Abstract

In aquatic ecosystems, phytoplankton growth may be limited by one or more essential nutrients, a phenomenon known as nutrient limitation. Nutrient limitation can vary spatially, including within and among systems, and temporally, including across seasons and years. For systems that are susceptible to eutrophication (the nutrient over-enrichment of water bodies), understanding patterns of nutrient limitation can be useful for water resource managers. We conducted phytoplankton growth bioassays in late February 2019 to analyze the nutrient limitation status of two water supply reservoirs (Oak Hollow Lake and City Lake), as well as an inflowing stream (Boulding Branch), in the North Carolina Piedmont. Two sites within each reservoir were assessed, including one site close to the water supply intake and one site further upstream. Each laboratory bioassay consisted of three nutrient addition treatments: nitrogen (N+), phosphorus (P+), and nitrogen and phosphorus (NP+), as well as a control. After 48 hr incubation under a growth light, chlorophyll a was measured in all samples, including initial samples taken prior to growth light incubation, and ANOVA was used to determine significant differences among treatments. The City Lake upstream site was P-limited, whereas the site closer to the intake was co-limited by N and P. The Oak Hollow downstream site closest to the water supply intake was P-limited while the Oak Hollow upstream site was not nutrient limited. Similarly, Boulding Branch was not nutrient limited. These results illustrate the spatial complexity of nutrient limitation. Further studies are needed to better understand this complexity, as well as examine seasonal and interannual patterns of nutrient limitation in these critical water supply reservoirs.