Net primary productivity (NPP) can be described as the rate at which producers store chemical energy (via photosyntehsis) minus the rate at which producers use chemical energy (via respiration). In other words, NPP is the rate at which energy for consumption by consumers is stored by producers.

 

The first graph displays the average net primary productivity, expressed in kilocalories per meters squared per year. The second graph shows the average world net primary productivity, measured in billions of kilocalories per year. The first graph represents the actual level of productivity for an ecosystem, while the second graph represents the level of productivity in relation to the amount of space that type of ecosystem occupies on the earth.

 

For example, in the first graph, the open ocean ranks toward the bottom in regards to NPP. However, in the second graph, the open ocean is the highest ranked. This is because oceans cover about 71% of the planet's surface, and while their individual NPP may be low, they cover such a large portion of the earth that the total world NPP is much, much higher.