The patches have little variety in their conditions. The concentration of compounds is at essentially 4% in every patch aside from the vents. Iron and sulfide is all that has any major change. And atmospheric conditions do not change at all. I propose the concentration of compounds is lower in the surface patches where you can essentially live forever from photosynthesis, to add a reason to stay in the depths.
and for the ice shelf sunlight should be much lower, maybe 8-12%, with a higher concentration of compounds to set a reason to be there.