What is primary productivity? Distinguish between GPP and NPP, and explain why the productivity of the oceans is low.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Define in one line. Primary productivity is the rate of biomass production by plants per unit area per unit time (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹).
2 · Explain GPP. The rate of production of organic matter during photosynthesis.
3 · Explain NPP. NPP = GPP − R; the biomass available to herbivores and decomposers.
4 · Name the factors. Plant species, environmental factors, nutrients and photosynthetic capacity.
5 · Explain the oceans. About 70% of the surface but only about 55 of 170 billion tons of NPP: light fades with depth and nutrients are scarce.
6 · Link to real life. A watered, fertilised paddy field has a far higher NPP than a dry desert patch.
7 · Close in one line. So only NPP, not GPP, can feed the rest of the ecosystem.

Scoring tip: Write NPP = GPP − R as an equation and give a unit with time in it. Many lose a mark by writing g m⁻² for productivity.