Coral Reef Abiotic Factors: Sunlight, Temperature, Salinity, and the Conditions That Shape a Reef

Short answer

• The abiotic factors for coral reefs are the nonliving physical and chemical conditions that decide where reefs can form and how well they grow. The main factors are sunlight, water temperature, depth, salinity, water movement from currents and waves, the hardness of the surface corals settle on, nutrient levels, water clarity, the chemistry of the water including pH, and dissolved oxygen. Reef building corals need warm, clear, shallow, sunlit seawater of normal salinity, with low nutrients and a hard surface to build on. The National Oceanic and Atmospheric Administration (NOAA) describes temperature and light as the primary factors that limit coral reef growth, and notes that reefs are restricted to warm tropical and subtropical seas with low nutrient levels. Because corals tolerate only a narrow range of each factor, a small change in one, such as water warming by a single degree above the usual summer high, can stress a whole reef. The factors also interact: depth controls light, sediment controls clarity, and currents control both nutrient delivery and waste removal, so no single factor explains a reef on its own.

What Abiotic Factors Are

In ecology, abiotic means nonliving. An abiotic factor is any nonliving physical or chemical part of an environment. On a coral reef, the abiotic factors are sunlight, water temperature, salinity, depth, currents, waves, the substrate or seafloor surface, nutrient concentrations, water clarity, pH, and dissolved oxygen. These are the conditions that living things must cope with. They are distinct from biotic factors, which are the living parts of the reef: the corals, algae, fish, invertebrates, and microbes. The living side of the reef, including the partnership between coral and its symbiotic algae, is covered in our companion article on biotic factors for coral reefs.

The distinction matters because the two sides do different jobs. Abiotic factors set the limits. A reef cannot form where the water is too cold, too dark, too fresh, or too cloudy, no matter how many larvae arrive. Biotic factors then decide what happens inside those limits: which corals grow fastest, which grazers keep algae in check, and which predators shape the community. A coral polyp is a living animal, but the sunlight that feeds the algae inside it is abiotic. A wave is abiotic, but the coral it breaks is biotic. Keeping the categories separate stops a common error: treating fishing, pollution, or coastal development as abiotic factors. Those are human pressures. Their effects show up by changing abiotic conditions, for example by adding sediment that clouds the water, or by changing biotic factors, for example by removing grazers, but the pressure itself belongs in neither category.

Abiotic factors also explain why reefs are found in some seas and not others. NOAA notes that about 25 percent of all marine species are found in, on, and around coral reefs, yet reefs themselves occupy only a small share of the ocean. That concentration of life exists only where the abiotic conditions line up: warm water, clear water, sunlight, and a hard surface, all at once, in the same place, for long enough for slow growing corals to build a reef. Change one factor and the reef changes with it.

Sunlight: The Energy That Sets the Rules

Sunlight is the first abiotic factor to understand, because it drives almost everything else on a shallow reef. Reef building corals depend on microscopic photosynthetic algae called zooxanthellae that live inside coral tissue. The algae capture sunlight and convert it into energy through photosynthesis, then pass much of that energy to the coral. NOAA’s Ocean Service explains that corals provide the algae with a protected environment, while the algae supply the coral with the products of photosynthesis. NOAA states that as much as 90 percent of the organic material photosynthetically produced by the zooxanthellae is transferred to the host coral tissue, and calls this transfer the driving force behind the growth and productivity of coral reefs. Note the scope of that figure: it describes transfer from algae to the coral host, not energy reaching every animal on the reef.

Because the algae need light, reef building corals need clear water that lets sunlight through. NOAA’s Ocean Service notes that reef building corals require clear water so that sunlight can reach their zooxanthellae, and that for this reason they are generally found only in waters with small amounts of suspended material, or water of low turbidity and low productivity. Cloudy water, whether from sediment, plankton blooms, or pollution, cuts the light supply and starves the partnership at the heart of the reef.

