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Cultural eutrophication within the Ottertail Power Plant cooling pond has led to frequent blooms of noxious algae, fish kills, and odor problems. The objectives of this project were to (1) estimate phosphorus and nitrogen loadings to the cooling pond and (2) evaluate seasonal phytoplankton dynamics and pond trophic state. Water chemistries and phytoplankton samples were collected monthly over the period January 1 to December 31, 1998 and 1999. Total nitrogen, total phosphorus, and phytoplankton counts were analyzed according to standard limnological methods. Sources of nutrient loading include Big Stone Lake water, fly ash pond return flows, domestic wastewater and overwintering waterfowl. Big Stone Lake water was found to contribute the greatest nitrogen load (4.8 g m-2 yr-1) while waterfowl were estimated to contribute the greatest total phosphorus load (0.54 g m-2 yr-1). An average volume of 3,532,766 m3/yr is pumped from Big Stone Lake into the Ottertail Cooling pond and contributes on average 76% of total nitrogen and 39% of total phosphorus loads. Overwintering waterfowl (average number = 13,464) contribute 27% of total nitrogen and 91% of total phosphorus load, respectively. Nitrogen: phosphorus ratios (by mass) average 3.2:1 in the cooling pond versus 15.7:1 in Big Stone Lake. Total phytoplankton cell counts averaged 39,099 cells/ml and ranged from 11,776 to 66,423 cells/ml. Diatoms, green algae and euglenophytes were found in great abundance during winter months (range = 0 to 30,248 cells/ml) while cyanobacteria predominated during the warmer summer months (range = 0 to 28,709 cells/ml) at all sites. High nutrient concentrations and low nitrogen to phosphorus ratios suggest that nitrogen may be limiting to algal productivity relative to phosphorus, favoring Cyanobacteria capable of fixing nitrogen during summer months.

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Proceedings of the South Dakota Academy of Science



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