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Phytoplankton are not only the main bearer of the nitrogen biological cycle, but also a key link driving the nitrogen cycle. However, most phytoplankton cannot directly utilize N₂, they must uptake nitrogenous nutrients (ammonium, nitrate, and urea) to meet their photosynthesis needs. We examined the uptake and assimilation of several nitrogenous substrates using stable isotope technique and identified the potential nitrogen transformations. Our results revealed that urea can meet the spring partial nitrogen requirement of phytoplankton, according to the random forest models, the greater uptake rate depending on the urease, Chl a, and nitrate concentrations, redundancy analysis demonstrated that the phytoplankton community composed of <i>Fragilaria</i>, <i>Ulothrix</i>, and <i>Synedra</i>. The uptake rates of nitrate and urea were comparable and lower than those of ammonium in summer, the major contributors were ammonium, urease, and dissolved oxygen (DO), and the assemblages were dominated by <i>Microcystis</i>, <i>Actinastrum</i>, and <i>Melosira</i>. The effects observed indicated that ammonium was taken up preferentially in autumn and phytoplankton (<i>Cyclotella</i>, <i>Chlorella</i>, and <i>Pseudanabaena</i>) appeared to be able to respond to changes in nitrogen forms by adjusting their community composition.