There is a vast amount of information in the open literature on the fate, behaviour, toxicity and bioaccumulation of PCBs in the aquatic environment . This profile only presents an outline of the data available, and the reader is directed to more extensive reviews such as US EPA (1984), CCME (1992) and WHO (1993) for a more detailed explanation of the data and reported values.

Polychlorinated biphenyls (PCBs) are mixtures of aromatic chemicals, manufactured by the chlorination of biphenyl in the presence of a suitable catalyst. The empirical formula for PCBs is C₁₂H_10-nCl_n where n may be any value from 1 to 10. PCBs with 5 or more chlorine atoms per molecule are referred to as 'higher chlorobiphenyls' and are relatively more persistent in the environment that 'lower chlorobiphenyls' which have four or fewer chlorine atoms.

Individual PCBs (commonly referred to as congeners, e.g. PCB 153) vary widely in their physical and chemical properties, according to the degree and position of chlorination. Most PCBs are slightly soluble in water and the solubility decreases with chorine content. Estimates of PCB solubility range from 2.7 to 15,000 µg l^-1 (CCME 1992).

PCBs have been widely used in industrial applications in view of their excellent thermal stability, strong resistance to both acid and base hydrolysis, general inertness, solubility in organic solvents, excellent dielectric properties, resistance to oxidation and reduction and nonflammability (CCME 1992).

PCBs can be considered to be among the most ubiquitous and resistant pollutants in the global ecosystem.

The principal route of PCB transport to the marine environment is from waste streams to receiving waters, downstream movement by means of solution and re-adsorption onto particles and by the transport of the sediment itself, until eventually reaching estuaries and coastal waters. The marine environment appears to be the ultimate and major sink for PCBs (CCME 1992).

WHO (1993) stated that in the oceans, levels of 0.05 - 0.6 ng l^-1 have been found.

Monitoring data for sediments and biota from the National Rivers Authority and the National Monitoring Programme Survey of the Quality of UK Coastal Waters are presented in Appendix D and indicate some elevated levels in certain estuaries.

In the aquatic environment, PCBs are usually found in much higher concentrations in sediments than in the overlying water. PCBs have a high affinity for suspended solids, especially those higher in organic carbon. This is supported by their low water solubility and high octanol/water partition coefficients (calculated Log Kow values range from 3.76 for biphenyl to 8.26 for decachlorobiphenyl) (CCME 1992).

PCBs, with the exception of some lower-chlorinated compounds, have low volatility and are soluble in organic solvents, particularly hydrocarbons. Temperatures in excess of 1,000 °C are required for their complete combustion.

Sorption to sediments is the predominate mechanism removing PCBs from the water column.

Individual PCBs vary widely in their susceptibility to biodegradation. PCBs with three or fewer chlorine atoms per molecule can be biodegraded by many organisms, whereas these with 5 or more chlorines are resistant to biodegradation and biotransformation. In general, PCB congeners have a low solubility in water, and high octanol-water partition coefficients, bioaccumulation potential, and resistance to biodegradation. The physical and chemical properties of PCBs cause their removal from water by sorption to suspended particles and bottom sediments (CCME 1992).

An exhaustive literature review on the toxicity of PCBs to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (WHO 1993). The most sensitive groups of organisms have been identified.

With regard to direct toxicity, WHO (1993) reported that results had been extremely variable with no consistent relationship between percentage chlorination or environmental conditions and toxicity. Over 96 hours, under static conditions, LC50 values have ranged between 12 µg l^-1 to >10 mg l^-1 for various aquatic invertebrate species. Flow-through studies showed increased toxicity. WHO (1993) reported a similar variation for PCB mixtures for fish with 96 hour LC50s varying between 0.008 and >100 mg l^-1. Long-term tests have shown that acute exposure, particularly in static conditions, considerably underestimates the toxicity of PCBs.

WHO (1993) also reported that for birds fed PCBs in their diet, there was evidence of egg-shell thinning, while for sea mammals, there was evidence that PCBs reduce their reproductive capacity. The main effect is on the implantation of the embryo but there can be physical changes to the female reproductive tract. Consequently, PCBs have been identified as an endocrine disrupting group of substances.

Accumulation of PCBs in sediments poses a potential hazard to sediment-dwelling organisms. Environment Canada has recently issued interim marine sediment quality guidelines and these include a guideline of 21.5 µg kg^-1 (dry weight) of total PCBs above which effects on sediment-dwelling organisms may occur.

The main concern over PCBs is their high bioaccumulation capacity.

PCBs are soluble in the lipids of biological systems and therefore tend to be bioaccumulated in fatty tissues (especially the higher chlorobiphenyls). BCFs of 200,000 and greater have been reported for fathead minnows Pimephales promelas (Duke 1971 and Neely 1977) and pink shrimp (Klien and Weisgerber 1976) and up to 1,000,000 in other organisms. Relatively low concentrations of PCBs in the aquatic environment can result in the accumulation of relatively high PCB levels in biota.

WHO (1993) quoted experimentally determined BCFs in various aquatic species, ranging from 200 to 70,000 or more. In the open ocean, there is bioaccumulation of PCBs in higher trophic levels with an increased proportion of higher chlorinated biphenyls in high-ranking predators.

The presence of high concentrations of PCBs or their residues in marine mammals have been suggested as the cause of (or contributing towards) pathological changes and reproductive failures in Baltic seals (Helle et al 1976), sealions, seals and beluga whales (Addison 1989); immunity suppression (and hence the possibility of being more susceptible to disease) in harbour porpoises (Kuiken et al 1994) and seals (Reijnders 1986; Swart et al, 1994); changes in the development stability of the Baltic grey seal (Zakharov and Yablokox 1990); and premature pupping in California sea lions (Delong et al 1973).

Potential effects include:

• toxicity of PCBs to invertebrates and fish in the water column;
• accumulation in sediments and potential hazard to sediment-dwelling organisms at concentrations greater than 21.5 µg kg^-1 (dry weight) of total PCBs;
• bioaccumulation of PCBs in fish, birds and Annex II sea mammals with known sublethal toxicological effects;
• endocrine disruption in birds and sea mammals posing a hazard to populations of these animals.

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