7.3 Energy Flow and Nutrient Cycling in Ecosystems

Energy Flow and Nutrients

Introduction to Energy Flow in Communities

Energy and nutrient dynamics are central to understanding how ecosystems function. This topic explores how energy enters, moves through, and exits biological communities, and how essential nutrients are cycled and conserved within ecosystems.

• Energy Flow: Describes the movement of energy from the sun through producers and consumers in an ecosystem.

• Nutrient Cycling: Refers to the movement and transformation of chemical elements essential for life.

The Physical Environment

Earth as a System for Matter and Energy

• Closed System for Matter: Earth does not gain or lose significant amounts of matter; elements are recycled.

• Open System for Energy: Energy enters as solar radiation and leaves as heat radiated into space.

• Solar Energy: Drives most biological processes, with highest input at the equator.

Primary Production

Gross and Net Primary Production

• Autotrophs: Organisms (e.g., plants, algae) that produce organic molecules from inorganic substances using energy from sunlight or inorganic chemicals.

• Gross Primary Production (GPP): Total amount of carbon fixed by autotrophs in an ecosystem.

• Net Primary Production (NPP): The energy remaining after autotrophs meet their own energy needs through respiration.

Equation:

$\mathrm{NPP = GPP - R}$

where $R$ is the energy used in respiration.

• Measurement: Often estimated by leaf surface area or biomass of autotrophs.

• Controls: Climate, nutrient availability, and autotroph biomass.

Vertical Structure and NPP in Forests

• Each additional leaf layer in a forest increases community carbon dioxide uptake, but with diminishing returns at higher layers.

• Photosynthesis rates peak during midday due to optimal sunlight angles.

Allocation of NPP in Biomes

• Biomes with low soil nutrients (e.g., tundra, temperate grasslands) allocate more NPP to roots for nutrient uptake.

• Tropical forests, despite poor soils, allocate more NPP to leaves due to light limitation in lower canopy layers.

Evapotranspiration and Plant Growth

Actual Evapotranspiration (AET)

• Definition: Total water evaporated and transpired from a landscape, measured in mm or ml per year.

• Correlation: Higher AET is associated with higher NPP and plant growth across terrestrial ecosystems.

Trophic Dynamics

Food Webs and Trophic Pyramids

• Trophic Levels: Hierarchical levels in an ecosystem, comprising producers at the base and various levels of consumers above.

• Trophic Pyramid: Visual representation of energy or biomass distribution among trophic levels; base is largest (autotrophs), apex is smallest (top predators).

Ecological Efficiency

• Definition: Percentage of energy transferred from one trophic level to the next.

• Consumption Efficiency: Proportion of available biomass ingested by consumers.

• Assimilation Efficiency: Proportion of ingested food absorbed by the digestive tract; higher in carnivores (up to 80%) than herbivores (20-50%).

• Production Efficiency: Proportion of assimilated biomass converted into new consumer biomass (e.g., offspring).

Stoichiometry and Nitrogen Content

• Stoichiometry: Quantifies the ratio of elements (e.g., C:N) in organisms.

• Predators generally have higher nitrogen content than herbivores, both in insects and vertebrates.

Energy Loss and the Second Law of Thermodynamics

• About 90% of energy is lost at each trophic transfer, mainly as heat (entropy increases).

• Not all consumed biomass is usable as energy.

Energy and Biomass Pyramids: Aquatic vs. Terrestrial Systems

Energy Pyramids

• Both terrestrial and aquatic ecosystems show greatest energy at the base (autotrophs), decreasing at higher trophic levels.

Biomass Pyramids

• Terrestrial ecosystems: Biomass pyramid mirrors energy pyramid; most biomass at the base.

• Aquatic ecosystems: Biomass pyramid can be inverted; primary producers (phytoplankton) have less biomass than consumers due to rapid consumption and turnover.

Patterns and Limitations of NPP

Terrestrial Systems

• Climatic Limitations: Temperature and precipitation are primary constraints on NPP.

• Photosynthesis is limited at high temperatures with low water (e.g., deserts) and at low temperatures (slow evapotranspiration).

• Nutrient Limitations: Nitrogen, phosphorus, calcium, and potassium can limit NPP; fertilization often increases productivity.

