Distribution and fate of polychlorinated biphenyls within the pelagic microbial food web

Sammanfattning: The composition and production of the plankton community in aquatic environments will in turn influence the food-web structure of larger predators, nutrient cycling, the sinking flux and, hence, the fate of hydrophobic organic contaminants. The aim of the research presented here was to study the distribution and fate of PCBs within the microbial food web, including bacteria, flagellates, ciliates, and phytoplankton. For this purpose mechanistic studies was conducted with three 14C-labelled PCBs as model substances; 4-chlorobiphenyl (UPAC # 3), 2,2',5,5'-tetrachlorobiphenyl (UPAC # 52), and 2,2',4,4',5,5'-hexachlorobiphenyl (UPAC # 153). In order to be able to interpret the behaviour and attempt to predict the fate of the PCBs the experiments were done in conjunction with estimates of biological and chemical parameters. Further, to study possible connections between the food web structure and the fate of PCBs experiments were performed with natural seawater collected at two different sites; the northern Baltic Sea (Sweden), and the Indian Ocean (outside Zanzibar Island, Tanzania). In addition to PCB experiments, seasonal and diel studies of the abundance and production of plankton organisms were conducted on Zanzibar Island with the aim of establishing carbon budgets for a rainy and a dry season.To follow the distribution of PCBs between different microorganism groups over time, the organism size fractions were separated by filtration and defined as bacteria (0.2-2 (m), flagellates (2-10 (m), and ciliates and phytoplankton (>10 (m). The largest fraction of the PCBs were sorbed to the bacteria, but the proportion of the PCB that partitioned into the larger organism groups generally showed a tendency to increase with hydrophobicity. The efficiency of the transport of PCBs from bacteria to higher trophic levels increased with chlorination, but was also found to be dependent on factors like production rates and the composition of the plankton community.In seasonal and diel studies of plankton communities in a tropical coastal ecosystem, the abundance of heterotrophic microorganisms did not significantly differ between the seasons, but the production rates for heterotrophic bacteria and flagellates were significantly higher during the rainy season. A high correlation between the abundance of heterotrophic nanoflagellates and picocyanobacteria suggests that picocyanobacteria was an important food resource for nanoflagellates. In both the seasonal and the diel study the carbon flow through the heterotrophic food chain increased with a factor of approximately 3 during the rainy season compared with the dry season. Similarly, the concentration of the hexachlorobiphenyl in the microplankton fraction was almost three times higher in experiments conducted during the rainy season than during the dry season. Results from the carbon budgets suggests that the microbial food web may be of particular significance as a food resource for higher trophic levels during the dry season.