Eutrophication driven by nutrient enrichment continues to alter primary productivity and ecosystem health in estuarine environments. Pamlico Sound, the largest lagoonal estuary in the United States, remains understudied despite its ecological and economic importance and its exposure to extensive nutrient inputs from the Neuse and Tar-Pamlico River Basins. To evaluate nutrient limitation and the role of different nitrogen (N) forms in shaping phytoplankton dynamics, two nutrient-addition bioassays were conducted in September and October 2024 using water collected from western Pamlico Sound. Treatments included three forms of N (NO₃, NH₄, and urea), phosphorus (P), combined N+P, and a control. Phytoplankton responses were assessed through chlorophyll a (chl a), diagnostic pigments analyzed by HPLC, pH, nutrient drawdown, and pigment-specific growth rates. Across both experiments, phytoplankton growth was strongly N-limited. P additions alone did not stimulate biomass accumulation or pigment-specific growth, and initial nutrient measurements showed extremely low ambient inorganic N. N-containing treatments consistently produced the highest pH increases and steepest nutrient uptake, indicating active CO₂ assimilation and stimulated production. Although total chl a did not differ significantly among N forms, diagnostic pigments revealed clear taxa-specific responses. Diatoms grew rapidly under all N forms, while chlorophytes showed a consistent preference for NO₃, and cryptophytes favored NO₃ and urea in September but responded comparably to all N sources in October. Cyanobacteria increased modestly but remained a minor component of the community. Seasonal differences in temperature, light, and initial biomass influenced the timing and magnitude of pigment responses, with September showing earlier and larger growth than October. These findings demonstrate that Pamlico Sound is strongly N-limited and that the form of N influences phytoplankton community composition, even when total biomass responses appear consistent. Because all tested N forms stimulated phytoplankton growth, nutrient management strategies must address both the concentration and chemical form of N delivered to the estuary. Reducing non-point source pollution from agriculture and animal operations, improving wastewater effluent monitoring, and limiting runoff during storm events will be critical for mitigating eutrophication, supporting balanced nutrient cycling, and preserving the ecological resilience of Pamlico Sound.
Nora Wood (Fri,) studied this question.