Impact of Coastal Currents and Eddies on Particle Dispersion in the Baltic Sea: A Lagrangian Approach to Marine Ecosystems

Saeed Hariri^1*Germo Väli²Markus Meier¹

• ¹Leibniz Institute for Baltic Sea Research (LG), Warnemünde, Germany
• ²Department of Marine Systems, Tallinn University of Technology, Tallinn, Harju County, Estonia

This study analyzes the dynamics of coastal currents and eddies in the Baltic Sea, focusing on their role in particle dispersion and ecosystem connectivity. Combining the General Estuarine Transport Model and Lagrangian methods, it examines both single and paired particledynamics, initially deployed in coastal areas of the Baltic Sea, for 2D and 3D simulations. Results show significant variability in transit times as it takes for 3D particles from the eastern coastal zone over 700 days to reach the central Gotland Basin, while those from the westerncoastal zone arrive 90 days faster. Longer transit times in the eastern coastal areas can influence the distribution of nutrients and pollutants, potentially exacerbating eutrophication, harmful algal blooms, and hypoxic conditions. In contrast, shorter transit times in the western Balticaccelerate dispersal, reducing localized impacts while increasing the spread of contaminants. In addition, (sub-)mesoscale eddies and vertical advection play a key role in particle transport, particularly in the northern Gotland Basin. Moreover, relative dispersion analysis shows aninitial nonlocal growth regime lasting up to 25 days in 3D but only 4–10 days in 2D, affecting connectivity between marine habitats. 3D simulation results show that, depending on the region, absolute dispersion exhibits ballistic growth for the first 7 days, followed by a transitionto a super-diffusion regime before normal diffusion sets in after 70-85 days. These findings highlight the role of physical processes such as eddies, coastal currents and mesoscale structures in shaping species dispersal, nutrient cycling, and pollution transport.