Final answer:
The main source of NPP in the open ocean is phytoplankton, which are limited by the scarcity of nutrients such as nitrogen and iron, relevant to the Atlantic and Pacific Oceans respectively. Ocean upwelling can boost phytoplankton growth when it occurs.
Step-by-step explanation:
In the open ocean, the Net Primary Productivity (NPP) is mainly from photosynthetic organisms such as phytoplankton. These organisms capture sunlight and carbon dioxide to create energy-rich compounds through photosynthesis. However, the productivity is limited due to the scarcity of nutrients such as nitrogen in the Atlantic Ocean and iron in the Pacific Ocean. The open ocean, also known as the oceanic or pelagic zone, is less productive compared to the neritic zone because there is a lack of a process like ocean upwelling that brings nutrients from the bottom to the surface, except in certain areas like the Humboldt Current.
The growth of phytoplankton can be significantly influenced by the availability of nutrients. In areas where nutrients are supplemented via natural processes like ocean upwelling, phytoplankton growth increases, which boosts NPP, serving as the basis of the food web that supports marine life from zooplankton to large mammals like whales.