a food pyramid for an aquatic ecosystem. At each trophic level, the size of the box shows the relative number of organisms or biomass. The amount of biomass decreases from bottom to top. Primary producers make up the majority of biomass in a food pyramid. At each trophic level, the organisms become larger and the populations become smaller than at the previous level. Figure 4-12 shows the movement of energy, relative biomass, and trophic levels.