Question

What do primary producers have available to convert into biomass? 
(a) 10% of secondary productivity
(b) Energy used for respiration
(c) Gross primary productivity
(d) Net Primary productivity 

Answer 1

Hint: The productivity that is available for the primary producers to convert into the biomass is equal to the difference between the total photosynthesis and respiration in the ecosystem. It is calculated by quantifying new organic matter produced and retained in a given period by living plants.

Complete answer:
Net primary productivity, or NPP, is gross primary productivity minus the rate of energy loss for metabolism and maintenance. In other words, it is the rate at which energy is stored by plants or other primary producers as biomass, and made available in the environment to consumers. 
- In ecology, productivity is the rate at which energy in the form of biomass is added to the organisms' bodies. Biomass is basically the sum of matter that's contained in a group of organisms' bodies. There are two types of productivity. They are the gross primary productivity and net primary productivity.
The gross primary productivity (GPP) is the rate at which solar energy is absorbed during photosynthesis in sugar molecules (energy absorbed per unit area per time) . Some of this energy is used by producers such as plants for metabolism / cellular respiration and some for growth (building tissues) .
After deducting the amount of energy required for the metabolism of the plant from the GPP, we obtain the Net primary productivity which is the amount of energy that gets transferred to the next trophic level.
- Plants usually absorb and transform about 1.3 to 1.6% of the solar energy that reaches the surface of the Earth and uses around a quarter of the energy collected for metabolism and maintenance. Thus, about 1 percent of the solar energy that reaches the surface of Earth (per unit area and time) ends up as net primary productivity. 
- Net primary productivity varies and depends on several factors. These include the input of solar energy, temperature, humidity, carbon dioxide levels, nutrient availability, and interactions of various communities with the environment. These factors affect the number of photosynthesizers present for capturing light energy and how efficiently they can perform their task.
So, the correct answer is ‘Net Primary Productivity’.

Note: From one trophic level to the next trophic level only 10% of energy will be transferred and 90% of the energy gets lost. This is called the 10 percent law. For instance, if the primary producers have 100% energy then the primary consumers will receive only 10% energy. In this way, the apex predators will receive only 0.1% of the energy. The rest amount of the energy will be released into the atmosphere in the form of the heat.