Reduced forms of nitrogen control the spatial distribution of phytoplankton communities: The functional winner, dinoflagellates in an anthropogenically polluted estuary.

The effects of reduced forms of nitrogen (NH and dissolved organic nitrogen (DON)) on the spatial distribution of diatoms and dinoflagellates in an estuarine-coastal water continuum were investigated from 2015 to 2019. The proportion of non-DIN in total nitrogen was utilized as an indicator of DON along with direct measurements of DON. While NO originated from Seomjin River, the abundant NH and DON occurred from Gwangyang Bay through Namhae. Diatoms were mostly confined to the upper estuarine system and dinoflagellates dominated in the regions with high levels of NH and DON. Generalized additive models also presented the different responses of diatoms and dinoflagellates to increases in NH and DON. Thus, our results highlight that diatoms dominate in NO-replete water with full access to the source and dinoflagellates take over the ecologically open niche in an anthropogenically polluted estuary with full access to reduced forms of nitrogen.