The Baltic Sea is the world’s least saline sea, with an average salinity¹ of just 7–8 grams per kilogram of water (0.7–0.8%), far below the global ocean average of 35‰. This is due to its semi-enclosed geography, high freshwater input² from rivers (e.g., the Neva and Vistula), limited exchange with the North Sea, and heavy rainfall. Its northern parts, like the Gulf³ of Bothnia, can even drop to 2‰. The low salinity supports unique ecosystems⁴ but also causes winter freezing.

世界上盐度最低的海是波罗的海，平均盐度仅为7–8‰（0.7–0.8%），远低于全球海水平均盐度（35‰）。成因包括半封闭地形、大量河流淡水注入（如涅瓦河、维斯瓦河）、与北海交换受限及降水丰富。其北部波的尼亚湾盐度可低至2‰。低盐度孕育了独特生态，但也导致冬季结冰。