On Spartina alterniflora litter and the trace metal biogeochemistry of a salt marsh

Abstract

The trace metals copper, nickel, zinc, manganese and iron, as associated with the aqueous surface microlayer retained on fallen Spartina alterniflora litter were studied in spatial and seasonal contexts. The metals were operationally defined as acetone soluble (organically associated), exchangeable, oxide (FeMn) associated, and total. Over 95% of all metals studied in two Delaware salt marshes were associated with the exchangeable and oxide classes. There was observed major spatial and seasonal variability in metal retention by the litter. Thus, litter some 12 m from tidal creek banks in the marsh was highest in metal content, which apparently reflects differences in the amounts of metal-rich microlayer reaching the litter with tidal water movements into litter-covered areas. Also, litter exhibited highest absolute quantities of trace metals studied in the winter season. This seasonal accumulation is biologically mediated, as sulphate reduction in anoxic marsh sediments releases H+ in the spring/summer and fall. The H+ released lessens retention of metals, especially iron, by the recumbent litter. For both marshes, and all seasons studied, the litter exhibited the following ranges in total metal content (dry weight basis): copper: 0·5–11 ppm; nickel: 2–11 ppm; zinc: 12–80 ppm; manganese: 40–350 ppm; iron: 1600–8100 ppm. The existence of two distinct chemical and physical types of microlayer in the salt marsh is reported. One is a ubiquitous, opaque brown, organic rich microlayer, the other, of more limited appearance, a thin, glassy, bluish-black material. The biogeochemical significance of both microlayer types is discussed, especially as the glass-like microlayer appears to result from oxidation of ferrous iron released from underlying anoxic sediments on a seasonal basis.