Why is most productivity on open oceans limited to surface waters?

Most productivity in open oceans is limited to surface waters primarily due to the availability of sunlight, which is essential for photosynthesis. Phytoplankton, the primary producers in marine ecosystems, thrive in the well-lit epipelagic zone (the upper layer of the ocean) where light penetrates. Below this layer, light diminishes rapidly, restricting photosynthetic activity and, consequently, the overall productivity of marine life. Additionally, nutrient availability tends to decrease with depth, further limiting productivity in the deeper ocean waters.