Life on the move: How traits and environment constrain marine species dispersal

The ocean hosts a vast diversity of life, from microscopic plankton to large marine mammals, with their distribution shaped by environmental factors and species-specific traits. While ocean currents largely determine the passive transport of plankton, some species can regulate their buoyancy or actively migrate, such as diel vertical migrators (DVM). Larger, more mobile species, like fish and marine mammals, navigate their environment with stronger swimming abilities. Understanding how marine species move and interact with their environment is crucial for predicting ecosystem shifts, especially as climate change alters ocean conditions. This thesis investigates how species traits and environmental factors influence the transport and connectivity of marine organisms using Lagrangian flow modeling. Three case studies explore the dispersal of different marine species: vertically migrating zooplankton, thermally constrained plankton, and cold-stunned Kemp’s ridley turtles. The first study (Chapter 2) examines DVM zooplankton in the Benguela upwelling system, comparing their movement to that of floating and sinking materials such as phytoplankton, nutrients, and biogenic matter. Results show that DVM zooplankton separate more rapidly from other materials, especially during the upwelling season. However, coherent ocean features like eddies and filaments trap different particle types together, potentially increasing interactions between zooplankton, their prey, and pollutants. Chapter 3 applies network theory to Lagrangian modeling to assess minimum time connectivity pathways in the Atlantic Ocean. Passive plankton can theoretically connect all Atlantic locations within three years, but connectivity time increases with depth. When thermal constraints are introduced, connectivity across the basin is reduced, emphasizing the role of environmental preferences in species dispersal. Chapter 4 investigates Kemp’s ridley turtle strandings in the Netherlands, using Lagrangian modeling to trace their drift pathways. Findings suggest that juvenile turtles become cold-stunned in the southern North Sea and experience temperatures below 12°C for up to a month. These results highlight the need for targeted rehabilitation and conservation strategies for this critically endangered species. Together, these studies demonstrate that both species traits and environmental constraints shape marine organism dispersal, influencing ecological interactions and habitat connectivity. As climate change alters ocean temperatures and circulation, species distributions will shift, affecting marine food webs and conservation planning. Future research should refine species-specific behavioral models and integrate dispersal with ecosystem dynamics to better inform marine conservation strategies.