Pyramid of Biomass and Pyramid of Energy

The pyramid of biomass represents the total standing mass of living organic material -- the standing crop -- present at each trophic level of a food chain, again usually measured per unit area at a given point in time, and typically expressed as dry weight in kilograms per unit area to allow fair comparison between trophic levels.

In most terrestrial ecosystems, the pyramid of biomass is upright, following the same general logic as the grassland pyramid of numbers: producers accumulate the greatest total biomass, since they form the base that all higher levels ultimately draw energy from, and biomass decreases at each successive trophic level above them. In a typical grassland food chain, for example, the standing biomass of grasses (producers) considerably exceeds that of the herbivorous mice or insects feeding on them (primary consumers), which in turn exceeds the biomass of the snakes preying on those herbivores (secondary consumers), which in turn exceeds the biomass of the owls or other top predators (tertiary consumers) -- a clear, broad-based upright pyramid.

Certain aquatic ecosystems, however, present a striking exception, giving an INVERTED pyramid of biomass. In a pond or in open ocean water, the producers are microscopic phytoplankton with extremely short individual lifespans and very rapid rates of reproduction and turnover -- they are consumed by zooplankton almost as quickly as new individuals are produced. As a result, although the RATE at which phytoplankton generate new biomass (their productivity) can be quite high, the standing biomass of phytoplankton actually present in the water at any single moment is often smaller than the standing biomass of the zooplankton and fish that feed on them, since those consumers, being larger and longer-lived individually, accumulate and retain biomass in their bodies for much longer than any individual phytoplankton cell survives. This produces a pyramid of biomass that is narrow at the base and widens above it -- the biomass equivalent of the inverted pyramid of numbers seen in the single-tree example, though arising here from a difference in generation time and turnover rate rather than from producer size. …