Problem: References. You should reference all the material that you have used for this report that is not your own work, including the module materials. If the source of the material has a formal reference (e.g. it is an item from the module materials, or is a book or a paper) then you should use the Harvard style as outlined in the Cite Them Right website. Box 1 Explaining primary production Primary production is the biomass (or the energy it represents) produced by vegetation within a defined area over a defined timescale. Gross Primary Production (GPP) is the total biomass produced. Net Primary Production (NPP) is GPP minus the portion used by plants in respiration (R), or NPP = GPP - R. Need Assignment Help?