when is the pyramid of biomass spindle shaped?

Pyramid of biomass is spindle shaped or inverted in case of aquatic ecosystem where organic matter present in producers ( phytoplanktons) is less than that of herbivores (plant eating fish) whereas carnivores (larger fish which eat other small fish, insects etc.) has the largest organic matter. This is because of increase in reproductive potential and survival of organisms as we move up the trophic level.