Toxicity of zinc, copper, and sediments to early life stages of freshwater mussels in the Powell River, Virginia

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Virginia Tech


The decline in mussel fauna of the Powell River has been attributed to extensive mining activities in headwater streams of Wise and Lee counties, Virginia. Surface mining causes erosion, sedimentation, and contamination of water with toxic substances from coal washing and waste piles. Historical water quality data of the Powell River have documented concentrations of metals at levels determined to be toxic to molluscs, specifically zinc (Zn) and copper (Cu). Acute toxicity tests with Zn, Cu, and mixtures of these two metals were conducted with glochidia and juvenile freshwater mussels. Effects of varying conditions such as water source, temperature, length of exposure, species, and lifestage were determined. Additionally, the effects of Powell River sediment on survival and growth of juvenile mussels were evaluated.

The Cumberland moccasin shell (Wedionidus conradicus) was the most sensitive species tested, with 24-hr and 48-hr LC50 values for glochidia ranging from 423 to 725 μg Zn/L. Glochidia of the pheasantshell (Actinonaias pectorosa) exhibited LC50 values from 274 to 2886 μg Zn/L, depending on test conditions. Similar results were obtained for glochidia of the rainbow (Villosa iris), with LC50 values ranging from 577 to 4123 μg Zn/L. Juveniles were more sensitive, with 48-hr LC50 values ranging from 360 to 1885 μg Zn/L for A. pectorosa, and 339 to 1122 μg Zn/L for V. iris, depending on test conditions. Juvenile mussels were affected by Zn at lower concentrations as evidenced by valve gaping and a swollen and nonresponsive foot. Copper was 5 to 15 times more toxic than Zn, with 48-hr LC50 values ranging from 52 to 156 μg Cu/L, and ECS5O values ranging from 25 to 115 μg Cu/L for juveniles of A. pectorosa. Copper appeared to exert a different toxic mode of action, as evidenced by closed valves and reduced siphoning. In general, sensitivities of early life stages of mussels to Zn and Cu increased with higher temperature, soft water, and length of exposure. At certain concentrations, Zn seemingly had an antagonistic effect (less than additive) when mixed with Cu. This effect was evidenced by reduced mortality of juveniles in Cu solutions when Zn was added at concentrations of about 400 to 800 μg/L. However, this antagonistic effect was not reflected in the percent of juveniles affected, which increased continuously with increasing metal concentrations.

Glass beads were found unsuitable as a control substratum for use in sediment testing with juvenile mussels. Results of sediment tests indicated that sediment in some areas of the Powell River may be toxic to juvenile mussels, and that toxicity may be linked to water quality. After 10 days, survival of juveniles in sediment collected downstream of a coal processing plant was significantly lower than survival in sediment upstream of the plant (p = 0.01). Further, survival in sediments with dechlorinated tap water was significantly higher than survival in sediment with river water (p = 0.0002). After 20 and 30 days, survival was similar among sites and water types. High metal concentrations in the river water appeared to contribute to toxicity, because juveniles in tap water displayed consistently better growth, and initially better survival than juveniles in river water and sediment. This toxicity was not apparent in sediments collected from the same sites less than two months later, suggesting the character of the sediments may change as new suspended sediment is deposited.

The USEPA water quality criteria for Zn (adjusting for water hardness) are 174 μg/L (acute) and 158 μg/L (chronic), whereas copper criteria are set at 28 μg/L (acute) and 18 μg/L (chronic). Powell River water samples collected during 1991 contained concentrations of Zn and Cu exceeding these criteria, as well as concentrations shown to have adverse effects on mussel populations. Results suggest that some metals are introduced into the river system in runoff, whereas Cu is being introduced as an episodic event. Intensive monitoring of water quality is needed to identify specific sources of metal pollution. If levels of heavy metal concentrations remain high, then the declining mussel populations of the Powell River will not recover, and endangered species may be extirpated from Virginia.