Concentration, stock and transport rate of heavy metals in a tropical red mangrove, Natal, Brazil

Abstract

This paper discusses distribution, dynamics and mass budget of heavy metals in a red mangrove forest of the Potengi River estuary, northern Brazil. Tidal hydrology, net primary productivity, decomposing rate of leaf and standing stock of leaf litter in a red mangrove forest were taken into consideration. The belowground roots represented the main mass budget of Fe, Zn and Pb (419.73 mol ha − 1 , 2.29 mol ha − 1 and 0.72 mol ha − 1 ). Differently, Al (683.05 mol ha − 1 ) was mostly present in aerial roots, Cd (1.33 mol ha − 1 ) in the branches and Ni in the trunks (4.43 mol ha − 1 ). Cr showed no significant difference among the structures of Rhizophora mangle (from 0.02 μmol ha − 1 to 0.96 mol ha − 1 ). The average litter fall value was 12.3 t ha − 1 year − 1 whereas the leaves contributed with 9.5 t ha − 1 year − 1 (78%), the reproductive parts with 1.4 t ha − 1 year − 1 (11%) and the branches with 1.4 t ha − 1 year − 1 (11%). The annual average transfer rates of heavy metals from the tree canopy to the sediment through leaf fall were: 0.56, 1.11, 0.01, 0.02 and 0.07 mol ha − 1 for Fe, Al, Pb, Zn and Ni respectively. Cu, Cr and Cd fluxes were less than 0.02 mol ha − 1 . An experiment on yellow leaf decomposition was carried out for different periods: 1, 2, 3, 7, 30, 60 and 90 days. The leaves in the litter bags lost 50% of their dry weight in 30 days. The rate of mass loss was faster in the first 7 days when the leaves lost 21.02% of their initial mass. Concentrations of heavy metals in the decomposing leaves showed a sharp rise from the beginning up to 90 days. However, since the residence time of leaves in the sediments was only 15 days, the leaf litter exported from the mangrove forest is still relatively low in metal concentrations. Considering that 7% of leaf fall (0.7 t ha − 1 year − 1 ) is exported to the marginal marine system, the average export of heavy metals through leaf litter is: 0.36, 0.02, 0.004, 0.01, 0.001, 0.005, 0.03 and 0.74 μmol ha − 1 year − 1 for Fe, Zn, Cr, Cu, Cd, Pb, Ni and Al, respectively. These data support the opinion that mangrove forest is an efficient biogeochemical barrier to the transport of heavy metals to Brazilian coastal areas.