Biophysical approaches to phytoplankton ecology: from parasitic infections to bloom rise and demise

[eng] Introduction: This thesis investigates Harmful Algal Blooms (HABs) from a physical perspective, aiming to understand the mechanisms that control their onset, evolution, and termination. HABs are complex phenomena in which physical, chemical, and ecological processes converge. Therefore, our approach integrates multiple scales of analysis to establish connections between microscopic factors and the macroscopic dynamics that drive these events. Research Content: The study was conducted across two main scales: 1. Macroscopic dynamics: HABs as passive biomass At the coastal scale, HABs were treated as a passive system subjected to environmental forcing. We analyzed the physical mechanisms that influence their distribution and persistence. This approach provides insight into how regional physical conditions can promote or inhibit the growth and accumulation of biomass. 2. Trophic controls at the microscopic scale The research also focused on ecological mechanisms operating at small scales, with particular emphasis on the role of microbial motility in predatory and parasitic interactions. Predation: We studied the chemotactic and chemokinetic responses of herbivorous flagellates to chemical compounds released by phytoplankton cells. These experiments reveal how chemical signaling modulates predator behavior and shapes predator-prey interactions within HABs. Parasitism: We performed microscopy experiments using a model system consisting of the generalist parasitoid Parvilucifera sinerae and several species of bloom-forming dinoflagellates. The aim was to determine whether these dinoflagellates develop motility-based strategies to avoid parasitic infection. The results suggest that adaptive behaviors may play a crucial role in the resilience of certain species against biological pressures. Conclusions: This work provides an integrated perspective on the processes that determine HAB dynamics. At the macroscopic scale, we demonstrate that coastal physical forcing can regulate the onset and intensity of blooms. At the microscopic scale, our findings highlight that motility in both predators and prey is a critical factor capable of modifying trophic interactions and influencing bloom development. The interdisciplinary approach adopted in this thesis establishes strong links between physical and ecological processes, emphasizing the need to address HABs as multiscale phenomena. Finally, the results provide mechanistic insights that may improve our ability to predict and manage these events, contributing to a better understanding of the impacts of HABs on coastal ecosystems increasingly affected by climate change.