Primary Productivity Definitions:

GROSS PRIMARY PRODUCTIVITY (GPP): • TOTAL energy captured by photosynthesis • All organic molecules produced by autotrophs • Before any energy is used by plants themselves • Units: energy/area/time (e.g., kcal/m²/year) or biomass (g/m²/year)

Equation: GPP = Total photosynthesis

NET PRIMARY PRODUCTIVITY (NPP): • Energy stored in plant biomass AFTER cellular respiration • Energy available to herbivores/decomposers • GPP minus plant respiration • Represents actual plant growth • What we can measure as biomass accumulation

Equation: NPP = GPP - R_plants

where R_plants = plant cellular respiration

Typically: NPP ≈ 50-60% of GPP (40-50% lost to plant respiration)

NET ECOSYSTEM PRODUCTIVITY (NEP): • Net carbon accumulation in ecosystem • NPP minus ALL heterotroph respiration • Includes consumers and decomposers • Can be positive (carbon sink) or negative (carbon source)

Equation: NEP = NPP - R_heterotrophs

OR

NEP = GPP - R_total (all organisms)

Relationships: GPP > NPP > NEP

GPP (100%) → NPP (50%) → NEP (variable, often ~10%)

Example with Numbers:

Tropical Rainforest: • GPP = 20,000 kcal/m²/year • Plant respiration = 10,000 kcal/m²/year • NPP = 10,000 kcal/m²/year • Heterotroph respiration = 9,000 kcal/m²/year • NEP = 1,000 kcal/m²/year (carbon sink)

Factors Affecting Productivity:

1. Light availability • More light → higher GPP • Tropical > temperate > polar

2. Temperature • Affects enzyme activity • Warm (optimal) > cold

3. Water availability • Rainforest > desert • Essential for photosynthesis

4. Nutrients • Nitrogen, phosphorus limiting • Fertilization increases NPP

5. Growing season length • Longer season → more production • Tropical (year-round) > temperate (seasonal)

Ecosystem Comparisons:

HIGHEST NPP: • Tropical rainforests • Swamps/marshes • Coral reefs • Estuaries

LOWEST NPP: • Deserts • Tundra • Open ocean (low nutrients)

NEP Significance:

Positive NEP (NEP > 0): • Carbon sink • Accumulating organic matter • Young forest, growing • Peat bogs

Negative NEP (NEP < 0): • Carbon source • Releasing CO₂ to atmosphere • Disturbed forest, decomposing • After clear-cutting

Zero NEP (NEP = 0): • Steady state • Mature forest (climax) • Production = decomposition • No net carbon accumulation

Measurement Methods:

1. Harvest method • Measure biomass accumulation • Destructive • Direct measurement of NPP

2. Gas exchange • Measure O₂ production or CO₂ uptake • Chamber or tower measurements • Can separate GPP and respiration

3. Remote sensing • Satellite data (NDVI) • Large-scale estimates • Correlates with chlorophyll

Human Impacts:

• Agriculture: Maximize NPP for crops • Deforestation: Reduces GPP, can make NEP negative • Climate change: Affects all productivity measures • Nutrient pollution: Can increase NPP (eutrophication)

Key Distinctions Summary:

GPP = Energy IN (photosynthesis) NPP = Energy stored by PLANTS (after plant respiration) NEP = Energy stored by ECOSYSTEM (after all respiration)

GPP - R_plants = NPP NPP - R_heterotrophs = NEP

The Carbon Cycle: Movement of carbon through Earth's systems (atmosphere, biosphere, hydrosphere, lithosphere).

MAJOR CARBON RESERVOIRS:

1. ATMOSPHERE (~800 Gt C) • CO₂ gas • CH₄ (methane) • Smallest reservoir but critical • Fast turnover

2. TERRESTRIAL BIOSPHERE (~2,000 Gt C) • Living biomass (plants, animals) • Soil organic matter (~1,500 Gt) • Plant biomass (~500 Gt)

3. OCEANS (~38,000 Gt C) • Largest active reservoir • Dissolved CO₂ • Marine organisms • Carbonate sediments • Deep ocean vs. surface ocean

4. FOSSIL FUELS (~4,000 Gt C) • Coal, oil, natural gas • Ancient organic matter • Locked underground

5. SEDIMENTARY ROCKS (>50,000,000 Gt C) • Limestone, chalk • Largest total reservoir • Very slow turnover (millions of years)

