The fate of trace metals in suspended matter in a mangrove creek during a tidal cycle

Abstract

The variation of heavy metal content in suspended matter (SM) during tidal cycles in a mangrove creek is described. The stable isotope of carbon was used as a tracer for sources of SM to the system. During tidal cycles three patterns of metal variability were found. The first, represented by Fe, showed an irregular and small variability throughout the cycle; a second, represented by Mn, exhibited a sharp increase in concentration during the rising tide, coincident with the greatest variation of pH and E h. The third pattern, including Cu, Cd, Pb, Ni, Cr and Zn, showed maximum concentration at the peak of the high tide, coincident with a shift in SM source. The stable isotope of carbon indicated that, during low tides, most of the organic carbon exported originated from mangrove plant detritus, while during the high tides organic carbon imported by the system was almost totally of marine origin. This shift of SM source is the principal parameter controlling metal fluxes through the system. Changes in water pH and E h, and manganese precipitation, can also serve as a secondary control. Although the results strongly suggest that the metallic load of marine SM is being immobilized by the mangrove environment, mass balance studies are necessary to show whether a net accumulation of metals is actually occurring.