Trace element storage capacity of sediments in dead P. oceanica mat from a chronically contaminated marine ecosystem

Posidonia oceanica mat is considered a long-term bioindicator of contamination. Storage and sequestration of trace elementsand organic carbon (Corg) were assessed in dead P. oceanica mat and bare sediments from a highly polluted coastal marine area (AugustaBay, central Mediterranean). Sediment elemental composition and sources of organic matter have been altered since the 1950s. DeadP. oceanica mat displayed a greater ability to bury and store trace elements and Corg than nearby bare sediments, acting as a long-termcontaminant sink over the past 120 yr. Trace elements, probably associated with the mineral fraction, were stabilized and trapped despitedie-off of the overlying P. oceanica meadow. Mat deposits registered historic contamination phases well, confirming their role as naturalarchives for recording trace element trends in marine coastal environments. This sediment typology is enriched with seagrass-derivedrefractory organic matter, which acts mainly as a diluent of trace elements. Bare sediments showed evidence of inwash of contaminatedsediments via reworking; more rapid and irregular sediment accumulation; and, because of the high proportions of labile organic matter, agreater capacity to store trace elements. Through different processes, both sediment typologies represent a repository for chemicals andmay pose a risk to the marine ecosystem as a secondary source of contaminants in the case of sediment dredging or erosion.