Coral Reef Ecology — Comprehensive Exam Notes

Upcoming Field-Trip & Logistical Reminders

• Weekend soft-sediment field exercise (Tupira Point & Pilot Bay)

  • For students driving to Pilot Bay: meet 12:00 noon sharp at Pilot Bay Boat Ramp (time moved 1 h later after re-checking tides).

  • For students nearer Tupira Point: arrive 11:00 AM to start as tide drops.

• Footwear & clothing

  • You will be standing ankle–mid-calf in water; feet inevitably wet.

  • Options: gumboots (can flood), wetsuit boots, sports shorts, roll-up trousers, quick-dry fabrics.

  • Bring spare clothes; bus has ample storage.

• Resources to print & bring

  • Fieldwork protocol + soft-sediment ID guides posted on Moodle.

Focus of Today’s Lecture

• Coral reef ecology, especially reef-building corals (hermatypic Scleractinians).

• Reef structure, biodiversity, interspecific interactions.

• Current threats (bleaching, predators, climate, run-off) & recovery dynamics.

• Role of Marine Protected Areas (MPAs) as a positive case study.

Global Distribution & “Coral Triangle” Hotspot

• Reef builders restricted to tropical 20 ° isotherm band: \approx\,23–29^\circ\,\text{C} surface water.

• Found in oligotrophic, sunlit, shallow (< 30 m) zones of Indian & Pacific Oceans.

• Coral Triangle (Malaysia → Indonesia → PNG → Solomon Is. → Philippines):

  • > 600 of \ge 800 known reef-building species occur here.

  • Nicknamed “marine Amazon” – extreme biodiversity within nutrient-poor ‘desert’ seas.

Reef Morphology & Darwin’s Volcanic Model

• Volcanic island emergent → cooling & subsidence produces:

  1. Fringing reef: coral attached to shore.

  2. Barrier reef: lagoon separates reef from sinking island.

  3. Atoll: island fully submerged; reef ring surrounds shallow lagoon; lagoon infilled by biogenic sand.

• Cross-section zonation

  • Fore-reef (seaward slope): high-energy waves; massive, robust corals (e.g. brain coral).

  • Reef crest: greatest wave impact; high recruitment.

  • Back-reef & lagoon: sheltered; branching, delicate forms; patch reefs.

Calcification & Growth Rates

• All key taxa (corals + coralline algae) precipitate CaCO₃ (calcite or aragonite) skeletons.

• Reef accretion both upward (toward light) & seaward.

• Growth relatively slow → vulnerable to sea-level rise out-pacing accretion.

Taxonomic Categories

• Hermatypic (reef builders): Order Scleractinia; rigid stony skeleton.

• Ahermatypic (soft corals): lack massive skeleton; solitary or colonial; present on reefs but do not build framework.

Coral Polyp Anatomy & Symbiosis with Zooxanthellae

• Polyp = mini-anemone inside skeletal cup.

• Zooxanthellae (symbiotic dinoflagellate algae) reside in gastrodermis.

• Mutualistic nutrient loop

  • Coral respiration provides \text{CO}2 + \text{H}2\text{O} → captured for photosynthesis.

  • Photosynthate (glucose, glycerol, amino acids) → \approx 90\% translocated to host; fuels tissue growth & CaCO₃ deposition.

  • Algae gain protection, stable light habitat, metabolic waste nutrients.

• Explains paradox of high productivity (up to 5000\ \text{g C m}^{-2}\text{ yr}^{-1}) in nutrient-poor water.

Reproduction Strategies

• Asexual: budding → clonal colony growth.

• Sexual: mass synchronous broadcast spawning (usually night; lunar-timed).

  • Reliant on precise environmental cues: seasonal temperature trend, photoperiod, lunar phase.

  • Ensures gamete encounter in sessile organisms, facilitates long-distance larval dispersal.

Environmental Requirements & Stressors (Goldilocks Zone)

• Light: must remain in photic zone (< 30 m, turbidity-dependent).

• Temperature: 23–29^\circ\,\text{C} typical, some Red Sea strains tolerate \ge 34^\circ\,\text{C}.

• Salinity: 32–40 ‰.

• Moderate water movement (gas exchange, nutrient delivery, larval supply).

• Intolerance to:

  • Prolonged emersion during low tides.

  • High sedimentation (clogs polyps + reduces light).

  • Rapid sea-level rise, ocean warming, acidification.

Competition for Space – “Silent Warfare”

• Limited suitable substrate → intense interspecific & intraspecific competition.

• Defensive/offensive mechanisms

  • Digestive filaments: corals extrude mesenterial filaments to digest neighbour tissue.

  • Sweeper tentacles: long nematocyst-laden extensions harpoon rivals.

  • Mucus toxins, over-topping growth, allelopathic chemicals.

