Total amount of energy stored by primary produces and therefore potentially available to or decomposers.
Net primary productivity is equal to gross primary productivity minus whatever fraction of gross primary productivity that is used by primary producers for their own metabolic needs. See also primary productivity.
Producers must divert a certain fraction of captured energy to catabolism versus anabolism. That diverted to anabolism more or less represents net primary productivity while that devoted to catabolism, to power short-term energy requirements, is more or less equal to gross primary productivity minus net primary productivity.
These losses could be, for example, to drive of organelles such as chloroplasts within cells, the opening and closing of stomata in plant leaves, or the flagellated swimming seen among many algae. Net primary productivity is whatever is available after such non-anabolic energy expenditures.
Net primary productivity can accumulate through growth of bodies or instead , but in either case it is the total biomass of the producers – whether they have increased in number or increased in individual size (or both) – that represents net primary productivity.