I believe the correct answer is preservation of heat.
Ice forms at the temperature of 0°C and lakes and rivers can experience a temperature of up to -2°C. Most marine organisms can survive up to 5°C. The lake usually freezes due to losing heat to the environment to form solid maintains heat in water bodies.
With the winter setting in, the water bodies will lose all the heat they have and when they lose heat until the absolute temperature of zero degrees Celsius, the water will undergo freezing forming a layer of solid ice on top of the lake. The lake only freezes in the upper layer due to it losing heat very slowly and in the end, the sheet of ice above the water will act as a wall that maintains the heat in the water below. This small amount of heat maintained is essential since it will enable the organisms to survive and live inside the lake in winter. Since ice is almost same to glass in transparency, it will allow the passing through of light that will further allow growth of marine plants and also marine animals such as fish. If the lake froze up to the bottom, the organisms would be trapped in a massive block of ice and with no movement and ability to move, most of them would die. This is the reason why you can see the Eskimos in the Northern Pole going ice fishing throughout the year with the cold temperatures and the fish are alive and well just as in other temperate conditions such as the Atlantic and Pacific coasts. The basic requirements for gemination of plants is light, water and nutrients all of which are supplied in the frozen lakes. The fish are also adapted to the cold conditions and some of them migrate to other regions or hibernate in their nests until the winter season is over.
Level: High School
Subject: Biology
Topic: Plants
