USA Biology Olympiad (USABO) Practice Exam

Which ecosystem has the lowest average net primary production per unit area?

1. Tropical rain forest

2. Savanna

3. Temperate forest

4. Open ocean

The correct answer is: Open ocean

The open ocean is characterized by a vast expanse of water where light penetration is limited, particularly in deeper regions. While the open ocean covers a significant portion of the Earth's surface and is crucial for global biogeochemical cycles, its average net primary production (NPP) per unit area is relatively low compared to terrestrial ecosystems such as tropical rainforests, savannas, and temperate forests. The factors contributing to the low NPP in the open ocean include nutrient availability, particularly nitrogen and phosphorus, which are often limiting in these vast areas. Although there can be regions of productivity, such as upwelling zones or coastal areas, the overall average across the entire ocean is low. In contrast, the other ecosystems listed, like tropical rainforests, have high levels of photosynthesis and biomass due to abundant sunlight and nutrient availability, leading to much higher average net primary production per unit area. Understanding these dynamics highlights the importance of nutrient availability and light in determining the productivity of different ecosystems, with the open ocean being the least productive when measured on a per unit area basis.