Leaf biochemical parameters in Avicennia marina (Forsk.) Vierh as potential biomarkers of heavy metal stress in estuarine ecosystems

Sediment loadings and leaf accumulation of the heavy metals copper (Cu), lead (Pb) and zinc (Zn) with accompanying changes in leaf chlorophylls' (a + b), carotenoids and the antioxidant enzyme peroxidase were examined in the grey mangrove Avicennia marina (Forsk.) Vierh, in order to determine the applicability of these parameters as biomarkers of heavy metal stress under field conditions. Copper was found to show limited accumulation to leaf tissue, following a linear relationship at lower sediment concentrations, with saturation at higher sediment Cu concentrations. Copper accumulation relationships to leaf tissue were maintained temporally, and increases in sediment Cu. salinity, and decreases in sediment pH and Zn contributed to the accumulation of Cu to leaf tissue. Lead showed a significant relationship between sediment and leaf Pb levels, but accumulation was minimal. Accumulation relationships for Pb were not maintained temporally, and high sediment Pb, low pH and organic content increased bioavailability and accumulation of Pb. Zinc was the most mobile of all metals and was accumulated to the greatest quantities in leaf tissue in a dose-dependant relationship. Some temporal variation in Zn accumulation occurred, and higher sediment pH, organic content Zn and Pb promoted leaf Zn accumulation. Leaf Cu and Zn showed the strongest relationship with peroxidase activity and to a lesser degree Pb. Zinc was the only accumulated metal to show relationship maintenance with peroxidase activity over time. It was found that peroxidase activity best reflects the total phytotoxic effect from the combined metal stress of all three accumulated leaf metals. The only significant photopigment relationship evidenced was that of leaf Zn with the chlorophyll a/b ratio, but was not maintained temporally. Peroxidase activity may be an appropriate biomarker for Zn or total metal accumulation in leaf tissue, and the chlorophyll a/b ratio a suitable biomarker of Zn accumulation though requires temporal monitoring under field conditions. (C) 2002 Elsevier Science Ltd. All rights reserved.