Accumulation of Selenium and Trace Metals on Plant Litter in a Tidal Marsh

Research in a San Francisco Bay intertidal marsh has shown the accumulation of Se and trace metals (Cu, Ni, Zn, Pb, Cd, Fe) on bagged litter of five common estuarine marsh plant species, over periods of months to a year. Metal concentrations increased as much as 150-fold relative to plant tissue initially placed in mesh litter bags. Concentrations of metals increased with the amount of time litter was submerged, though mass accumulation was moderated by the concomitant decay of the litter. The association of very fine particulate matter with the litter, as determined based on Si concentrations, was the main mechanism for enrichment. Further accumulation occurs during winter months via the sorption onto or precipitation with Fe-rich phases, particularly in lower marsh plants. The resultant annual fluxes of Se and most metals to sediments via litter were estimated to be equivalent to between 1 and 16% of the primary flux on suspended particulate matter and may be an important mechanism for scavenging dissolved and colloid-associated metals. Previous studies, which ignored the particulate contribution, may have incorrectly assumed the predominance of sorption and co-precipitation mechanisms. The degree to which litter bags enhance fine sediment trapping over natural conditions needs to be evaluated.