Diatoms vs dinoflagellates: a temporal network analysis of bloom impacts on phytoplankton diversity and community structure in French coastal waters.

Understanding phytoplankton community responses to bloom events is essential as these have major implications for biogeochemical cycles and marine food webs, but can also be harmful to ecosystems, the economy, and public health. Diatoms and dinoflagellates are the two most common bloom-forming classes, with distinct ecologies and bloom dynamics. Using time series from a long-term phytoplankton monitoring survey (REPHY) of French coastal waters analyzed with ecological association networks and diversity metrics, we investigated how blooms of diatoms and dinoflagellates affect diversity and community structure within phytoplankton communities. We highlight distinct responses: an increase in species richness during diatom blooms and a decrease during dinoflagellate blooms. However, both bloom types resulted in a decrease in Shannon and Pielou indices. Temporal association network modularity and association strength were also differently impacted between the two bloom types. However, high-level association types and composition in association networks remain stable, suggesting an important role of their proportions in ecosystem functioning and resilience. We propose several hypotheses for these findings based on known ecological and biological processes that remain to be tested individually in dedicated studies. Our work underlines the importance of distinguishing the different types of blooms and studying the role of interactions between phytoplankton for a better understanding of their dynamics.