Upwelling is when cold, deep water rises toward the surface

Upwelling is when cold, deep water rises toward the surface. This process brings nutrients that are found deeper in the ocean to the surface, which is often nutrient poor.

Deep ocean water is more nutrient-rich than simply because things (nutrients, plankton carcasses, fish carcasses) in the ocean sink.

Upwelling brings those lost/sunk nutrients back to the surface, which creates "blooms" of algae and zooplankton, which feed on those nutrients. These blooms then become feeding grounds for plankton feeders, then fish, etc, sustaining that lives near the surface.