Effects of Nitrogen and Phosphorus Fertilization on Primary Productivity: A Case History of Grayson Lake, Kentucky

Although fertilization has been a common practice to increase fish production in low nutrient lakes and ponds, applicability of the practice in larger reservoirs is questionable. Under the assumption that increased algal production moves up the food web to fish, we fertilized Grayson Lake, Kentucky, surface water with high N to P ratio fertilizer during springs 1994 and 1995. April through July 1994 we added 1 kg P/ha and 22 kg N/ha over a 9-week period to approximately 162 ha of lake surface area. During 1995 we increased loading to 7 kg P/ha with 22 kg N/ha. Weekly photic zone water samples were taken directly after fertilization (within 1 day). Water quality and primary production were assessed in fertilized and unfertilized lake regions. Alkalinity, inorganic nitrogen, soluble reactive phosphorus, and chlorophyll a concentrations were not significantly higher in the fertilized areas (N = 40 in 1994, N = 50 in 1995, P > 0.1 Mann-Whitney test). Secchi depth was also unaffected by fertilization. The large volume and rapid hydrologic renewal probably contributed to the lack of increased primary production after fertilization. Although total P always averaged greater than 57 µg/liter, chlorophyll a never averaged higher than 4.7 µg/liter. Secchi depths averaged less than 1.7 m, suggesting non-algal turbidity was high. We conclude that fertilization is an ineffective technique to increase fish production in large reservoirs (e.g., kilometers long) because factors other than nutrient concentrations, such as temperature, light, available habitat, and top-down predation, may determine primary productivity and fish survival.

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