Changes in nitrogen forms and levels affect biogeochemical processes, but little is known about the quantitative effect of ambient nitrogen on phytoplankton growth in biogeochemical kinetics. Based on a new experimental data analysis method, we investigated the phytoplankton growth and nitrogen uptake processes of typical phytoplankton subjected to different nitrogen forms in the Bohai Sea. We employed a nonlinear differential model to fit the kinetic processes of phytoplankton growth, death, and nutrient uptake considering multiple equations and parameters. The maximum nitrogen uptake rate (k up), half-saturation coefficient (k s), and maximum phytoplankton growth rate (k G) were obtained based on the multivariate nonlinear differential mothed. The k up and k s were related with k G for different ambient nitrogen forms (urea, ammonium, and nitrate) in the N sufficiency experiment group I (SGI) treatment, showing that phytoplankton grew faster in the presence of ammonium and nitrate than in the presence of urea, owing to quicker nitrogen uptake rate. The k up , k s , and k G in the N-Starvation experiment group II (SGII) treatment were lower than that in the SGI treatment, indicating higher nutrient affinity of phytoplankton at lower nitrogen concentrations. Hence, we can infer that eutrophic algae such as Nitzschia closterium may easily become the dominant species in the SGI treatment with abundant nitrogen supply and high k G , k up , and k s. In contrast, oligotrophic algae such as Leptocylindrus danicus and Asterionellopsis glacialis may easily become the dominant species in the SGI or SGII treatments with an insufficient supply of ambient nitrogen and low k G , k up , and k s. Plain Language Summary The ecosystem is vulnerable in coastal waters, suffering by human activities associated with anthropogenic nutrients, such as harmful algae bloom. The forms and levels of nutrients affect phytoplankton growth, while little is known about the biogeochemical kinetics. We investigated the phytoplankton growth and nitrogen uptake kinetics of typical phytoplankton subjected to different nitrogen forms (e.g., urea, ammonium, and nitrate) in the Bohai Sea. We proposed a multivariate nonlinear differential fitting method to analyze ecological experimental data, considering multiple kinetic equations and parameters of phytoplankton growth, death, and nutrient uptake. Based on the in situ batch culture experiment, the phytoplankton grew faster in ambient ammonium and nitrate than in ambient urea in the N sufficiency experiment group conditions, whereas, they grew and acquired nutrient significantly lower in the N starvation experiment group conditions. The phytoplankton domain community was different in different ambient nitrogen forms and levels, where eutrophic algae such as Nitzschia closterium might easily become the dominant species in the N sufficiency treatment with abundant nitrogen supply, whereas, oligotrophic algae such as Leptocylindrus danicus and Asterionellopsis glacialis might easily become the dominant spe...

The shift of diatom dominance to combination of diatom and dinoflagellate dominance in the Bohai Sea (BS), China might be associated with the change of dissolved nitrogen (DN). This study combines field investigation, ship‐based microcosm experiments, a nitrogen biogeochemical model, and statistical analysis to reveal the impact of DN loading on the dominance of diatoms and dinoflagellates in the BS, considering the quantitative relationship between the algal growth/death and nitrogen migration/transformation processes. The diatom to diatom‐dinoflagellate shift was significantly associated with the DN regime shift of oxidized nitrogen (NO3‐N) to reduced nitrogen (NH4‐N and dissolved organic nitrogen [DON]) since the mid‐1990s. This phenomenon was supported by microcosm and numerical experiments: (a) Gonyaulax verior as a dominant dinoflagellate in the BS efficiently absorbed/assimilated NH4‐N, and it can also utilize DON derived from centric diatoms; Chaetoceros densus as dominant centric diatoms in the BS was able to absorb/assimilate NO3‐N, and it can also utilize DON derived from the Xiaoqing River; (b) The change in dinoflagellates/diatoms ratios was reproduced with the change in DN regime from the 1980s to the 2010s by the proposed nitrogen biogeochemical model based on localized parameters in the BS. Moreover, mixotrophism and stronger survival stability, associated with the faster NH4‐N and DONalgal uptake and slower decomposition of G. verior, also supported the shift from diatoms to diatoms–dinoflagellates during high variations in nutrient supply. This study infers the phytoplankton regime shift mechanism associated with the DN structure in coastal waters, providing a credible scientific basis for the shift from water quality‐based to ecosystem‐based environmental management.