Final answer:
The formation of sea ice and icebergs increases the density of the surface water as salts are concentrated in the remaining liquid water. Water is densest at 4°C, providing an insulated habitat for aquatic life. The low density of ice allows it to float, preventing bodies of water from freezing solid.
Step-by-step explanation:
When sea ice and icebergs form, the process leads to an increase in the density of the surface water. This is because as water freezes, it excludes the salts that are in the water, concentrating them in the remaining liquid water. This concentration of salts increases the water's density. Additionally, water is densest at 4°C, which often lies below the layer of ice. The liquid water underneath the ice, enhanced in saltiness and thus denser, can still be a viable habitat for aquatic life through the insulation provided by the ice layer on top.
Bodies of water freeze from the top down because ice is less dense than liquid water, creating a protective surface layer that insulates the underwater ecosystem. This fascinating characteristic of water allows ecosystems in lakes, ponds, and oceans to survive in colder climates where ice formation is a regular occurrence.