The rate of production of organic matter during photosynthesis is called Gross Primary Productivity (GPP) of the ecosystem.

When respiratory losses (R) is subtracted from GPP, we get Net Primary Productivity (NPP). Net primary productivity is the available biomass for the consumption of heterotrophs.

GPP = 300 kcal

R = 20% of GPP = 20/100x300 = 60 kcal

NPP = GPP - R

= 300-60

= 240 kcal