Abstract-In order to estimate fluxes of carbon within the microbial community, seawater was collected in a coastal area off Zanzibar Island, Tanzania, during a rainy season (November 1994) and a dry season (September 1996). Diel experiments were conducted in a 24-L polycarbonate bottle and samples were retrieved every third hour over a period of 30 to 33 h. Abundance and production rates of bacteria, nanoflagellates, and microplankton were determined. To determine possible connections between the fluxes of carbon and the fate of 2,2Ј,4,4Ј,5,5Ј-hexachlorobiphenyl International Union of Pure and Applied Chemistry (IUPAC) no. 153, a time-course experiment was conducted after the diel experiment. The initial PCB partitioning was similar in the two experiments, but the subsequent distribution among the organism groups was different. In the rainy season experiment, the PCB concentration in the largest size fraction was almost three times higher than during the dry season experiment. This was in line with results from the diel experiments where the carbon flux through the microbial food web was approximately three times higher during the rainy season than during the dry season experiment. These results suggest that the transfer rate of 2,2Ј,4,4Ј,5,5Ј-hexachlorobiphenyl through the microbial food web is coupled to the carbon flux.

To study the uptake mechanisms of 2,2',4,4',6,6'-hexachlorobiphenyl (HxCB 153) in microplankton with different feeding strategies, two laboratory communities were used. Trophic transfer of HxCB 153 was tested in a heterotrophic microbial food web consisting of bacteria, flagellates, and ciliates. Passive uptake was tested in a community consisting of algae and bacteria. The experiments were conducted over 6 d, and samples were retrieved daily. In the heterotrophic food web, a significant increase of the HxCB 153 concentration was observed in the top predator level (ciliates, p < 0.05), where the concentration doubled during the 3 d of the experiment. A concomitant decrease was observed in the ciliate prey (flagellates), indicating that HxCB 153 was redistributed due to trophic transfer. On average, 33% of the HxCB 153 was sorbed to ciliates over the course of the experiment. In the experiment with algae and bacteria, the HxCB 153 concentrations were relatively stable over time. The largest fraction partitioned into bacteria (57%), whereas only 4% of the HxCB 153 sorbed to the algae. The uptake was 8-fold higher in ciliates than in algae, although the algal biomass was 10-fold higher. The results imply that trophic transfer may be a significant transport route of more hydrophobic organic contaminants (HOCs) in plankton communities. In transfer models, this pathway may be taken into consideration so that the transport rate of HOCs to higher trophic levels is not underestimated.

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Pelagic bacteria represent the potentially most predominant biological, particulate sorptive surface for hydrophobic organic contaminant (HOC) and constitute a food source for the microheterotrophic food web, which conceivably continues up to pelagic fish. However, no data have been reported on HOCs in bacteria. Therefore, this fraction (0.2-2 µm) was isolated with a new technique in situ in the Baltic Sea followed by HOC analysis (HRGC-MS). Results show bacterial concentrations (PAHs and PCBs) in the same order as or higher than the larger particulate fraction (2-90 µm), which clearly illustrates the significance of heterotrophes for the distribution and dynamics of HOCs in marine waters.

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