Results and Discussion

Metal concentrations in sediment and macrophyte samples are provided in Tables 1 and 2. Mean metal concentrations in sediments showed the following order Cu > Zn > Cr > Ni > Pb > Cd, whereas the order in macrophytes was Cu > Zn > Cr > Pb > Ni > Cd. All studied metals in sediment samples were above the threshold effect level (TEL), whereas Cd, Cr and Cu were above the probable effect level (PEL) and world common trace metal range levels in lake sediments (WCTMRL). More than half of the sampling points reported Cr above the PEL. Veeranam Lake and Bellandur Lakes had Cr concentrations similar to this study, whereas Renuka Lake, Yercaud Lake, Akkulam Veli Lake, and Kodaikanal Lake showed higher concentrations of Cr (Balamurugan et al. 2014; Das et al. 2008; Gopal et al. 2017; Ramachandra et al. 2018; Suresh et al. 2012; Swarnalatha et al. 2014). Almost all samples had Cu above the WCTMRL (Table 1). Nickel concentrations in current samples were within PEL and WCTMRL levels. Overall, the north shoreline (V12, V21) and inlet (V5) sediment samples had the highest concentration of studied metals. This is because of the untreated effluents entering the lake through these points. Middle samples (V36, V37) had lower concentration of metals.

Cadmium concentrations were within toxic limits as reported by Kabata-Pendias (2010), in all macrophyte samples. A study by Jumbe and Nandini (2009) at Varthur Lake reported mean Cd concentration of 8 mg kg−1 in E. crassipes, which is higher than results from the current study. Findings from another study in A. philoxeroides and E. crassipes reported 5.64 and 5.79 mg kg−1 Cd (Jha et al. 2016), which were higher values of Cd than this study. Similarly, high concentrations of Cd were reported in E. crassipes shoot (128 mg kg−1) by Singh et al. (2016), and 9.13 mg kg−1 and 5.59 mg kg−1 in root and shoot of E. crassipes (Singh et al. 2017) all of which were much higher than those values observed in the present study. Rana and Maiti (2018) reported Cd concentration of 2.3 mg kg−1 and 1.9 mg kg−1 in shoot and root (respectively) of C. esculenta, which is again higher than present study results. Chromium is generally considered toxic for plants because it may alter N metabolism by affecting protein formation (Bonanno and Cirelli 2017). The concentration of Cr ranged from 34 to 54.8 mg kg−1, with a mean value of 42.33 mg kg−1 in macrophytes (Table 2). Typha angustifolia root had the lowest Cr concentrations, while E. crassipes shoot had higher Cr concentrations; however, all samples exceeded toxic levels (Kabata-Pendias 2010). Jha et al. (2016) observed Cr concentration of 41.5 mg kg−1 in A. philoxeroides in Kolkata wetlands, which is consistent with the current study. Rana and Maiti (2018) recorded Cr concentrations of 11.9 mg kg−1 and 7.6 mg kg−1 in root and shoot (respectively) of C. esculenta at a natural wetland contaminated with coke oven effluent. These reported values are lower compared to the present study. Chromium concentrations of 8.06 mg kg−1 and BDL in root and shoot (respectively) of E. crassipes at Kanjli wetland were observed by Singh et al. (2017) which were low compared to the current study. Copper in macrophytes ranged from 21.3 to 263.5 mg kg−1, with the maximum concentration found in A. philoxeroides shoot material (Table 2). A study by Chatterjee et al. (2011) in Table 1 Comparison of metal (mg kg−1) in sediments of Varthur Lake, Bangalore, with other Indian lakes and standard values

TEL threshold effect level, PEL probable effect level, WCTMRL world common trace metal range in lake sediment

East Calcutta wetlands, West Bengal, reported Cu concentrations of 23.2 mg kg−1 and 12.5 mg kg−1 in E. crassipes and C. esculenta, which are below values observed in the current study. Meitei and Prasad (2016) observed lower levels of Cu than the present study in E. crassipes (5.6 mg kg−1 in root and 2.5 mg kg−1 in shoot), A. philoxeroides (4.8 mg kg−1 in root and 0.5 mg kg−1 in shoot) and C. esculenta (1.3 mg kg−1 in root and 0.8 mg kg−1 in shoot). Rana and Maiti (2018) reported Cu concentration of 80.7 mg kg−1 in C. esculenta root, and Kumar et al. (2008) observed Cu level of 104.2 mg kg−1 in T. angustifolia which are both higher compared to current results. Lead is not an essential element for plant metabolism and is considered among the most toxic metals, even at low concentrations (Bonanno and Cirelli 2017; Prasad 2004). Macrophyte Pb concentrations ranged from 8.7 to 56.7 mg kg−1, with a mean value of 21.9 mg kg−1 (Table 2). Colocasia esculenta root had lowest Pb concentration, while A. philoxeroides shoot had the highest Pb concentration. Singh et al. (2016) reported a Pb concentration of 256 mg kg−1 in E. crassipes shoot which is 10 times higher than present results. Similarly studies by Jha et al. (2016) (26.97 mg kg−1 in shoot and 80.95 mg kg−1 in root) and Singh et al. (2017) (83.42 mg kg−1 in root and 79.99 mg kg−1 in shoot) showed higher concentrations of Pb in E. crassipes compared to the current study. Findings from Mazumdar and Das (2015) reported Pb concentrations of 21.6 mg kg−1 in shoot and 27.9 mg kg−1 in root

