information:filtering, structural:reformatting
Waters

Hudson Bay has a lower average salinity level than that of ocean water. The main causes are the low rate of evaporation (the bay is ice-covered for much of the year), the large volume of terrestrial runoff entering the bay (about 700 km (170 cu mi) annually, the Hudson Bay watershed covering much of Canada, many rivers and streams discharging into the bay), and the limited connection with the Atlantic Ocean and its higher salinity. Sea ice is about three times the annual river flow into the bay, and its annual freezing and thawing significantly alters the salinity of the surface layer. Although its exact effects are not fully understood currently, the cyclonic storms in the bay are responsible for synoptic variability of salinity along the coast.

One consequence of the lower salinity of the bay is that the freezing point of the water is higher than in the rest of the world's oceans, thus decreasing the time that the bay remains ice-free. The increase of river inflows during the winter has decreased the season of sea ice by more than one month since the 1960s.

The lower salinity of the bay also has effects on the distribution and prevalence of common marine life such as micro algae. Research has shown that the lower salinity of the Hudson Bay limits the growth of micro algae, which causes a notable change in biomass along the bay's salinity gradient.