Final answer:
As ice forms near the poles, the salinity of the remaining water increases because the salt is left behind in the liquid water. The ice floats because it's less dense than water, allowing marine life to exist in the warmer water below the ice layer.
Step-by-step explanation:
When ice forms near the poles, the salinity of the remaining seawater increases. This happens because as the sea water freezes, the ice crystallizes, leaving most of the salt behind in the unfrozen water. This process of freezing causes the water beneath the ice to become saltier and denser. The density of water is greatest at 4°C, and as the water gets colder towards the surface and turns to ice, the ice floats due to being less dense than the water. This unique property of water, where ice floats on the surface, allows aquatic life to survive underneath during the winter months in relatively warmer water.