Why is the productivity of the oceanic zone is with low nutrients?

The productivity of the oceanic zone is low in nutrients primarily due to the limited availability of essential elements like nitrogen, phosphorus, and iron. These nutrients are often found in higher concentrations in coastal areas where upwelling occurs, but in the open ocean, they are sparse. Additionally, the vastness of the oceanic zone means that phytoplankton, which are the primary producers, have to compete for the limited nutrients available, leading to lower overall productivity compared to more nutrient-rich environments.