Over the past few decades, the contamination of aquatic ecosystems by heavy metals has become a major environmental concern throughout the globe (Dahshan et al., 2013; Sanyal et al., 2015; Mitra et al., 2018; Uzoekwe and Aigberua, 2021).Widespread discharge of effluents from domestic wastewater, industrial effluents, and agricultural runoffs has continuously added heavy metals into the aquatic environment resulting in serious threats to aquatic ecosystem health (Dhamodharan et al., 2019; Ouyang et al., 2006; Sutherland, 2010; Has-Schon et al., 2006). After entering into an aquatic ecosystem, heavy metals are quickly precipitated over sediment (Ghosh et al., 2016; 2018), rendering the sediment a sink of heavy metals. In recent years, high concentrations of heavy metals have been detected in the sediment of rivers, lakes, ponds, and many other freshwater bodies in India (Antizar-Ladislao et al., 2015; Mandal et al., 2018; Mayanglambam and Neelam, 2020; Goyal et al., 2022) indicating increasing anthropogenic fluxes of heavy metals into these water bodies. Although heavy metals are quickly precipitated over sediment, there exists a dynamic interaction of heavy metals between the sediment and the overlying water. Heavy metals are accumulated in the body of aquatic organisms, either through direct accumulation from water, interstitial water of sediment soil, or through the food chain depending on the microhabitat occupied by the species as well as on the dynamic interaction of heavy metals between sediment and water (Dhamodharan et al., 2019; Yujun et al., 2011; Ghosh et al., 2016; 2018; 2021). The heavy metals are persistent, do not degrade over time, and thus become dangerous to aquatic organisms when their quantities in the body of the organisms exceed permissible limits (Renu et al., 2021; Sahajahan et al., 2022).
Little is known about the dynamics of heavy metals deposition between sediment and water and their subsequent transfer to fish and other aquatic ecosystems in East Kolkata Wetlands (EKW), a 12000 ha area of mostly sewage-fed wetlands located in the eastern edge of the metropolitan city Kolkata. EKW was declared a Ramsar site (No. 1208) in August 2002. The untreated wastewater containing city sewage of Kolkata and effluents of an innumerable number of tanneries and other industries feed into EKW systems (Chatterjee et al., 2007; Adhikari et al., 2009; Mondal et al., 2022). A significant part of EKW, comprising about 4000 ha, is used as the world’s largest wastewater-fed aquaculture (WFA), in which wastewater is purified through dilution and sedimentation, and the fish is grown at the expense of dissolved nutrients contained in the wastewater. The EKW is an important freshwater fisheries resource. The WFAs of EKW produce more than 10,915 metric tons of fish annually, which are most popular among fish consumers and are thus sold fresh in huge quantities to Kolkata and greater Kolkata fish markets (Bunting et al., 2010; Kumar et al., 2023). In recent years the anthropogenic flux of pollutants to WFAs in EKW has tremendously increased due to the expansion of Kolkata city, resulting in increased city sewage load as well as an increase in industrial activities within EKW. Therefore, it is urgent to reveal the possible rate of transfer of heavy metals from water and sediment of these WFAs to fish reared in them. Once in the aquatic environment, heavy metals are accumulated in different aquatic organisms, transferred to higher trophic levels through the food chain, and potentially can perturb ecosystem functioning (Colla et al., 2021). Fish cannot escape the harmful effects of pollutants entering into aquatic environments (Yarsan et al., 2013), and thus serves as a useful bio-monitoring agent of heavy metal pollution in the aquatic ecosystem (Rashed, 2001; Osman, 2012; Kumar et al., 2019; Akila et al., 2022). Some heavy metals tend to biomagnify through the food chain in aquatic ecosystems (Nally et al., 2023), and fish residing on the top of the food chain serves as a useful bio-monitoring agent to evaluate patterns of heavy metal transfer in the aquatic ecosystem (Kumar et al., 2019; Akila et al., 2022). Moreover, the concentrations of heavy metals in fish bodies provide a clue to possible health hazards of the human consumers (Ali et al., 2022). Once the heavy metals enter into the body of fish, either through direct uptake from water or through the food chain, they are distributed across various tissues for storage and removal (Alam et al., 2002; Canbek et al., 2007; Ahmed et al., 2010; Islam et al., 2015). Plenty of reports are available on the pattern of bioaccumulation of heavy metals in gills, liver, kidney, gut, and muscle tissues as well as in the whole body of fish (Kucuksezgin et al., 2002; Lewis et al., 2002; Ghosh et al., 2016; 2018; 2021). The concentrations of heavy metals in gills represent the concentrations of heavy metals in water, whereas the concentrations in the liver reflect the storage of heavy metals (Yılmaz, 2003). It has been reported that the bioaccumulation of heavy metals in the muscle does not take place actively (Romeo et al., 1999; Sunlu et al., 2001). But muscle is the principal part of a fish that human consumes, and therefore, the concentration of heavy metals in muscle highlights possible human health hazards arising from the consumption of such fish.
This study deals with the accumulation of Cr in the fish, Oreochromis niloticus reared in some selected WFAs of the EKW situated close to a leather tannery complex. Tanneries are one of the principal sources of Cr contamination of the aquatic ecosystem in India (Chattopadhyay et al., 2000; Fahim et al., 2006; Chowdhury et al., 2015; Kokkinos and Zouboulis, 2021; Rahman et al., 2022). The leather tannery industry produces several kinds of waste, including trimmings from chrome-tanned leather, chromium sludge, and shavings from chrome-tanned leather, which principally contains Cr. High concentration of Cr has been reported from different species of fish harvested from EKW (Dutta et al., 2022; Kumar et al., 2023). Oxidative stress is the main reason behind Cr producing cytotoxic, cardiotoxic, and genotoxic effects in fish (Balakrishnan et al., 2013; Taju et al., 2017; Tian et al., 2018; Renu et. al., 2021). Cr also invades mitochondria and the nucleus, resulting in DNA damage and altering cellular regulations at these sites (Venter et al., 2015).
The objective of this study was to evaluate the pattern of deposition of Cr in water and sediment in four WFAs of EKW and the mode of transfer of the metal to different tissues, such as gills, liver, intestine, kidney, and muscle, in the freshwater fish, Oreochromis niloticus reared in these WFAs. We attempted to identify the variables responsible for transferring Cr from water and/or sediment to fish. We applied structural equation modeling (SEM) for this purpose. SEM explores theoretical and empirical relationships of variables in a network form and identifies the underlying processes or unmeasured ‘latent’ variables that influence the transfer and concentration of metals in different compartments (Skrondal and Rabe-Hesketh, 2004; Lupi et al., (2019). SEM is now used to handle complex variable interactions inherent in pollutant transfer (Pollman, 2014).