Abstract: | The effect of nutrient enrichment on the distribution of polychlorinated biphenyl's (PCBs) in the microbial food web and the
residence time of PCBs in seawater was studied in an experimental mesocosm system. Two 5 m high temperature and light controlled
mesocosm tubes (⊘ = 0,5 m) were filled with seawater from the northern Baltic Sea. Inorganic phosphorus and nitrogen were
added daily to one mesocosm, while the other served as a control. Experiments were conducted at 5, 10 and 20°>C. Three 14C-labelled PCBs of different degree of chlorination were added to subsamples of the mesocosms: 4 chlorobiphenyl (MCB), IUPAC
# 3; 2,2′,5,5′-tetrachlorobiphenyl (TCB), IUPAC # 52 and 2,2′, 4,4′,5,5′-hexachlorobiphenyl (HCB) IUPAC # 153. The biomasses
and growth rates of the microorganisms as well as the sedimentation rate of particulate organic material increased with nutrient
enrichment. The size distribution of the microorganisms changed with nutrient status, from dominance of picoplankton (< 2
μm) in the control towards increased importance of micro (> 10 μm) and nanoplankton (2– 10 μm) in nutrient enrichment. The
specific growth rate of the bacterial community was found to be more temperature dependent than that of the phytoplankton
community. The relative proportion of PCBs in the >2 μm fraction was observed to be in the order MCB < TCB < HCB, while the
opposite distribution prevailed in the < 2 μm fraction. We hypothesize that this is due to the combined effect of the different
Kow values of the PCBs and a different composition of the particulate organic carbon in the > 2 μm and < 2 μm fractions (e.g.
different lipid composition). The residence time of the PCBs in the mesocosm generally decreased with nutrient enrichment,
but was dependent on the degree of chlorination of the PCB. Our results indicate that the transport of organic pollutants
up through the food web is more important in nutrient poor than in nutrient rich waters and that the importance of sedimentation
is higher in eutrophic ecosystems.
This revised version was published online in July 2006 with corrections to the Cover Date. |