Effects of Conductivity, Nitrogen, and Phosphorus on Phytoplankton in Lake Mattamuskeet

Lake Mattamuskeet is a 16,000-hectare lake in eastern North Carolina negatively impacted by cultural eutrophication and resulting phytoplankton blooms. The purpose of this project was to explore a possible remediation for the lake by examining the effects of conductivity changes on phytoplankton concentrations in the presence of nitrogen and phosphorus in this lake. Lake water was collected during a late summer bloom; samples were dosed with phosphorus, nitrogen, and both nitrogen and phosphorus, and subsequently dosed with various concentrations of salts to vary conductivities, and incubated for five, ten, or fifteen days. After incubation, the samples were filtered; the filters were frozen and later measured for chlorophyll a concentrations, which were used to tabulate phytoplankton concentrations. While there were no significant differences in chlorophyll a concentrations for samples with different conductivities, samples dosed with nitrogen showed significantly higher concentrations of phytoplankton than those with phosphorus, suggesting that nitrogen, rather than the generally accepted phosphorus, is a limiting nutrient in Lake Mattamuskeet. Conductivity had no discernible effects on phytoplankton regardless of the presence of nitrogen or phosphorus, despite the tendency of ions to bind nitrogen- and phosphorus- containing compounds. Light microscopy was used to identify phytoplankton species present in samples; five species were identified. In addition, species composition showed no relationship with conductivity. These data indicate that, as Mattamuskeet National Wildlife Refuge attempts to restore the lake, conductivity manipulation may not be a viable answer to the nutrient loading or phytoplankton problems and nitrogen reduction must be considered. The data also provide new perspectives on managing other fresh or slightly brackish lakes subject to cultural eutrophication and changing sea levels.