Final answer:
The open ocean, despite its vast area, has lower life density per unit area due to nutrient scarcity. Phytoplankton, limited by nitrogen or iron availability, are key to photosynthesis but when they die, their nutrients sink and are not easily recycled without processes such as ocean upwelling.
Step-by-step explanation:
The open ocean, also known as the pelagic or open oceanic zone, covers a vast area and is responsible for a significant portion of the Earth's Net Primary Productivity (NPP). Despite this, the open ocean does not contain as much life per unit area compared to other parts of the ocean such as the neritic zone. This is due to several factors, primarily the scarcity of nutrients necessary for life.
Nutrients in the ocean are crucial for supporting the food chains, starting with phytoplankton, which perform around 40 percent of all photosynthesis on Earth. Although these microscopic organisms are abundant, growth is often limited by the availability of essential nutrients like nitrogen and iron, which vary depending on the region of the ocean. In the Atlantic, growth is limited by nitrogen, while in the Pacific, it is iron that is in short supply.
Moreover, when organisms die in the open ocean, their bodies sink to the ocean floor, sequestering the nutrients and energy contained within them. Without processes such as ocean upwelling, which can bring these nutrients back to the surface, the open ocean remains a relatively less productive area. Ocean upwelling is a process where deep ocean waters rise to the surface, bringing with them nutrients that can be used by living organisms. This explains why the open ocean can have a high NPP overall but still have low life density.