Materials and methods
Study area
Varthur Lake located at 12.9407° to 12.9566° N and 77.67189° to 77.7476° E is the second largest lake in the Bangalore city (BBMP) (fig. 1). It is situated at Varthur ward in Bangalore east with an area of 180.8 hectares (447 acres 14 guntas) and spread across Ammanikere and Bellandur Khane villages. The catchment of Varthur lake is around 279 square kilometers with 96 cascaded lakes. Land use analyses study using temporal (1970 to 2016) remote sensing data showed an increase in built-up (paved surfaces: buildings, roads, etc.) from 3.8% (1973) to 89% (2016) with a sharp decline in vegetation (58.7% to 6.1%), water bodies (4.5% to 1.2%) and other (open lands, agriculture) land uses (33.1% to 5.0%) in the catchment [56].
Macrophyte and sediment sample collection
Representative samples of macrophytes and sediment were collected from the inlet to outlet regions of the lake, following quadrat based transact method (quadrat of 1 m2) (fig. 2). Three to five samples were collected in each quadrat. Random sampling method was used for collection of macrophytes. The major species of macrophytes in this lake were Eichhornia crassipes and Alternanthera philoxeroides. Macrophytes were identified based on the standard taxonomic literatures [57] and stored in polythene bags. Approximately 1 kg of sediment (in triplicates) was collected at a depth of 0-20 cm at each sampling locations with the help of a cylindrical PVC cores with 5 cm of internal diameter.
Sample preparation and analysis
Collected macrophytes were washed to remove adhered sediments and epiphytes and segregated based on species. Shoot and root of each sample was separated, and oven dried at 60º C for 2-3 days until the attainment of constant weight. It is then powdered using mixer/grinder and sieved to get fine powders. The replicates of each sample were used for nutrient analysis. The samples were analyzed for C, H and N using TRUE-SPEC CHN Analyzer. Sediments were air dried and sieved to remove coarse debris. Sediment samples were pulverized using a mortar and sieved to get fine powder. Nutrient analyses in sediment samples were assessed through TRUE-SPEC CHN Analyzer like macrophytes.
Fig. 1 Study Area – Varthur Lake, Greater Bangalore, Karnataka State, India
Fig. 2 Sampling locations of macrophytes and sediments in Varthur Lake
Heavy metal analysis
Pulverized sediment sample (0.5 g) was acid digested with 3:1 (v/v) HCl-HNO3, evaporated to 2 ml, filtered using 0.45µm filter paper and diluted to 50 ml using double distilled water [58] for heavy metal analysis. Macrophyte samples were acid digested with triacid mixture (HNO3:H2SO4:HClO4 in 5:1:1) until transparent solution was obtained. Flame Atomic Absorption Spectrophotometry (GBC Avanta version 1.31) was used for analysis of six heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in samples along with the reference reagent blanks and standards (Merck, USA).
Bio-concentration factor (BCF)
Bio-concentration factor (BCF) in macrophytes is the ratio of heavy metal concentration in the plant to that in the sediment at a sampling location (equation 1). Higher values of BCF indicate that the easy assimilation of heavy metal by macrophytes from sediments and the higher possibility of heavy metal redistribution in the environment [59]. BCF expresses the ability of a plant to uptake a specific element from sediments and subsequent accumulation in its tissues. Higher BCF values imply of good bioaccumulation or accumulation capability of macrophytes. A BCF value higher than one indicates that a particular plant species is aiding as a hyper-accumulator of trace elements [60].
Heavy metal content in macrophytes
Bio-concentration factor (BCF) = Heavy metal content in macrophytes/Heavy metal content in sediment ----------------------------1
Heavy metal content in sediment
Translocation factor (TF)
Translocation factor (TF) describes the efficiency of a plant to translocate metal from its root to shoot and is computed as the ratio of concentration (mg/kg) of metal in plant shoot to the concentration of the same metal in plant root (equation 2). Higher TF values indicate higher capacity of mobility [61].
Heavy metal content in shoot of macrophytes
Translocation factor (TF) = Heavy metal content in shoot of macrophytes/Heavy metal content in root of macrophytes -------------------------------2
Heavy metal content in root of macrophytes
