Algae often float on surface of water during day but sink down during night due to

Algae often float on the surface of the water during the day but sink during the night due to the evolution and trapping of oxygen bubbles during the day in their photosynthesis.
Algae produce oxygen during the day during photosynthesis. The oxygen that is released gets trapped between the filaments of the algae, making it lighter and causing it to float to the top of the pond or surface of water bodies.