Recovery of a power plant cooling reservoir ecosystem from selenium bioaccumulation

Hyco Reservoir, a North Carolina power plant cooling impoundment, was impacted by selenium toxicity during the late 1970s. Selenium inputs via coal ash discharge resulted in bioaccumulation through the aquatic food chain which caused reproductive failure and population declines of bluegill, other Lepomis spp., largemouth bass, crappie, yellow perch, and sucker species. Following these declines, green sunfish, satinfin shiner, gizzard shad, eastern mosquitofish, and redbelly tilapia dominated the fish community. During 1990, the power plant converted to a dry fly ash handling system that reduced selenium loading into the reservoir. Construction costs for this system were approximately $48 million and annual operating costs have been approximately $1 million/year. Mean selenium concentrations in reservoir waters at the power plant discharge declined from 8.8 μg/l to 3.2 μg/l within one year of the system operation, and this decline was consistent with a priori mass balance modeling predictions. Depuration of selenium from sediments and tissues of plankton, benthos, and fish has been variable since the system operation. Significant selenium declines have occurred in tissues of aquatic organisms in the eight-year recovery period while little change has been observed in sediments. Recovery of the fish community has been rapid, particularly in areas upstream of the ash pond discharge, and confirmed population-level modeling predictions using a Leslie-matrix demographic model. Diversity of the fish community increased throughout the reservoir and species dominance shifted from a green sunfish and satinfin shiner-dominated system to a bluegill-dominated system within five years after the system operation. Increased bluegill and green sunfish hybridism also occurred during early recovery and coincided with a low abundance of spawning bluegill adults. Sport fisheries have re-developed for largemouth bass and crappie and strategies for managing the recovering fishery have included periodic stock assessments, creel surveys and harvest restrictions, prey fish stocking, and continued contaminant monitoring. This pollution abatement system has been successful in reducing selenium loading into the reservoir and prompting recovery of the aquatic ecosystem.