Ciliate grazing control of a spring bloom in a temperate fjord

Plankton communities underpin marine ecosystem functioning, yet the mechanisms driving the onset and termination of spring phytoplankton blooms are not fully understood. In particular, the role of microzooplankton grazing and the impact of mixotrophic versus heterotrophic ciliates on phytoplankton dynamics. This study aimed to quantify ciliate grazing and phytoplankton growth rates throughout a spring bloom in Roskilde Fjord, Denmark; and determine whether nutrient depletion or microzooplankton grazing was the primary driver of bloom culmination. Surface water was collected during March 2017, and multiple dilution experiments were conducted to estimate phytoplankton growth and grazing mortality rates. Phytoplankton community structure was assessed by flow cytometry, while ciliate morphotypes and nutritional modes were identified using FlowCam imaging. Despite initially high nutrient concentrations, a marked decrease in DIN and DIP occurred over the study period. Phytoplankton biomass, dominated by cryptophytes, reached its peak by mid-March before declining. Concurrently, ciliate assemblages shifted from mixotrophic to heterotrophic dominance. Grazing rates frequently exceeded phytoplankton growth rates after the first experimental day, indicating strong top-down control. The lack of significant differences in growth between nutrient-amended and unamended treatments suggested grazing, rather than nutrient depletion, predominantly constrained phytoplankton accumulation. These findings highlight how ciliate grazing can exert a critical influence on spring bloom dynamics, even under moderate nutrient conditions. Understanding these trophic interactions is essential for predicting the resilience and stability of coastal marine ecosystems in the face of changing environmental conditions. • A community shift from mixotrophic to heterotrophic ciliates played a decisive role in controlling phytoplankton bloom succession and bloom termination • The shifting trophic strategies of organisms highlight the importance of individual-level nutritional flexibility for ecosystem stability. • Microzooplankton resilience (mixotrophy to heterotrophy) maintain community function despite environmental fluctuations