MAJOR CARBON FLUXES (Gt C/year):

NATURAL Processes:

1. Photosynthesis (~120 Gt/yr) • Atmosphere → Terrestrial biosphere • CO₂ + H₂O → glucose + O₂ • Removes CO₂ from atmosphere

2. Cellular Respiration (~120 Gt/yr) • Terrestrial biosphere → Atmosphere • Glucose + O₂ → CO₂ + H₂O • Returns CO₂ to atmosphere • Nearly balances photosynthesis

3. Ocean-Atmosphere Exchange (~90 Gt/yr each direction) • CO₂ dissolves in ocean • CO₂ released from ocean • Temperature-dependent • Cold water absorbs more CO₂

4. Ocean Photosynthesis (~50 Gt/yr) • Phytoplankton fix CO₂ • Base of marine food web

5. Ocean Respiration/Decomposition (~50 Gt/yr) • Returns CO₂ to ocean

6. Sedimentation (~0.2 Gt/yr) • Dead organisms sink • Form sediments • Very slow removal from cycle

7. Volcanic Emissions (~0.1 Gt/yr) • CO₂ released from Earth's interior • Slow addition to cycle

8. Weathering (~0.3 Gt/yr) • Chemical breakdown of rocks • CO₂ consumed • Forms carbonates

HUMAN Alterations:

1. FOSSIL FUEL COMBUSTION (~9 Gt C/yr) • Burning coal, oil, gas • Transfers ancient carbon to atmosphere • NEW input (not balanced) • Largest human impact

   Process: Fossil fuels → Atmosphere C (solid/liquid) + O₂ → CO₂

2. DEFORESTATION (~1-2 Gt C/yr) • Reduces photosynthesis • Releases stored carbon • Soil carbon exposed to decomposition • Double impact: less uptake + more release

3. LAND USE CHANGES • Agriculture replaces forests • Reduces carbon storage • Soil degradation

4. CEMENT PRODUCTION (~0.5 Gt C/yr) • Heating limestone releases CO₂ • CaCO₃ → CaO + CO₂

5. OCEAN ACIDIFICATION • Ocean absorbs ~2.5 Gt C/yr of human emissions • CO₂ + H₂O → H₂CO₃ (carbonic acid) • Decreases pH • Harms coral reefs, shellfish

Net Result: • Human activities add ~10 Gt C/yr to atmosphere • Ocean absorbs ~2.5 Gt C/yr • Terrestrial biosphere absorbs ~2.5 Gt C/yr • Atmosphere accumulates ~5 Gt C/yr • CO₂ concentration rising (~420 ppm in 2024, was ~280 ppm pre-industrial)

Consequences:

1. Climate Change • CO₂ is greenhouse gas • Global warming • Weather pattern changes

2. Ocean Acidification • Threatens marine life • Coral bleaching • Shell formation impaired

3. Altered Ecosystem Function • Photosynthesis rates change • Plant growth patterns shift • Migration of species

4. Positive Feedbacks • Warming → permafrost thaw → CH₄ release • Warming → less ocean CO₂ uptake • Warming → increased respiration

Natural Carbon Sinks: • Oceans (largest) • Forests (especially tropical) • Soil organic matter • Peatlands

Carbon Cycle Time Scales: • Fast cycle: Atmosphere ↔ Biosphere (years to decades) • Slow cycle: Sediments ↔ Rocks (millions of years) • Human activities have accelerated the fast cycle dramatically

Key Insight: Natural carbon cycle was roughly balanced. Human activities (fossil fuels, deforestation) have unbalanced it, causing atmospheric CO₂ to rise and driving climate change.

