Why does biomass decrease as you move up the pyramid which result into biomagnification?

With less energy at higher trophic levels, there are usually fewer organisms as well. Organisms tend to be larger in size at higher trophic levels, but their smaller numbers result in less biomass. Biomass is the total mass of organisms at a trophic level.

Hope this helps! Please correct me if I'm wrong :)

not sure if this what   you'r looking for would be percent composition