INTRODUCTION

Aquatic ecosystem quality is of vital concern for living beings as it provides habitats for a variety of flora and fauna, recharges ground water, augments and maintains stream flow, recycles nutrients, purifies water etc. The components of aquatic ecosystem and their working pattern are highly dependent on the catchment structure and the land use pattern in the catchment (Ramachandra and Ahalya, 2001).

Aquatic ecosystems worldwide are being severely altered or destroyed at a rate greater than any other time in human history and faster than they are being restored. Some of these losses occur through intentional exploitation of resources. Other losses occur cumulatively through lack of knowledge or fragmented approach in resource management (UNEP, 1994). The capacity of rivers and their biota to maintain any substantial degree of ecological integrity and to perform ecosystem services, such as pollution dilution and water quality protection, is under immense pressure from large diversions and regulation.

Maintenance and enhancement of economically valuable aquatic ecosystem functions especially floodwater storage and conveyance, pollution control, ground water recharge, anq fisheries and wildlife support have all too often been largely ignored in aquatic resource management. The amount of water entering as precipitation that ends up in the stream depends greatly on the characteristics of the catchment, such as catchment geomorphology, geology, soil type and development and vegetation types and extent of cover (Dodds, 2002). Water falling on the ground may infiltrate the soil or run overland. In catchments with permeable surface, water infiltrating the soil percolates down to the water table. Streams arise where the land surface intersects the water table and groundwater from the water table usually comprises a major part of the stream discharge. Perennial streams are maintained by the ground water flow during times of little or no rainfall (Tideman, 1996). Ecological processes within catchments exert a strong control on the inputs of organic and inorganic chemicals, both particulate and dissolved, into the down slope streams. The disruption of inputs from the catchments is a reliable signal of disturbance. Natural disturbing forces on catchments include fire, cyclones, grazing, defoliation by insects etc., while human-generated/induced disturbances consist of forces such as encroachment, conversion of forest to agricultural land, timber harvesting, livestock grazing and land clearing. It may be due to developmental projects or population pressure on the natural resources (Downes et al., 2002).

Water related developmental projects like hydroelectric power projects have both direct and indirect effect on the river basin and attendant socio­economic condition of the region. Because of their strategic location in the mid-hills (lower contour elevation) and valley bottom, storage reservoirs invariably inundate populated valleys, large forest tracts, and wildlife habitat endowed with rich biodiversity of immense conservation significance (Abbasi, 2000). There are mainly two types of disturbances caused by hydroelectric power projects like submersion and change in basin morphometry and biotic disturbances. Change in catchment practices gives rise to alteration in the physicochemical and biological properties of streams and rivers.

In aquatic ecology, the disturbance is most commonly conceived as being due to physicochemical and biological factors. The alteration in the physicochemical characteristics of aquatic ecosystem is the result of inputs of pollutants from point and non-point sources. The non-point sources of pollution are very difficult to assess and manage. Remediation is also difficult because it usually requires measures to be implemented over a large scale (Gilpin, 1996). In this study the source of pollution was non-point sources like agricultural runoff, bank and catchment soil erosion, diversion of water from rivers for irrigation and poor catchment agricultural practices in the Sharavathi river catchment.