Question

Among the following ecosystems, which has the least Net Primary Production (NPP)?

• A. Savanna
• B. Estuaries
• C. Open ocean
• D. Agricultural land

Hint:

Don't confuse total global NPP with NPP per unit area. The open ocean has a huge total NPP because it covers \textasciitilde70\% of the Earth, but its productivity per square meter is extremely low. Estuaries and tropical rainforests have the highest NPP per unit area.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Net Primary Production (NPP) is the rate at which photosynthetic producers create net useful chemical energy. It is the total primary production (GPP) minus the energy used by the producers for their own respiration. The question asks to identify the ecosystem with the lowest NPP per unit area.
Step 2: Detailed Explanation:
Let's compare the productivity of the given ecosystems:

Savanna: These grasslands have moderate productivity, limited by seasonal rainfall.

Estuaries: These are among the most productive ecosystems in the world. The constant influx of nutrients from rivers combined with ample sunlight creates ideal conditions for high NPP.

Open ocean: The open ocean has a very low NPP per unit area. This is primarily because it is nutrient-limited (especially nitrogen and phosphorus) and light can only penetrate the shallow surface layer (photic zone). While its vast area means it contributes significantly to global NPP, its productivity rate per square meter is extremely low, comparable to a desert.

Agricultural land: The productivity of agricultural land can be very high due to subsidies of water, fertilizers, and pesticides. However, it is generally less productive than a healthy estuary or tropical rainforest.

Step 3: Final Answer:
When comparing NPP on a per-unit-area basis, the open ocean is the least productive due to severe nutrient limitation.