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.
Highlights
Environmental mercury contamination is a global issue
The results obtained in the current study from the lower stretch of Hugli (Ganges) River along with Indian Sundarban wetlands reveal that HgT concentration in sediments are currently low due to a number of factors such as geomorphological positions, tidal and riverine inputs despite increasing anthropogenic factors associated with the growth of population and industry around Calcutta
The HgT values in surface sediments are well below the prescribed ERL value (0.15 μg g-1), sediment dwelling biota are high in terms of HgT in their body tissues
Summary
In the past two decades, the concentration of Hg has been determined across the globe in numerous environmental matrices, including air, water, sediment, and biota (Driscoll et al, 2013; Taylor et al, 2012). Primary consumers feeding directly from organic material in sediments receive mercury in lower levels (Choy et al, 2010). The organisms feeding on these primary consumers accumulate mercury in higher concentrations (DesGranges et al, 1998; Edmond et al, 2010; Wolfe et al, 1998). In this way the mercury concentrates through trophic transfer from lower to higher trophic levels in the aquatic food web. The highest body burden of mercury concentration could be expected in the top predator fishes and human beings (NRC, 2000) accumulation factors of almost 5000 that of sedimentary mercury have been recorded (Sharma, 1993)
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