Figure 6c. Arsenic decreasing at Terrebonne Bay sites. 



that can change in a consistent fashion 

 with time. 



There are natural factors, such as the 

 difference between mussels and oys- 

 ters, and salinity, and possibly repro- 

 ductive state and season, that can affect 

 concentrations (Phillips, 1980). Some 

 of this is accounted for in the Mussel 

 Watch Project by sampling in the same 

 season every year, always collecting the 

 same species at a site, and seeking mol- 

 lusks of a certain size. Other factors, 

 such as salinity, are monitored but can- 

 not be controlled. There is ample rea- 

 son for attributing interannual varia- 

 tions in concentration to natural factors, 

 but it is difficult to attribute temporal 



correlations with natural factors. The 

 strong temporal correlations between 

 concentrations and time that were found 

 in the five-year data set do, most likely, 

 imply a human influence at those sites. 



LONGER-TERM TRENDS 



Lauenstein et al. (1990) found decadal 

 trends in lead concentrations by com- 

 paring NS&T data from 1986 through 

 1 988 with data from analyses of mussels 

 and oysters collected in 1976 through 

 1 978 by a previous "mussel watch" pro- 

 gram (Goldberg et al., 1983) sponsored 

 by the U.S. Environmental Protection 

 Agency (EPA). Fifty sites were com- 

 mon to both programs and, at 39 of them, 



25 



