EFFECT OF NUTRIENT ENRICHMENT ON THE DISTRIBUTION AND SEDIMENTATION OF POLYCHLORINATED-BIPHENYLS (PCBS) IN SEAWATER

The effect of nutrient enrichment on the distribution of polychlorinat ed biphenyl's (PCBs) in the microbial food web and the residence time of PCBs in seawater was studied in an experimental mesocosm system. Tw o 5 m high temperature and light controlled mesocosm tubes (empty set = 0.5 m) were filled with seawater from the northern Baltic Sea. Inorg anic phosphorus and nitrogen were added daily to one mesocosm, while t he other served as a control. Experiments were conducted at 5, 10 and 20 degrees C. Three C-14-labelled PCBs of different degree of chlorina tion 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 grow th rates of the microorganisms as well as the sedimentation rate of pa rticulate organic material increased with nutrient enrichment. The siz e distribution of the microorganisms changed with nutrient status, fro m dominance of picoplankton (< 2 mu m) in the control towards increase d importance of micro (> 10 mu m) and nanoplankton (2-10 mu m) in nutr ient enrichment. The specific growth rate of the bacterial community w as found to be more temperature dependent than that of the phytoplankt on community. The relative proportion of PCBs in the > 2 mu m fraction was observed to be in the order MCB < TCB < HCB, while the opposite d istribution prevailed in the < 2 mu m fraction. We hypothesize that th is is due to the combined effect of the different K-ow values of the P CBs and a different composition of the particulate organic carbon in t he > 2 mu m and < 2 mu m fractions (e.g. different lipid composition). The residence time of the PCBs in the mesocosm generally decreased wi th nutrient enrichment, but was dependent on the degree of chlorinatio n of the PCB. Our results indicate that the transport of organic pollu tants up through the food web is more important in nutrient poor than in nutrient rich waters and that the importance of sedimentation is hi gher in eutrophic ecosystems.