-oxygenation of the water. This can affect organisms such as fish and other aquatic plants, and the decay of the latter can lead to a strong odour (Anon., 2010).
Genus Lemna has about 15 species distributed in both the hemispheres of the world, represented in Pakistan by 5 species (Hashmi and Omer, 1986). These species are mostly small and rarely exceeding 5mm in length. Usually gregarious and often forms green floating mats on the surface of still or slowly flowing water (Cook et al., 1974). In D.I.Khan district 2 species (Lemna aequinoctialis Welw. and L. gibba L.) were found during investigation.
Spirodela, duckweed with 4 species (Cook et al., 1974), is mostly distributed in temperate and tropical zones as represented in Pakistan by 1 species (Hashmi ' Omer, 1986). In D.I.Khan district it is also found on the surface of water.
Note: Lemna gibba resembles L. aequinoctialis but it is easily differentiated from the later in being gibbous beneath. L. aequinoctialis is flat on both surfaces.
Spirodela polyrrhiza may be confused with Lemna spp. which are smaller and have only a single root per frond. In Spirodela polyrrhiza roots are 2–18 per frond.
Duckweeds can accumulate large amount of mineral nutrients in their tissues. Concenteration as high as 7% N in dry mass of Lemna gibba and 2.9 % P in dry mass of Spirodela polyrrhiza obtained under laboratory conditions has been reported. When growing in wastewater, concentration of N may reach the values up to 7.3% and concentration of P may exceed 2.5%. However, concentration higher than 4% N and 1% P on dry mass basis are commonly found in natural wetlands (Vymazal and Kröpfelová, 2008)
Ricciocarpus, a liverwort, comprises 1 species, Ricciocarpus natans (L.) Corda, found free floating on surface of water in the warmer parts of the world (Cook et al., 1974). In our research area it is very common growing with duckweeds (Lemna spp., Spirodela sp.).
These aquatic weeds (as mentioned above) are responsible for lowering quantity as well as quality of water. They cause taste and odour problems and also increase biological oxygen demand because of organic loading (Lancar and Krake, 2002).
Marsilea, a fern, has about 65 species distributed throughout the warmer regions of the world. The species are aquatic and semi aquatic (Cook et al., 1974). Seedlings remain submerged and adults float as emergent plants (John et al., 2003). Several species are regarded as troublesome weeds in rice fields and irrigation ditches (Cook et al., 1974). In D.I.Khan district it is represented by 1 species, Marsilea quatrifoliata L. It is perennial, rooted at the bottom of the soil, leafy, heterosporous floating fern (Ahmad and Younus, 1979).
Nelumbo is a single genus of family Nelumbonaceae. It has 2 species, N. pentapetala (Walt) Fernald and N. nucifera Gaerth. The latter is distributed in the West Indies, C. America, Southern and Eastern parts of N. America and the other in S. E. Asia, India and Iran (Cook et al., 1974). In Pakistan 1 species, N. nucifera (lotus) is found. The flowers are very large and showy and considered sacred by Hindus. Whole plant is holy to Budhists (Qaiser, 1993). Although the flowers and buds of lotus are 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 (John et al., 2003).
The lotus (N. nucifera) is a very beautiful plant cultivated for food purpose in D.I.Khan on small scale near Indus River. The seeds are eaten. The rhizomes (locally known as bhen) are often sold in the market. These are commonly sliced and cooked with or without meat.
Nymphoides a genus with about 20 species distributed in temperate and topical regions of both the hemispheres. It is represented in Pakistan by 3 species. 2 species (Nymphoides cristata and N. peltatum) were recorded in flora of Pakistan (Qaiser, 1977) and 1 species (Nymphoides indica) reported by the author as a new record for Pakistan (Marwat et al., 2009). Represented in D.I.Khan district by 2 species (Nymphoides indica and N. cristata). Both the species are quick growing rooted floating emergent. 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 which then provides a high quality organic matter to the soil (Sanil and John, 2010).
Note: Nymphoides indica (L.) Kuntze is closely related to N. cristata (Roxb.) O. Ketze, but differs from the latter in having petals of unusual feathery edges. In N. cristata to be continued.
(Roxb.) O. Ketze the petals have no feathery edges (Marwat et al., 2009).
Charm of boating, swimming, bathing and other recreations in water is minimized due to infestation of water bodies' with such floating weeds (Lancar and Krake, 2002).
Pistia comprises only 1 species (Pistia stratoites L.) distributed throughout the tropical and subtropical regions of the world (Nasir, 1978), also commonly found in our area (D.I.Khan). It is a free floating plant that grows in a wide variety of aquatic habitats. In many regions it has become a serious pest (Cook et al., 1974).
P. stratoites is one of the most problematic weeds in the world and along with Salvinia to be continued. molesta is particularly troublesome in regions with subtropical climate (Lancar and Krake, 2002). P. stratiotes mats clog waterways, making boating, fishing and almost all other water activities impossible. Its mats degrade water quality by blocking the air-water interface and greatly reducing oxygen levels in the water, eliminating underwater animals such as fish. Water lettuce mats greatly reduce biological diversity: mats eliminate native submersed plants by blocking sunlight, alter emerged plant communities by pushing away and crushing them, and also alter animal communities by blocking access to the water and/or eliminating plants the animals depend on for shelter and nesting (Ramey and Peichel, 2001).
Potamogeton has about 100 species which are cosmopolitan. It occurs in large variety of habitat (Cook et al., 1974). About a dozen of species are reported from Pakistan (Aziz and Jaferi, 1975). Five species were recorded from D.I.Khan district during extensive survey of the area out of which 4 species are submerged and 1 species, Potamogeton nodosus Poiret is rooted with both floating and underwater leaves (Aziz and Jaferi, 1975). P. nodosus has been reported to be a nuisance in canals and ditches (Cook et al., 1974). The plant is used medicinally by local people. Plant is boiled in water and its poultice is applied on dislocated joint to make it soft and then properly adjust in original position easily. The seeds are eaten by birds (Marwat et al., 2008).