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Abstract
Potential toxic metal pollution in mangroves has attracted extensive attention globally; however, the seasonal variation of potential toxic metals in mangrove wetlands is still poorly understood. Herein, we investigated the variation of content as well as chemical speciation of typical metals (Pb, Cr, Zn and Cu) in the sediments from the Zhangjiang Estuary mangrove wetland, China. The potential risk of metal contamination was also investigated. Compared to the wet season, we found that sediment metal content was higher in the dry season. Mangrove sites show accumulated significant metals than does the mudflat both in wet and dry seasons. Geo-accumulation (Igeo) shows moderate pollution, probably because of the dilution as result of runoff and tidal hydrodynamics in the wet season. Increased concentrations of all metals in the acid-soluble fraction and decreased metal contents in the residue fraction were found in the dry season. Risk assessment indicated that the concentrations of Pb poses a higher environmental risk in the dry season. These results can increase awareness of metal pollution in the dry season and provide information for potential toxic metal management in mangrove wetlands.
Highlights
Mangroves are widely distributed in more than 100 countries in the tropical and subtropical regions of the world [1]
We found that there were no significant differences in heavy metal contents between bulk soil and mud flat in most of the sampling sites during wet season
Potential toxic metal contents (Pb, Cr, Cu, and Zn) in the Zhangjiang Estuary mangrove sediments (Figure 2) both in wet and dry season are lower than of mangroves exposed to intense anthropogenic activities in Shenzhen (China) [17], Divar Island and South Gujarat
Summary
Mangroves are widely distributed in more than 100 countries in the tropical and subtropical regions of the world [1]. Mangrove sediments could serve as a reservoir of potential toxic metals, buffering their release and bioavailability in the land–sea interface [4]. The content of potential toxic metals in mangrove sediments is concerning. Potential toxic metals in mangroves could form complexes with particulate organic carbon, Fe-Mn hydroxides and sulphides [4,5,6]. When the wetlands are flooded by the tide, potential toxic metals can be released from sediments into pore-waters by dissolution of bearing minerals under anoxic conditions [7,8]. Sequential extractions provide useful tools for the operational assessment of metals in different geochemical fractions, and have an excellent performance in revealing the influence of sediment characteristics in mangrove wetlands [9,10]
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