||Sea ice is a key component of polar and sub-polar seas, including the Baltic Sea. It not only affects climate, from global to local scales, but also strongly modifies the conditions in the marine environment through freezing and eventual melt in several ways. The role of snow in sea ice growth is presumed to be high in the Baltic Sea, although very little quantitative information is available on the contribution of precipitation to ice growth in the Baltic Sea. Even less is known about the chemistry of sea ice and its role as a moderator of (bio)geochemical cycling and budgets of elements in the Baltic Sea. The aim of this Thesis is to shed new light on these matters.The importance of snow and precipitation on the growth of Baltic Sea ice is vital. For the first time, the stable oxygen isotopic composition of ice, snow and water were used to quantify the amount of snow and precipitation incorporated into the sea ice cover. Results show that meteoric ice (precipitation transformed to ice) constitute up to 35% of the total sea ice mass, much higher than reported elsewhere. Furthermore, the formation of superimposed ice (refrozen snow melt or rain) seem to be important as well, presumably due to the mild ice climate conditions in the region.The atmospheric load is also important in the chemical budgets of several elements, in particular nitrogen and lead, in the sea ice cover. The sea ice cover acts as a buffer for accumulated substances until its inevitable melt. However, the accumulated nutrients are transported through the ice at times when the ice becomes permeable. This, in turn, makes the accumulated nutrients available in the lower ice layers as well as in under-ice water, potentially affecting algal growth. The chemical budgets and the seasonal cycling of elements with selective rejection/retention during freezing of seawater are potentially affected, especially in the shallow regions of the Bothnian Bay.