• Long-term outcome: patch mosaics; evolutionary trade-off between aggression & growth form.

Reef-Associated Fauna & Functional Roles

• Fish diversity far exceeds structural expectation; examples per reef region:

  • GBR (~1625 spp), Fiji (~1000 spp), Philippines (~1900 spp).

• Herbivores (parrotfish, surgeonfish) “lawn-mow” fleshy algae → prevent algal overgrowth.

• Predators (wrasses, triggerfish) control invertebrate coral eaters.

• Loss of key functional groups destabilises coral–algae balance.

Natural & Anthropogenic Disturbances

Crown-of-Thorns Starfish (COTS)

• Voracious coral predator; episodic population explosions → whole-reef devastation.

• Drivers of outbreaks

  • Removal of natural predators (bumphead wrasse, triton snail, lobsters) via overfishing/collecting.

  • Enhanced larval survival from nutrient run-off + warmer water.

• Management tools

  • Diver injection programs (must avoid stabbing; induces spawning).

  • eDNA surveillance for early detection; ROVs with AI recognition + toxin injection.

ENSO Event (1982–83) Western Pacific Case Study

• El Niño lowered sea level, raised SST by \sim2–3^\circ\,\text{C}, increased storminess.

• Up to 90 % coral mortality; subsequent urchin boom delayed recovery (decades).

Coral Bleaching

• Heat stress → expulsion of zooxanthellae → white skeleton visible.

• Analogy: heat waves act like “underwater forest fires.”

• Filmed in documentary Chasing Coral; current events on Ningaloo Reef (news 2023/24).

Jamaican Phase Shift Example

• Late 1980s: overfishing removed ~80\% of reef fish biomass.

• Hurricanes + coral diseases + nutrient influx.

• Coral cover dropped from \sim52\% → \sim5\%; algae now ~90\% cover.

• Represents shift to new stable algal-dominated state; recruitment window for corals closed.

Multiple Stressor Framework & Red Sea Core Study

• Stressors operate at different scales:

  • Global: warming, acidification, cyclone frequency.

  • Local: overfishing, sediment, nutrient run-off.

• Layering pushes reefs past resilience threshold → irreversible phase shift.

• Red Sea Porites drill-core analysis

  • Chronology back to 1920; stress bands (dark CT bands) mark disturbance years.

  • 2015: universal stress bands despite not being hottest year.

  • Massive nitrogen run-off combined with elevated SST (sub-threshold) produced bleaching.

• Key takeaway: local water-quality management can mitigate global heat stress impacts.

Marine Protected Areas (MPAs) – Evidence of Success

• Five criteria for effective MPA (Edgar et al. 2014):

  1. No-take fishing ban.

  2. Enforcement effective (not paper parks).

  3. > 10 yrs old (ecological maturation).

  4. > 100 km² size.

  5. Isolation from direct land influence.

• Outcomes compared to fished sites:

  • Target fish biomass ↑ dramatically, followed by non-target species.

  • Crown-of-thorns outbreaks ↓ frequency.

  • Coral cover % significantly higher within reserves.

• GBR zoning plan (AIMS) – exemplar but faces sociopolitical challenges from fisheries sector.

Ethical, Socio-Economic & Practical Implications

• Reefs supply food security (fishery nursery grounds), shoreline protection, tourism revenue, pharmaceutical bio-prospects.

• Loss undermines economies of many tropical nations; cultural heritage at risk.

• Management must blend climate action (mitigation), watershed practices (run-off control), and ecosystem-based fisheries regulations.

Key Numerical & Formulaic References

• Thermal tolerance: 23–29^\circ\,\text{C}\;(common); stress threshold \approx32^\circ\,\text{C} (Red Sea case).

• Salinity requirement: 32–40\ \text{‰}.

• Primary productivity peak: \le 5000\;\text{g C m}^{-2}\text{ yr}^{-1}.

• Coral Triangle species count: >600/800 spp.

Study & Exam Check-List

• Explain Darwin’s fringing → barrier → atoll succession.

• Compare hermatypic vs aher­matypic corals (skeleton, symbiosis, habitat role).

• Diagram coral polyp + zooxanthellae nutrient cycling.

• Discuss Goldilocks constraints (light, T°, salinity, turbidity) and how climate change alters each.

• Outline mechanisms and ecological consequences of coral bleaching.

• Evaluate effectiveness & limitations of COTS management options.

• Trace Jamaican reef phase shift as a case study of compounded stressors.

• Interpret stress-band core data: what environmental reconstructions are possible?

• List 5 attributes of successful MPAs and relate them to GBR outcomes.

Recommended Reading

• Barnes & Hughes “An Introduction to Marine Ecology” – Coral chapters.

• Nybakken & Bertness “Marine Biology: An Ecological Approach.”

• Primary papers cited in lecture (full bibliography on final slide of original PPT).