Bio accumulation and bio transportation of heavy metals along a water-soil-plant-firefly (Coleoptera: Lampyridae: Luciolinae) food chain: a case study from the southern Gangetic plains, West Bengal, India

Abstract

Cadmium (Cd), Chromium (Cr), Copper (Cu), Iron (Fe), Lead (Pb), Zinc (Zn) were estimated along a water-soil-plant-firefly food chain for understanding bioaccumulation and biotransformation at the study site, i.e., areas surrounding a surface water body exposed to multidirectional anthropogenic usage (bathing, cleaning, washing) and periodic exposures of domestic sewerages and runoffs from nearby agricultural areas. In the food chain components, the trend of accumulation followed Fe > Zn > Cu > Pb > Cr > Cd. Chromium (Cr) and lead (Pb) levels were higher in sediment (80 ppm and 34.4 ppm respectively). Lower N/P ratio (0.08) together with high pH (7.8± 1.2) in water indicated the eutrophic conditions. Dissolved metals (<1.0 ppm) were lower at the sediment, from soil-water interface some fractions entered the macrophytes and through trophic cascades got bio-accumulated in the study insects, Sclerotia aquatilis (Coleoptera: Lampyridae: Luciolinae). Essential heavy metals, Fe, Zn and Cu levels were higher both in fireflies and their associated plants. Higher levels Pb and Cd (0.027 and 0.047 ppm respectively) may result from mixing of agriculture runoffs and road side leachates. Excessive Cr and Pb were marked in Pistia stratiotes, 15.1 and 13.4 ppm, followed by Croton bonplandianum 3.27 and 2.28 ppm, and Alternanthera philoxeroides 2.96 and 4.55 ppm, respectively. However, Alternanthera philoxeroides represented the rich source of Zn (649 ppm). Prolonged courtship activities of males in aquatic and macrophytic environment, led to asymmetric deposition of essential heavy metals Fe, Zn and Cu (higher levels in males 458, 149.7, 16.6 ppm, than those in females 208, 121, 13.8 ppm) in fireflies. Higher level of Pb in fireflies (8.88 in males and 5.98 ppm in females) may be sourced by direct or indirect utilisation of Pista stratiotes (13.4 ppm), which acted as an efficient nursery ground for larval instars of Scl. aquatilis and provided vital resource for the completion of their life cycle. This research highlights the utility of fireflies as potential indicator of heavy metal pollution.