A good way to understand this is through an ocean world thought experiment. If a hypothetical planet existed which was covered in a 100km deep ocean, all nutr

A good way to understand this is through an ocean world thought experiment. If a hypothetical planet existed which was covered in a 100km deep ocean, all nutrients would steadily fall to the bottom of the ocean - where there would be minimal to no energy to make use of the nutrients. You'd eventually be left with a surface layer containing only the minimal amounts of nutrients which diffusion pulls back up to the surface. In order for this hypothetical water world to host a thriving ecosystem, you would need a large moon or heavy vulcanism to ensure that the oceans remain mixed.