Unravelling protist trophic complexity and prey-size dynamics in Central Baltic spring blooms

The trophic mode is one of the most important functional traits of organisms, determining their position in the food web and their role in the ecosystem. Under the classical concept, phytoplankton was considered to consist exclusively of phototrophic and microzooplankton exclusively of heterotrophic organisms. However, it is now increasingly recognized that mixotrophy (e.g. combining photo- and phagotrophy) occurs among both photo- and microzooplankton species, and that in extension, trophic diversity and relationships among plankton are expected to be more extensive and complex than previously thought. To enhance understanding of plankton trophic modes, diversity and relationships during spring bloom period in Bornholm Basin, central Baltic Sea, we categorized here the temporal succession in (1) the aquatic protist community by trophic modes, and (2) the community composition in terms of taxonomic groups and feeding mode, and its correlation with environmental factors and available prey-size. Our results show that the trophic mode composition of the community changed drastically over the course of the spring season, representing a high trophic complexity and more complex dynamics than previously suggested. The heterotrophic community was characterized by a high diversity of species and groups, with heterotrophic ciliates showing a clear seasonal succession in body size-classes, switching from the smallest sized-fraction (10-20 µm) in winter to an increasingly amount of larger-sized species of 30-55 µm and >55 µm with progression of the spring period. Changes in ciliate community composition were correlated with sea surface temperature, shifting from a cold-associated to a warm-associated community over the course of the spring season. Results further suggest that in communities including a larger mixotrophic component, size trait-based trophic relations between heterotrophic groups and their prey are complex, potentially due to similar prey-size preferences among heterotrophic and mixotrophic species. Overall, our findings emphasize the importance of accounting for the trophic modes of species to enhance the understanding of trophic relations and dynamics within bloom events.