IELTS Listening 實戰 10 海洋生物學演講（Part 4 風格）

Good afternoon. Today's lecture focuses on one of the most urgent ecological crises of our time — coral reef bleaching. By the end of this session, you should understand the biology of the phenomenon, the primary causes, and the range of responses being tried globally.

Let me begin with what a coral actually is. Many people assume corals are plants, but they are in fact animals — closely related to sea anemones and jellyfish. Each coral polyp is a small, soft-bodied creature that secretes a hard calcium carbonate skeleton, and it is these skeletons, accumulated over thousands of years, that form the reef structures we see.

Now, here's the crucial detail. Each polyp houses tiny algae — called zooxanthellae, z-o-o-x-a-n-t-h-e-l-l-a-e — living inside its tissues. The relationship is symbiotic: the algae provide the coral with up to ninety percent of its energy through photosynthesis; in return, the coral provides the algae with shelter and nutrients. Crucially, it is these algae that give coral its vivid colours.

When corals become stressed — particularly by elevated sea temperatures — they expel the algae. Without the algae, the coral loses both its colour, turning white or pale, and most of its food source. This is bleaching. A coral can survive bleaching if conditions return to normal within a few weeks, but if stress is prolonged, the coral starves and dies.

The main driver of bleaching is ocean warming. An increase of just one degree Celsius above the long-term summer average, sustained for four weeks, is enough to trigger mass bleaching. The 2016 event on the Great Barrier Reef killed an estimated thirty percent of corals there. Subsequent events in 2017, 2020, and 2022 have further damaged the reef.

A secondary driver is ocean acidification — as oceans absorb carbon dioxide, their pH drops, and it becomes harder for corals to build their calcium carbonate skeletons. There's also localised pressure — pollution from agriculture, especially nitrogen runoff, and sediment from coastal construction. These local stressors, while less severe than warming, compound it — a coral already stressed by sediment is more vulnerable to a temperature spike.

Let me turn now to responses. I'll cover four main strategies.

The first is marine protected areas, or MPAs — regions where fishing and extraction are banned or restricted. Well-enforced MPAs allow reef ecosystems to recover some resilience, though they do nothing to stop warming itself.

The second is coral gardening — essentially reef restoration. Fragments of healthy coral are grown in underwater nurseries, often on rope lines, and then transplanted back to damaged reefs. This is slow and expensive, but some projects in the Caribbean have restored thousands of square metres.

The third is more experimental: assisted evolution, sometimes called selective breeding. Scientists identify coral varieties that survived bleaching events and breed them, hoping to create strains with higher heat tolerance. Early results from projects in Hawaii and Australia are promising, but scaling this up is a major challenge.

And the fourth — perhaps most important but least glamorous — is climate action, reducing the global greenhouse gas emissions that drive ocean warming in the first place. Without this, the other three strategies are essentially buying time.

Before I close, one piece of context. Coral reefs cover less than one percent of the ocean floor, yet they support around a quarter of all marine species. Their loss would be catastrophic — not just ecologically, but for the three hundred million people worldwide who depend on reef fisheries and tourism for their livelihoods.

Next week we'll look in detail at one specific case study — the Chagos reef recovery project in the Indian Ocean. Please read chapter four of the course pack beforehand.

Questions 1-6 — Complete the notes. Write NO MORE THAN TWO WORDS AND/OR A NUMBER.

Coral Reef Bleaching — Lecture Notes

What is a coral?

• An animal, related to sea anemones and (1) _______
• Builds a (2) _______ skeleton over thousands of years

Zooxanthellae

• Provide coral with up to (3) _______ % of its energy
• Also give coral its (4) _______

Bleaching trigger

• A rise of just (5) _______ °C sustained for (6) _______ weeks

Questions 7-10 — Complete the summary. Write NO MORE THAN TWO WORDS.

Four responses are currently being tried. Marine protected areas restrict (7) _______ activities and allow ecosystems to recover. Coral gardening involves growing fragments in (8) _______ and transplanting them. Assisted evolution uses (9) _______ breeding to create heat-tolerant strains. Without global (10) _______ action, however, the other strategies only buy time.