Coral reefs: distribution, conditions, threats, management

Coastal Environments – Coral Reefs (Cambridge A‑Level Geography 9696)

Learning Objective

Explain the global distribution, the six core environmental conditions, the main threats and the range of management approaches for coral reefs, and evaluate their effectiveness using appropriate case‑study evidence.

1. Global Distribution of Coral Reefs

Coral reefs are confined to a narrow tropical‑subtropical latitudinal band (≈ 30° N – 30° S). Within this band sea‑surface temperatures (SST) remain between 20 °C and 30 °C and sufficient photosynthetically active radiation (PAR) penetrates the water column – both essential for the symbiotic zooxanthellae.

Why reefs are limited to 30° N – 30° S

• Temperature: Outside the band water is too cold for zooxanthellae photosynthesis or too warm, causing chronic bleaching.
• Light: High‑latitude waters receive insufficient PAR at depth.
• Seasonal stability: Minimal temperature and daylight variation support continuous growth.

2. Six Core Environmental Conditions (as in the syllabus)

3. Major Threats to Coral Reefs

3.1 Climate‑change related threats (syllabus expects four separate impacts)

• Sea‑surface temperature (SST) rise – Global mean SST has risen ~0.13 °C per decade (1981‑2020). Sustained warming forces corals to expel zooxanthellae, causing bleaching.
• Ocean acidification – Increased atmospheric CO₂ lowers seawater pH (projected drop of ~0.2 units by 2100) and reduces aragonite saturation (Ω_ar) by ~30 %, slowing CaCO₃ deposition.

  Key chemical series (syllabus expectation):

  $$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^- \rightleftharpoons 2\text{H}^+ + \text{CO}_3^{2-}$$

  Fewer CO₃²⁻ ions → lower Ω_ar → slower reef calcification.

• Sea‑level rise – Current global average rise ≈ 3.3 mm yr⁻¹. Deeper water reduces the amount of light reaching shallow reef flats, potentially limiting photosynthesis.
• Increased storm intensity – Climate models predict a ~5 % increase in the frequency of Category 4–5 cyclones. Stronger waves break reef framework and resuspend sediments, raising turbidity.

3.2 Local anthropogenic impacts

• Coastal development & land‑based runoff – Adds sediments and nutrients, raising turbidity and encouraging macro‑algal overgrowth.
• Over‑fishing – Removes herbivorous fish (e.g., parrotfish) that control algae, leading to phase shifts from coral‑dominated to algal‑dominated states.
• Destructive fishing methods – Blast fishing and cyanide poisoning directly kill corals and associated fauna.

3.3 Physical disturbance

• Tourism pressure – Trampling, anchoring, and reef‑side construction damage reef structure.
• Storm events (see climate‑change sub‑point) – Intensified storms increase physical breakage.

3.4 Pollution

• Plastic debris, oil spills, and chemical contaminants (e.g., pesticides) cause direct toxicity and smothering.

4. Management & Conservation Strategies

Effective management integrates **hard‑engineering**, **soft‑engineering**, **policy**, and **community‑based** actions.

Evaluation of Management

• Ecological indicators – % live coral cover, species richness, recruitment rates, aragonite saturation (Ω_ar).
• Socio‑economic indicators – tourism revenue, fishery yields, community participation rates.
• Compliance & enforcement – number of patrols, prosecutions for illegal activity, zoning‑compliance percentages.
• Adaptive management – regular monitoring feeds back into policy (e.g., GBRMPA’s 2022 “Reef 2050” plan integrates new bleaching thresholds).

Case‑Study Evaluation: Great Barrier Reef Marine Park (GBRMP)

Successes

• Protection of > 34 % of reef area through multiple zones.
• Long‑term monitoring shows stable fish biomass in no‑take zones.
• Water‑quality improvement programmes have cut nitrogen loads by ~30 % since 2000.

Ongoing challenges

• 2016/2017 mass bleaching – > 50 % of surveyed reefs bleached despite MPA status.
• Continued land‑based runoff from agriculture in the Fitzroy basin.
• Enforcement gaps in remote outer‑shelf zones.

Overall, the GBRMP demonstrates that MPAs can safeguard biodiversity, but without addressing climate‑change drivers and upstream land‑use, ecological resilience remains limited.

5. Quick Audit of the Notes Against the 9696 Syllabus (Topic 8.3 – Coral Reefs)

6. Summary Checklist for Exam Revision

• Identify the five main reef regions and the marginal‑zone example; explain why reefs are limited to 30° N – 30° S.
• Recall the six core environmental factors and be able to describe a cause‑and‑effect chain (e.g., low turbidity → high PAR → healthy zooxanthellae).
• Explain how rising SST causes bleaching and how ocean‑acidification chemistry reduces aragonite saturation and calcification.
• List at least three local threats and match each with a specific management response (e.g., over‑fishing → no‑take zones).
• Distinguish hard‑engineering (artificial reefs) from soft‑engineering (water‑quality improvement) and give an example of each.
• Describe community‑based management and its relevance to “diversity, equality and inclusion”.
• Know the key evaluation criteria for MPAs and be prepared to discuss the GBR case‑study (successes vs. remaining challenges).