Polycyclic aromatic hydrocarbons (PAHs) are like metals in 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 multitude of human activities, from coal and wood 

 burning to waste incineration, create PAH compounds in excess of those that would exist naturally. In 

 addition, human production, transport, and use of oil release more PAHs to the environment, on a 

 globally averaged basis, than does natural seepage (NRC, 1985). Because they are relatively more 

 concentrated in oil than in combustion products, 2- and 3-ring PAH compounds, especially those with 

 alkyl groups on a benzene ring such as methyl- and dimethylnaphthalene and methylphenanthrene 

 (Table 1), are sometimes classified separately from the higher molecular-weight 4- and 5-ring PAH 

 compounds. Since high concentrations of both types of compounds tend to be found in the same loca- 

 tions, 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, 

 copper, nickel, selenium, and zinc can be toxic at high concentrations, they are also essential in small 

 quantities to the maintenance of life (Nielsen, 1988). 



SAMPLING SITES 



The NS&T Mussel Watch Project is national in scale and sampUng sites should be representative of 

 large areas rather than the small-scale patches of contamination commonly referred to as "hot spots." 

 To this end, no sites were knowingly selected near waste discharge points. Furthermore, since the 

 Mussel Watch Project is based on analyzing indigenous mussels and oysters, a site must support a 

 sufficient population of these moUusks to provide annual samples. 



NS&T sampling sites are not uniformly distributed along the coast. Within estuaries and embayments, 

 they average about 20 kilometers (km) apart, while along open coastlines the average separation is 70 

 km. Almost half of the sites were selected in waters near urban areas, within 20 km of population 

 centers in excess of 100,000 people. This choice was based on the assumptions that chemical contami- 

 nation is higher, more likely to cause biological effects, and more spatially variable in these waters than 

 in rural areas. 



By 1993, 255 sites had been sampled, but not all sites in all years. The numbers sampled in each year 

 from 1986 through 1993 were 145, 147, 174, 185, 214, 192, 195, and 169, respectively. All are plotted 

 in Figure 1 and listed in Table 2, which indicates years of sampling. Temporal trend detection improves 

 as more years are sampled, so trends discussed here are based on data from the 154 sites sampled in at 

 least six of the eight years. 



SPECIES COLLECTED 



Since no single species of mollusk is common to all coasts, it has been necessary to collect seven 

 different ones: the blue mussel Mytilus edulis on the East Coast from Maine to Cape May, NJ; the 

 American oyster Crassostrea virginica from Delaware Bay southward and throughout the Gulf of 

 Mexico; the mussels M. edulis and M. californianus on the West Coast; the oyster Ostrea sandvicensis 

 in Hawaii; the zebra mussel Dreissena polymorpha at sites in the Great Lakes; the mangrove oyster 

 Crassostrea rhizophorae in Puerto Rico; and the smooth-edged jewel box Chama sinuos at the one site 

 in the Florida Keys. 



