Mangroves P2-3
Exam Information (Context from Start of Lecture)
(Not mangrove content, but included for completeness)
Exam is online, 24‑hour window (9am–9am), but designed to take ~4 hours.
Two sections:
Section A: 2 questions → choose 1 (from this lecturer’s material).
Section B: 3 questions → choose 1 (from other lecturers: Martin, Roxanne, Roberts).
1000 words per answer (±10%).
Questions are based on a figure (graph, diagram, etc.) that you must interpret.
Figures are unique to the exam → prevents AI‑generated answers.
Only material covered in lectures will be examined.
Always consider who wrote the question → answer using their taught content.
2. Recap: Mangrove Diversity & Structure
Mangroves contain:
Different tree morphologies
Rhizophoraceae: stilt roots, straight trunks
Others resemble temperate trees (e.g., oak‑like forms)
Skinny, tall, or squat species depending on environment
Faunal diversity
Fully marine: fish, shrimp
Semi‑marine: crabs, marine snails
Terrestrial: insects, birds, snakes, lizards
Tidal influence shapes zonation and species composition.
Structural diversity influences:
Ecosystem functioning
Coastal protection
Sediment stabilisation
Carbon storage
3. Blue Carbon – Introduction
3.1 What is Blue Carbon?
Term coined in late 1990s (IUCN report).
Refers to carbon captured and stored by marine ecosystems, including:
Mangroves
Salt marshes
Seagrass beds
Kelp forests
Algal reefs
Marine sediments (seabed carbon)
3.2 Why the interest?
Originally inspired by terrestrial forests (e.g., Amazon) as carbon sinks.
Scientists asked: Do coastal wetlands store carbon as effectively?
Answer: Yes—often far more effectively.
4. Carbon Sequestration Rates: Mangroves vs Other Systems
4.1 Early evidence (2011)
Mangroves, salt marshes, seagrass beds store carbon much faster than terrestrial forests.
Measured as g C m⁻² yr⁻¹ (or tonnes per hectare per year).
Mangroves consistently outperform tropical forests.
4.2 Updated evidence (2021)
Compared against:
Peatlands
Boreal forests
Tropical forests
Mangroves still show the highest sequestration rates.
4.3 Economic valuation (2019)
Blue carbon ecosystems valued in USD per hectare.
Mangroves: up to $91,000 per hectare in carbon sequestration value.
Seagrass: ~12,000 (overestimated in older literature).
Mangroves = most valuable blue carbon ecosystem.
5. Why Mangroves Store So Much Carbon
5.1 Carbon enters via photosynthesis
CO₂ → fixed into biomass (wood, leaves, roots).
50% of wood dry weight = carbon.
Dead wood and litter contribute to long‑term storage.
5.2 Particle trapping
Mangroves trap suspended particles because:
Tides bring in water with suspended organic matter.
Water slows as it enters the forest → particles settle.
Organic particles (detritus, seagrass fragments, terrestrial material) accumulate in sediment.
This is a major driver of carbon storage.
5.3 Soil carbon dominates
Carbon pools in mangroves:
Above‑ground biomass (trees, branches, dead wood)
Surface soil + roots
Deep soil layers → largest carbon pool
Mangrove soils:
Can be metres deep
Accumulate carbon for thousands of years
Continuously build upward (no “reset” like terrestrial forests)
5.4 Slow decomposition
Mangrove sediments are:
Waterlogged
Anoxic (low oxygen)
Consequences:
Microbial breakdown is slow.
Anaerobic pathways are less efficient → carbon persists longer.
5.5 Low methane emissions
Unlike freshwater wetlands:
Marine sediments contain high sulphate.
Sulphate inhibits methanogenesis → low methane release.
Makes mangroves climate‑positive carbon sinks.
6. What Forest Properties Increase Carbon Storage?
6.1 Tree density
More trees → more biomass → more carbon.
Dense roots slow water → more particle trapping.
6.2 Species composition
Some species trap sediment more effectively.
Root architecture influences sedimentation rates.
6.3 Forest age
Older forests store more carbon.
Mangrove carbon accumulation increases with age (unlike terrestrial forests which plateau).
6.4 Forest structure
Complex root systems → more friction → more sediment deposition.
6.5 Hydrodynamics
Moderate tidal energy enhances sediment delivery.
Storms can:
Erode sediment
Deposit new sediment from offshore
→ Storm effects are not straightforward.
7. Threats to Mangroves
7.1 Global loss
Less than 50% of historical mangrove cover remains.
Past loss rates: 1–2% per year (now slowed).
2004 Asian tsunami highlighted mangrove value → increased protection.
7.2 Regional patterns
Largest absolute losses: Indonesia, Malaysia, Myanmar
But these countries also have the largest mangrove areas.
7.3 Proportional losses (since 2000)
Highest proportional losses in:
Malaysia
Central America
Some Caribbean regions
7.4 Species‑level threats
Southeast Asia: high species richness → low % threatened
Central America: only ~3 species → local loss = 100% species loss
7.5 Causes of mangrove decline
Aquaculture expansion (shrimp farming)
Land conversion (dikes, agriculture)
Urban development
Wood extraction (fuel, building materials)
Pollution (especially oil spills)
Oil blocks root pores → trees suffocate
Nigeria is a major example
Climate change
Sea‑level rise
Storm damage
Temperature stress
8. Why Mangrove Loss Matters
Loss of blue carbon storage → CO₂ released
Loss of coastal protection
Loss of biodiversity
Loss of fisheries habitat
Loss of livelihoods in developing countries
Loss of ecosystem services valued at billions globally
9. Summary
Mangroves are:
The most powerful blue carbon ecosystem
Exceptional at long‑term carbon storage
Threatened by human activity, especially in developing nations
Critical for climate mitigation, biodiversity, and coastal protection
Their carbon storage capacity depends on:
Forest age
Tree density
Species composition
Sediment trapping efficiency
Hydrodynamic conditions
Despite slowing deforestation rates, mangroves remain highly threatened, especially in regions with low species diversity and high development pressure.