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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 Fragilaria, Ulothrix, and Synedra. 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 Microcystis, Actinastrum,
and Melosira. The effects observed indicated that ammonium was taken up preferentially in autumn
and phytoplankton (Cyclotella, Chlorella,
and Pseudanabaena) appeared to be able
to respond to changes in nitrogen forms by adjusting their community composition.