Ever since I was a little girl, I always marveled at how perfect nature is. One such simple phenomenon is this special property of water which is the building block of probably most of life on earth. Water has highest density at 4 degree Celsius and least dense as ice. It's a simple property of water we learn in physics. But it has infinitely bigger significance in the aquatic life. This simple physics is the building block of aquatic life in colder regions.
Densest water at 4 degree Celsius sinks to the bottom of water bodies. The water at the top is less dense, either hotter or colder than 4 degree. In colder places, the water at the top can further cool down and form ice, which will again float at the top, having lesser density. So even in extreme cold situations, even if the water at the top freezes, the water at the bottom stays in liquid form, preserving all aqautic life. This difference in temperature in different depths also leads to water currents and oxygen flow throughout the water body. Colder water holds more dissolved oxygen. Hotter water with dissolved oxygen at the top changes temperature and sinks to the bottom as it approaches 4 degree, carrying with it life sustaining oxygen throughout the water body.
There are many such apparently simple phenomena together that make life possible on the planet. That is perfection, meant to remain together and function together to perform the magic of life.