Total Nitrogen Shapes Diversity of Bloom-Forming Dinoflagellates in the Baltic Coastal Waters

The impact of nitrogen on harmful algal blooms (HABs) and functions of biota in marine ecosystems under eutrophication is a topical issue of growing importance. The article aimed at describing the diversity of planktonic bloom-forming dinoflagellates in the SW Baltic Sea coastal waters under variable eutrophication. The analysis of 44 year-long database revealed 82 dinoflagellate species and demonstrated diversity patterns of ten common bloom-forming species, including seven mixotrophs from the genera <i>Prorocentrum</i>, <i>Dinophysis,</i> and <i>Ceratium</i>, under variable eutrophication evaluated using total nitrogen (TN) content in water. Based on the Intermediate Disturbance Hypothesis (IDH), we presumed those coastal waters with total nitrogen concentrations that are optimal to dinoflagellates to host greater taxonomic diversity compared to areas with non-optimum TN content. The results showed that the highest dinoflagellate species richness was associated with much lower TN concentrations than the optimum values for these species. Thus, our findings disagreed with the IDH. We suggested and discussed possible reasons of this inconsistency, including algal growth rates and disturbance frequency. We also updated the classification of eutrophication levels in the Baltic Sea based on the distribution of TN content and diversity of HAB-forming dinoflagellates. The results can contribute to predictive assessment of HABs under growing eutrophication.