The Hydrophobic Component of Terrestrial Dissolved Organic Nitrogen Promotes Shifting Diatom–Dinoflagellate Dominance in the Bohai Sea

Human-induced dissolved organic nitrogen (DON) input is impacting coastal ecosystems globally, and the shift from diatoms to dinoflagellates may be associated with increasing DON concentrations and changing DON compositions. Based on field microcosm experiments, changes in three-dimensional fluorescence component and extracellular leucine aminopeptidase activity were used to reveal the effects of DON on diatom and dinoflagellate growth. Additionally, a Nutrients–bi-Phytoplankton–Detritus biogeochemical model was employed to elucidate the kinetic mechanism of DON on the shift from diatoms to dinoflagellates in the Bohai Sea (BS). Our results revealed that shifting diatom–dinoflagellate dominance was associated with the hydrophobic component of terrestrial DON. Dinoflagellates (i.e., Karenia mikimotoi ) efficiently assimilated humic-like substances in hydrophobic DON; whereas, diatoms (i.e., Chaetoceros spp.) efficiently utilized the protein-like components in hydrophilic DON. The reason for this was the higher extracellular leucine aminopeptidase activity of dinoflagellates compared to that of diatoms, which enabled them to degrade humic-like substances and protein-like components more effectively. The modeling study clarified that the DON composition, particularly the proportion of hydrophobic DON, regulated the shift from diatoms to dinoflagellates. Our study provides insight into the mechanisms underlying phytoplankton regime shifts in the BS and valuable guidance for decreasing eutrophication by controlling terrestrial DON inputs and compositions. Key Points Karenia mikimotoi can efficiently assimilate humic-like substances Chaetoceros spp. can efficiently utilize protein-like components Karenia mikimotoi has higher extracellular leucine aminopeptidase activity than Chaetoceros spp. Plain Language Summary Human-induced nitrogen input is causing major changes in coastal ecosystems worldwide and posing a threat to human well-being. In the Bohai Sea (BS), the increase in DON concentrations and changes in its composition have been linked to harmful algal blooms and a shift from diatom to dinoflagellate dominance. This study investigated the mechanism behind the shift from diatoms to dinoflagellates using field experiments, a biogeochemical model, and techniques for analyzing organic chemicals. Our findings indicated that dinoflagellates (represented by Karenia mikimotoi ) can efficiently assimilate humic-like substances in hydrophobic DON, and that diatoms (represented by Chaetoceros spp.) can effectively utilize protein-like components in hydrophilic DON. This difference arose because of higher extracellular leucine aminopeptidase activity in dinoflagellates compared to diatoms, enabling them to degrade humic-like substances and protein-like components more effectively. Our modeling study further revealed that the DON composition, particularly the proportion of hydrophobic DON, regulated the shift from diatoms to dinoflagellates. These insights into the mechanisms driving phytoplankton regime shifts in the BS provide valuable guidance for mitigating eutrophication by controlling terrestrial DON inputs and compositions.