Table 2 Mean, range, and critical concentration of metal in macrophytes of Varthur Lake, Bangalore

of C. esculenta, which is higher compared to the current study. Suthari et al. (2017) observed Pb concentration of 383.3 mg kg−1 in A. philoxeroides root, which is more than ten times higher compared to current findings. Yadav and Chandra (2011) observed Pb content of 32.5 mg kg−1 in root of T. angustifolia which is almost two times higher compared to the current study. Chatterjee et al. (2011) in C. esculenta and Ramachandra et al. (2018) in T. angustifolia, A. philoxeroides and E. crassipes reported similar Pb concentrations as in the current study. Nickel is another toxic metal that may affect plant growth, metabolism and physiology. In the macrophyte samples the concentration of Ni ranged from 3.5 to 17.1 mg kg−1, which was below toxic levels (Table 2) (Kabata-Pendias 2010). Studies by Ramachandra et al. (2018) (T. angustifolia, E. crassipes, A. philoxeroides) and Yadav and Chandra (2011) (T. angustifolia) reported similar Ni concentrations to those of the current study. The range of Zn in macrophytes ranged from 14.8 to 155.5 mg kg−1 (mean 64.76 mg kg−1), with A. philoxeroides accumulating highest Zn concentrations and E. crassipes accumulating the lowest (Table 2). Higher accumulation of Zn was reported in the same macrophytes by Chatterjee et al. (2011), Yadav and Chandra (2011), Singh et al. (2017), and Mazumdar and Das (2015). A similar Zn concentration range was seen in T. angustifolia, A. philoxeroides, and E. crassipes by Ramachandra etal. (2018) and C. esculenta by Meitei and Prasad (2016). Alternanthera philoxeroides and E. crassipes had most of the metals in higher concentration among the studied macrophyte samples. The metal accumulation strategy of macrophytes was studied using BCF and TF. The BCFs ranged from 0.5 to 1.45, 0.38 to 1.28, 0.19 to 0.28, 0.75 to 0.9, 0.13 to 0.27, and< 0.35 to 1.14 for Pb, Zn, Ni, Cr, Cd, and Cu, respectively. The order of BCF for metals was Zn > Pb > Cr > Cu > Ni > Cd. Alternanthera philoxeroides had higher BCFs for Pb, Zn, and Cu, which was similar to that reported by Suthari et al. (2017). Typha angustifolia had higher BCFs for Ni and Cd which were similar to results obtained by Yadav and Chandra (2011). The BCF for Cr was highest for C. esculenta. The TFs ranged from 0.9 to 1.45, 0.22 to 0.71, 0.45 to 1.13, 0.7 to 1.32, 0.25 to 1.5, and 0.65 to 1.43 for Pb, Zn, Ni, Cr, Cd, and Cu, respectively. The TFs for metals was Pb > Cr > Cu > Cd > Ni > Zn. Colocasia esculenta had higher TFs for Pb, Zn, and Ni than other macrophytes. Similar TFs were observed for Pb and Zn by Mazumdar and Das (2015) in C. esculenta. The TFs for Cd and Cu were highest in A. philoxeroides, and T. angustifolia had the highest TF for Cr. Phytostabilization acts as an efficient remediation technique when plants show high BCF and low TF (BCF > 1 and TF < 1), while phytoextraction is suitable when both BCF and TF > 1 (Yang et al. 2015). Thus, A. philoxeroides and E. crassipes had potential for phytostabilization of Pb and Zn, respectively. Lead and Zn phytostabilization by E. crassipes was reported earlier by Jha et al. (2016) and Meitei and Prasad (2016), respectively. Copper was accumulated mainly by phytoextraction by A. philoxeroides. Nickel, Cr and Cd had higher mobility in C. esculenta,T. angustifolia, and A. philoxeroides, respectively, as BCF < 1 and TF > 1. The present study revealed metal concentration and accumulation capabilities of sediments and macrophytes from Varthur Lake, Bangalore. Studied metals were higher in inlet and shoreline regions of the lake. Sediments were sinks for metals. The phytoremediation ability of macrophytes were revealed through metal accumulation. Alternanthera philoxeroides and E. crassipes remediated most metals through phytostabilization and phytoextraction.