Almost all food chains begin with photosynthesis, and photosynthesis occurs in sunlight. So marine plants will only live near the surface where light can penetrate, and of course that will keep herbivores, and their predators, up at the surface too. But once these organisms die, or shed scales, eggs, leaves, shells, or feces, they and their byproducts will sink down toward the sea floor.
Upwelling occurs when surface currents move away from one another, or when winds push surface water away from the shore. This draws deeper water upward to replace the surface water.
The deep water in the global conveyer belt flows across the seafloor all the way from the North Atlantic before it reaches Antarctica, so it has a lot of time to pick up a lot of nutrients. There are a lot of surface currents flowing around the continent - the southern limbs of the Pacific, Atlantic, and Indian Ocean gyres and the Circumpolar and Subpolar Currents - to stir up the water. And there are likely to be strong polar winds blowing offshore, which would also drive upwelling.
In summer, the sun shines non-stop, which would allow photosynthesis 24 hours a day.