Metal accumulation in the tissues and shells of Indothais gradata snails inhabiting soft and hard substrata in an acidified tropical estuary (Brunei, South East Asia)

Acidification of estuaries affects the ecology and distributions of species and communities, and alters the properties and availabilities of contaminants to organisms. We studied metal pollution in an acidified, turbid and eutrophic South East Asian estuary (Sungai Brunei estuarine system (BES), Brunei Darussalam, Borneo). The estuary is significantly acidified by monsoon rain drainage through pyrite-rich (FeS2) sediments, as well as by heterotrophic metabolism, such that pH varies between 5.8 and 8.3. Contamination relates mainly to domestic pollution, with the estuary draining from adjacent urban areas. We determined the accumulation of Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn in an intertidal snail (Indothais gradata) commonly found in the BES. This snail inhabits both hard and soft substrata, allowing for comparisons of metal accumulation (i) between substrata (habitats), (ii) between shells and soft tissues and (iii) among sample stations along the estuarine gradient. We found that substratum type had no effect on metal burden, and that essential metals occurred in greater concentration in the tissues compared to the shells. However, importantly, shell metal accumulation was considerably more discriminatory among the sample stations and was well correlated among different metals, indicating that shells may represent better biomonitor tools under the circumstances of relative low contamination levels in small tidal estuaries. Greatest contamination occurred at one station (Bandar), with levels falling off upstream and downstream of this station, indicating a limited mixing effect of freshwater inflow or seaward tidal forcing. Relative to the other metals, Fe accumulation was the greatest, consistent with the naturally high Fe(OH)3 soil levels in the region. There was no obvious effect of acidified water raising metal burden in the BES, when compared with published data for other estuaries; however, this requires more critical consideration in the light of global marine acidification. This study contributes to understanding of the complexity of metal biomonitoring of acidified South East Asian estuaries.