Nitrogen form regulates phytoplankton community composition and stoichiometry in estuarine and coastal waters

River discharge delivers large amounts of nitrate (NO₃^-) into estuarine and coastal waters, shaping the nutrient structure and phytoplankton community composition. Little is known about the effect of nitrogen forms on the composition and stoichiometry of phytoplankton community in the ocean. It was hypothesized that NO₃^- enrichment shifted phytoplankton community composition by triggering diatom growth and thereby diminished nitrogen to phosphorus uptake ratios (N:P) of phytoplankton community, ultimately increasing the dissolved inorganic nitrogen to phosphorus ratios (DIN:DIP) in ambient seawater. To test this hypothesis, nutrient enrichment bioassay with different nitrogen forms (NO₃^- vs. ammonium, NH₄⁺) was conducted in a wide range of regions from phosphorus-limited Pearl River estuary to the nitrogen-limited shelf in the northern South China Sea. Our results showed that diatoms and dinoflagellates exhibited stronger growth responses to NO₃^- and NH₄⁺, respectively. Nitrogen forms mediated the phytoplankton community composition, causing shifts in phosphorus uptake and altering nutrient stoichiometry in the seawater. Phytoplankton grown on NO₃^- had higher demands for phosphorus, resulting in lower N:P uptake ratios of phytoplankton than grown on NH₄⁺. Hence, high riverine NO₃^- input triggered the growth of diatoms and enhanced the DIN:DIP ratios in the seawater. However, nitrogen forms had little effect on the DIN consumption and carbon fixation by phytoplankton. The findings highlight that nutrient management strategies should consider not only the total DIN loading but also the forms of nitrogen inputs-regulating riverine NO₃^- or sewage NH₄⁺ may alter phytoplankton composition and nutrient stoichiometry, even if total DIN removal remains similar.