Effects of Nutrients and Light on Phytoplankton Growth in Albemarle Sound, North Carolina

Oligohaline zones of estuaries are often subject to harmful algal blooms. Albemarle Sound and its tributaries in northeastern North Carolina comprise the largest oligohaline lagoon in the United States. In recent summers, toxigenic cyanobacterial blooms threaten the Albemarle Sound ecosystem and its value as an important commercial fishery. Nutrients and light are essential resources for phytoplankton. Understanding their effects on phytoplankton growth is important for estuarine water quality management. Chapter one of my dissertation explores how phytoplankton biomass and nutrients have changed over the past few decades and which areas of Albemarle Sound are prone to algal blooms. Long-term trends in nutrients and chlorophyll a from the 1970s to 2020 were identified using the generalized additive model. Both total nitrogen and chlorophyll a concentrations in Albemarle Sound have more than doubled in the past 20 years. These parallel increases in nutrient concentrations and phytoplankton biomass are indicative of nutrient-stimulated phytoplankton production. Chapter two examines which nutrient(s) limit phytoplankton growth and whether N₂ fixing cyanobacteria will replace non-N₂ fixers and fix more N₂ for growth when ambient nitrogen is scarce. We conducted nutrient addition bioassay experiments to determine the nutrient limitation status of phytoplankton and assessed N₂ fixation potential through direct measurements of N₂ fixation rates and by microscopic quantification of N₂ fixing cyanobacteria. Nitrogen was the primary limiting nutrient for phytoplankton. During cyanobacterial blooms with low in situ dissolved inorganic nitrogen concentrations, N₂ fixation rates were high and were further stimulated by experimental phosphorus additions. Chapter three investigates whether periodic salinity intrusion events affect phytoplankton light limitation and whether the critical depth criterion can predict the onset of blooms in the shallow temperate estuary. I measured photosynthetic rates under different irradiances to determine the light limitation status of phytoplankton and measured phytoplankton production and respiration to calculate the critical depth. Phytoplankton production was light-limited even in summer when solar radiation was high. Salinity intrusion resulted in shallower mixed layer depth and higher light availability, thereby partially alleviating light limitation. The dominant algal genus changed from dominance by the high-light adapted cyanobacteria, Dolichospermum, during the salinity intrusion to the low-light adapted cyanobacteria, Pseudanabaena, after the salinity intrusion ended. Additionally, the critical depth was much deeper than the mixed layer depth in summer with high bloom activity, consistent with predictions derived from the critical depth theory. Overall, the findings of widespread nitrogen limitation and stimulation of N₂ fixation by phosphorus suggest that both nitrogen and phosphorus inputs should be managed to control algal blooms in Albemarle Sound. Furthermore, understanding phytoplankton responses to changing light conditions will help explain bloom dynamics and improve the predictability of blooms.