Difficulty: Introductory | Prerequisites: Part 1 study notes (Coral Reef Ecosystem Structure), basic understanding of pH.
This is part two of two sets of study notes on Caribbean coral reefs. It covers the abiotic (non-living) factors that damage reef ecosystems: storms, rising ocean temperatures, ocean acidification, and human land-use changes such as logging and sewage discharge. You should already know the ten key reef species, the difference between producers and consumers, and how zooxanthellae power corals.
Coral reefs are hammered by three categories of abiotic stress: ocean conditions (stronger storms, rising temperatures, falling pH), land-use runoff (logging adds sediment, agriculture and sewage add nutrients), and disease. Each stressor hits corals directly and triggers cascading effects through the food web, with staghorn coral consistently the most vulnerable species.
Coral bleaching
The process by which corals expel their zooxanthellae under stress (usually high water temperature), turning white and losing their primary energy source. Think of it as the coral evicting its solar panels: the coral is still alive at first, but without energy it usually dies.
Ocean acidification
The process by which ocean water becomes more acidic as it absorbs increasing levels of carbon dioxide from the atmosphere. In simple terms, more CO2 in the air means more CO2 dissolved in the sea, which lowers the pH and makes it harder for corals and shellfish to build their skeletons.
pH
A scale measuring how acidic or basic a solution is. The scale runs 0 to 14: below 7 is acidic, above 7 is basic. Normal ocean water is slightly basic (around 8.1). As pH drops toward 7.6, corals struggle to build their calcium carbonate skeletons.
Sediment
Small dirt and soil particles that wash into the ocean from land, especially after logging or construction removes ground cover. Sediment clouds the water, blocking the sunlight corals need.
Nutrients (in this context)
Nitrogen and phosphorus compounds that enter coastal water from sewage, agriculture, or soil erosion. Low levels are normal; high levels cause algae blooms that outcompete corals for space and light.
Caribbean reefs are regularly hit by hurricanes. The two coral types in this model respond very differently:
Staghorn coral has a branching structure. Its branches snap easily in strong wave action, making it highly vulnerable to storm damage.
Boulder star coral has a massive, rounded shape. It is far more resistant to physical breakage.
When storm severity is set to 50% in the Gizmo, staghorn coral populations decline sharply while boulder star coral holds relatively steady. Sponges may increase because they face less competition for space.
Caribbean water temperatures normally range from 24 to 30 degrees Celsius. When temperatures rise above this range (e.g., to 33 degrees Celsius in the Gizmo), corals undergo bleaching.
The mechanism:
Heat stress causes corals to expel their zooxanthellae.
Without zooxanthellae, the coral loses its colour (turns white) and its main energy source.
The coral colony usually dies.
Staghorn coral is more sensitive to bleaching than boulder star coral. When corals die, the entire food web suffers: fish species drop from 442 to roughly 294 because coral provides both habitat and the structural foundation of the reef.
As atmospheric CO2 rises, the ocean absorbs more of it, lowering the pH (making the water more acidic). Normal ocean pH is around 8.1. At pH 7.6 in the Gizmo:
Staghorn coral populations decline dramatically.
Corals and other marine invertebrates need more energy to build their calcium carbonate exoskeletons in acidic water.
Fish species present on the reef drop significantly.
The underlying chemistry: corals build skeletons most efficiently when the water is slightly basic. Lower pH means less available carbonate for skeleton-building, which weakens existing structures and slows growth.
Logging removes the vegetation that anchors soil on land. Without root systems holding the earth in place, rain washes soil into rivers and out to sea.
At 50% logging in the Gizmo:
Sediment load rises from 5 ppt to 155 ppt.
Nutrient load rises from 4 ppt to 8 ppt.
Water clarity drops from 94% to 75%.
The reduced water clarity blocks sunlight from reaching zooxanthellae, weakening corals. Queen angelfish are hurt the most by logging. Algae benefit because they thrive in higher-nutrient water.
Sewage discharge sharply increases the nutrient load in coastal water. At 50% sewage in the Gizmo:
Sediment load: 18 ppt.
Nutrient load: 10 ppt.
Water clarity: 86%.
The primary effect of excess nutrients is algae overgrowth. Algae populations explode because they feed directly on dissolved nutrients. As algae spreads, it smothers corals and takes over reef surfaces.
The two algae-eating species, parrotfish and long-spined sea urchins, increase in response because their food supply grows.
In 1983, a disease wiped out most long-spined sea urchins across the Caribbean. In the Gizmo, setting sea urchin infection rate to 100% while sewage is at 50% shows the consequences:
Sea urchin populations collapse immediately.
Without sea urchins grazing algae, algae populations grow unchecked.
Corals are smothered and decline further.
This experiment demonstrates a keystone-species effect: the sea urchin's role in controlling algae is so important that losing it destabilises the entire reef. Parrotfish alone cannot compensate.
Staghorn coral has declined by over 90% since the 1970s and is now critically endangered. The reasons stack up:
Fragile branching structure breaks in storms.
High sensitivity to bleaching from warm water.
Vulnerable to acidification.
