Why are open oceans often referred to as nutrient-poor regions?

Open oceans are often referred to as nutrient-poor regions primarily due to the limited availability of nitrogen and iron, which are essential nutrients for phytoplankton and other marine life. Unlike coastal areas, where nutrient runoff from land can stimulate high levels of productivity, the open ocean has lower concentrations of these critical nutrients.

In many parts of the open ocean, especially in areas away from land, the supply of nitrogen is particularly low. This scarcity leads to reduced phytoplankton growth, which forms the base of the marine food web. As a result, the overall biological productivity is lower compared to more nutrient-rich environments.

Iron, while present in trace amounts in seawater, is often a limiting factor in many ocean regions, especially in higher latitudes and certain open ocean zones. Its limited availability can inhibit the growth of phytoplankton, further contributing to the characterization of the open ocean as nutrient-poor.

The other options do not accurately capture the main reasons for this classification. High levels of biodiversity do not directly correlate with nutrient availability but rather suggest a wealth of life that can thrive under right conditions. Excessive nitrogen and iron contradict the understanding of nutrient limitations in the open ocean, and an abundance of organic material typically implies a richer,