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

Abstract

The effects of reduced forms of nitrogen (NH₄⁺ 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 NO₃^- originated from Seomjin River, the abundant NH₄⁺ 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 NH₄⁺ and DON. Generalized additive models also presented the different responses of diatoms and dinoflagellates to increases in NH₄⁺ and DON. Thus, our results highlight that diatoms dominate in NO₃^--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.

Keywords: Anthropogenically polluted estuary; Diatoms; Dinoflagellates; Dissolved organic nitrogen; NH(4)(+); Nitrogen.