Heavy metals in wetlands of southwestern India: from sediments through invertebrates to migratory shorebirds

Heavy metal pollution in Indian wetlands is rising due to industrial, agricultural and urban development activities. Shorebirds occupy upper trophic levels and are therefore especially vulnerable to heavy metal pollution. We evaluated the concentration of heavy metals (zinc, copper, cobalt, chromium, lead and cadmium) in 22 common species of migrant shorebirds (220 shorebird dropping samples) with diverse foraging behaviors, in their different prey (55 prey samples) and in the sediments (90 sediment samples) in different habitat types (mudflats, mangroves and sand beaches) between 2019 and 2021. Further, we analyzed a total of 10 biofilm samples from mudflats and mangroves. We detected relatively low concentrations of heavy metals in the sediments (Zn concentration range: 9.11–40.91 mg/kg; Cu: 5.74–21.38 mg/kg; Co: 2.00–4.04 mg/kg; Cr: 4.05–41.03 mg/kg; Pb: 1.02–7.19 mg/kg; Cd: 0.56–4.35 mg/kg). However, we measured relatively high concentrations of heavy metals in invertebrate prey species (Zn concentration range: 84.72–224.74 mg/kg; Cu: 26.63–170.36 mg/kg; Co: 13.98–14.42 mg/kg; Cr: 14.78–98.16 mg/kg; Pb: 18.95–157.29 mg/kg; Cd: 9.33–60.56 mg/kg). In addition, we found high concentrations of heavy metals in shorebird droppings (Zn concentration range: 41.33–58.8 mg/kg; Cu: 31.42–52.11 mg/kg; Co: 36.34–55.68 mg/kg; Cr: 52.3–68.21 mg/kg; Pb: 25.94–43.13 mg/kg; Cd: 5.53–16.4 mg/kg). It is evident that concentration of heavy metals increased successively moving from sediment to prey to shorebird species, likely through trophic transfer. The biofilm samples contained very high concentrations of Cr, Pb and Cd (22.64, 28.09 and 18.46 mg/kg respectively) which could be harmful to biofilm grazing shorebirds. Since bioaccumulation of heavy metals entail risks in living species, we suggest that increasing concentrations may detrimentally affect physiological processes in invertebrates and shorebirds. There is an urgent need to identify the sources of pollution and to reduce the discharge of heavy metals and other pollutants into coastal and inland wetlands.