Introduction

In recent times urbanization has become prominent worldwide which has become an ever expanding process that inculcates alteration in natural ecosystem’s integrity, litho-morphological characteristics, surrounding air and water quality in that particular microclimate. Urban lakes play a pivotal role in maintenance of the homeostasis in the system. These water bodies modulate temperature, regulate local hydrological cycle, detoxify water, increases the primary productivity of the system through algal photosynthesis and help in improving environmental conditions for life. Primarily algae in these urban lakes use the light energy to synthesize carbohydrates from carbon dioxide in dissolved form in water by photosynthesis. The source for the C source is the bacterial respiration in the lake unlike the green plants. As one gram of oxygen is liberated for approximately each gram of carbohydrate produced, a measurement of the oxygen production is an indirect estimate of the rate at which energy is stored by photosynthetic processes. During the process of photosynthesis light energy is used by green plants to synthesize carbon-containing organic materials from carbon dioxide as resented by light. The light energy is incorporated by chlorophyll into carbohydrate, which is converted into the components of protoplasm. The rate at which green plants produce carbohydrates is called primary productivity (Odum, 1959). The parameters like algal chlorophyll content, algal biomass, primary productivity help in assessing the status of urban lakes. Chlorophyll content gives a measure of the growth, spread and quantity of algae. The community structures reveal the assemblage of the algal community with respect to the quality of the water under varying levels of stress. The primary productivity in terms of day net photosynthetic productivity gives the quantum of the C put in the form of food by the photosynthetic apparatus of the algae.

Unplanned rapid urbanization in Bangalore in recent times has resulted in either disappearance of lake ecosystems or deteriorated the lake water quality impairing the ecological processes. The urban water bodies in India as lakes are experiencing deterioration through varying degrees of environmental stress due to encroachments, eutrophication (especially from domestic effluents), and siltation. There has been a dramatic increase in the population during the last century which is devoid of corresponding development of civic facilities, leading to deterioration of these urban water bodies. The main causes for the impaired conditions of the lakes can be summarized as: pollutants from fixed point sources like nutrients from wastewater from municipal and domestic effluents, organic, inorganic, and storm water runoff. Pollutants from non-point sources as nutrients through fertilizers from agricultural areas as run off, organic pollution from human settlements located along the periphery of the lakes and reservoirs (Reddy and Char, 2006). Small water bodies are abundant (Downing et al., 2006) and have disproportionately high hydrologic and nutrient processing rates (Smith et al., 2002). Water is an essential component for life on earth, which contains minerals extremely important in human nutrition. However, the dramatic increase in population has resulted in an enormous consumption of the world’s water reserves (Ho et al., 2003). Natural contamination of water resources mainly results from normal geological phenomena such as ore formation (Al Fraij et al., 1999). Nonetheless, human activities are a major factor in deciding the quality of the surface waters through atmospheric pollution, effluent discharges, use of agricultural chemicals, eroded soils, and land use (Sillanpaa et al., 2004). Eutrophication is most often the result of an elevated supply of nutrients, particularly nitrogen and phosphorus, to surface waters that results in enhanced production of primary producers, particularly phytoplankton and aquatic plants. The abundance of organic compounds, toxic chemicals, nitrites, and nitrates in potable water may cause adverse effects on human health such as cancer, other human body malfunctions, and chronic illnesses (Ikem et al., 2002). Therefore, it is essential to constantly monitor water quality used for drinking purposes (Virkutyte and Sillanpaa, 2006).

Freshwaters receive most inorganic nutrients and other toxic substances generated by both the domestic and industry as waste and released into the environment. Although aquatic ecosystems are operational with a variety of physicochemical and biological mechanisms to eliminate or reduce adverse effects of such compounds, toxicants may evoke changes in development, growth, reproduction, and behavior and may even cause death of freshwater organisms (Rand et al., 2003). The water bodies as lakes, tank and reservoirs proves to be an excellent candidates for evaluating the health of the ecosystem and proved to be a good material for the study of functional aspects of the ecosystem in terms of photosynthetic productivity under varying levels of anthropogenic stress. There has been a lot of work ascribing the diversity of plankton with relation to water quality, but the photosynthetic dynamic’s of the lake systems is seldom been mentioned. The present work was carried out on two urban lakes Varthur and Bellandur in the city and a reservoir at the outskirts of the city T. G. Halli. The lakes were checked for its nutrient status with reference to physicochemical parameters. The community structure analysis analysis together with determination of the day net phtotsynthesis at various sampling locations was carried out.

The main objective of the study was to find out the difference in photosynthetic productivity in terms of measurable variables between the lakes at varied anthropogenic stress and a to compare across organic and nutrient stress conditions conducted during the period from  Jan-Aug 2010.