Effects of internal phosphorus loading on nutrient limitation in a eutrophic reservoir

While lake and reservoir management has historically focused on controlling external nutrient loads to improve water quality, internal mechanisms can also contribute to the processes of eutrophication. We assessed how the release of phosphorus (P) from anoxic sediments in the hypolimnion of a eutrophic reservoir affected epilimnetic nutrient concentrations and ratios. We also conducted nutrient bioassay experiments to determine if water column total nitrogen:total P (TN:TP) ratios could be used to predict nutrient limitation in the reservoir. We estimated that anoxic sediments from the lacustrine zone of the reservoir released 7.1 mg P/m2/day into the reservoir during stratification. This internal load was an important source of P to the epilimnion of the reservoir that helped to lower TN:TP ratios and create N limiting conditions following thermocline erosion. With respect to the enrichment bioassays, we found that nutrient limitation varied both spatially and temporally in the reservoir with observed periods of no nutrient limitation, N limitation, P limitation, and N and P co-limitation. However, corresponding water column TN:TP ratios correctly identified the limiting nutrient in less than 50% of the nutrient bioassays. As such, total nutrient ratios should be used with caution when trying to predict nutrient limitation in individual systems.