The dynamics of heavy metals through litterfall and decomposition in a red mangrove forest

The cycling of Zn, Mn and Fe through production, decomposition, and export of litter was studied at the Itacurussá Experimental Forest, a red mangrove forest in Southeast Brazil. The total litterfall was 8.69 t ha-1 yr-1. The heaf litter represented 56% to 100% of the total litterfall. The metal concentrations in the fallen leaves were: Mn = 230 ± 50 g g-1; Fe = 116 ± 44 g g-1 and Zn = 5.5 ± 1.0 g g-1 (n = 15). The average transfer rates of heavy metals from canopy to sediment through leaf fall were: 1.39 kg ha-1 yr-1 for Mn, 0.70 kg ha-1 yr-1 for Fe, and 0.03 kg ha-1 yr-1 for Zn. These rates represent 4.5%, 4.0%, and 57.0% of the total forest biomass reservoir for Zn, Fe and Mn, respectively. There was no accumulation of the metals in the first 10 days of decomposition, and since the residence time of leaves in the sediments was less than 6 days, litter exported from the forest had relatively low metal concentrations. Since 7% of the leaf fall (0.42 t ha-1 yr-1) is exported to the sea, we estimated an average export of heavy metals throught leaf detritus as: Mn = 0.097 kg ha-1 yr-1, Fe = 0.049 kg ha-1 yr-1 and Zn = 0.002 kg ha-1 yr-1. The export of metals through leaf fall represents less than 0.01% of the total sediment reservoir. We conclude that mangrove ecosystems are probably efficient biogeochemical barriers to the transport of metal contaminants in tropical coastal areas.