Competes poorly against algae when nutrient levels rise.
No single stressor is the cause. The combination of climate change, land-use runoff, and disease (removing the sea urchins that would otherwise keep algae in check) has been devastating.
Students often think coral bleaching means the coral is already dead. Bleaching is the loss of zooxanthellae; the coral is still alive initially but will die if conditions do not improve.
Students sometimes confuse pH direction. A lower pH number means more acidic, not more basic. Ocean acidification means pH is going down, not up.
Students sometimes think logging harms reefs because wood falls into the water. The real mechanism is that removing vegetation causes soil erosion, which sends sediment and nutrients into the ocean, reducing water clarity.
Students often assume that more nutrients are always good for an ecosystem. On a coral reef, excess nutrients fuel algae overgrowth that smothers corals. Corals thrive in low-nutrient, clear water.
⚠️ Know the mechanism of coral bleaching: heat stress > zooxanthellae expelled > coral loses colour and energy source > coral dies. This is a high-frequency exam question.
⚠️ Be able to explain why staghorn coral is more vulnerable than boulder star coral to both storms and bleaching.
⚠️ Understand ocean acidification: more atmospheric CO2 > ocean absorbs CO2 > pH drops > harder for corals to build calcium carbonate skeletons.
⚠️ Know the cascade from land-use change to reef decline: logging/sewage > sediment/nutrients in water > reduced clarity + algae overgrowth > coral decline > fewer fish species.
⚠️ The sea urchin die-off experiment is a classic keystone-species question. Be ready to explain why removing sea urchins leads to algae takeover and reef collapse.
⚠️ Remember the healthy reef baseline values and how they change under stress (e.g., water clarity drops from 94% to 75% with logging at 50%).
True or False: Coral bleaching means the coral has already died. Answer: False. Bleaching is the expulsion of zooxanthellae. The coral is alive but starving, and will die if conditions do not improve.
Fill in the blank: As atmospheric CO2 increases, ocean pH ________ (rises/falls). Answer: falls (the ocean becomes more acidic).
True or False: Logging harms coral reefs because wood physically damages corals. Answer: False. Logging removes vegetation that prevents erosion, sending sediment and nutrients into the ocean, which reduces water clarity.
Fill in the blank: The long-spined sea urchin's primary role on the reef is to control ________ populations. Answer: algae.
True or False: Staghorn coral is more resistant to storms than boulder star coral. Answer: False. Staghorn coral's branching structure is fragile and breaks easily. Boulder star coral's massive, rounded shape resists storm damage.
Q: Describe the process of coral bleaching. What causes it, and why does it usually kill the coral?
A: When water temperatures rise above the normal range (above roughly 30 degrees Celsius in the Caribbean), corals expel the zooxanthellae living in their tissue. This causes the coral to turn white (bleach). Because zooxanthellae provide the coral's main energy source through photosynthesis, losing them means the coral starves. Unless temperatures drop and the zooxanthellae return, the colony dies.
Q: Explain how ocean acidification damages coral reefs. Include the role of carbon dioxide in your answer.
A: Rising atmospheric CO2 is absorbed by ocean water, forming carbonic acid and lowering the pH. At lower pH, corals and other marine invertebrates must spend more energy to build their calcium carbonate exoskeletons. Growth slows, existing structures weaken, and coral populations decline. In the Gizmo, reducing pH to 7.6 caused a sharp drop in staghorn coral and a reduction in fish species from 442 to 294.
Q: How does logging on land affect a coral reef that may be many kilometres away?
A: Logging removes vegetation whose roots anchor soil. Without that ground cover, rain erodes soil into rivers, which carry sediment and nutrients to the coast. The sediment clouds the water, blocking sunlight that zooxanthellae need for photosynthesis. The nutrients fuel algae growth, which outcompetes coral for reef space.
Q: Why are long-spined sea urchins considered a keystone species on Caribbean reefs?
A: Sea urchins are the primary grazers of algae on the reef. When sea urchins were wiped out by disease in 1983, algae populations exploded and smothered corals, because parrotfish alone could not eat enough algae to compensate. The loss of one species cascaded through the entire food web.
Q: Give at least three reasons why staghorn coral is more vulnerable than boulder star coral to environmental stress.
A: (1) Its branching structure snaps easily in storms, while boulder star coral's rounded shape resists breakage. (2) Staghorn is more sensitive to temperature-driven bleaching. (3) Staghorn coral has declined more sharply under ocean acidification. (4) When nutrients rise and algae spreads, staghorn's branching structure is more easily overgrown.
This material connects back to Part 1 (Coral Reef Ecosystem Structure). The food web from Part 1 explains why losing one species (such as sea urchins) cascades through the entire system here.
Coral bleaching and ocean acidification connect to climate science and the carbon cycle. Understanding how atmospheric CO2 dissolves in seawater is relevant to chemistry (acid-base equilibria) and Earth science courses.
The keystone-species concept demonstrated by the sea urchin die-off appears in general ecology, where students study trophic cascades, and in conservation biology, where it informs prioritisation of species protection.
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