1.8 Primary Productivity

Producers (plants, algae and some bacteria) capture light energy and store it as chemical energy in sugar. The rate at which they do this is called primary productivity. It's usually measured as energy or mass per area per time, like kcal/m²/yr (kilocalories per square meter per year) or g/m²/yr.

Gross primary productivity (GPP) is the total amount of energy producers capture. But producers need energy too, so they use some of that sugar in their own cellular respiration. The energy they use is called respiration loss (R).

Net primary productivity (NPP) is what's left: NPP = GPP − R. NPP is the energy actually stored in plant tissue, so it's the energy available to herbivores, decomposers and everything else. You can rearrange the equation: GPP = NPP + R, and R = GPP − NPP.

Think of GPP as a paycheck before expenses and NPP as the money left to spend. A forest with high GPP but very high respiration might store less than you'd expect. NPP is also what people harvest when they cut timber or grow crops, so it sets an upper limit on how much food and wood an ecosystem can supply.

Productivity depends on sunlight, water, temperature and nutrients. Ecosystems with lots of all four have the highest NPP per square meter.

• Highest on land: tropical rainforests (warm, wet, sunny all year).
• Highest in water: estuaries, salt marshes, swamps and coral reefs (shallow, sunny, nutrient-rich).
• Lowest: deserts (no water), tundra (cold, short growing season) and the open ocean (few nutrients at the surface).
• The open ocean has low NPP per square meter, but it covers so much area that its total productivity is enormous.

In water, productivity drops with depth because water absorbs sunlight. Different wavelengths (colors) are absorbed at different depths. Almost all red light is absorbed in about the top meter. Blue light travels farthest, but it gets below about 100 m only in the very clearest water. Aquatic producers have adapted to this: many algae, like red algae, have extra pigments that capture the blue and green light found deeper down. Below the photic zone there isn't enough light for photosynthesis at all, so producers live near the surface or in shallow water. Cloudy (turbid) water cuts productivity even more.

Because photosynthesis makes oxygen and respiration uses it, scientists can measure aquatic productivity by tracking dissolved oxygen (DO). In the light and dark bottle method, water samples with algae go into a clear bottle and a dark bottle, and both are left for a set time.

The dark bottle gets no light, so only respiration happens and DO drops. The drop equals respiration. The light bottle has both photosynthesis and respiration, so its change in DO equals NPP. Adding them gives GPP: GPP = NPP + respiration.