An inverted pyramid of biomass is found in which ecosystem?

The correct answer is Option (1) → Aquatic ecosystem (sea).

In most ecosystems the pyramid of biomass is upright: producers have more biomass than herbivores, and herbivores more than carnivores. In the sea the pyramid of biomass is generally inverted, because a small standing crop of phytoplankton supports a much larger biomass of consumers such as fishes.

Option (1) Aquatic ecosystem (sea). — Correct. In the sea the biomass of fishes far exceeds that of phytoplankton, so the pyramid of biomass is inverted.

Option (2) Forest ecosystem. — Incorrect. In a forest the producers (trees) have far more biomass than the animals that feed on them, so the biomass pyramid is upright.

Option (3) Desert ecosystem. — Incorrect. A desert has a small biomass at every level, but the producers still have more biomass than the consumers; the pyramid is upright.

Option (4) Grassland ecosystem. — Incorrect. In a grassland the grass has more biomass than the herbivores and carnivores, so the pyramid of biomass is upright.

NCERT Extract:
Ecological Pyramids – “In most ecosystems, all the pyramids, of number, of energy and biomass are upright, i.e., producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. … The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton.”

Figure 12.4 (c) Inverted pyramid of biomass-small standing crop of phytoplankton supports large standing crop of zooplankton