• Succession: NPP peaks at intermediate stages of ecological succession, where biomass and diversity are highest.

Aquatic Systems

• Limiting Factors: Light (photic zone) and nutrients (mainly nitrogen in marine, phosphorus in freshwater).

• Coastal margins and upwelling zones have highest NPP due to nutrient availability.

Global Patterns

• NPP is highest in tropical regions (0° latitude) due to year-round sunlight and precipitation.

• Oceans and terrestrial ecosystems show distinct latitudinal patterns in NPP.

Nutrients and Nutrient Cycling

Essential Nutrients

• Five elements (C, O, H, N, P) make up 93–97% of biomass in living organisms.

• Limiting Nutrients: Nitrogen and phosphorus are most often in shortest supply relative to demand.

Nutrient Cycling

• Involves use, transformation, movement, and reuse of nutrients in ecosystems.

• Critical for maintaining productivity and ecosystem function.

Nitrogen Cycle

• Importance: Nitrogen is essential for amino acids, nucleic acids, chlorophyll, and hemoglobin.

• Atmospheric nitrogen ($N_2$) is abundant but biologically unavailable due to strong triple bonds.

• Nitrogen Fixation: Conversion of $N_2$ to biologically usable forms by cyanobacteria, soil bacteria, bacteria in root nodules (e.g., legumes), lightning, and the Haber-Bosch process.

• Human activities (e.g., fossil fuel combustion, fertilizer production) add reactive nitrogen to ecosystems.

Phosphorus Cycle

• Importance: Phosphorus is a component of ATP, DNA, RNA, bones, and phospholipids.

• Not common in atmosphere; mainly found in rocks and sediments as solid phosphates.

• Released by weathering of rock; mycorrhizal fungi aid plant uptake.

• Decomposition returns phosphorus to the environment.

Table: Comparison of Nitrogen and Phosphorus Cycles

Feature	Nitrogen Cycle	Phosphorus Cycle
Main Reservoir	Atmosphere ($N_2$ gas)	Rocks and sediments (solid phosphates)
Limiting in	Marine systems (usually)	Freshwater systems (usually)
Key Biological Process	Nitrogen fixation (by bacteria, lightning, Haber-Bosch)	Weathering, mycorrhizal uptake
Human Impact	Fertilizer, fossil fuels, eutrophication	Fertilizer, eutrophication

Human Impacts: Agriculture and Eutrophication

Haber-Bosch Process

• Artificial nitrogen fixation using high temperature and pressure to produce ammonia for fertilizer.

• Supports a significant portion of global food production but consumes about 1% of global energy.

• Has dramatically increased nitrogen and phosphorus input to terrestrial systems since the 1960s.

Eutrophication

• Definition: Transformation of an ecosystem from low to high nutrient levels, often due to human activity (fertilizer runoff).

• Causes rapid increase in primary production, followed by proliferation of consumers (e.g., zooplankton).

• Excess organic matter sinks and is decomposed by bacteria, leading to oxygen depletion (hypoxia) and mass die-offs ("dead zones").

• Major dead zones occur in estuaries worldwide, such as the Mississippi River delta.

Decomposition and Nutrient Release

Role of Decomposition

• Breakdown of organic matter, releasing carbon dioxide and recycling nutrients.

• Essential for soil formation and maintaining soil properties.

• Decomposers include bacteria, fungi, and animals (e.g., vultures, family Cathartidae).

Summary Table: Key Concepts in Energy Flow and Nutrient Cycling

Concept	Definition	Key Points
GPP	Total carbon fixed by autotrophs	Controlled by climate, nutrients, biomass
NPP	GPP minus respiration	Available for growth and consumption
Ecological Efficiency	Energy transfer between trophic levels	~10% per level; includes consumption, assimilation, production efficiencies
Limiting Nutrient	Nutrient in shortest supply relative to need	Nitrogen (marine), phosphorus (freshwater)
Eutrophication	Increase in nutrient levels, often human-caused	Leads to hypoxia and dead zones

Additional info: The notes expand on the original slides by providing definitions, equations, and context for key ecological concepts, and by summarizing the processes in tables for clarity.