Primary productivity can be defined as the rate at which primary producers (e.g. green plants) traps and store solar ' radiation in the form of biomass. This is measured in terms of weight (g~2) and in terms of energy (kcal m ~2) per year in a given time.
So, primary productivity varies from ecosystem to ecosystem and the ecosystem which possesses more producer will be more productive in terms of primary productivity. So, young forest grow quicker than older mature forest and are more productive in terms of productivity. The shallow polluted lake and alpine meadow will be less productive because of less number of producers and high amount of dead organic matter.