Threats to coral reefs from increasing concentrations of dissolved carbon

dioxide

By Miguel Rodriguez

What is a Coral Reef?• Coral reefs are diverse underwater ecosystems held together by calcium carbonate

exoskeletons secreted by corals, which support and protect the coral polyps.• Most coral reefs are built from stony corals, which in turn consist of polyps that

cluster in groups.

- “polyps” are tiny marine animals that stay fixed in one place and are the main structure of a coral reef. Each polyp is connected by living tissue to form a community. Each polyp has a ring of tentacles shaped like a cup around a central opening. The tentacles are like long arms with tips that can sting. They are used either for defense or to capture zooplankton for food.

Close up of polyps arrayed on a coral

• Reefs grow best in warm, shallow, clear, sunny and agitated waters.

• They are therefore most commonly found at shallow depths in tropical waters

The Wildlife• Coral reefs occupy less than 0.1% of the world's ocean surface, yet

they provide a habitat for 25% of all marine species, including fish, molluscs, worms, crustaceans, echinoderms sponges, tunicates and other cnidarians.

• The Great Barrier Reef is the largest coral reef. It is made up of over 2 900 individual reefs and 900

Islands stretching for over 2 600 kilometres off the northeast coast of Australia. It is home to more than 1,600 types of fish, 133 types of shark and ray and 30 species of whale and dolphin tart.

Why are Coral Reefs so special?• The Great Barrier Reef provides the pockets of space for small fish to spawn (release

eggs into the water) away from larger predators.• The vibrancy of the reef provokes awe from its visitors who are used to less exotic

wildlife.• Coral reefs deliver ecosystem services to tourism, fisheries and coastline protection.

The global economic value of coral reefs has been estimated to be between US $29.8 billion[5] and $375 billion per year.

• Coral reefs protect shorelines by absorbing wave energy, and many small islands would not exist without their reefs to protect them.

• Coral reefs are a source of medication: some painkillers come from coral reefs and the marine aniamls as do drugs used for arthritis, Alzheimer's disease, heart disease and viruses. Research is still in process

• Cold, deep-water reefs, are home to marine organisms that produce potent molecules that might inspire new anti-cancer therapies

Coral Reefs also form a major part of the Carbon Cycle

Coral reefs are sites of two important processes: the organic carbon metabolism (photosynthesis & respiration) and the inorganic carbon metabolism (calcification & dissolution of calcium carbonate).

Because of their high rates of calcification, coral reefs fix about half of all the calcium entering the sea into calcium carbonate. Calcification releases carbon dioxide as it involves the precipitation of calcium carbonate from bicarbonate and calcium in seawater.

In addition, while coral reefs absorb large amounts of carbon dioxide (per unit area) during photosynthesis, they generally release almost equivalent amounts via respiration, resulting in little net storage.

The Threats to Coral Reefs• Cyanide fishing for aquarium fish(spraying a sodium cyanide mixture in order to stun the fish) -Cyanide concentration slows photosynthesis in zooxanthellae, which results in coral reefs losing colour as well as eliminating one of their major food sources• Sunscreen use- Some commonly used chemical sunscreen

ingredients cause coral bleaching• overuse of reef resources e.g. overfishing disrupting the natural food chains

• Blast fishing(using explosives) = destroys coral reefs

Harmful land-use practices, including urban (e.g. sewage) and agricultural runoff (fertilizers) and water pollution, which can harm reefs by encouraging excess algal growth which blocks out sunlight for coral reef (can’t photosynthesize so it dies)• Coral Mining- humans use the first layer of coral reefs as road-fill or cement

which causes long-term physical damages• Climate Change-makes the water warmer and coral can’t live in water

temperatures that are not within 26-27°C-When coral is subjected to higher temperatures than it is used to, the symbiosis between it and algae that powers it breaks down, leaving the coral reef as a chalky, brittle ghost of what it once was. Once the coral bleaches and dies, so does its wildlifeThe Great Barrier Reef specifically has been affected by diver’s fins slapping against the coral, anchors scraping the reef, and the rampant scavenging of shellfish disturbing its food chain.

Threats of Coral Reefs Continued…

Ocean Acidification-the current most worrying threat

• Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide from the earth’s atmosphere.

• Over 500 billion tonnes of CO2 released by humans since the start of the industrial revolution have been dissolved in the oceans.

• pH of surface layers of the earth’s oceans in the late 18th century ≈ 8.1179 currently ≈ 8.069, which represents 30% acidification.• Reef-building corals deposit calcium carbonate un their exoskeletons do they

need to absorb carbonate ions from seawater. • The concentration of carbonate ions is low in seawater because they are not

very soluble.• Dissolved CO2 makes the carbonate concentration even lower as a result f

some interrelated chemical reactions: CO2 + H2O -> H2CO3 -> H+ + HCO3

−

H+ + CO32 –

Ocean Acidification-the current most worrying threat

• If the carbonate ions concentrations drop it is more difficult for reef-building corals to absorb them to make their exoskeletons

• Also, if seawater ceases to be a saturated solution of carbonate ions, existing calcium carbonate tends to dissolve, so existing exoskeletons of reef-building corals are threatened.

• Volcanic vents in the Gulf of Naples have been releasing carbon dioxide into the water for thousands of years, reducing the pH of the seawater. In this area of acidified water there are no corals, sea urchins or other animals that make their exoskeletons from calcium carbonate. In their place other organisms like invasive algae and sea grasses flourish. Unfortunately this could be the disheartening future for coral reefs around the world if carbon dioxide emissions continue to rise…

• Loss of tropical coral reefs to acidification could cost $1 trillion by 2100 in terms of lost shoreline protection and lost revenues for the tourism and food industries.