A pyramid of biomass shows the total mass of living organisms at each trophic level, per unit area (or volume, for aquatic habitats), at one moment in time. It is usually given as dry mass in g m−2.biomass = mean mass of one organism × number of organisms Dry mass is used because the water content of organisms varies a lot and changes with conditions. To find it, samples are dried in an oven at about 80 °C (low enough that organic matter does not burn) and weighed repeatedly until the mass is constant. The organisms must be killed, so only small samples are taken and the results scaled up.Pyramids of biomass solve the main problem with pyramids of numbers: they take the size of organisms into account. One oak tree and the thousands of caterpillars feeding on it give an inverted pyramid of numbers, but an upright pyramid of biomass, because the tree is far heavier than all its caterpillars.Worked example: 40 caterpillars have a mean dry mass of 0.05 g, so their biomass is 40 × 0.05 = 2 g.Pyramids of biomass can still be inverted. In the English Channel in winter, the mass of phytoplankton present at one moment is less than the mass of zooplankton feeding on them. Phytoplankton reproduce so quickly that a small standing crop can feed a larger mass of consumers, but a snapshot of biomass does not show this rapid turnover. A pyramid of energy, which includes time, would be upright.