Introduction

Wetlands being the transition zone of land and water, plays a significant role in nutrient cycling, treatment of water, attenuation of floods, maintaining stream flow, recharge ground water, moderate local microclimate, provision goods (fish, fodder, fuel, drinking water, etc.) and services (regulating, cultural, etc.) to the dependent population [1]. Sustained discharge of untreated or partially treated sewage has been altering the chemical integrity of aquatic environment by enriching the system with nutrients, leading to the eutrophication of urban water bodies [2]. Wastewater generated in the domestic and industrial sectors consists of chemical ions, nutrients and heavy metals [3, 4, 5, 6].

Lakebed provide a platform for sediment deposition, which trap heavy metals [8], aiding in the remediation as well as regulating the biological processes. Sediments act as sink of nutrients [7] and analyses of sediments would reveal extent and history of eutrophication. The nutrient budget, ecology, trophic status and rate of evolution of lakes is influenced by plant detritus and sediments. The particulate detritus of plants are the primary source of organic matter and total organic carbon and small volume is contributed by animal and other sources [9]. Sediment –water interactions are important because of higher sediment surface per volume of water in shallow lakes [10].

Plant communities (macrophytes) in wetlands [11] act as nutrient sink by uptake of elements released by sediment to water column, which will influence water chemistry. Assessment of the chemical composition in the macrophytes provide an information about the uptake ability of plants to nutrients [12], nutrients availability for metabolism and the nutrient value of the plants [13]. The ability of macrophytes to uptake nutrients and metals from soil and water forms the basis of phytoremediation [14]. Nutrient composition or accumulation in the tissues is an important feature for identifying the ecological strategy of the plant species and this aids in predicting the competitive complex interactions among the plant communities [15, 16] and aboveground biomass stores higher proportion of nutrients [17]. Phytoremediation capability of aquatic macrophytes have been studied earlier by researchers [18, 19, 20, 21, 22, 23, 24, 25, 26, 27] and hence they are being used in monitoring the status of an ecosystem (bio-monitoring).

Heavy metals have increased enormously in the environment from anthropogenic sources due to industrialization and enhanced agricultural activities (pesticides, etc.). Heavy metals in the environment have been posing challenges due to the hazardous properties such as toxicity, persistence, accumulation in the biological organism leading to biomagnification in food webs [28, 29, 30, 31, 32, 33] , which further get transformed into more toxic compounds [34] posing serious challenges to biotic health. The occurrence of toxic pollutants in water bodies (lakes, ponds, streams and rivers) would affect the health of population who depend on these water sources to meet their daily requirements (water, fish, food, etc.). Consumption of water and wetland goods laded with metals would lead to the accumulation in the kidneys, liver and bones of humans, resulting in chronic disruption of metabolic activities and lead to cardiovascular, neurological and renal diseases [35, 36]. Table 1 provides the sources and toxic effects of heavy metals on plants and humans. Bottom sediments, plants and other organisms in polluted wetlands contain heavy metals [37] due to bioaccumulation. Analyses of spatial distribution of heavy metals in sediments and macrophytes of wetlands aid in tracing the sources and the extent of contamination, which is useful in remediation and prudent management of water bodies.

Wetlands are distributed across various topographic and climatic regimes and support diverse and unique habitats in India, [38]. Due to inadequate management many of the wetlands in urban and rural areas are subject to anthropogenic pressures, including pollution from industry and households; land use changes in the catchment; tourism; encroachments; and over exploitation of their natural resources [38]. Bangalore is located at an altitude of 920 meters above mean sea level, delineating three watersheds, viz. Hebbal, Koramangala-Challaghatta and Vrishabhavathi watersheds (Figure 1). The undulating terrain in the region has facilitated creation of a large number of tanks for the traditional uses of irrigation, drinking, fishing, and washing.

Bangalore, being a part of peninsular India, had the tradition of harvesting water through surface water bodies to meet the domestic water requirements in a decentralised way. After independence, the source of water for domestic and industrial purpose in Bangalore is mainly from the Cauvery River and ground water. Untreated sewage is let into the storm water drains, which progressively converge at the water bodies. Varthur lake is the second largest lake in Bangalore. It is a part of a system of interconnected tanks and canals, i.e. three chain of lakes in the upstream joins Bellandur lake with a catchment area of about 149 square kilometres (14979 Hectares) and overflow of this lake gets into Varthur lake and from where it flows down the plateau and joins Pinakini river basin 39]. Thus, Varthur lake receives all the surface runoff, wastewater and sewage from the Bangalore South taluk (about 40% of Bangalore city sewage). Sustained inflow of untreated sewage and effluents (from industries) has contaminated the lake resulting in eutrophication [40] as the inflow of pollutants has surpassed the lake’s assimilative capacity. This has led to algal bloom with extensive growth and spread of invasive macrophytes, resulting in malodor decline of dissolved oxygen [40]. Hence, the current research investigates the level of nutrients (carbon and nitrogen) and predominant heavy metals (cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn)) concentrations in the Varthur lake through analysis of representative sediment and macrophyte samples.

Table 1 The sources and effects of heavy metals exposure