Technical Article What Limits the Productivity of Acid Mine Drainage Treatment Ponds? Jeffrey A. Simmons, Jonah M. Long, and Joshua W. Ray Biology Dept, WV Wesleyan College, 59 College Ave, Buckhannon, WV, USA 26201; corresponding author’s e-mail: simmons@wvwc.edu Abstract. Acid mine drainage (AMD) treatment ponds are very common in the U.S. Appalachian coal region and are the main source of many headwater streams. Though the water that discharges from these ponds generally meets state and federal water quality standards, there is a distinct lack of productivity in most of these ponds. Our first objective was to compare the productivity of chemically-treated, biologically-treated, and untreated AMD ponds with uncontaminated (reference) ponds. Next, we used principal component analysis and multiple regression of 20 physicochemical characteristics of these ponds to resolve which factor(s) were responsible for inhibiting productivity. We discovered that chemically-treated AMD ponds and untreated AMD ponds exhibited significantly less gross primary productivity (GPP) than reference ponds; biologically-treated ponds (containing AMD that has passed through a wetland) did not vary significantly from reference ponds. Chemically-treated ponds also had significantly less net primary productivity (NPP) than reference ponds. Community respiration did not vary among the pond types. Our test results indicated that soluble reactive phosphate concentration explained most of the variance in both GPP and NPP. Apparently, phosphate availability, not metal toxicity, regulated phyto- plankton productivity in these ponds. Key words: Heavy metals; mine drainage; phosphorus; phytoplankton; primary productivity Introduction There are literally hundreds of acid mine drainage (AMD) treatment ponds distributed throughout the Appalachian region of the U.S.A. Since the passage of the Surface Mining Reclamation and Control Act in 1977, construction of treatment ponds has been required at all coal mining operations where AMD occurs. AMD treatment ponds serve as holding tanks to allow mine operators to reduce the acidity and heavy metal concentrations before the water is discharged into streams. Although the water in many of these ponds has been successfully treated, visual inspection reveals that algal productivity remains low. So what limits primary productivity in AMD treatment ponds in which acidity and heavy metals have been reduced? AMD treatment ponds are usually constructed in series, with the water quality improving with each successive stage of treatment. Most commonly, treatment consists of adding alkaline chemicals, such as lime (CaCO 3 ), anhydrous ammonia (NH 3 ), or sodium hydroxide (NaOH). Constructed wetlands have also been used successfully to reduce acidity and contaminant levels, particularly Fe, Mn, and Al (Hedin and Nairn 1993; Skousen et al. 1995; Karathanasis and Johnson 2003). Sulfate reduction and limestone dissolution in wetlands generate alkalinity that neutralizes acidity in mine drainage. As the pH and redox potential of the mine drainage are increased, metals tend to form insoluble hydroxides, precipitate out of solution, and settle to the bottom of the pond. The slow-moving water and long residence times of treatment ponds and wetlands facilitates the precipitation of Fe, Mn, and Al hydroxides. At the last pond in the series where the water is discharged into a nearby stream, the water quality must meet permitted effluent standards, which usually include a circumneutral pH and low metal concentrations. Theoretically, phytoplankton and aquatic macrophytes should thrive in the sunny, warm waters of these final treatment ponds during the growing season, but usually there is no visually discernible difference between these ponds and untreated AMD ponds. Apparently, some factor inhibits the growth of aquatic plants in these circumstances. This observation is troubling because it suggests that treated pond water may continue to prevent algal growth after it is discharged into streams, thereby reducing stream productivity and altering the stream carbon cycle. Considering that approximately 10% of the streams in Appalachia receive either treated or untreated AMD inputs, this is a potentially serious threat to our stream ecosystems (Herlihy et al. 1990). Although some pond discharges are quickly diluted upon entering a stream, in other cases, the pond is the sole or predominant source of a stream. One possible reason for the apparent inhibition is that even though the heavy metal concentrations in pond water are low enough to meet state or federal water quality criteria (usually based on epidemiological and Mine Water and the Environment (2004) 23: 44–53 ©IMWA Springer-Verlag 2004