We know that as a planet increases in size, its surface gets covered by oceans (more water is captured by gravity and shape is more spherical). Therefore, a planet larger than Earth can not have a large or significant landmass.
For Earth, oceans were very important for the origin of life. However, because most of the ocean seabed does not get sunlight and there are not enough nutrients in the surface of the oceans, life is not as dense in water as it is on land. This drastically reduces the ability for a larger planet to host civilization.
My question is this: What are potential realistic ways life can evolve to get energy and nutrients simultaneously (like plants) while living in an ocean? Such life must be able to proliferate enough to make dense ecosystems like forests and grasslands that gave rise to civilization.
I have read that potentially, volcanic systems could host life. But I do not believe that would be enough energy for large amounts of life especially considering that large planets have less activity afaik.
P.S: I am assuming that dense ecosystems are necessary for civilization. Is that true?