5.3.3 Biomass

Measuring Biomass

1. Dry Mass:

• Biomass is measured in terms of dry mass rather than wet mass because water content can vary between organisms.
• To obtain dry mass, a sample is dried in an oven at low temperature (usually 80°C) until a constant mass is achieved.

2. Mass of Carbon:

• On average, carbon content is assumed to make up about 50% of the dry mass.

Measuring the Energy in Biomass

The chemical energy stored in biomass can be determined using calorimetry, specifically with a bomb calorimeter:

3. A sample of known dry mass is placed in the calorimeter and burned in an atmosphere of pure oxygen.
4. The calorimeter is surrounded by water, and the heat released by the burning sample causes the temperature of the water to increase.
5. The temperature change is measured, and the energy content of the sample is calculated using the formula:

$$\text{Energy (J)} = \text{mass of water (g)} \times \text{specific heat capacity of water (4.18 J/g°C)} \times \text{temperature change (°C)}$$

Uses of Biomass Data

1. Estimate Productivity:

• Biomass data is used to calculate the net primary productivity (NPP) and gross primary productivity (GPP) of ecosystems.

2. Compare Ecosystems:

• Biomass provides insight into the energy transfer and efficiency between trophic levels.

3. Conservation & Management:

• Helps in understanding energy flow and planning sustainable practices for agriculture and ecosystem management.

Key Terms

• Gross Primary Productivity (GPP): The total energy fixed by plants in photosynthesis.
• Net Primary Productivity (NPP): The energy remaining after plants have used some for respiration; available to herbivores and decomposers.

$$\text{NPP} = \text{GPP} - \text{Respiration}$$