Sunlight also explains the shape of a reef. Corals grow fastest where light is strongest, near the surface, and growth slows as light fades with depth. A reef slope is therefore not uniform: the shallowest corals often grow in flat, spreading forms that catch maximum light, while deeper corals may grow in flatter or more open shapes that make the most of weaker light. The same species can look different at different depths, because light, not genetics alone, shapes its growth form. Light is also seasonal. In winter, when days are shorter and the sun sits lower, photosynthesis slows and coral growth can slow with it. A reef is a solar powered system, and its calendar follows the sun.

Water Temperature: Warm, But Only Within Limits

Water temperature is the second primary limiting factor NOAA names, and it is the one with the narrowest margin for error. Reef building corals need warm water, but only within a tight band. Figures for that band differ between sources, so this article follows one authority throughout. NOAA’s Coral Health and Monitoring Program states that reef building corals are restricted to a seawater temperature range of 18 to 36 degrees Celsius, and that optimal reef growth occurs between 26 and 28 degrees Celsius. Other study materials often quote narrower figures, such as 23 to 29 degrees Celsius, which describe typical reef conditions rather than the full range corals can tolerate. The NOAA figures are used here as the governing range, and every temperature statement in this article follows them.

The lower end of the range is a hard limit. NOAA’s Coral Reef Watch program explains that most tropical or subtropical corals cannot grow in oceans where the water temperature dips below 18 degrees Celsius (64 degrees Fahrenheit) for extended periods in winter. That is why reef building corals are absent from cold seas, even where light and hard surfaces are plentiful. Below that threshold, reef building slows and largely stops.

The upper end is more dangerous, because the margin above normal is tiny. NOAA’s Coral Reef Watch program explains that corals start getting stressed if the water gets only 1 degree Celsius warmer than the highest temperature expected in summertime, a level called the bleaching threshold. At that point, stressed corals expel their zooxanthellae, losing both their color and a major food source, in the process called coral bleaching. Bleached corals are not yet dead, and they can recover if temperatures fall quickly, but prolonged heat can kill them. NOAA measures accumulated heat stress in degree heating weeks, and reports that significant bleaching can be expected when heat stress reaches 4 degree heating weeks, while widespread bleaching with mortality of heat sensitive corals is likely at 8 degree heating weeks or higher.

Temperature also controls the pace of reef life. Within the tolerated range, warmer water generally supports faster coral growth and faster metabolism in reef animals, which is one reason tropical reefs are so productive. But the same warmth that speeds growth also speeds stress when it overshoots. A reef that has lived for decades at 27 degrees Celsius may bleach at 29 degrees, a change a swimmer would barely notice. That sensitivity is why sea surface temperature is the most closely watched abiotic factor on reefs worldwide, and why the animals on the coral reef depend so heavily on conditions they cannot control.

Depth: How Deep the Light Reaches

Depth matters mainly because it controls light. NOAA’s Coral Health and Monitoring Program explains that light intensity decreases exponentially with depth in seawater, and that coral symbiont photosynthesis and coral calcification fall with it. The program states that coral reefs are therefore limited to depths of less than 20 meters, assuming water clarity is satisfactory, because below that depth light intensity is not sufficient to promote the growth rates needed for reef development. Other guides cite deeper outer limits, up to 50 or 70 meters, for reef building corals. The difference is one of definition: corals can occur deeper, but the fast growth that builds a reef framework needs the bright light of the shallows. This article follows the NOAA framing, which ties depth to the growth rate a reef needs, not merely to coral presence.

The depth limit is not fixed. In exceptionally clear water, usable light reaches deeper, and reef development can extend further down. In murky water, the limit rises toward the surface. Depth therefore works together with clarity: a reef in clear oceanic water may grow deeper than a reef near a muddy river mouth, even at the same latitude. Divers notice this directly. The brightest, most densely coral covered slopes are usually in the top 10 to 20 meters, while deeper slopes hold fewer, more widely spaced corals.