ENERGY FLOW vs. NUTRIENT CYCLING:

ENERGY FLOW:

Pattern: ONE-WAY, LINEAR • Sun → Producers → Consumers → Lost as heat • Does NOT cycle • Continuously input from sun • Continuously lost as heat • Must have constant external source

Characteristics:

1. Enters as light energy (photosynthesis)
2. Converted to chemical energy (glucose, ATP)
3. Transferred through food chain
4. Lost at each trophic level (~90% loss)
5. Ultimately ALL energy dissipated as heat
6. Cannot be recycled
7. Heat energy unavailable to organisms

Laws of Thermodynamics: • 1st Law: Energy cannot be created or destroyed (only converted) • 2nd Law: Energy conversions increase entropy (disorder) → Some energy always lost as heat → Heat is most disordered form → Cannot be converted back to useful form

Visual: SUN → Plants → Herbivores → Carnivores ↓ ↓ ↓ ↓ HEAT HEAT HEAT HEAT (end) (end) (end) (end)

NUTRIENT CYCLING:

Pattern: CYCLICAL, CIRCULAR • Nutrients move between biotic and abiotic components • Continuously recycled • Same atoms used over and over • No new input needed (except small additions) • Limited supply must be shared

Characteristics:

1. Elements (C, N, P, etc.) cycle
2. Move between organisms and environment
3. Decomposers return nutrients to soil/atmosphere
4. Producers take up nutrients again
5. Chemical form changes but atoms conserved
6. Can be limiting factors (scarce nutrients limit growth)
7. Biogeochemical cycles

Visual: Plants ← Soil ↓ ↑ Animals → Decomposers ↓ ↑ Waste → (recycle) ↑_______↓

KEY DIFFERENCES:

WHY ENERGY FLOWS ONE-WAY:

1. Thermodynamics • 2nd Law: Energy quality decreases • Heat is lowest quality (high entropy) • Cannot spontaneously increase quality • Like water flowing downhill - can't flow back up

2. Chemical Nature • Energy stored in chemical bonds • When bonds broken, energy released as heat • Heat disperses to environment • Too diffuse to recapture

3. Biological Reality • No organism can use heat energy • Photosynthesis requires light, not heat • Each metabolic process loses energy as heat • Cumulative losses prevent cycling

WHY NUTRIENTS CYCLE:

1. Conservation of Matter • Atoms are not destroyed • Only change form/location • Carbon atom in your body was once in dinosaur, tree, etc.

2. Limited Supply • Fixed amount of elements on Earth • Must be reused • Otherwise life would run out

3. Decomposer Role • Break down dead organic matter • Release nutrients back to environment • Makes nutrients available again • Essential link in cycle

4. Chemical Transformations • Nutrients change chemical form • But same elements present • Example: N₂ → NH₃ → NO₃⁻ → protein → NH₃ → N₂ • Nitrogen atom conserved through cycle

IMPLICATIONS:

1. Ecosystem Sustainability • Energy: Need constant sun input (not sustainable without) • Nutrients: Can sustain indefinitely if cycles intact

2. Human Impact • Energy: Fossil fuels (one-time use, becomes heat) • Nutrients: Pollution disrupts cycles (eutrophication) • Must maintain nutrient cycles

3. Trophic Structure • Energy limits food chain length (runs out) • Nutrients don't limit length (cycle back)

4. Ecosystem Productivity • Energy: Usually abundant (from sun) • Nutrients: Often limiting (fixed supply) • Nutrient addition can increase productivity

5. Evolution • Organisms evolved to capture energy efficiently • Organisms evolved to recycle nutrients • Decomposers critical for nutrient cycling

EXCEPTIONS/QUALIFICATIONS:

1. Energy can be "stored" temporarily • Biomass, fossil fuels • But eventually all becomes heat • Not true cycling

2. Nutrients can be "lost" • Sedimentation (geological time) • Leaching from ecosystem • But eventually return (weathering, uplift) • Much slower than biological cycling

ANALOGY: • Energy like money you spend (gone forever) • Nutrients like money in circulation (changes hands but exists)

Key Principle: Energy and nutrients both essential, but behave fundamentally differently due to laws of thermodynamics (energy) vs. conservation of matter (nutrients)!

The Nitrogen Cycle: Movement of nitrogen through atmosphere, soil, water, and organisms.

