Introduction

The world’s ecosystems vary dramatically in their productivity, both in terms of how much biomass they produce per unit area and in their total contribution to global net primary production (NPP). Understanding these differences is crucial for ecology, conservation, and climate science. This article explores which ecosystems have the highest total net primary production and ranks them according to their global contribution.

What Is Net Primary Production (NPP)?

Net primary production (NPP) is the amount of biomass generated by primary producers (plants, algae, and cyanobacteria) after accounting for the energy used for their own respiration. NPP is typically measured in units such as grams of carbon per square meter per year or billion kilocalories per year for global totals. It represents the energy available to support all other life in the ecosystem, from herbivores to top predators.

Ranking Ecosystems by Total Global NPP

1. Open Oceans

• Area:
  Approximately 70% of the Earth’s surface.

• NPP per unit area:
  Low to moderate, but the vast area means the total global NPP is the highest of any ecosystem57.

• Key producers:
  Phytoplankton (microscopic algae).

2. Tropical Rainforests

• Area:
  Much smaller than the open ocean, but still significant.

• NPP per unit area:
  Very high—tropical rainforests are the most productive terrestrial ecosystems per unit area15.

• Key producers:
  Trees, shrubs, and other plants.

3. Northern Coniferous (Boreal) Forests

• Area:
  Large, especially in the northern hemisphere (e.g., Canada, Russia, Scandinavia).

• NPP per unit area:
  Low compared to tropical rainforests, but the large area means total NPP is still substantial17.

• Key producers:
  Coniferous trees (e.g., pine, spruce, fir).

4. Estuaries

• Area:
  Very small—narrow transition zones between rivers and oceans.

• NPP per unit area:
  Extremely high, but the small area means total global NPP is much less than the other ecosystems listed15.

• Key producers:
  Mangroves, seagrasses, and algae.

Why Do Open Oceans Top the List?

Despite having relatively low NPP per square meter, the open ocean covers such a vast area that its total global NPP is the highest of any ecosystem57. Phytoplankton, the primary producers of the open ocean, are highly efficient at using available nutrients and light, and their rapid turnover allows them to support large marine food webs.

Why Are Tropical Rainforests Second?

Tropical rainforests are the most productive terrestrial ecosystems per unit area, thanks to abundant rainfall, warm temperatures, and high biodiversity15. However, their total area is much smaller than the open ocean, so their global NPP is lower.

Northern Coniferous Forests and Estuaries

• Northern coniferous (boreal) forests:

  • Large area, but lower productivity per unit area than tropical rainforests.

  • Still contribute significantly to global NPP due to their size.

• Estuaries:

  • Extremely productive per unit area, but their small size limits their total global contribution.

Common Misconceptions

A common misconception is that the most productive ecosystems per unit area (like tropical rainforests or estuaries) also have the highest total global NPP. In reality, the open ocean dominates global NPP because of its vast size, even though its productivity per unit area is relatively low57.

Another misconception is that all forests contribute equally to global NPP. In fact, tropical rainforests are much more productive per unit area than boreal or temperate forests.

Conclusion

The correct descending order of total global net primary production for the listed ecosystems is:

Open Oceans > Tropical Rainforests > Northern Coniferous Forests > Estuaries

This order reflects the balance between productivity per unit area and the total area covered by each ecosystem.

Summary Table

Correct Answer:
(2) Open Oceans > Tropical rain forests > Northern coniferous forests > Estuaries