What term describes the total gain in biomass per unit area per unit time fixed by primary producers?

The correct term for the total gain in biomass per unit area per unit time fixed by primary producers is net primary productivity (NPP). NPP measures the amount of light energy converted into chemical energy (biomass) by photosynthetic organisms, after subtracting the energy used for respiration.

In contrast, gross primary productivity (GPP) refers to the total amount of organic matter produced by photosynthesis before any energy is used or lost through respiration. It represents the maximum potential productivity of an ecosystem but does not account for the energy that is utilized by the producers themselves for metabolic processes.

Thus, net primary productivity specifically highlights the biomass accumulation that contributes to growth and reproduction of the plant biomass, making it crucial for understanding energy flow and sustainability within ecosystems.