The Bioavailable Age of DON: A New Paradigm for Explaining Diatom-Dinoflagellate Succession in Coastal Seas

Predicting phytoplankton community shifts is vital for safeguarding coastal ecosystem services, but traditional nutrient paradigms cannot explain dynamics in anthropogenically affected seas under changing hydrology. The role of dynamic dissolved organic nitrogen (DON), with poorly understood chemical evolution and bioavailability, remains unresolved. Combining coastal field surveys and a year-long laboratory aging experiment in Laizhou Bay, China, we show DON aging regulates diatom-dinoflagellate succession. Field data linked spring dinoflagellate dominance to elevated DON/DIN ratios and humification signatures. Laboratory tests revealed terrigenous chicken manure-derived DON transforms from labile protein-like to recalcitrant humic-like substances with aging. Diatoms thrived on fresh DON but were inhibited by aged DON (with downregulated nitrogen assimilation genes), while dinoflagellates thrived in aged DON through upregulated ‘endocytosis-enzymatic hydrolysis’. Laizhou Bay’s prolonged water residence time (> 90 days) facilitates in-situ DON aging, driving seasonal succession. We propose the “DON bioavailable age” paradigm, linking hydrodynamics, molecular transformation, and niche partitioning, offering a predictive tool for managing phytoplankton dynamics and eutrophication globally.