Dynamic colloid-water partitioning of pyrene through a coastal Baltic spring bloom

Colloidal organic particles constitute the dominant portion of particulate organic matter in surface seawater, but their influence on the phase specia tion and bioavailability of hydrophobic organic compounds (HOCs) is sparsel y evaluated. Studies on colloid-water partitioning have been focused on oth er regimes and have largely been performed on chemically defined subportion s of total colloids such as the humic fraction. Available estimates of coll oid-water partition coeffficients (K-coc) are highly variable and not easil y explained by regularly applied K-ow-K-oc relationships. Here, pyrene was partitioned to bulk natural colloids isolated using cross-flow ultrafiltrat ion techniques from the surface water of a coastal bay. A key objective was to elucidate biogeochemical controls on the changing colloid-sorbent quali ties over the course of the dynamic allochtonous-autochtonous transition of a well-constrained boreal coastal spring bloom. The pyrene K-coc was found to decrease from 12.9 +/- 0.9 x 10(3) L-w/kg(oc) in the terrestrial runoff dominated regime to values around 2.9 +/- 0.7 x 10(3) L-w/kg(oc), once phy toplankton production became the governing source of organic matter to the surface waters. The changing K-coc was well correlated with the molar extin ction coefficient at 280 nm of the colloidal organic carbon. This study sup ports other reports of an improved prediction of HOC phase speciation throu gh this simple molecular proxy of the "quality" of organic sorbents. While being poor sorbents on a carbon atom basis, relative to soils and sediments , coastal marine colloids, by their shear abundance, may significantly atte nuate the truly dissolved exposures of HOCs with log K-ow above 5.