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CHAPTER ONEINTRODUCTIONBackground of the studyPhytoplankton are the autotrophic components of the plankton community and a key part of oceans, seas, and fresh water basin ecosystems. The name comes from the Greek word phyton, meaning plant and planktons, meaning wanderer or drifter. They are microscopic free floating plants that drift with water currents. They consist of the assemblage of small plants having no or very limited power of locomotion, they are therefore more or less subject to distribution by water current.. They are a multiple group of oxygenic organisms that account for nearly half of the global primary production (Field et al., 1998) and also play indispensable roles in other biogeochemical cycles in the ecological system (Falkowski, 2012). They have high diversity, with several thousand species already documented and many remaining to be explored (Moon-vander Staay et al., 2001). Their equivalent spherical diameter (ESD) can range from less than 1 µm for cyanobacteria such as Prochlorococcus to more than 1 mm for some giant diatoms (morris et al,. 2011). Furthermore, their physiology differs substantially even within the same genera or species and the role of intraspecific variability in population dynamics and biogeochemical cycle remains to be investigated (Strzepek and Harrison, 2004, and Biller et al., 2015). Phytoplankton can range in size and shape, and since they are photosynthezing autotrophic organisms, they inhabit waters exposed to sunlight. Although each organism is microscopic, in sufficient numbers, phytoplankton can be observed as coloured patches at the surface of bodies of water, or where two current meet, due to the presence of chlorophyll. Phytoplankton are often culture to support aquaculture, and are critical for controlling carbon dioxide and oxygen levels in the earth’s atmosphere since the Precambrian era. It is estimated that phytoplankton are responsible for as much as 85% of the oxygen in the atmosphere ( Berber and kothavala 2001).The plankton study is a very useful tool for the assessment of water quality in any type of water body of river. Phytoplankton communities give more information on changes in water quality than mere nutrient concentration or chlorophyll concentration. Water quality is an ensemble of physical, chemical and biological characteristics of the given water (Straskraba and Tundisi 1999) following this knowledge, eutrophication of fresh water is regarded as a water quality issue which results in the deterioration of the aquatic environment and impacts on water usages. The diversity of phytoplankton communities in natural fresh water and brackish habitat has been shown to increase ecosystem stability and resource use efficiency (Ptacnik et al., 2008). Plankton communities are broadly classified into two basic categories: phytoplankton and zooplankton. The phytoplankton organisms are exclusively of plant origin and are thus autotrophy being the primary producer from the lowest trophic level in the food chain of fresh water ecosystem and play a key role in biodiversity of the aquatic ecosystem. They are ecologically significant as they form the basic link in the food chain of all aquatic animals, (Mistra et al., 2001). Habitat quality specificity of the different members of phytoplankton is seen to be reflected in their distribution and occurrence in relation to the quality of water where they do live (Saha et al., 2000). The abundance of phytoplankton in the water column reflects the influence of the environmental factors and their bio – processes (Suthers et al., 2009). Phytoplankton growth and periodicity are known to be limited by physical and chemical variations. Phytoplankton are bio indicators of water quality, some algae such as Microcystis, Anabaena, Aphani zomenon, cylindrospermopsis are known to produce toxins ( sen et al,. 2013). A bloom of the above general may result in high risk to health. They can cause odour, alter the taste of water and may most likely cause decolouration or form large mats that can interfere with boating, swimming and fishing (Borgh, 2004) and general biological processes of the aquatic system.Phytoplankton satisfy conditions to qualify as suitable indicators, in that they are simple, capable of quantifying changes in water quality, applicable over large geographic areas and can also furnish data on background conditions and natural variability (Lee, 1999). However, phytoplankton are found mostly in euphotic zones, this is because they are photosynthetic plants and they need sunlight for their survival, the depth at which they can survive is about 180m. Apart from sunlight, nutrient availability plays an important role in phytoplankton growth. Examples of food elements needed for the survival of phytoplankton include hydrogen, oxygen, carbon, phosphorus, sodium, potassium, calcium, magnesium etc. If any of these nutrients is absent, it is called “limiting nutrient”. Most limiting nutrients include phosphorus and nitrogen. If their concentrations should increase, phytoplankton abundance will increase too. Also, temperature and thermal stratification of water bodies have indirect and direct effects on phytoplankton. Certain phytoplankton have movement adaptations while, however, cyanobacteria are the only active groups that move up and down to capture the available nutrients and thus they form more frequent and adverse blooms (Jeppeson, 2009). Phytoplankton are mainly grazed by herbivorous zooplankton. Cyanobacteria are not edible to herbivorous zooplankton because of its toxicity (Belinger,2010). Natural plankton communities are composed of planktons of different sizes and shapes and these changes temporally and spatially as a response to environmental changes. One or more algal assemblage could be used as indicators of water quality (Mohapatra and Mohanty, 2010). The use of algae as aquatic environmental indicator has long been documented (michellutti et al., 2001, zenetos, 2002). The diversity of phytoplankton communities in natural freshwater and brackish habitat has been shown to increase ecosystem stability and resource use efficiency (ptacnik et al., 2008).

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