e-Proceedings of the 2 nd International Scientific Conference on Design and Management of Port Coastal and Offshore Works Thessaloniki, Greece, May 24-27 2023 65 Circulation pathways in Thermaikos Gulf based on field and model Lagrangian experiments Y. Androulidakis 1,2,* , C. Makris 1 , Y. Krestenitis 1 , V. Kolovoyiannis 2 , V. Baltikas 1 , Z. Mallios 1 1 Division of Hydraulics and Environmental Engineering, School of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Central Macedonia, 54124, Greece 2 Laboratory of Physical and Chemical Oceanography, Department of Marine Sciences, University of the Aegean, Mytilene, Lesvos Island, 81100, Greece * Corresponding author: iandroul@civil.auth.gr Abstract The Northern Thermaikos Gulf (NTG; Figure 1a) is a semi-enclosed coastal region of the Aegean Sea facing severe pollution events, characterized by anthropogenic and natural stresses such as intense industrial and agricultural activities, urban outflows, and several nutrient-rich river discharges. The hydrography and the hydrodynamic circulation patterns of NTG are revisited in this paper based on the findings of an integrated observational-modeling study, conducted during an annual cycle from June 2021 until May 2022. The main goal of the study is to investigate the environmental conditions that determine the renewal and water quality of the semi-enclosed basin. An operational system for predictions of ocean circulation and Lagrangian pathways to support search and rescue and first responders during pollution accidents over NTG was also developed providing daily 3-day forecasts. Keywords Drifters, Hydrodynamic circulation, Delft3D, Operational system. 1 INTRODUCTION The Northern Thermaikos Gulf (NTG) is located at the tip of the northwestern Aegean Sea (eastern Mediterranean Sea; Figure 1a). Thessaloniki, the second largest city in Greece with population of 1.1 million habitants, is located along the gulf's northern coast. Two large (Axios and Aliakmonas) and two smaller (Gallikos and Loudias) rivers (Figure 1a) supply freshwater into the NTG along its western coast, containing large quantities of nutrients (Karageorgis et al. 2005). Several smaller outflows (e.g., the Halastra irrigational drainage channel network and Anthemountas intermittent river; Figure 1a) periodically discharge overflowing waters, containing agricultural (drainage), urban (treated and untreated wastewater) and industrial (liquid chemicals and heavy metals) pollutants. The main pollution pressure of NTG, related to these inputs, is eutrophication, which is strongly controlled by the mesoscale circulation patterns and the renewal capability of the gulf (Androulidakis et al. 2021). The principal objective of this study is to describe the seasonal distribution of the water masses' physical properties, the main circulation patterns, and the variability of the physical connectivity pathways, based on a systematic observational approach (six field campaigns with a 2-monthly temporal step) supported by outputs from three-dimensional (3- d) numerical simulations combined with Lagrangian particle modeling, covering a recent annual cycle (June 2021 - May 2022). Figure 1. (a) Location of NTG, river system, drainage basins, and Delft3D-Thermaikos model domain. Components of the observational platform: (b) "AEGEAN" drifter, (c) "CARTHE" drifter, (d) Workhorse ADCP, and (e) SBE 19plus CTD