HUMIC AND FULVIC-ACIDS - SINK OR SOURCE IN THE AVAILABILITY OF METALSTO THE MARINE BIVALVES MACOMA-BALTHICA AND POTAMOCORBULA-AMURENSIS

Humic acids (HA) and fulvic acids (FA) are common forms of organic mat ter in marine sediments, and are routinely ingested by deposit- and su spension-feeding animals. These compounds may be a sink for metals, im plying that once metals are bound to humic substances they are no long er available to food webs. A series of experiments was conducted to qu antitatively examine this premise using 2 estuarine bivalves from San Francisco Bay, USA: the suspension feeder Potamocorbula amurensis and the facultative deposit feeder Macoma balthica. HA and FA, isolated fr om marine sediments, were bound as organic coatings to either hydrous ferric oxides (HFO) or silica particles. Cd and Cr(III) were adsorbed to the organic coatings or directly to uncoated HFO and silica particl es. Pulse-chase laboratory feeding experiments using Cd-109 and Cr-51( III) were then conducted to determine absorption efficiencies of Cd an d Cr for individual specimens using each of the particle types. The re sults demonstrated that: (1) absorption of Cr(III) from all types of n on-living particles was consistently low (< 11%). Ingested Cd showed g reater bioavailability than Cr(III), perhaps due to differences in met al chemistry. (2) Bivalves absorbed Cd bound to uncoated HFO or silica particles (i.e. with no HA or FA present). (3) The presence of organi c coatings on particles reduced Cd bioavailability compared with uncoa ted particles. (4) Both geochemical and biological conditions affected the food chain transfer of Cd. The data suggest that in marine system s inorganic and organic-coated particles are predominantly a sink for Cr in sediments. In the transfer of Cd to consumer animals, inorganic particles and humic substances can act as a link (although not a highl y efficient one) under oxidized conditions.