Introduction

Tropical urban water bodies have become prone to increased anthropogenic stress especially due to constant sewage ingress leading to poor water quality. Often the management of these water bodies in urban areas of the developing countries is largely neglected to a major extent. Due to nutrient enrichment these water bodies are increasingly becoming eutrophied, resulting in very frequent algal blooms and rapid macrophyte growth and with periodic successions. The macrophyte and macrophyte that tolerate these stressed conditions out-compete the native species and reduce endurance to local conditions. In this competitive exclusion principle, the superior competitor species dominates a constant environment and lead to a low community richness and diversity. Contradicting this principle, phytoplankton communities generally consist of a wide variety of species in a believed stable environment, which was described as paradoxical (Hutchinson, 1961). The Hutchinson’s dilemma named ‘‘Paradox of the Plankton’’ was explained by non equilibrium conditions due to the fluctuations of the environment (Hutchinson, 1961), related to annual cycles of physical, chemical and biological factors. Most of the water body systems are extremely sensitive to these environmental changes resulting in unstable conditions and discontinuously advantageous habitat features for different species (Reynolds, 1997; Calijuri et al. 2002). Studies about population dynamics and ecological succession have shown that a fluctuating environment results in phytoplankton communities comprising of different species at different seasons (Sommer et al. 1986; Talling, 1987; Giani, 1994; Figueredo and Giani, 2001). Climatic conditions play a major role in the dynamics of stressed aquatic systems. In tropical urban lakes, that are anthropogenically more stressed the oscillations of total radiation and water temperature are relatively smaller, but seasonal patterns of phytoplankton communities are not negligible (Talling, 1987; Grover and Chrzanowski, 2006). These environments also exhibit seasonal changes in climate (especially related to light, temperature and precipitation) that induce modifications in the physico-chemical characteristics of the water and influences the phytoplankton dynamics (Costa and Silva, 1995; Figueredo and Giani, 2001). Sedimentation, grazing, light, CO2, nutrient concentrations act as forces responsible for the selection of phytoplankton species (Anneville et al. 2004). The final phytoplankton composition results from the balance between gains and losses within the pool of species adapted to survive in that particular environment (Reynolds, 1997; Zohary, 2004). It is therefore interesting to understand how stressed aquatic sewage fed systems respond to reasonably constant sewage composition but significantly varying environmental conditions.

Bellandur and Varthur lake, situated in the south of Bangalore, were built nearly a millennia ago to store natural run off water for drinking and irrigation purposes (Government of Karnataka, 1990). Today large scale developmental activities and unplanned urbanization in the lake catchment area have resulted in reduced catchment yield. Inefficient primary feeder channels feeding the lake have also contributed to fresh water shortfall. However, this shortage has been adequately supplemented by an increased sewage inflow into the lakes causing nutrient enrichment. Bellandur and Varthur lakes have been receiving about 40% of Bangalore city sewage for over 50 years resulting in eutrophication. There are substantial algal blooms, dissolved oxygen depletion, malodor generation and an extensive growth of water hyacinth that covers about 70 to 80% of the lake in the dry season. Sewage brings in large quantities of C, N and P which are trapped or transformed by the system.  Despite being stressed these lakes function as facultative lagoons and bring down the COD/BOD and nutrient levels to satisfactory levels. The phytoplankton communities, with dominant species like Chlorella in Varthur and Microcystis in Bellandur, show seasonal transition in preference to the water quality. Here we have examined the role of seasons, phytoplankton dynamics and macrophyte cover on the functioning of the lake to remove the organic load and render the discharged water capable of being re-used.