Particle-water partitioning of PCBs in the photic zone: a 25-month study in the open Baltic Sea.

From previous laboratory and field studies, it remains unclear whether partitioning of hydrophobic organic contaminants (HOCs) to phytoplankton from water is kinetically limited or may be treated as an equilibrium process. Here, we report on the partitioning of polychlorinated biphenyls (PCBs) to particulate organic carbon (POC), dominated by planktonic primary production, in the open Baltic Sea during a 25-month period. The organic carbon-normalized partition coefficient (Koc) was corrected for temperature, salinity, and sorption to filter-passing organic carbon. At all 21 sampling occasions, the log Koc-log Kow regression was significantly linear, despite a large variation in biogeochemical parameters such as POC concentration and composition, primary production, and phytoplankton species composition. These data strongly suggest that partitioning of PCBs to POC in temperate surface waters is equilibrated and therefore not kinetically limited by factors such as rapid phytoplankton growth rate or large cell size. The partitioning of PCBs to the POC was described throughout seasonal cycles by log Koc = 0.88 +/- 0.07 log Kow + 0.90 +/- 0.47 (95% confidence interval). The slope of the log Koc-log Kow regression for the single sampling occasions varied between 0.56 and 1.25, and there was a seasonal variation in the POC sorbent quality (e.g., log Koc for PCB 28 varied between 5.5 and 6.9; median 5.9). These variations reflect the variability in structural composition of the POC pool in such pelagic waters. Being able to predict particle-water partitioning of HOCs significantly reduces the required complexity of both food web uptake models and predictions of POC-mediated export of HOCs to the deep ocean.