NCERT Class 12 Environment & Ecology — Chapter 2: Ecosystem

The correct option is The standing crop of phytoplankton is small but supports a large biomass of zooplankton..

Explanation

Ecological pyramids represent the trophic structure of an ecosystem. The pyramid of biomass indicates the total dry weight of living matter (standing crop) present at each trophic level at a particular time. Unlike terrestrial ecosystems, which typically have upright biomass pyramids, aquatic ecosystems often exhibit an inverted pyramid of biomass.

Detailed Analysis:

• The standing crop of phytoplankton is small but supports a large biomass of zooplankton. is Correct: In marine environments, the producers are primarily microscopic phytoplankton. These organisms have a very small standing crop (mass at any specific moment) compared to the zooplankton and fish that feed on them. However, phytoplankton have an extremely high reproductive rate (turnover), allowing a small mass to support a much larger biomass of long-lived consumers (zooplankton). This results in a pyramid with a narrow base and a wide top.
• Zooplankton have a much faster turnover rate than phytoplankton. is Incorrect: The statement reverses the biological reality. Phytoplankton have a much faster turnover rate than zooplankton. It is this rapid replacement that allows the small producer biomass to sustain the ecosystem.
• Producers are largely absent from open marine waters. is Incorrect: Producers are not absent; they are the foundation of the marine food web. The inversion is due to the weight comparison, not the absence of organisms.
• The energy pyramid is inverted in the sea due to high efficiency. is Incorrect: The pyramid of energy is always upright in all ecosystems, including oceans. Energy flow is unidirectional and decreases at each trophic level due to heat loss (Second Law of Thermodynamics); it can never be inverted.

Key Takeaway:
The marine pyramid of biomass is inverted because the standing crop of producers (phytoplankton) is smaller than that of consumers (zooplankton), a structure made possible by the high reproductive turnover of the producers.