## 2 Materials and Methods
### Oceanographic Factors  
The characteristics of seawater in the Baltic Sea comprise properties that are not subject to significant change in a daily cycle but are rather shaped over longer time periods, such as seasons or decades. The Baltic Sea, enclosed by land and connected to the North Sea by several relatively shallow Danish Straits, has little interchange of waters with the Atlantic Ocean. This geographical location creates relatively isolated hydrological conditions affecting the physical and chemical properties of Baltic Sea waters [78; 79]. The basic property of the Baltic Sea is salinity. Seawater contains more than 30 different chemical compounds, mainly salts, as well as microelements, such as iodine, copper, phosphorus, gold, and silver. Moreover, it contains diluted gases, mainly nitrogen, oxygen, and carbon dioxide, with traces of hydrogen sulfide and organic compounds, which may be present in the benthic zone [80; 81]. Compared to oceans, the Baltic Sea is characterized by low salinity due to restricted interchange of waters with the North Sea and a considerable inflow of water from rivers. The average salinity of surface waters amounts to ca. 7% and increases towards the Danish Straits, where it reaches a level of ca. 12% [82; 83]. The greatest differences in salinity are observed in the vertical direction. Salt water, which is heavier, sinks; thus, at a depth of ca. 100 m, salinity may reach ca. 19% [14]. Variable salinity levels strongly affect the density of water and, consequently, the circulation and layering of the Baltic's waters. The latter have an impact on both the Baltic's ecosystem and its hydrological and meteorological conditions, which in turn are of key importance to safe navigation and effective operation of the LNG infrastructure [84]. Mean salinity values in the Gdarsk Deep are lower. In 2022, the difference between the maximum (7.54, January) and minimum (7.21, February) mean salinity value was 0.33. Salinity in the Vistula Lagoon stands at a mere 1-3% [85]. A relatively stable salinity of ca. 6% of the Bay of Pomerania has an impact on the salinity of the Szczecin Lagoon. In winter, strong winds and increased water levels cause a backwater current effect from the Baltic Sea to the Szczecin Lagoon through the River Swina, which temporarily increases salinity in the northern part of the Szczecin Lagoon to 6%, where mean salinity in the area is ca. 2% [86]. Thermal conditions, i.e., temperature distribution across water layers, is one of the crucial factors affecting changes in the Baltic's ecosystem. The Baltic is characterized by large seasonal temperature fluctuations, which depend on the amount of sunshine, heat exchange with the atmosphere, and the surface structure of the water [87; 88; 89]. In summer, the temperature of surface water may reach 20-25 \({}^{\circ}\)C, especially in shallow areas, such as the Gdarsk Deep or the Vistula Lagoon. As a result of the heating up of waters, distinct thermal layers are formed, where warmer surface water is separated from cooler deep-sea water with the thermocline--a transitional layer in which the temperature drops rapidly as the depth increases [90; 91; 92; 93]. In winter, the situation reverses. The temperature of surface water drops to 0-2 \({}^{\circ}\)C, and, in the northern and eastern parts of the Baltic, the surface of the sea freezes. The thickness of the ice depends on the weather, including air temperature and wind force, which may cause crumbling and accumulation of ice [94; 95]. Shallow areas, such as the Bay of Pomerania and the Vistula Lagoon, are characterized by the greatest variability of temperatures, as they are more prone to heating up and cooling down than deeper water areas. Thermal properties of waters determine circulation, layering, and hydrological and meteorological conditions, which in turn affect both the ecosystem and business activity, including the operation of the LNG infrastructure in the region [85].