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...