The hydrophobic component of terrestrial dissolved organic nitrogen promotes shifting diatom–dinoflagellate dominance in the Bohai Sea

Human-induced nitrogen input is changing coastal ecosystems globally, and threatening the human the well-being associated with them. Dissolved organic nitrogen (DON) has gradually become the main form of nitrogen in the Bohai Sea (BS). Increasing DON concentrations and changing DON composition have been associated with harmful algal blooms and a regime shift from diatom to dinoflagellate dominance. However, the underlying mechanisms are still not well explored. Here we investigate the diatom–dinoflagellate dominance shift mechanism based on field microcosm experiments, the Nutrients–bi-Phytoplankton–Detritus (NbPD) biogeochemical model, and organic chemical analysis techniques (e.g. three-dimensional fluorescence spectrum of dissolved organic matter and activity determination of extracellular leucine aminopeptidase) in the BS. We found that shifting diatom–dinoflagellate dominance was associated with the hydrophobic component of terrestrial DON. Dinoflagellates, such as <i>Karenia mikimotoi</i>, can efficiently assimilate the humic-like substances in hydrophobic DON, whereas diatoms (<i>Chaetoceros </i>spp.) can efficiently utilize the protein-like components in hydrophilic DON. The reason for this is the higher extracellular leucine aminopeptidase activity of dinoflagellates compared to diatoms, which enables them to degrade humic-like substances and protein-like components more effectively. The modeling study revealed that DON composition, especially the proportion of hydrophobic DON, regulates the regime shift from diatoms to dinoflagellates. Our study offers insight into the mechanisms underlying phytoplankton regime shifts in the BS, which is valuable for decreasing eutrophication by controlling the input and composition of terrestrial DON.