The multitude of species complicates comparisons among sites, because different species can have 

 different chemical concentrations even if the surrounding environments are identical. At sites in Long 

 Island Sound where it was possible to sample both mussels and oysters, M. edulis and C. virginica, two 

 trace elements, Cu and Zn, were enriched in oysters by more than a factor of 10 relative to mussels, 

 while Pb was more than three times higher in mussels. For other elements and for organic compounds 

 no strong species-effect was observed. Similarly, at a site off the Columbia River two species of mus- 

 sels, M edulis and M. calif omianus, were sampled. In that case there were no important concentration 

 differences for any element or organic compound. 



There has been some discussion recently among malacologists over whether the West Coast organism 

 called M. edulis is actually M. galloprovincialis in California and M. trossulus towards the north. In 

 fact, the three species may be strains of a single Mytilus species (Seed, 1992). Given this uncertainty, 

 the mussels collected at the Columbia River site may have been M trossulus or even M. galloprovincialis 

 instead of M. edulis. However, the lack of concentration differences between two Mytilus species at 

 that site has been taken to validate comparisons among aU marine mussels collected in the program. 



DATA AVAILABILITY 



Except in the Great Lakes, moUusks are collected in the months of November through March with each 

 site occupied within 30 days of a prescribed date. In the Great Lakes, collections are made in August. 

 From 1986 through 1991, six separate composite samples of 20 oysters or 30 mussels were collected at 

 each site. Three composites were homogenized for trace element analyses and three for trace organic 

 analyses. That protocol still applies whenever a site is sampled for the first time. Otherwise, since 1992 

 only two composite samples have been collected; one for each analytical category. The concentrations 

 used in this report are the arithmetic means of three concentrations measured prior to 1992 or the single 

 value measured since then. An appendix, available upon request, contains all the mean concentrations 

 of elements or of groups of organic compounds. A computer disk with all the raw data is available, and 

 the raw data can also be obtained on the Internet using the Universal Resource Locator at http://www- 

 orca.nos.noaa.gov/projects/nsandt/nsandt.html. 



LOCATIONS WHERE CONCENTRATIONS ARE HIGH 



Using the 1990 data, because it included more sites than any other single year, O'Connor (1992) calcu- 

 lated the following "high" concentrations as the mean plus one standard deviation of the lognormal 

 distribution of concentrations among sites. 



1 



The "highs" for copper and zinc must be calculated separately for mussels (mu) and oysters (oy). 

 ^The "high" for lead in mussels is 4.3 ppm and 0.96 in oysters, the value listed is the lowest public 

 health guideline (see following page). 



