Freshwater mussel (Unionidae) shells document the decline of trace element pollution in the regional watersheds of Chicago (Illinois, USA)

The present study analyzed trace element concentrations from the shells of native freshwater mussels collected from the headwater tributaries of the Illinois River (USA). These analyses were conducted to determine whether (A) anthropogenic enrichment could be observed and (B) whether enrichment had decreased following the enactment of Clean Water Act legislation in the 1970s. Collections archived in museums allowed comparison of the pre-Columbian period, the mid-20th century, and the early 21st century. The element Cu was consistently elevated above pre-Columbian baselines, while the elements As, Cd, Fe, and Zn were elevated in some collections. Although higher than baseline, concentrations Zn consistently declined from the mid-20th century to modern times, although differences occurred at individual sites. There was evidence for food web influence: the element Mn was significantly negatively correlated to sediment primary productivity as indexed by shell δ¹³C and Cu was positively correlated to trophic position as reflected by shell δ¹⁵N. Zn correlated to stream order across all time periods. We conclude that a mixture of historic factors affecting pollution control and land use patterns in the watershed led to sometimes conflicting effects on trace element bioaccumulation in mussel shells.