Abiotic and biotic factors influencing the decline of native unionid mussels in the Clinch River, Virginia

Declining unionid populations in the Clinch River are of concern due to the high endemism in the diverse fauna of the Cumberlandian region. Increase in agricultural and mining activities, as well as in industry and urbanization, are coupled with unionid declines throughout the watershed. In many reaches of the Clinch River, mussel populations exist which fail to show recruitment suggesting that this is the weak link in the complex life cycle. Two possible factors which could endanger the sensitive juvenile stage are the presence of sediment toxicants or adult Corbicula fluminea in the depositional areas, the preferred habitat of the juveniles.

Before investigating the impacts of these factors, it was necessary to characterize the relationship of the juveniles with the sediment they inhabit. Observations of feeding behavior using videotape, dye studies in a feeding chamber, and gut content analysis were used to determine mechanisms of feeding, the primary food source, and the origin of substances taken up by juveniles. Exposure to sediment came not only through direct contact, but also through filtration of interstitial water and sediment-associated fine particulate organic matter. Juveniles used pedal locomotory and pedal sweep feeding behaviors to facilitate movement of particles into the pedal gape.

Intermittent sediment toxicity was found in laboratory bioassays using Daphna magna and Chironomus riparius. These data, along with fluctuating metals in the Clinch River sediments, indicated that acute insults existed from which recovery would depend on the frequency, intensity and duration of the events. Field studies revealed that the intermittent toxicity is reflected in the community structure of benthic macroinvertebrates and impairs growth of juvenile unionids in-situ studies. The intermittent toxicity which may be associated with rain events impairs stream biota and may prevent recruitment of juvenile unionids.

The presence of adult C. fluminea in sediments was found to decrease juvenile unionid growth and recovery from test sediments and to increase mortality and resuspension of juveniles into the water column. Both the presence of sediment-bound toxicants and C. fluminea may be contributing to unionid bivalve declines in the Clinch River, Virginia.