ES- Module 6: Gross and Net Primary Productivity

How do photosynthesis and respiration affect energy flow?

Photosynthesis: Photosynthesis is the process by which plants, algae, and some bacteria convert solar energy into chemical energy in the form of glucose (C₆H₁₂O₆). In this process, they take in carbon dioxide (CO₂) and water (H₂O) and use sunlight to produce glucose and oxygen (O₂). This process is the foundation of energy flow in ecosystems because it provides the chemical energy (stored in glucose) that supports nearly all life forms. Photosynthesis captures energy from the sun and converts it into a form that organisms can use.

Respiration: Cellular respiration is the process by which organisms (plants, animals, fungi, bacteria) break down glucose to release stored chemical energy. In this process, glucose is combined with oxygen to produce carbon dioxide, water, and energy (in the form of ATP). While photosynthesis captures energy, respiration releases it for use in metabolic activities like growth, reproduction, and maintenance.

Together, photosynthesis brings energy into ecosystems, while respiration releases energy to support life processes. The balance between these two processes influences the overall flow of energy within ecosystems

What are the calculations for gross primary productivity (GPP) and net primary productivity (NPP)?

Gross Primary Productivity (GPP):

GPP is the total amount of energy captured by producers (such as plants and algae) through photosynthesis in a given area over a specific time period. It represents the total energy assimilated by the producers before any energy is used for respiration.

GPP=TotalEnergyCapturedthroughPhotosynthesis measured in kgC/m2/year

Net Primary Productivity (NPP):

NPP is the energy that remains after producers use some of the energy for their own cellular respiration (R). NPP represents the energy that is available to consumers in the ecosystem (e.g., herbivores and decomposers).

NPP=GPP−R

Where:

GPP is the total energy captured.

R is the energy used for respiration by the producers.

NPP is important because it is the portion of energy that can flow through the food chain to support herbivores, carnivores, and decomposers.

Why does primary productivity have low efficiency?

Primary productivity has low efficiency due to several factors:

Energy Loss in Photosynthesis:

Only a small fraction of the sun's energy (about 1-2%) is captured by plants and other producers through photosynthesis. Most of the sunlight is either reflected, absorbed as heat, or passes through the plant without being used.

Respiration Losses:

A significant portion of the energy captured by producers is used for their own cellular respiration to power their life processes, leaving less energy for growth and reproduction (NPP).

Limitations of Photosynthetic Pathways:

Photosynthesis is limited by environmental factors such as water availability, light intensity, temperature, and nutrient availability. Suboptimal conditions reduce the efficiency of energy capture.

Energy Transfer Inefficiency:

Not all of the energy captured by producers is transferred to herbivores. Some of the energy is stored in forms that are difficult for consumers to digest, such as cellulose and lignin in plant tissues.

Why are some ecosystems much more productive than others?

Several factors contribute to variations in productivity among different ecosystems:

Availability of Solar Energy:

Ecosystems near the equator, such as tropical rainforests, receive more sunlight year-round compared to ecosystems at higher latitudes. This increased solar energy boosts photosynthesis and leads to higher GPP and NPP.

Temperature:

Warmer temperatures enhance enzyme activity involved in photosynthesis and other biological processes. Ecosystems in tropical and temperate regions are generally more productive than colder environments like tundra or boreal forests.

Water Availability:

Adequate water supply is essential for photosynthesis and plant growth. Wet ecosystems, such as rainforests and wetlands, are highly productive, while deserts, where water is scarce, have much lower productivity.

Nutrient Availability:

Nutrient-rich ecosystems, such as estuaries and agricultural land, tend to have high productivity because plants have access to the essential nutrients they need to grow. In contrast, ecosystems with poor soil quality, like deserts or nutrient-poor marine environments, tend to have lower productivity.

Length of Growing Season:

Ecosystems with long growing seasons (like tropical rainforests) can capture more energy over time, resulting in higher productivity. In contrast, ecosystems with short growing seasons (like tundra or temperate forests) have limited time for energy capture.

These factors combine to make ecosystems such as tropical rainforests and estuaries some of the most productive, while deserts and tundra are among the least productive.