1. Introduction |
Lentic ecosystems are one of the most productive ecosystems in the biosphere and play a significant role in the ecological sustainability of the region. Their ecosystem service functions are equally important in terms of sustainable economic and social development. However, continuous inputs of various forms of chemical pollution from a variety of human activities have seriously deteriorated the health status of many lake ecosystems. If this trend continues, it may not only affect human health and social-economic development, but also lead to the collapse of lake ecosystems themselves. (Goldman, et al., 1983; Constanza et al., 1997; Westman, 1977; Rapport et al., 1998). They constitute an essential component of human civilisation, meeting crucial needs to sustain life on earth, such as water (agriculture, drinking, etc.), food (protein production, fodder, etc), biodiversity (diverse flora and fauna), energy (firewood wood, etc), recreation (tourism), transport, water purification, flood control, pollutant sink and climate stabilisers (Ramachandra and Ahalya, 2004; DEP, 2002). Lakes are under increasing threat due to the separate, but often combined impact of identifiable point sources such as municipal and industrial wastewater, and non-point degradation like urban and agricultural run-off within a lake’s watershed. Major degrading factors include excessive eutrophication due to nutrient and organic matter loading; sedimentation due to inadequate erosion control in agriculture, construction, logging and mining activities; removal of native vegetation in the catchment; introduction of exotic species; acidification from atmospheric sources and acid mine drainage; and contamination by toxic (or potentially toxic) metals such as mercury and organic compounds such as polychlorinated biphenyls (PCBs) and pesticides. In addition, physical changes at the land-lake interface (such as. draining of riparian wetlands) and hydrologic manipulations (such as damming outlets to stabilise water levels) have major impacts on the structure and functioning of these ecosystems (Prasad et al., 2002, Ramachandra, et al., 2005a, 2005b, Wetzel et al., 1991)
Lakes have played a major role in the history of Bangalore serving as an important drinking and irrigation sources. They occupy about 4.8% of the city’s geographical area (640 sq. km) covering both urban and non-urban areas. Bangalore has many man-made wetlands but has no natural wetlands. They were built for various hydrological purposes and mainly to serve the needs of irrigated agriculture (Ramachandra and Ahalya, 2001). The spatial mapping of water bodies in the district revealed the number of waterbodies to have decreased from 379 (138 in north and 241 in south) in 1973 to 246 (96-north and 150-south) in 1996. Table 1 provides the distribution of tanks by taluks in Bangalore. Spatio-temporal analyses reveal 35% decline in number of waterbodies (Figure 1) due to urbanisation and industrialisation.
Bangalore city is located over ridges delineating four watersheds, viz. Hebbal, Koramangala, Challaghatta and Vrishabhavathi watersheds (spanning over Bangalore north and south taluks). The undulating terrain in the region has facilitated creation of a large number of tanks providing for the traditional uses of irrigation, drinking, fishing and washing. This led to Bangalore having hundreds of such water bodies through the centuries. Even in early second half of 20th century, in 1961, the number of lakes and tanks in the city stood at 262. These, and open spaces generally, were seriously affected however with the enhanced demand for real estate and infrastructure` consequent to urbanisation. The built-up area in the metropolitan area was 16 % of total in 2000 and is currently estimated to be around 23-24 %. Official figures for the current number of lakes and tanks vary from 117 to 81, but recent remote sensing data gives a different picture altogether, showing only 33 lakes visible, out of which only about 18 are clearly delineated while another 15 show only faint signs of their former existence. The tanks were reclaimed for various purposes such as residential layouts, commercial establishments, sport complexes, etc. Consequent to these, decline in groundwater table (20 m to 150 m), flooding, changes in micro climate were noticed (Ramachandra,. et al., 2002a, 2002b). This highlights the need for appropriate conservation, restoration and management measures. In this regard a study was undertaken to characterise Varthur lake. The study consisted of three parts (i) Morphometric survey – to provide the information on the depth, surface area, width, etc of the lake ((Mutreja, 1986)); (ii) Water quality survey – to elucidate the quality of lake water and the nearby groundwater (APHA, 1985; NEERI, 1988; Development Alternatives 2000; DWIIH,1995); (iii) Socio-economic survey – to assess the dependency of the nearby residents on the lake ecosystem (Edward, et al., 1997).
