Seasonal Nitrogen Uptake Dynamics and Harmful Algal Blooms in the York River, Virginia

During a 2-year study of planktonic nitrogen (N) nutrition, temporal variability of (1) ambient nutrient concentrations; (2) uptake rates of ammonium (NH4+), nitrate (NO3−), nitrite (NO2−), urea, and amino acids (AA) in three size fractions (> GF/F, > 5 μm, and 5–0.2 μm); (3) NH4+ regeneration and NO3− regeneration (nitrification); and (4) an unexpected bloom of Alexandrium monilatum were examined. Dissolved organic N (DON) was the most abundant form of fixed N. High concentrations of NH4+ and NO2− were detected during the late summer and fall, reaching maximums of 9.9 and 7.6 μmol N L−1, respectively. The highest uptake rates were for NH4+ at all stations, size fractions, and seasons sampled and ranged from 34 to 80% of total absolute N uptake. The magnitude of uptake rates in the > GF/F fraction generally followed the pattern of NH4+ > NO3− > urea > AA > NO2− with some exceptions when urea uptake rates were higher than NO3−. Rates of NH4+ regeneration and nitrification often exceeded uptake rates, indicating autochthonous pathways for nutrient loading. Exceptionally high dinoflagellate biomass was found in late summer and corresponded with harmful algal blooms. Kinetic curves measured during an A. monilatum bloom showed high Vmax (33.7 ± 2.7 × 10−3 h−1) and high Ks (7.3 μmol N L−1) for NH4+ suggesting that it can rapidly utilize high concentrations when available but may be outcompeted by other phytoplankton when concentrations of NH4+ are low. However, A. monilatum demonstrated that it is capable of using a diverse suite of N substrates, giving it a potential competitive advantage under diverse nutrient conditions.