Below the sunlit zone, corals do not disappear. They change strategy. The Smithsonian Ocean program explains that deep sea corals live where light is absent, at around 200 meters and deeper, and in waters as cold as minus 1 degree Celsius. Deep sea corals have no zooxanthellae at all and must catch all their own food, using tentacles armed with stinging cells to capture drifting particles called marine snow. Some form reefs hundreds of miles long. They prove that corals as animals can live almost anywhere in the ocean, while reef building corals, the kind that form the tropical reefs people visit, are confined to the warm, bright shallows by their dependence on sunlight.

Salinity: Salt Water, in Balance

Salinity is the salt concentration of seawater, usually measured in parts per thousand. Normal ocean salinity is about 35 parts per thousand, meaning about 35 grams of dissolved salt per kilogram of seawater. Reef building corals need seawater close to that normal level, and they are sensitive to change. Figures for the tolerated range vary between sources, so this article again follows one authority. NOAA’s Coral Health and Monitoring Program states that optimal reef building coral calcification occurs at normal oceanic salinities of 33 to 36 parts per thousand. Broader tolerance figures, such as 32 to 42 parts per thousand, appear in study materials and describe the wider range corals may survive, not the range where they build reef best. The NOAA figures are used here as the governing range.

Corals are marine animals, and their cells are adapted to a stable salt balance. When salinity falls, for example after heavy rain or where a river pours fresh water into the sea, corals must spend energy maintaining their internal salt balance, and growth slows. Prolonged exposure to fresh or brackish water can kill them. That is why reefs are rare near large river mouths, even in the tropics, and why flood plumes from coastal storms can damage inshore reefs. When salinity rises, for example in a shallow lagoon where evaporation concentrates salt under a hot sun, corals face the opposite stress, and again growth suffers.

Salinity is one of the most stable abiotic factors on an open ocean reef, which is part of why reefs do well far from shore. It changes most where land meets sea: estuaries, river mouths, and coastal lagoons after rain. A reef’s salinity story is therefore often a story about its coastline. Reefs near undisturbed coasts with little runoff enjoy steady salinity. Reefs near cleared land, farms, or cities may face repeated pulses of fresh, sediment laden water after storms, each one a short stress event that adds up over a season.

Water Movement: Currents and Waves

Water movement, from currents and waves, is an abiotic factor that works both for and against a reef. NOAA’s Coral Health and Monitoring Program notes that coral reefs do best in areas with moderate and continuous wave activity, which is necessary to wash vital nutrients across the reef and to carry away waste. Currents deliver food, oxygen, and coral larvae, and they remove sediment, mucus, and waste products that would otherwise smother corals. A reef in gently moving water is fed and cleaned at the same time.

Currents also connect reefs to each other. Coral larvae drift on currents for days or weeks before settling, so the current pattern decides which reefs can reseed which neighbors after damage. A reef downstream of a healthy reef may recover faster than an isolated reef, because larvae arrive on the current. Fish larvae travel the same way, which is one reason the food chain coral reefs support is tied to water movement as well as to light and temperature. The animals that eat, and are eaten, on a reef arrive and depart on moving water.

Too much movement, however, is destructive. Storm waves can break branching corals, scour reef surfaces, and bury corals under shifting sand. Cyclones and hurricanes are the most dramatic example: a single storm can flatten decades of coral growth on an exposed reef slope, while a sheltered lagoon reef nearby escapes with minor damage. The reef’s shape reflects this balance. Exposed reef fronts often hold sturdy, massive corals that resist waves, while sheltered lagoons hold delicate branching corals that would shatter on the front. NOAA also notes that healthy reefs can absorb up to 97 percent of a wave’s energy, buffering shorelines from currents, waves, and storms. The reef protects the coast, but only while the reef itself survives the waves.

Water movement also interacts with temperature. Currents can bring cooler water from deeper layers up onto a reef, easing heat stress during a warm spell. Conversely, still water during a calm, hot period lets surface temperatures climb, raising bleaching risk. That is why two reefs in the same region can bleach differently in the same heatwave: the one with better water flow may escape, while the one in still water bleaches.

