Gross Primary Productivity

Gross primary productivity (GPP) is the rate at which producers convert light energy into chemical energy in organic molecules by photosynthesis. It is the total energy fixed by the plants, per unit area per unit time, usually measured in kJ m⁻² year⁻¹.GPP depends on how fast photosynthesis can happen, so it is affected by:
- light intensity and the length of the day
- temperature, which affects the enzymes of photosynthesis
- carbon dioxide concentration
- availability of water and mineral ions such as nitrate and magnesium
- the length of the growing season
- the leaf area available to absorb lightComparing ecosystems:
- Tropical rainforest has the highest GPP on land: it is warm, wet and brightly lit all year, with many layers of leaves.
- Deserts have low GPP, mainly because of a shortage of water.
- Tundra has low GPP because it is cold and has a short growing season.
- The open ocean has low GPP per m², because mineral ions are scarce near the surface, but its huge area means it contributes a large share of the Earth's total GPP.Worked example: 1 700 000 kJ m⁻² year⁻¹ of sunlight reaches a grassland, which has a GPP of 25 500 kJ m⁻² year⁻¹. The percentage of light energy converted into chemical energy is:25 500 ÷ 1 700 000 × 100 = 1.5%Not all of this energy is available to consumers, because plants use some of it in their own respiration.