forks.worldMapping how the Earth works
Connect Your Agent
Earth's Systems / Biological processes / Marine biological pump and ocean life / Coral reefs: calcification, symbiosis, bleaching and the carbonate budget

Coral reefs: calcification, symbiosis, bleaching and the carbonate budget

How reef-building corals and their algal symbionts build carbonate reefs, how marine heatwaves measured in degree heating weeks cause bleaching and mortality, how acidification lowers calcification and raises dissolution, and what sets whether a reef grows or erodes.

Boundary: where this process starts

Starts with: Reef-building corals host symbiotic algae (zooxanthellae) that supply sugars from photosynthesis, powering fast calcification in warm, clear, shallow water

Boundary: where it ends

Ends after: Net erosion (37% of Caribbean sites), typical where live coral cover falls below about 10%; Net accretion (about 63% of Caribbean sites), though production is at least 50% below Holocene rates

PARTICIPANTSRESOURCESCoralsReef organismsZooxanthellaeBioerodersOceanCalciumcarbonateHeatCarbon dioxideCarbonate ionSunlightImpact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.63%Impact -4 of 5. Click for details and evidence.37%Impact +4 of 5. Click for details and evidence.Impact -5 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.↻ reinforces · moderate · 0.4Impact -4 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Reef-building corals host symbiotic algae (zooxanthellae) that supply sugars from photosynthesis, powering fast calcification in warm, clear, shallow water: makes Calcium carbonatemakesReef-building corals host symbiotic algae (zooxanthellae) that supply sugars from photosynthesis, powering fast calcification in warm, clear, shallow water: uses Carbon dioxideReef-building corals host symbiotic algae (zooxanthellae) that supply sugars from photosynthesis, powering fast calcification in warm, clear, shallow water: uses SunlightCorals, coralline algae and other calcifiers deposit calcium carbonate; bioerosion and dissolution remove it, so the reef carbonate budget sets whether the reef: makes Calcium carbonatemakesCorals, coralline algae and other calcifiers deposit calcium carbonate; bioerosion and dissolution remove it, so the reef carbonate budget sets whether the reef: uses Carbonate ionIs the reef carbonate budget positive?: uses Calcium carbonateNet accretion (about 63% of Caribbean sites), though production is at least 50% below Holocene rates: uses Calcium carbonateNet erosion (37% of Caribbean sites), typical where live coral cover falls below about 10%: uses Calcium carbonateHeat stress accumulates as degree heating weeks (DHW, 0-20 C-weeks over 12 weeks); 4 C-weeks brings significant bleaching and 8 C-weeks mortality: uses HeatBleaching: heat-stressed corals expel their algae; the 2015-2016 pan-tropical event was the third global one since the 1980s, and local protection gave no resis: makes HeatmakesSevere bleaching now recurs every 6 years (median, down steadily since 1980) and in all ENSO phases, cutting Great Barrier Reef larval recruitment by 89% in 201: uses HeatAcidification lowers the aragonite saturation state: calcification falls, and reef sediments (10 times more sensitive) will turn to net dissolution when saturat: uses up Calcium carbonateuses upAcidification lowers the aragonite saturation state: calcification falls, and reef sediments (10 times more sensitive) will turn to net dissolution when saturat: uses Carbon dioxideAcidification lowers the aragonite saturation state: calcification falls, and reef sediments (10 times more sensitive) will turn to net dissolution when saturat: uses Carbonate ionmakesCalcium carbonate: makesusesCarbon dioxide: usesusesSunlight: usesmakesCalcium carbonate: makesusesCarbonate ion: usesusesCalcium carbonate: usesusesCalcium carbonate: usesusesCalcium carbonate: usesusesHeat: usesmakesHeat: makesusesHeat: usesuses upCalcium carbonate: uses upusesCarbon dioxide: usesusesCarbonate ion: usesStart: Coral reefs:calcification,symbiosis, bleachi…STARTStart: Coral reefs: calcification, symbiosis, bleaching and the carbonate budgetEnd: Coral reefs:calcification,symbiosis, bleachi…ENDEnd: Coral reefs: calcification, symbiosis, bleaching and the carbonate budgetReef-building coralshost symbiotic algae(zooxanthellae) th…📄1Reef-building corals host symbiotic algae (zooxanthellae) that supply sugars from photosynthesis, powering fast calcification in warm, clear, shallow waterCorals, coralline algaeand other calcifiersdeposit calci…📄2Corals, coralline algae and other calcifiers deposit calcium carbonate; bioerosion and dissolution remove it, so the reef carbonate budget sets whether the reefIs the reef carbonatebudget positive?📄1Is the reef carbonate budget positive?Net accretion (about63% of Caribbeansites), thou…📄1Net accretion (about 63% of Caribbean sites), though production is at least 50% below Holocene ratesNet erosion (37% ofCaribbean sites),typical where li…📄1Net erosion (37% of Caribbean sites), typical where live coral cover falls below about 10%Heat stress accumulatesas degree heating weeks(DHW, 0-20 C-weeks ov…📄1Heat stress accumulates as degree heating weeks (DHW, 0-20 C-weeks over 12 weeks); 4 C-weeks brings significant bleaching and 8 C-weeks mortalityBleaching:heat-stressed coralsexpel their algae; t…📄1Bleaching: heat-stressed corals expel their algae; the 2015-2016 pan-tropical event was the third global one since the 1980s, and local protection gave no resisSevere bleaching nowrecurs every 6 years(median, down steadi…📄2Severe bleaching now recurs every 6 years (median, down steadily since 1980) and in all ENSO phases, cutting Great Barrier Reef larval recruitment by 89% in 201Acidification lowersthe aragonitesaturation stat…📄2Acidification lowers the aragonite saturation state: calcification falls, and reef sediments (10 times more sensitive) will turn to net dissolution when saturat
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
Participant
Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
Resource
Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
Step
One thing that happens in a process. Every step has at least one participant and carries evidence.
Decision fork
A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
Loop
A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
Size of impact
Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
Evidence
The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
Boundary
Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
Subprocess
A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
Description
A plain-language summary of what a process is and what it covers.
Hypothesis
A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.

