Coral reefs

what are the 4 conditions necessary for the formation of coral?

• temperature

• depth

• salinity

• exposure to air

temperature

• are only found in waters bounded by the 20°C isotherm

• cannot develop below 18°C or above 30°C-35°C (coral bleaching happens at high temps)

depth

• cannot develop in water deeper than 50-70m

• usually grow in 25m or less

• this is why reefs are restricted to the margin of the continents or islands

• is necessary due to light restrictions as sufficient light must be available to the symbiotic zooxanthellae in the coral tissue for photosynthesis

salinity

• intolerant of salinities deviating from that of normal seawater

• so are not found where inshore waters are subject to continuing influxes of freshwater from river discharges

exposure to air

• can secrete mucus to prevent desiccation

• but are killed by long exposure to air

% of sea floor coverage - % of ocean creatures

• less than 1% of the ocean floor

• habitat for 25% of ocean creatures

• 75% of ocean species born or raised in coral reefs

rising sea levels

• 30-40% of sea level rise due to thermal expansion

• decreases salinity

• decreased temp. of deeper corral (is deeper + reduces light)

• increases temperature

Darwin’s theory of the development of reefs

• took an evolutionary approach to reef development, believing that all three types started the same

• He believed that the land in the ‘middle’ would eventually be eroded and reduced over time

• leading to an increasing ‘gap’ between the land and the reef

• coral growth is most vigorous on the outer edge of the reed (waves, oxygenated water, food availability

• eventually, the land would disappear entirely, resulting in the atoll

Murray’s theory on development of corral reefs

• base of the reef consisted of a submarine hill or plateau rising from the ocean floor

• these reached within 60m of the sea surface and consisted of either sub-surface volcanic peaks or wave worn stumps

• as a fringing reef grows, pounded by breaking waves, masses of coral gradually accumulate on the seaward side, washed there by the waves

• are cemented into a solid block

biology of coral

• most coral have zooxanthellae algae living symbiotically with the coral polyps, zooxanthellae provide the coral with food

• the coral provides access to the light for zooxanthellae

• are animals, made up of calcium carbonate or limestone that is developed by coral polyps

3 different types of reef

• fringing reefs

• barrier reefs

• Atoll reef

fringing reef

Are low, narrow bands of coral next to the coast, at about low tide level.

consist of a platform of coral which is connected to the land and which has grown out from it.

the surface of the coral platform is usually slightly concave and is filled by a shallow lagoon (between land and outward edge of reef)

this lagoon is often less than 500m wide

the seaward edge of the reef is often the highest part because this is where the breaking waves oxygenate the water and make food available for the polyps.

the outer edges of the reef slope steeply down into the sea beyond

e.g. coral coast of Fiji and reefs along coast of Mauritius

barrier reef

similar structure to fringing reefs, but are usually several km from the land are always over 0.5km away.

are separated from the land by wide, deep lagoons which are two deep for the polyps to live.

however, floor of the lagoon is usually made of old coral, indicating that these barrier reefs may have originated as fringing reeds which slowly grew out from the coastline.

seaward edge of the reef is often the highest part and the outer edges of the reef slope steeply down in the sea beyond.

e.g. Great Barrier Reef, Aus, made up of over 3000 sections of reef separated by channels.

Atoll reefs

narrow, circular reefs broken by channels. They surround a deep, circular lagoon.

e.g. Suvadiva Atoll and Gan Atoll in the Maldives. Are also very common in the South Pacific Ocean - bikini Atoll, Marshall Islands

distribution of coral reefs

• all in linear band +/- 30° N/S of equator

• Intense clusters around NE Australia and equatorial coastlines

• randomly scattered in Pacific Ocean due to volcanic islands

• unevenly spread around the world

(when describing distribution use T(CLUES)EAM

Trends - loose/strong, positive/negative correlation

Clustered, linear, uneven, scattered

Evidence - use specific place names to support

Anomalies - find features that disagree with the trend

Manipulate - make simple calculations if data present)

facts and figures

100,000 to 300,000 years for a barrier reef to reach full maturity

1/5 of all coral in the world has died since 2015

what are the threats to corral reefs?

• global warming

• sea level rise

• pollution

• physical damage

global warming - threat

Rising sea temps

• will stress coral that is already operating at the limits of its tolerance

• 2016 9 month marine heatwave destroyed over 30% if BR

• rising temp may allow for new growth areas

• direct impact

Ocean acidification

• occurs as more CO2 is absorbed by the water

• this prevents coral from forming strong exoskeletons (calcium carbonate) which leaves it vulnerable to predation and unattractive to algae

• 30% increase in acidity since industrial revolution

Storm surges

• natural stressor for reefs, however increased sea temps are likely to lead to a higher frequency and magnitude events

• more likely to damage fringe reef nor barrier or atoll

• indirect impact

global warming - scale + severity

Global

rising sea temps

• tends to lead to a slow decline through increased bleaching, some species more tolerant than others (red sea corrals)

• will impact all reefs

acidification

• bleaching will occur and new coral growth will be limited, reefs can quickly decline (quicker than because of sea level change)

