In eutrophic ecosystems, primary production may significantly affect the behaviour of toxicants or the accumulation in the aquatic food chain. Little is known about the potential of extracellular carbon, e.g., exudates, to scavenge hydrophobic chemicals. In a laboratory Selenastrum capricornutum culture with 30% extracellular carbon, phytoplankton-water bioconcentration factors (BCF) and exudate-water association factors (KDOC) were measured for two chlorobenzenes and 4 PCBs (total log Kow range 4.6-7.4). The apparent logBCFs were measured after 3 day batch equilibration and varied linearly with logKow with a slope of 0.63. The KDOC values for exudates from the same batch were measured using a gas purge method and were very high and more or less equal to Kow: logKDOC=logKow −0.057 (r2=0.926, n=6). The strong association with exudates limits the bioavailability of PCBs in laboratory cultures and natural aquatic systems.

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