Multitudinous dissolved organic nitrogen (DON) enters seawater from Ulva prolifera green tides impacting phytoplankton community structure in the Yellow Sea. Field investigations and cultural experiments on U. prolifera-derived DON aging revealed its regulatory effects. The green-tide outbreak area of the Southern Yellow Sea exhibits a seasonal cycle where dominance shifts from diatoms to dinoflagellates effected by young to aging U. prolifera-derived DON from summer to spring. Diatom abundance rose significantly following the summer green tide outbreak, fueled by young, protein-rich DON released by U. prolifera. After autumn and winter aging, U. prolifera-derived DON was dominant from protein-like to humic-like components, and dinoflagellates bloomed in the green tide outbreak area. An aging U. prolifera-derived DON adding culture experiment of Chaetoceros curvisetus and Prorocentrum minimum proved that old and young DON promoted dinoflagellate and diatom growth, respectively. Diatoms preferred 0 and 12 days-aged DON (DON