Primary productivity limitations in relatively low alkalinity, high phosphorus, oligotrophic Kentucky reservoirs

Many Eastern Kentucky reservoirs lack algal biomass commensurate with their macronutrient concentrations. Fisheries managers sometimes suggest fertilizing these reservoirs with nitrogen (N), phosphorus (P) and carbon (C) to enhance fish production. To assess which factors could be limiting phytoplankton growth, we conducted nutrient bioassay experiments in two low alkalinity (<17 mg CaCO3/L), high P (>30 μg/L), low chlorophyll (<3 μg/L chlorophyll a) reservoirs. We examined productivity (estimated as chlorophyll a concentrations) in situ in 4-L bags using various additions of N, P, and C. Results suggest primary productivity appears to be primarily limited by temperature, light-penetration (turbidity), and phosphorus. We observed that P was generally the limiting nutrient for algal productivity if the water was clear and temperature was >10 °C. We observed a secondary N limitation at high P loading. Under high loading with N + P, available carbon could become limiting in our closed system. Adding N + P in warm waters resulted in hypereutrophic algal concentrations (sometimes >100 μg/L chlorophyll a), and could create conditions favoring nuisance algal species-neither of which would be useful for growing game fish. Adding nutrients to surface freshwaters is generally not advisable, nor a good management practice.