On the ecological duality between ciliates and dinoflagellates across marine ecosystems.

Marine ciliates and dinoflagellates are key microzooplankton groups in oceanic food webs. A prevailing ecological framework suggests that ciliates dominate under cool, mixed conditions typical of late winter and early spring, whereas dinoflagellates prevail in warmer, stratified waters during late spring and summer. This review highlights how temperature, stratification, nutrient dynamics, prey composition, turbulence and top-down control shape seasonal and regional patterns. While the ciliate-dinoflagellate succession is often observed in temperate seas, it is not universal. Polar regions exhibit compressed seasonality, while tropical systems show weak seasonality, often dominated by mixotrophic dinoflagellates. The widespread occurrence of mixotrophy in both groups complicates this duality, allowing species to persist across contrasting environmental conditions. Ultimately, the relative dominance of ciliates or dinoflagellates reflects a context-dependent interplay of multiple drivers rather than a fixed seasonal rule. As climate change intensifies ocean stratification and alters nutrient regimes, understanding these dynamics becomes critical to predict shifts in plankton communities and their consequences for marine biogeochemical processes and ecosystem functioning.