CHAPTER ONE
INTRODUCTION
1.1 Background of study
Macrophytes are the conspicuous plants that dominate wetlands, shallow lakes, and streams. Macroscopic flora includes the aquatic angiosperms (flowering plants), pteridophytes (ferns), and bryophytes (mosses, hornworts, and liverworts). Macrophytes may be floating, floating-leaved, submerged, or emergent (Sculthorpe, 1967), and may complete their life cycle in water (still and flowing) or on hydric soils (inundated and non-inundated).
Aquatic macrophytes are considered photosynthetic organisms of freshwater habitats, comprising vascular plants, aquatic bryophytes and macro algae, easily seen with the naked eye and are normally found growing in or on the surface of water, or where soils are flooded or saturated long enough. Aquatic macrophytes as primary producers and habitat providers are important component of river ecosystems.
They play an important role in the structure and functioning of freshwater ecosystems (Wetzel, 2001; Hrivnak et al., 2009; Tamire and Mengistou, 2012). The macrophytes serve as a base of aquatic food-chains, besides they also actively contribute to the promotion and maintenance of food webs and services in freshwater ecosystems (Scheffer and Jeppesen, 2007; Smith, 2011).
Aquatic macrophytes also act as important bioindicators of environmental conditions and long term ecological changes in water quality (Lacoul and Freedman, 2006; Solimini et al. 2006). The function of macrophytes in these ecosystems is related to their structural attributes like species composition, distribution, abundance and diversity which in turn relies on various environmental factors such as light, water, temperature, substrate composition, disturbance, competitive interactions, herbivory, epiphyte loading, water levels, quality of the lake water and sediment nutrients (Kaul et al., 1978; Pandit, 1984, 1992; Barko and Smart, 1986; Duarte et al., 1986; Lodge, 1991; Cronk and Fennessey, 2001; Wetzel, 2001; Capers, 2003; Pankhurst, 2005; Jaikumar et al., 2011; Siraj et al., 2011; Feldmann, 2012; Tamire and Mengistou, 2012). Apart from these, factors such as composition and properties of sediments also seem to have significant effect on the distribution of certain Macrophytic species (Dawson and Szoszkiewicz, 1999; Heegaard et al., 2001). Vegetative and clonal reproduction is considered to be the major mechanism for population growth and dispersal of macrophytes because sexual reproduction and genetic recombination are often subordinate strategies (Wetzel, 2001). The efficient reproduction strategies and good dispersal capabilities are the two factors that help some aquatic macrophytes to become cosmopolitan in distribution and display high levels of polymorphism and phenotypic plasticity in response to variations of environmental factors. (Sculthorpe, 1967; Barratsegretain, 1996; Santamaria, 2002).
1.2 AQUATIC MACROPHYTE
Aquatic macrophytes are aquatic photosynthetic organisms, large enough to be seen with the naked eye, that actively grow permanently or periodically submerged below, floating on, or growing up through the water surface. Aquatic macrophytes do not belong to one distinct taxonomic group but rather form a collection of many plant taxa. The term “aquatic macrophytes” is commonly used for all macroscopic forms of aquatic vegetation; it includes macroscopic algae (stoneworts and the alga, Cladophora), some ferns and mosses (pteridophytes) and many flowering plants (angiosperms). These plants require special adaptations for living in submerged water, or at the water surface. The most common adaptation is aerenchyma but floating leaves and finely dissected leaves are also common. Aquatic plants can only grow in water or in soil that is permanently saturated with water. They are therefore a common component of wetlands.