Comparison of heavy metals concentration in sediments and vegetative organs of two species of Grey and Red mangrove

Mangrove forests of Iran; like anywhere else; are ecologically sensitive areas and exposed to a wide range of anthropogenic threats. Considering the ecological importance of Khor-e-Azini wetland as the only Rizophora mucronata (Lam) habitat in Iran and also due to the lack of information on heavy metals pollution in this habitat, relevant investigation is of utmost importance for their better management. Hence, the aim of this study were to investigate the concentration of heavy metals in sediments and vegetative organs of Avicennia marina (Forssk) Vierh and R. mucronata and compare their concentrations with international standards to reveal the contamination status of this habitat. The ultimate goal was to provide information for pollution control at the source and more efficiently protect these sensitive ecosystems.

After a reconnaissance visit to the mangrove habitat, a six-hectare stand composed of A. marina and R. mucronata was selected and sampling was conducted using line sampling method. In the selected stand, three 300-meter transects, 100 meters apart from each other, were formed. Also six plots (20×20 m2) were selected along transects at equal distances. To collect samples, 10 plots from among the 18 plots were selected in a complete random manner. Then, 5 to 10 trees higher than three meters were selected randomly for sampling of sediment, root and leaf of mangrove trees. Heavy metals concentrations were measured using an atomic absorption spectrophotometer. Also, the mean concentrations of heavy metals in sediments, roots and leaf of A. marina and R. mucronata were analyzed using Independent Sample T-test.

The mean concentrations of heavy metals in sediments and organs of A. marina and R. mucronata showed no significant differences for Ni and Zn in sediments, roots and leaves of the two species, but the concentration of Cu in sediments (66.60; 39.91 µg/g), roots (70.99; 33.70 µg/g) and leaf (72.51; 20.85 µg/g) in the two species was significantly different at confidence level 95%. Also, comparison of concentration of Zn, Cu and Ni in sediment samples of A.marina and R. mucronata with international and national standards showed that of Zn was lower than the international standard, but higher than that the standard set for the Persian Gulf sediments. In addition, the concentration of Cu in sediments for the two species was higher than that of the international standard. Compared with the standard of Persian Gulf sediments, Cu concentration in A. marina habitat was much higher and in R. mucronata habitat, somewhat higher than the standard. The concentration of Ni was much higher than the international and national standards.

Our results indicated that the concentration of Ni in the sediments of A.marina and R. mucronata were critically high and that of Cu was in a warning status based on the standard of Persian Gulf sediments. Although no significant differences were detected for Zn and Ni concentrations in sediments and plant organs of A.marina and R. mucronata, Cu concentration in sediments, roots and leaf for the two species was significantly different with values for A.marina being higher than that in R. mucronata.