INTRODUCTION:

The Madivala lake (tank) is today one of the largest and important waterbody with a predictably promising future as a wetland habitat for Bangalore city. It served many local needs for agricultural, domestic and other economic activities in the past (Chakrapani, B.K. 1989). Initially the water spread of this lake was about 114 hectares. Urbanisation leading to change in water quality (pollution), change in land use, encroachments in its natural catchment etc., is prevalent.

In this context it is necessary to understand ecological changes that have occurred over a period to understand habitat's dynamic changes caused by urban influences. This include changes in physico-chemical and biological characteristics of the waterbody; changes influenced from continual inputs from urban pressures like domestic and industrial waste water entry; changes seen in the activities/landuse in the catchment, etc. An attempt is made in this paper based on monitoring during 1982 to 1995 with reference to water quality and plankton

STUDY AREA

Madivala lake with a bed level of +880m MSL, located in the south taluk of Bangalore urban district (12°52' N and 77°38' E). This lake was constructed in 1893 and has been a major source for irrigation and domestic utilities (Chakrapani, B.K. 1989). The tank water ceased to be potable since about twenty five years as a consequence of the city's growth, and has been exposed to changing utilitarian priorities over recent times.

METHODS:

The lake was monitored in 1984, 1989 and 1995 (Chakrapani, B.K. 1989; Chakrapani et al, 1990; Krishna et al, 1996) to project ecological changes and suggest measures for its protection and ecological restoration. The physico-chemical and biological analyses were carried out using APHA (1981) and literature (Edmonson, 1959; Needham & Needham, 1962; Allen Pentecost, 1984; Michael R.G., 1973; Lakshman et al, 1986).

RESULTS AND DISCUSSION:

Lakes in and around Bangalore have been facing many ecological threats. Plankton diversity among other groups of organisms are also severely threatened (Chakrapani B.K. 1995), which has affected the ecology of organisms at the lower trophic levels. Since the early eighties, information regarding water quality (5 physical, 23 chemical parameters and 7 metal ions) at different times [1982-84 (Chakrapani, B.K. 1989), 1989 (Chakrapani et al, 1990), 1995 (Krishna et al, 1996) and 1996 (Jayaram C. 1997)] are available. Important results are:

Regarding plankton of this habitat, the taxonomically identifiable units (TIU's) in all, for both phytoplankton (PP) and zooplankton (ZP) have been on record since 1982. The overall diversity has generally been on the decline. An improvement in plankton form diversity during 1989 could probably be attributed to a "change in diversity" phase. After 1989 the forms occurring have been predominantly different. The TIU's, during 1982, 1984, 1989 and 1995 have been; 37, 35, 53 and 30 respectively for both PP and ZP together; 10, 7, 25 and 11 respectively for PP and 27, 25, 28 and 19 respectively for ZP. During and after 1989, an increase in the diversity of myxophyceae and bacillariophyceae were observed. Though bluegreen algae reduced in form diversities by 1995, a dominance shift from microcystis species to spirulina species and a general decrease in all other groups among PP was observed (Krishna et al, 1996; Krishna et al 1995). Among ZP, a general decrease in all groups except protozoans was observed (Krishna et al, 1996). Among ZP there was a shift to forms, associated with polluted waters (Krishna et al, 1996). Further, among a number of lakes surveyed during 1995, this habitat showed some noticeable shifts in planktonic forms, representative of polluted waters (Krishna et al 1995). All the above trends are indicative of a poor diversity of forms as well as evidence for pollution on account of anthropogenic pressures.

From observations concerning other habitats, interesting relationships have been seen to emerge between lower and higher trophic levels like plankton and birds (Chakrapani and Krishna, 1994).

PROSPECTS FOR AND EFFORTS AT ECORESTORATION:

Urbanization in Bangalore has influenced the resource use and ecology of this lake as in case of many other aquatic habitats in the region. The investigations (in relation to this habitat in terms of ecological changes; protective and restorative efforts etc.), provide a deeper understanding regarding natural, man made and other environmental dynamics associated with the existence of lake habitat in growing urban situations.

Pre 1970's: Water in the lake was potable (till early 1980's). Lake witnessed normal resource usage - irrigation, fish, domestic use, recreation, etc. Sewage from Jayanagar and surrounding areas entered the lake.

Early 1980's: Comprehensive ecological studies were carried out from 1982-84 (Chakrapani, B.K. 1989). Attempts were made to understand the ecology and dynamics of plankton, water quality and other groups of organisms.

Based on these investigations, following recommendations were made:

(a) Prevention of direct entry of untreated sewage/waste inflow into the tank. Sewage/sullage inflows were continuous from the inlet at the western end. In addition, sewage from pipes running along the western shore of the lake also entered the waterbody.
(b) Provisions of preliminary treatment facility to reduce sludge, suspended solids, BOD and silt loads.
(c) Controlling mosquito larvae through introduction of suitable fish species.
(d) Control of water hyacinth (Eichorrnia species) to be initiated either mechanically or biologically.
(e) Measures to improve shallow aquatic regions to support a richer diversity of macrophytes.
(e) Ensure shore protection as well as ecological desiltation.
(f) Measures to be initiated to preserve, protect and improve a shore area of at least 200 to 250m from high water level (HWL) which could lead to habitat improvement

In 1986, the Laxman Rao committee's report (Lakshman et al, 1986) brought in a phase of rethinking regarding the lake resources in Bangalore urban. The positive aspects of the need to preserve these habitats were raised in many context. This has led to the transfer of lakes in the Bangalore urban district to the forest department for protection, preservation and to implement improvement measures.

At present, the result is seen in the form of status surveys, protective fencing, development of forest nurseries, proposals for lake conservation measures (Krishna et al 1995), formation of lake protection committees. In addition to the loss of 41 lakes, these investigations highlight the need for an integrated approach involving aspects like monitoring, research, awareness creation, along with eco-management efforts. It is also important that people's involvement would also form a crucial input for managing these habitats.

ACKNOWLEDGEMENT:

Thanks are due for all the voluntary help from fellow birdwatchers in the field and for all cooperation from many people, institutions/organisations and some of their staff (especially Mrs. Maria Joseph for typing the report). Encouragement from Prof. S. Ravichandra Reddy, Prof. Katre Shakuntala and from Dr. T.V. Ramachandra, are gratefully acknowledged. The CSIR is thanked for its associateship.

REFERENCES:

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