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

## TL;DR

The study found that lower nitrogen levels in the Baltic Sea correlate with higher diversity of harmful dinoflagellates, contradicting expectations from the Intermediate Disturbance Hypothesis.

## Contribution

The study updates eutrophication classifications in the Baltic Sea based on dinoflagellate diversity and nitrogen levels.

## Key findings

- Maximal dinoflagellate species richness occurs in areas with the lowest total nitrogen concentrations.
- Findings contradict the Intermediate Disturbance Hypothesis by showing higher diversity at lower disturbance levels.
- Updated eutrophication classifications were proposed based on dinoflagellate bloom patterns and nitrogen content.

## Abstract

This study examined the diversity of dinoflagellates in coastal waters of the Baltic Sea at different eutrophication levels since many of these microplankton organisms form harmful algal blooms that deteriorate the environment. The analysis of long-term (44 years) database revealed altogether 82 dinoflagellate species; among them, ten species (seven mixotrophs and three heterotrophs) were most common, abundant, and regularly formed blooms. The discovered dinoflagellate diversity patterns correlated with eutrophication levels assessed by total nitrogen content in water. Maximal species richness was observed in the areas with the lowest nitrogen concentrations, i.e., at the smallest eutrophication. The findings were discussed in relation to the postulates of the Intermediate Disturbance Hypothesis. The results allowed updating the classification of eutrophication levels in the Baltic coastal waters, thus contributing to adequate assessment of the linkage between harmful dinoflagellate blooms and the process of eutrophication.

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 Prorocentrum, Dinophysis, and Ceratium, 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.