Evaluating Nutrient Limitation of Phytoplankton Growth in North Inlet Estuary, SC

The biolimiting nutrients nitrogen (N) and phosphorus (P) are vital components of molecules essential to life. In marine systems, N:P ratios tend to follow the “Redfield ratio” of 16:1, which is often used to infer nutrient limitation of phytoplankton biomass. Traditionally, estuaries are thought to be N-limited (N:P < 16), but there have been increasing instances of P-limitation (N:P > 16) in coastal waters worldwide. Over the last few decades, nutrient loading in North Inlet Estuary (NIE) has changed such that dissolved inorganic nitrogen (DIN):dissolved inorganic phosphorus (DIP) ratios have increased by ca. 130%. This increase suggests that the estuary may be transitioning to N and P colimitation or primary P-limitation. We hypothesized that P would be the primary limiting nutrient of phytoplankton biomass and community composition in NIE for the summer of 2023. Dissolved inorganic nitrogen (DIN, 20 μmol l-1 N), low phosphate (LP, 5 μmol l-1 ), and high phosphate (HP, 20 μmol l-1 ) combined DIN+LP, or combined DIN+HP were added to water samples collected at Clambank Landing in NIE on a monthly basis. Changes in phytoplankton biomass (chl a) and community composition were measured via High Performance Liquid Chromatography (HPLC) to determine if nutrient additions were indicative of limitation by that nutrient. N was the single or primary limiting nutrient for all bioassays, with potential P co-limitation. Shifting nutrient ratios and limitation status can impact trophodynamics and nutrient mitigation strategies may be employed to avoid cascading effects in estuarine food webs.