Substrate: A Hard Place to Start

Substrate is the surface on which a reef grows, and for corals it must be hard. Coral larvae, called planulae, drift in the water until they find a suitable surface, settle on it, and begin to secrete a calcium carbonate skeleton. They need a firm, stable surface: rock, dead coral skeleton, or other hard material. Soft sand or loose mud will not do, because a young coral anchored in shifting sediment is easily buried or washed away. The first corals on a new site therefore colonize rock or old reef, and each generation builds on the skeletons of the last. Over centuries, that accumulation becomes the reef framework itself.

Substrate also shapes where reefs can begin. A rocky coastline or a submerged limestone platform is a good starting point. A broad sandy plain is not, even if the water above it is warm, clear, and sunlit. This is one reason reefs are patchy: the right water conditions may stretch for hundreds of kilometers, but reefs form only where a hard foundation exists within those waters. Once a reef is established, its own dead skeleton becomes new substrate, so the reef can grow outward and upward as long as conditions stay favorable.

The quality of the substrate matters too. A clean, bare rock surface invites settlement. A surface coated in thick algae, sediment, or slime may repel larvae or prevent them from attaching firmly. Grazers help here: by scraping algae from reef surfaces, fish and sea urchins keep patches of clean rock open for new corals to settle. In that way, a biotic factor, grazing, maintains the abiotic factor, available substrate. The two sides of the reef system are inseparable, and substrate is one of the clearest places where they meet.

Nutrients: The Paradox of Poor Water

Coral reefs present a puzzle: they are among the most productive ecosystems on Earth, yet they grow in some of the most nutrient poor waters in the ocean. NOAA’s Coral Health and Monitoring Program states that reef building corals are limited to oligotrophic, or low nutrient, subtropical and tropical seas, and that optimal calcification occurs in low nutrient level water. The explanation lies with the zooxanthellae. The Smithsonian Ocean program explains that the symbiosis between coral and its algae is what allows reef building corals to thrive even in waters that are nutrient poor, because the algae supply nutrients through photosynthesis. The reef does not need rich water, because it makes its own food from sunlight.

That does not mean nutrients are irrelevant. Corals and their algae need small amounts of nitrogen, phosphorus, and other nutrients, and currents deliver them in a slow, steady supply. The reef is efficient at recycling: waste from one organism becomes food for another, and the same nutrients pass through many bodies before leaving the system. What harms reefs is excess nutrients, not the modest amounts they need. Runoff carrying fertilizer from farms, sewage from towns, and eroded soil from cleared land can flood a reef with nitrogen and phosphorus. The extra nutrients feed fast growing algae, which overgrow corals, shade their zooxanthellae, and block new larvae from settling. The balance tips from coral dominated to algae dominated, and the reef changes character.

Research on the Great Barrier Reef shows how sharp that tipping point can be. A large study by the Australian Institute of Marine Science, using data from 150 reefs and more than 2,000 water quality stations, found critical water quality thresholds of 10 meters for Secchi disk depth, a measure of water clarity, and 0.45 micrograms per liter of chlorophyll, a stand in for nutrient levels. Beyond those thresholds, seaweed cover increased rapidly and coral diversity fell. The study found that seaweed cover showed the strongest response to poor water quality, increasing fivefold with declining water clarity. It also found that the water on 22 percent of reefs, about 647 reefs, did not meet water quality guidelines, and predicted that improving water quality in those areas would reduce seaweed by 39 percent and increase the number of coral species by 13 percent. Nutrients are therefore an abiotic factor with a narrow safe band: too little and the reef starves, too much and algae take over, and the reef needs the low, steady middle.

Water Clarity and Sediment

Water clarity is closely tied to nutrients, but it deserves its own place, because it acts directly on light. Clear water lets sunlight reach corals. Turbid water, clouded with suspended sediment, plankton, or pollutants, blocks light before it reaches the reef. Even a thin layer of settled sediment on a coral can smother it, blocking light to its zooxanthellae and forcing the polyp to spend energy clearing itself with mucus and ciliary movement. Repeated sedimentation weakens corals, slows growth, and makes them more vulnerable to disease and bleaching.

