The ecological pyramids represent the trophic structure and also trophic function of the ecosystem. In many ecological pyramids, the producers form the base and the successive trophic levels make up the apex. The pyramid of biomass for an aquatic ecosystem is inverted; the base is much smaller than the structure it supports.
Biomass refers to the total weight of dry matter present in an ecosystem at any one time. The pyramid of biomass indicates the gradual reduction in biomass at each trophic level from base to top.
In terrestrial environment, the basic trophic level is occupied by the autotrophic plants. The herbivores occupy the next
trophic level. The total biomass of producers is much more than the total biomass of herbivores. Likewise the total biomass of the third level or primary carnivores or secondary consumers will be less than the herbivores and so on. Similarly in the marine ecosystem the phytoplankton occupy the basic trophic level and the next trophic level is occupied by zooplankton. The third and fourth levels are occupied by carnivores such as worms and molluscs and carnivorous fishes respectively. Here too, the biomass falls in a graded manner from the basic trophic levels to higher trophic levels.
Thus the pyramids of biomass are generally upright. But in case of parasites, the pyramid of biomass is inverted.
Similarly if organisms of lower levels are much smaller than those of higher levels, biomass pyramid may be inverted. In a pond, size of the producers is very small and that of the consumers is large. Hence, biomass shows an increase towards the higher levels, making the pyramid inverted in shape.