<p>Phytoplankton play a vital role in nutrient biogeochemical cycling, and their elemental ratios covary with available inorganic nutrients; however, the underlying interaction patterns and mechanisms remain unclear. Here, based on large-scale and long-term field surveys together with culture experiments, we found a stoichiometric convergence over time between phytoplankton nitrogen (N):carbon (C) molar ratios and dissolved inorganic N:CO₂ molar ratios. Functional genomics evidence demonstrated that this N-C stoichiometric convergence was driven by phytoplankton community succession with its matched N and C metabolism. A theoretical framework was proposed and proved applicable for depicting this convergence. The convergence speed was faster with the greater difference between phytoplankton and water N:C molar ratios. Moreover, replacing CO₂ with dissolved inorganic C could alter the direction of convergence. This stoichiometric convergence characterizes the response of phytoplankton elemental ratios to inorganic nutrient dynamics and represents an evolutionary pathway for phytoplankton community assembly over time.</p>