Organic enrichment is one of the most common forms of disturbance in European regional seas, leading to ecosystem alterations such as increases in primary productivity. These marine basins are also contaminated by a vast number of different hydrophobic organic compounds (HOC's) e.g. PCB's, from throughout their multinational catchment areas. In the Baltic Sea both eutrophication and contaminants have been identified as two of the largest environmental threats (SNV, 1993). Little however, is known of their possible interactions. This study aims to trace the fate and ecological effect Of 15N labelled phytoplankton organic fractions: (i)dissolved organic material (ii)particulate organic material and (iii)organic aggregates from two eutrophic species and thus the flow of material from producers to consumers in an established microcosm system. Interactions between organic material and sediment bound PCB's will be of primary importance and assessed by the binding potentials of radiolabelled PCB's to different phytoplankton organic fractions and their subsequent ecological partitioning and bioavailability.