degradation. The pesticide DDT is me- 

 tabolized to DDE and DDD in the envi- 

 ronment, but those compounds degrade 

 very slowly under environmental con- 

 ditions. 



The three butyltin compounds, aggre- 

 gated as tBT, are found in mollusks 

 because tributyltin (TBT) has been used 

 as an antifouling agent in the paint com- 

 monly used on ships and some underwa- 

 ter marine facilities. Its use on vessels 

 under 75 feet long was banned in 1988. 

 Tributyltin degrades to dibutyltin and 

 then to monobutyl tin, which itself does 

 not persist, so unlike the chlorinated 

 compounds, tBT should degrade rela- 

 tively quickly (Seligman et al., 1988). 

 Consequently, the NS&T Program 

 should find substantial decreases in tBT 

 concentrations during the next several 

 years. 



Polycyclic aromatic hydrocarbons 

 (PAHs) are similar to metals in the sense 

 that they occur naturally. They are found 

 in fossil fuels such as coal and oil. Their 

 existence, however, is also attributable 

 to humans because they are produced 

 when organic matter is burned. A mul- 

 titude of human activities, from coal and 

 wood burning to waste incineration, cre- 

 ate PAH compounds in excess of those 

 that would exist naturally. In addition, 

 human production, transport, and use of 

 oil releases more PAHs to the environ- 

 ment, on a globally averaged basis, than 

 does natural seepage. Because they are 

 relatively more concentrated in oil than 

 in combustion products, 2- and 3-ring 



compounds, especially those with alkyl 

 groups on a ring such as methyl- and 

 dimethylnaphthalene and methyl- 

 phenanthrene (Table 1), are sometimes 

 classified separately from the higher 

 molecular-weight 4- and 5-ring com- 

 pounds. Since high concentrations of 

 both types of compounds tend to be 

 found in the same locations, all PAH 

 compounds have been combined into a 

 single group in this report. 



All of these trace metals and groups of 

 organic compounds can be acutely or 

 chronically toxic to marine life and to 

 humans under some conditions. On the 

 other hand, while the elements arsenic, 

 chromium, copper, nickel, selenium, and 

 zinc can be toxic at high concentrations, 

 they are also essential to the mainte- 

 nance of life (Nielsen, 1988). 



DIFFERENCES BETWEEN 

 SPECIES IN CHEMICAL 

 CONCENTRATIONS 



One use of the Mussel Watch data is to 

 compare chemical concentrations among 

 sites. However, it is inappropriate to 

 compare concentrations of some ele- 

 ments in mussels with those in oysters. 

 This was demonstrated through analy- 

 ses of mussels (M. edulis) and oysters 

 (C. virginica) collected at one site in 

 Long Island Sound. The results shown 

 in Figure 2 indicate that, despite being 

 exposed to the same environment, the 

 species do not accumulate all chemicals 

 to the same extent. Concentrations of 

 copper, zinc, and silver are more than 1 