Storm Surges

• along with tsunamis, single events can devastate small areas of reef

Sea level rise - threat

reduce access to light and so zooxanthellae growth reduced, thus inhibiting coral

coastal squeeze

sea level rise - scale + severity

Global

• as with rising sea temps, likely to lead to a slow decline with shallow growing fringe corals and deeper corals most affected

• coastal developments and sea level rise will reduce coral habitats as they can’t migrate landwards

pollution - threat

agricultural

• runoff from farming releases fertilisers and pesticides into the water

• these can cause algae blooms of harmful species and also kill off pests that are actually helpful to the coral e.g. sea lice

• provides food for plankton, which provides food for crown of thorns

• runoff containing lots of soil will create cloudier waters, inhibiting photosynthesis

industrial

• general water pollution can come from industry, however the greatest threat comes from incidents such as oil spills or chemical leaks

• e.g. shipping lanes near the GBR

pollution - scale + severity

local

Agriculture

• after storm events we can see significant bleaching events

• crown of thorns tend to increase 3yrs after a storm on GBR and they can destroy large areas in weeks

Industrial

• these incidents can cause total destruction of a particular reef but are usually limited in areal extent

physical damage - threat

Fishing

• blast fishing, cyanide (often SE Asia) and trawler nets all damage coral

Tourism

• divers, souvenir sellers, increased run off, boats anchoring, diesel etc. all impact coral

• 2 million tourists every year to the GBR

• increase in tourism leads to increase in coastal development - sediment in the sea

• 4,000k - 6,000k of sun cream entering ocean around coral reefs annually

Crown of thorns starfish

• adult COTS can consume up to 10m² of coral a year

Physical damage - scale + severity

local

Fishing

• can destroy entire reed systems in local areas

Tourism

• tends to cause localised damage

Crown of thorns star fish

• total destruction of local coral reefs

• can cause 55% live coral to less than 5% in weeks

how much coral cover has the GBR lose in 30yrs

50%

management strategies - global warming

Management

• Allen Coral Atlas, a high-res satellite tool that monitors coral reefs.

• Cop 28, 2023, first call for a global shift away from fossil fuels/Paris agreement, limit global temperature increase well below 2^oc Ideally below 1.5^oc

Evaluation

• Its unlikely that even our best efforts to reduce global warming are going to happen in time to save many of our coral reefs

• More likely that we will find ways in which to farm corals more able to survive these higher temperatures and CO2 levels such as those found in the Red Sea and then transplant them to save reefs

management strategies - sea level rise

management

• same as global warming

• Allen Coral Atlas, a high-res satellite tool that monitors coral reefs.

• Cop 28, 2023, first call for a global shift away from fossil fuels/Paris agreement, limit global temperature increase well below 2^oc Ideally below 1.5^oc

Evaluation

• SL rise presents a global threat to coral as few areas will escape destruction. However, it is a relatively slow processes with rises of only a few mm to cm a year at most which does give some margins for coral to adapt.

• This could result in shallow growing corals in fringing reefs migrating landwards or disappearing altogether. While unlikely to destroy coral in its own, there is no doubt that this has the potential to significantly reduce the biodiversity and health of our reefs.

management strategies - pollution

Management

• small scale reef management is taking place on the GBR to cull the COTS - only really on tourist areas where funding is available

• Egypt has legislation to manage industrial waste - however release of polluted waste water from Ras Shukeir oil terminal (heavy metals, 40,000 cubic metres a day) suggests the legislation isn’t actually implemented

Evaluation

• doesn’t address the cause of the problem - if pollution continues - COTS can keep growing (can link back to tourism)

• as food demand increases it is likely that despite a drive for sustainability, fertiliser and pesticides will increase and more land will be cultivated. Linked into the greater frequency in storms and this more run off, this is likely to become an increasing problem

management strategies - physical damage

Management

• Fishing

  • sustainable fishing programmes - Philippines (NGO + gov) have introduced CFUs (coral fishery units), small local in-situ coral farms tended by villagers.

  • are currently 186 CFU and more than 15,000 fragments have already been produced (aim is 30,000)

• Tourism

  • Hide-away Resort in the middle of the Coral coast, Fiji - only swim in reef at high tide, glass bottom boats for tourists to view reef, designated reef walk path

• COTS

  • small scale reef management is taking place on the GBR to cull the COTS - only really on tourist areas where funding is available (offsets damage done by tourists?)

Evaluation

• Fishing

  • without local community buy in and a replacement protein source, local level destruction unlikely to stop - CFUs bring more fish to area

  • global fishing legislation should prevent overfishing in the future

• Tourism

  • overall very successful in HICs but less so in LICs where enforcement is weak

Coral nurseries

Coral Vita has started growing coral using land-based nurseries

By using a laboratory process called ‘microfragmenting’ coral growth can be achieved at 50 times the natural rate

enabled the restoration of certain coral species, e.g. Great Star corals, which would otherwise grow too slowly to be feasible for restoration using ocean-based nurseries

Future reefs program

Aus, after mass bleaching in 2016

University of technology Sydney + local reef-tour company, collect 12 species of coral that had survived the bleaching and grow them on mesh platforms in a sandy lagoon

after growing the coral for months, the coral fragments were planted back onto the reef, in the hope that it would sped up the recovery process, and when the next heatwave strikes there will be a greater chance of the coral surviving (as the species that survived the last one)