Assessment of Total Mercury (HgT) in Sediments and Biota of Indian Sundarban Wetland and Adjacent Coastal Regions

Abstract

The distribution of total mercury (HgT) in surface sediments (0-5 cm; n = 12; particle size < 63 μm) and representative biota (benthic polychaetes, bivalve mollusks and finfish) were observed in the Sundarban mangrove wetland and adjacent regions nearby the Indian Ganges river estuary. Relatively low concentrations of HgT were measured in sediments ranging from 0.008 μg g-1 to 0.056 μg g-1. There exist sharp differences in HgT accumulation in biota which revealed the following decreasing trend: polychaetes > fish > bivalve mollusks. These variations are related to a number of intrinsic (size, age and sex) and extrinsic (pH and salinity) factors together with the accumulation mechanisms intrinsic to each species for mercury. An organ-specific HgT accumulation in bivalve mollusks was evidenced with the following decreasing order: visceral mass > siphon > adductor muscle > mantle > gill, with a maximum value of 0.42 μg g-1 in Sanguinolaria acuminata. Fishes showed wide efficiency in HgT accumulation in dorsal muscle, and of most concern, one species presented HgT above 0.05 μg g-1 levels, the prescribed limit established by European Union. The benthic polychaetes showed extreme variations of HgT in their body tissues, with the maximum value of 0.603 μg g-1 in Dendronereis heteropoda which is above the European Union threshold value. The authors strongly recommend further monitoring to investigate the source of toxic metals, including Hg which may originate from diverse potential sources such as industrial discharges, agricultural runoff and sewage sludge from upstream of the Ganges River Estuary.