Energy Flow and Biogeochemical Cycles in Ecosystems

Energy Flow in Ecosystems

Fundamentals of Energy Flow

Energy enters the terrestrial ecosystem primarily in the form of sunlight. Plants (producers) convert light energy to chemical energy through photosynthesis. This chemical energy is stored in organic compounds and transferred through the ecosystem via consumption and decomposition.

• Producers: Organisms (mainly plants) that convert solar energy into chemical energy.

• Consumers: Animals that obtain energy by eating plants or other animals.

• Decomposers: Bacteria and fungi that break down dead organic matter, releasing chemical energy.

• Energy Transfer: At each trophic level, energy is lost as heat due to metabolic processes.

Primary production is the conversion of solar energy to chemical energy by producers. Net primary production (NPP) is the amount of new organic material added to an ecosystem over a given period, representing the stored chemical energy available to consumers.

Net Primary Production in Various Ecosystems

Different ecosystems vary in their net primary production, which is the rate at which producers create usable chemical energy.

Ecosystem	Net Primary Production (NPP)
Open ocean	Low per unit area, but high total due to vast size
Algal beds and reefs	High
Tropical rain forest	Very high
Temperate forest	Moderate
Desert	Low
Temperate grassland	Low to moderate

Energy Supply Limits the Length of Food Chains

When organic material is transferred from one trophic level to the next, only a fraction of the energy is stored; most is lost as heat. Typically, only about 10% of the energy at one trophic level is transferred to the next.

• Energy Pyramid: Illustrates the cumulative loss of energy with each transfer in a food chain.

• 80–95% of energy at one trophic level never transfers to the next.

Biogeochemical Cycles

Overview of Chemical Cycles

Chemical cycles in ecosystems involve both biotic (living) and abiotic (nonliving) components. These cycles are called biogeochemical cycles and include the movement of elements such as carbon, nitrogen, and phosphorus through the environment.

• Abiotic Reservoirs: Nonliving sources where chemicals accumulate or are stored (e.g., atmosphere, soil, rocks).

• Biotic Reservoirs: Living organisms that incorporate chemicals into organic compounds.

Carbon Cycle

The carbon cycle describes the movement of carbon among the atmosphere, organisms, and the earth. Carbon is a major ingredient of all organic molecules and cycles globally.

• Photosynthesis: Plants, algae, and cyanobacteria convert atmospheric CO2 into organic compounds.

• Cellular Respiration: Returns CO2 to the atmosphere.

• Decomposition and Burning: Release CO2 from dead organisms and fossil fuels.

Equation for Photosynthesis:

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Equation for Cellular Respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

Human activities, such as burning fossil fuels, increase atmospheric CO2 levels, impacting global climate.

Phosphorus Cycle

The phosphorus cycle depends on the weathering of rock to release phosphate ions (PO43–) into the soil. Phosphorus is essential for nucleic acids, ATP, and other biomolecules.

• Weathering: Gradually adds inorganic phosphate to soil.

• Assimilation: Plants absorb phosphate; animals obtain it by eating plants.

• Decomposition: Returns phosphate to soil from animal waste and dead organisms.

• Leaching and Runoff: Transfers phosphate to aquatic ecosystems, where it may become limiting.

Process	Description
Weathering of rock	Releases phosphate ions into soil
Assimilation	Plants absorb phosphate; animals eat plants
Decomposition	Returns phosphate to soil
Leaching/runoff	Transfers phosphate to aquatic systems

Phosphate availability is often low and can limit plant growth.

Nitrogen Cycle

The nitrogen cycle depends on bacteria to convert atmospheric nitrogen (N2) into forms usable by plants. Nitrogen is a crucial element for proteins and nucleic acids.

• Nitrogen Fixation: Bacteria convert N2 gas to ammonia (NH3).

• Nitrification: Ammonia is converted to nitrites (NO2–) and then nitrates (NO3–).

• Assimilation: Plants absorb nitrates; animals obtain nitrogen by eating plants.

• Denitrification: Bacteria convert nitrates back to N2 gas, returning it to the atmosphere.

Equation for Nitrogen Fixation:

$N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2$

Most atmospheric nitrogen is unavailable to living organisms until fixed by bacteria. The nitrogen cycle is essential for soil fertility and ecosystem productivity.

Summary Table: Major Biogeochemical Cycles

Cycle	Main Reservoir	Key Processes	Importance
Carbon	Atmosphere, fossil fuels, biomass	Photosynthesis, respiration, decomposition, combustion	Organic molecules, energy flow
Phosphorus	Rocks, soil, water	Weathering, assimilation, decomposition, runoff	Nucleic acids, ATP, cell membranes
Nitrogen	Atmosphere, soil	Nitrogen fixation, nitrification, assimilation, denitrification	Proteins, nucleic acids

Additional info:

• Biogeochemical cycles are essential for recycling nutrients and maintaining ecosystem health.

• Human activities can disrupt these cycles, leading to environmental issues such as eutrophication and climate change.