Spatial Variation of Heavy Metal Accumulation in the Sediments Adjacent to Different Mangrove Plant Species within a Mixed Mangrove Stand

Selective accumulation of heavy metals in mangrove plant parts have been reported earlier; however, varying heavy metal accumulation in the sediments adjacent to different mangrove plant species is rarely reported. We hypothesized that the pattern of heavy metal accumulation could vary in the adjacent sediments of different mangrove plant species. We compared the heavy metal accumulation (As, Co, Cu, Cd, Cr, Pb, Ni, and Zn) in the mangrove sediments adjacent to Avicennia marina (AM), Bruguiera gymnorrhiza (BG), Rhizophora mucronata (RM), and Excoecaria agallocha (EA) from a mixed mangrove stand within the Indian Sundarbans. Metal concentrations (except Cr) varied significantly among the four mangrove sites. The highest concentrations of As, Cd, and Pb were observed in EA sites, Cu and Zn, in BG sites, followed by Co and Ni, in AM sites. Contamination factor, enrichment ratio, and geo-accumulation indices indicated significant Cd pollution in all the sites; however, the highest magnitudes of these indices were observed in EA sites. These indices showed very high Cu and Zn pollution in BG sites. Pollution load index and potential ecological risk index depicted overall 'deterioration of sediment quality' and very high risk in all the sites. Principal component analysis and cluster analysis also showed that metal accumulation was significantly different in the four sites. Overall, the results indicated that the heavy metal type and concentration in mangrove sediments could vary depending on the mangrove plant species thriving in its adjacency. However, characterizing the reasons behind such differential accumulation needs further study.