Aquatic biota living in urban watersheds, are chronically exposed to a complex mixture of contaminants from various sources, including municipal wastewater effluents and road runoff. This study examined the general condition, immune function and contaminant load of wild freshwater mussels collected from a large urbanized river over three consecutive field seasons. Four study sites along the Grand River (ON) were selected to represent an incremental contaminant exposure, such that mussels collected from the final downstream site were exposed to the cumulative inputs from 11 municipal wastewater treatment plants and road runoff from four cities. Wild mussels collected downstream of the urban area had significantly lower (p<0.05) condition factor and did not live as long (significantly reduced mean age) as the mussels collected upstream of the cities. There appears to be a trend of increasing proportions of gravid females at the downstream sites, but whether this trend indicates feminization of the mussel population or is simply an artifact of sampling effort is unclear. An examination of hemocyte phagocytosis revealed a pattern of increasing immune activity at the downstream sites, but only in one of the years sampled. The significant and cumulative increase in Cu, Pb, Zn, Al, Cr, and Ni in the gills of downstream mussels indicates that metals are bioavailable in this ecosystem and that tissue concentrations increase with multiple urban inputs. While the complex nature of the exposure prevents identification of the cause(s) of the observed effects, some contaminants such as ammonia and chloride reach levels known to be toxic to freshwater mussels at the downstream sites. These results indicate that chronic exposure to multiple contaminants negatively impacts mussel health and longevity and corroborates previous assumptions that waterborne contaminants contributed to the decline of the freshwater mussel populations in this watershed.
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