Sediment reaches reefs from land and from the sea. Rivers carry soil eroded from cleared farmland, construction sites, and deforested hillsides. Coastal dredging stirs up the seafloor. Storms resuspend sand and mud already on the bottom. Inshore reefs near river mouths live with chronic turbidity, while offshore reefs in deep oceanic water enjoy the clearest conditions. That gradient helps explain why the healthiest, most coral dominated reefs are often the furthest from shore, even though inshore reefs sit in equally warm water.

The same AIMS study cited above treats clarity as a decisive factor: when Secchi disk depth, the depth at which a white disk disappears from view, fell below 10 meters, seaweed cover rose sharply and coral diversity fell. Clarity is therefore not a cosmetic concern for divers. It is a working abiotic factor that decides how much light reaches the reef, how much algae grows, and how much coral can build. Protecting water clarity, by keeping soil and nutrients on land, is one of the most direct ways to protect the light supply a reef depends on.

pH and Ocean Chemistry

The chemistry of seawater, especially its pH and carbonate content, is an abiotic factor that decides whether corals can build their skeletons. Corals build from calcium carbonate, in the form called aragonite, which they extract from seawater. When seawater chemistry favors aragonite formation, corals build easily. When it does not, building becomes harder, growth slows, and existing skeletons can weaken.

NOAA’s Ocean Acidification Program explains the key measure, aragonite saturation state. When the saturation state is above 1, aragonite can form or stay intact. When it falls below 1, aragonite will dissolve and become less available for building shells and skeletons. NOAA’s Science on a Sphere program adds that corals and other calcifiers are more likely to survive and reproduce when the saturation state is greater than 3, and become stressed when it falls below 3. Seawater absorbs carbon dioxide from the atmosphere, and more carbon dioxide in the water lowers pH and lowers carbonate availability. That process, ocean acidification, therefore shifts the chemistry against coral building.

The scale of the change is already measurable. NOAA Research reports an international study, published in the journal Global Change Biology, which found that ocean acidification has significantly compromised 40 percent of the global surface ocean and 60 percent of the subsurface ocean to a depth of 200 meters, measured by declines in aragonite saturation state. The same study estimated that some tropical and subtropical coral reefs have lost 43 percent of their suitable habitats. Those are study findings reported by NOAA, not a prediction about any single reef, and conditions vary by location. But they show that pH is not a background detail. It is a changing abiotic factor, and its direction, toward lower pH and lower carbonate availability, makes reef building harder.

Local chemistry varies too. In lagoons and on reef flats, photosynthesis during the day can raise pH slightly as algae consume carbon dioxide, while respiration at night lowers it. Corals therefore experience daily chemical swings, and healthy reefs have long coped with them. The concern is the long term shift in the baseline, which narrows the margin corals have to work with.

Dissolved Oxygen

Dissolved oxygen is the oxygen gas dissolved in seawater, and reef animals need it to breathe. Fish extract it through gills, and coral polyps and other invertebrates absorb it from the water around them. Reefs are usually well oxygenated, because waves and currents mix air into the water and photosynthesis by algae and seagrasses releases oxygen during the day. At night, when photosynthesis stops but respiration continues, oxygen levels can dip, especially in sheltered lagoons with little water movement.

Oxygen interacts with temperature in a way that matters for reef health. Warm water holds less dissolved oxygen than cold water, so a reef during a heatwave faces two stresses at once: higher temperature and lower oxygen. Low oxygen can also follow algal blooms, when the decay of dead algae consumes oxygen faster than it is replaced. A reef with good water movement rarely faces serious oxygen shortage, because currents constantly renew the supply. A still, warm, nutrient rich lagoon is the setting where oxygen stress is most likely. Dissolved oxygen is therefore best understood as a supporting factor: rarely the main limit on a healthy reef, but a real stressor when heat, still water, and excess nutrients combine.

