Materials and Methods

Study area

Varthur lake (12°57′23.56″ to 12°56′33.99″ N, 77°43′08.71″ to 77°44′41.72″ E) is the second largest lake located towards the south of Bengaluru District in Karnataka, India and is built by the Ganga Kings over a thousand years ago to meet the local demand of domestic and irrigation water (figure 5a). It covers a water-spread area of 190 ha and is the main source of water for irrigation of the nearby agricultural fields. The lake falls is located in Koramangala - Challaghatta (K&C) watershed and is a part of interconnected lakes (namely, Byappanahalli, Challaghatta, Agara, Bellandur, Haralur, Kasavanahalli, Kaikondanahalli, Doddanakundi, Vibhuthipura, Kundalahalli, Chinnappanahalli and Varthur) and canals that receive all the surface runoff, wastewater and sewage from the Bengaluru South taluk and finally drains into the Dakshina Pinakini River¹². This had increased the pollution load of Varthur lake. The average annual rainfall of Bengaluru is 859 mm and temperatures vary from 14°C (minimum during December to January) to 33°C (maximum during March to May). There are two rainy periods, i.e. from June to September (south-west monsoon) and November to December (north-east monsoon).

Fig. 5a: Varthur lake - Sampling locations of water	Fig. 5b: Froth / foam formation in Varthur lake

Fertilizers and insecticides are being used in the lake catchment for cultivation of horticultural (Amaranthus, Coriander, Turnip, Palak, Spinach etc.), floriculture and agricultural crops (Maize, Paddy) and the runoff from the catchment with the nutrients reaches Varthur lake. Sustained inflow of untreated sewage, industrial effluents along with the nutrient rich catchment run-off has enhanced the nutrient content in the lake leading to eutrophication with the profuse growth of macrophytes covering the major part of the lake and foam/froth formation at outlets. Macrophytes in the lake include species such as Eichhornia crassipes, Alternanthera philoxeroides, Colocasia esculenta, Cyperus sp., Ipomoea aquatica, Ludwigia sp. and Typha sp. that occupied large area of the lake. Heavy metals like copper, zinc, chromium, lead, nickel and cadmium were found to accumulate in lake sediments and in macrophytes in Varthur lake¹³. Decomposition of algae, fish and macrophytes releases a variety of organic compounds into the water body, which act as surfactants (foaming agents) with a hydrophilic end and hydrophobic hydrocarbon chain at the other end. Most surfactants originate from the detergents, oil and grease released from household or industries. These agents rise to the lake surface and interact with water molecules thus, reducing the surface tension of water. When the surface tension decreases, air mixes with the water molecules and foaming agents resulting in bubble formation. These bubbles aggregate together and forms foam in lakes. Foam generated in Varthur lake is sticky, white in color and spread a foul smell to the surrounding area (figure 5).

Water quality assessment

Water samples (V1 and V2 from two outlet points of the lake) and foam samples (V2) were collected from Varthur lake in clean disinfected bottles (figure 5). In situ parameters like water temperature (WT); total dissolved solids (TDS); electrical conductivity (EC) and pH were measured on-site using handheld probes (Eutech: PCSTestr 35). Dissolved oxygen (DO) was determined on-site by Winkler’s method¹⁴. The parameters like total alkalinity (TA); total hardness (TH); calcium (Ca); magnesium (Mg); chloride (Cl); chemical oxygen demand (COD); biochemical oxygen demand (BOD); sodium (Na); potassium (K); nitrate (N) and orthophosphate (OP) of water and foam samples collected from Varthur lake were analysed in the laboratory according to the standard protocols as per APHA¹⁴.

Plankton identification: The plankton (phytoplankton and zooplankton) samples collected using standard plankton net (of mesh size 63 mm and 30 cm diameter) was transferred to clean sterile container and were preserved by adding 2 ml of 5% formalin. The planktons were identified microscopically according to the standard keys^15,16.