Why Nitrogen is Limiting (Paradox): • 78% of atmosphere is N₂ gas • BUT most organisms CANNOT use N₂ directly • N≡N triple bond extremely strong • Requires huge energy to break • Only certain bacteria can fix N₂ • "Plenty" in air but "unavailable"

Usable Forms: • NH₃/NH₄⁺ (ammonia/ammonium) • NO₃⁻ (nitrate) • Organic nitrogen (proteins, nucleic acids)

Stages of Nitrogen Cycle:

1. NITROGEN FIXATION Converting atmospheric N₂ to usable forms

   Biological fixation: • Nitrogen-fixing bacteria • N₂ → NH₃ (ammonia) • Enzyme: nitrogenase • Requires lots of ATP (energy expensive)

   Types of nitrogen fixers: a) Free-living soil bacteria • Azotobacter, Clostridium • Live independently in soil

   b) Symbiotic bacteria • Rhizobium in legume root nodules • Peas, beans, clover, alfalfa • Mutualism: bacteria get sugars, plant gets nitrogen

   c) Cyanobacteria • Aquatic environments • Some rice paddies (Anabaena)

   Abiotic fixation: • Lightning: N₂ → NOₓ • Industrial (Haber process): N₂ + H₂ → NH₃ (fertilizer) • ~50% of N fixation now human-caused!

2. NITRIFICATION Conversion of ammonia to nitrate (two steps)

   Step 1: Ammonia → Nitrite • By Nitrosomonas bacteria • NH₄⁺ → NO₂⁻ (nitrite) • Aerobic process

   Step 2: Nitrite → Nitrate • By Nitrobacter bacteria • NO₂⁻ → NO₃⁻ (nitrate) • Most usable form for plants

   Both steps are chemosynthesis (bacteria get energy from oxidation)

3. ASSIMILATION Uptake of nitrogen by organisms

   • Plants absorb NH₄⁺ and NO₃⁻ from soil • Synthesize amino acids, proteins, nucleic acids • Animals eat plants (or other animals) • Incorporate nitrogen into biomolecules • Nitrogen becomes organic nitrogen

4. AMMONIFICATION (Decomposition) Return of nitrogen to soil

   • Decomposers break down dead organisms, waste • Proteins, nucleic acids → NH₃/NH₄⁺ • By bacteria and fungi • Releases ammonia back to soil • Can be nitrified again or used directly

5. DENITRIFICATION Return of nitrogen to atmosphere

   • Denitrifying bacteria (Pseudomonas, Clostridium) • NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ • Use nitrate as electron acceptor (anaerobic respiration) • Occurs in waterlogged, oxygen-poor soils • Completes the cycle • Farmers try to prevent this (lose nitrogen)

Cycle Summary: N₂ (atmosphere) ↓ fixation NH₃/NH₄⁺ (soil) ↓ nitrification NO₃⁻ (soil) ↓ assimilation Organic N (organisms) ↓ ammonification NH₃/NH₄⁺ (soil) ↓ denitrification N₂ (atmosphere)

Human Impacts:

1. EXCESS Nitrogen (Major Problem)

   a) Fertilizer Production • Haber-Bosch process • Doubled available nitrogen on Earth • Runoff into waterways

   b) Fossil Fuel Combustion • NOₓ emissions • Acid rain • Air pollution

   c) Legume Cultivation • Increased biological fixation

2. Consequences of Excess:

   a) Eutrophication • Excess nutrients in water • Algal blooms • Oxygen depletion (dead zones) • Fish kills

   b) Groundwater Contamination • High nitrate in drinking water • Health hazard (blue baby syndrome)

   c) Air Pollution • NOₓ → smog • N₂O greenhouse gas (300× stronger than CO₂)

   d) Soil Acidification • Nitrification produces H⁺ • Lowers soil pH

   e) Loss of Biodiversity • Favors fast-growing species • Outcompete native plants • Simplifies ecosystems

Ecological Importance:

• Nitrogen in proteins (enzymes, structural) • Nitrogen in nucleic acids (DNA, RNA) • Nitrogen in ATP, chlorophyll • Essential for all life • Often THE limiting nutrient in ecosystems

Adaptations to Low Nitrogen:

1. Carnivorous plants • Venus flytrap, pitcher plants • Get nitrogen from insects • Live in nutrient-poor bogs

2. Mycorrhizae • Fungi help plants absorb nutrients • Include nitrogen

3. Legume-Rhizobium symbiosis • Plants "farm" nitrogen-fixing bacteria

Agricultural Applications:

• Crop rotation with legumes • Adds nitrogen to soil naturally • Reduces fertilizer need • Example: Corn → soybeans → corn

Key Insight: Nitrogen is abundant but "locked up" in N₂. Life depends on nitrogen-fixing bacteria to convert it to usable forms. Humans have dramatically altered the nitrogen cycle through fertilizer production and fossil fuel use, causing environmental problems.