How the Factors Work Together

No abiotic factor acts alone, and the interactions are often what decide a reef’s fate. Light and depth are linked, because depth controls how much light reaches a coral. Temperature and oxygen are linked, because warm water holds less oxygen. Nutrients and clarity are linked, because excess nutrients grow plankton and algae that cloud the water. Salinity and sediment are linked, because the same river flood that freshens the water also delivers the sediment that blocks light. Currents touch all of them, delivering nutrients, larvae, and oxygen, and removing waste and sediment.

A single event can pull several factors at once. A tropical cyclone brings heavy rain that lowers salinity, floods that carry sediment and nutrients, waves that break corals, and clouds that cut light, all in the same week. A marine heatwave brings high temperatures that stress corals, calm seas that let surface water warm further, and low oxygen as warm water holds less gas. A bleaching event is therefore never only a temperature story, even when temperature is the trigger. The reef that survives is usually the one where the other factors are in its favor: good water flow, clear water, normal salinity, and a healthy grazer community keeping algae in check.

The same logic applies in reverse. Improving one factor can ease pressure from another. Clearer water lets more light reach deeper corals, partly offsetting the light lost to depth. Better water flow brings cooler water during a heatwave, partly offsetting high temperature. Stable salinity after a dry season lets corals recover from a flood pulse. Reef management therefore works on the factors people can influence, mainly water quality and local pressures, even though no one can control the temperature of the open ocean.

Where Reefs Can Grow, and Where They Cannot

Put the factors together and the map of the world’s reefs makes sense. Reef building corals need warm water between roughly 18 and 36 degrees Celsius, clear sunlit shallows less than about 20 meters deep for fast reef growth, normal ocean salinity, low nutrients, and a hard surface. Those conditions occur together mainly in tropical and subtropical seas, which is why reefs cluster in the warm belt around the equator and are absent from cold, dark, or muddy coasts at the same latitude. Reefs are also absent where large rivers pour fresh, sediment laden water into the sea, and where cold currents bring water below 18 degrees Celsius.

The Great Barrier Reef shows both the pattern and its limits. The Great Barrier Reef Marine Park Authority, in its Outlook Report, records that average sea surface temperatures in the region have risen by 0.94 degrees Celsius since 1900, and that mass coral bleaching events occurred in 2016, 2017, 2020, 2022, and 2024. During the 2024 event, sea surface temperatures peaked at 2.5 degrees Celsius above historical summer maximums in the southern region, and extreme bleaching, where more than 90 percent of corals on a reef are affected, was recorded in all three regions of the marine park for the first time. The 2024 event was the seventh recorded mass bleaching event for the reef. Those figures describe one reef system, not all reefs, but they show what happens when the temperature factor moves outside the narrow band corals tolerate, even on the world’s largest reef system. The seasonal weather that shapes those temperatures is covered in our companion article on the weather of the Great Barrier Reef.

The same map logic explains why reefs matter beyond their own boundaries. NOAA notes that healthy reefs can absorb up to 97 percent of a wave’s energy, protecting shorelines and the communities behind them. A reef is therefore not only a habitat shaped by abiotic factors. It is also a structure that changes the abiotic conditions around it, calming waves, trapping sediment, and sheltering lagoons. The factors shape the reef, and the reef shapes its surroundings in return.

Frequently Asked Questions

What are the abiotic factors of a coral reef?

The abiotic factors of a coral reef are the nonliving physical and chemical conditions that affect it: sunlight, water temperature, depth, salinity, water movement from currents and waves, substrate, nutrient levels, water clarity, pH, and dissolved oxygen. They are distinct from biotic factors, which are the living organisms on the reef. The main factors are light and temperature, which NOAA describes as the primary limits on reef growth, together with clear shallow water, normal salinity, low nutrients, and a hard surface to build on.

What temperature do coral reefs need?

Following NOAA’s Coral Health and Monitoring Program, reef building corals are restricted to seawater temperatures of 18 to 36 degrees Celsius, with optimal growth between 26 and 28 degrees Celsius. Most corals cannot grow where water dips below 18 degrees Celsius for extended periods in winter. At the warm end, NOAA’s Coral Reef Watch program sets the bleaching threshold at 1 degree Celsius above the highest summertime mean temperature. Other sources quote narrower ranges, such as 23 to 29 degrees Celsius, which describe typical reef conditions rather than the full tolerated range.

Why do coral reefs need sunlight?

Coral reefs need sunlight because reef building corals depend on zooxanthellae, microscopic photosynthetic algae living inside coral tissue. The algae use sunlight to make food through photosynthesis and pass much of it to the coral. NOAA states that as much as 90 percent of the organic material the algae produce is transferred to the host coral. Without enough light, the algae cannot feed the coral, growth slows, and the reef cannot build. That is why reef building corals live in clear, shallow water, and why sediment or plankton blooms that block light harm reefs.

Why do coral reefs grow in nutrient poor water?

Coral reefs grow in nutrient poor water because their food comes mainly from sunlight, through the zooxanthellae inside coral tissue, rather than from nutrients dissolved in the water. The Smithsonian Ocean program explains that this symbiosis lets reef building corals thrive even in waters that are nutrient poor. The reef recycles nutrients efficiently, passing them through many organisms. Excess nutrients harm reefs, because they feed fast growing algae that overgrow corals, shade them, and block new coral larvae from settling.

How deep can coral reefs grow?

Following NOAA’s Coral Health and Monitoring Program, reef development is generally limited to depths of less than 20 meters where water clarity is satisfactory, because light falls off quickly with depth and growth rates below that are too slow to build a reef framework. Corals can occur deeper, and deep sea corals live where light is absent, around 200 meters and deeper, but those corals have no zooxanthellae and catch their own food instead of relying on sunlight. They do not build the shallow tropical reefs this article describes.

What happens when abiotic factors change?

When abiotic factors move outside the range corals tolerate, reefs suffer. Water warming just 1 degree Celsius above the usual summer high can trigger coral bleaching, and prolonged heat can kill corals. Extra nutrients and sediment cloud the water, cut light, and let algae overgrow corals. Lower pH and lower aragonite saturation make it harder for corals to build skeletons. Freshwater floods lower salinity and stress corals. The factors interact, so a reef facing several changes at once, such as heat plus still water plus murky runoff, is at greater risk than a reef facing one change alone.

Sources & references

• National Oceanic and Atmospheric Administration, Coral Health and Monitoring Program. What Are Coral Reefs? https://bennington.edu/sites/default/files/sources/docs/Sherman_CHAMP%20-%20What%20are%20Coral%20Reefs.pdf
• National Oceanic and Atmospheric Administration, National Ocean Service. What is Zooxanthellae? Corals Tutorial. https://oceanservice.noaa.gov/education/tutorial_corals/coral02_zooxanthellae.html
• National Oceanic and Atmospheric Administration, Coral Reef Watch. Tutorial: More About Coral Bleaching. https://coralreefwatch.noaa.gov/product/5km/tutorial/crw02b_more_bleaching.php
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• National Oceanic and Atmospheric Administration, Ocean Service. Coral Reefs Fast Facts. https://coast.noaa.gov/states/fast-facts/coral-reefs.html
• National Oceanic and Atmospheric Administration, Ocean Acidification Program. OA Indicators Explained. https://oceanacidification.noaa.gov/oa-indicators-explained/
• National Oceanic and Atmospheric Administration, Science on a Sphere. Ocean Acidification: Saturation State. https://sos.noaa.gov/catalog/datasets/ocean-acidification-saturation-state-climate-model-ssp1-sustainability/
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• Smithsonian Ocean. Ancient Architects of the Ocean: How Stony Corals Have Survived Through Earth's Changes. https://Ocean.si.edu/ecosystems/coral-reefs/ancient-architects-ocean-how-stony-corals-have-survived-through-earths
• Great Barrier Reef Marine Park Authority. Outlook Report: Sea Temperature. https://outlookreport.gbrmpa.gov.au/values/3-ecosystem-health/32-physical-processes/326-sea-temperature
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