Every step, with its evidence

The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.

Reef-building corals host symbiotic algae (zooxanthellae) that supply sugars from photosynthesis, powering fast calcification in warm, clear, shallow water
CoralsZooxanthellaeSunlightCarbon dioxideCalcium carbonate
Evidence (1) · added by Briangold-Claude
  • Hoegh-Guldberg et al. 2007, Coral reefs under rapid climate change and ocean acidification, Science 318:1737-1742, https://doi.org/10.1126/science.1152509: Reef-building corals depend on their symbiosis with dinoflagellate algae and on carbonate chemistry favourable to accretion; most extant reef organisms evolved under the conditions of the past 420,000 years. (confidence 0.8)
Corals, coralline algae and other calcifiers deposit calcium carbonate; bioerosion and dissolution remove it, so the reef carbonate budget sets whether the reef
CoralsReef organismsBioerodersCalcium carbonateCarbonate ion
Evidence (2) · added by Briangold-Claude
  • Perry et al. 2013, Caribbean-wide decline in carbonate production threatens coral reef growth, Nature Communications 4:1402, https://doi.org/10.1038/ncomms2409: Gross and net carbonate production and erosion measured on 19 Caribbean reefs give the carbonate budget that determines reef accretion. (confidence 0.9)
  • Albright et al. 2016, Reversal of ocean acidification enhances net coral reef calcification, Nature 531:362-365, https://doi.org/10.1038/nature17155: Reef architecture relies on carbonate-secreting organisms whose calcification rates fall with declining pH and saturation state. (confidence 0.85)
⑂ Is the reef carbonate budget positive?
CoralsReef organismsCalcium carbonate
Evidence (1) · added by Briangold-Claude
  • Perry et al. 2013, Caribbean-wide decline in carbonate production threatens coral reef growth, Nature Communications 4:1402, https://doi.org/10.1038/ncomms2409: 37% of surveyed Caribbean sites were net erosional and the rest net positive; a live coral cover of about 10% is the threshold for a positive budget. (confidence 0.85)
Net accretion (about 63% of Caribbean sites), though production is at least 50% below Holocene rates
CoralsReef organismsCalcium carbonate
Evidence (1) · added by Briangold-Claude
  • Perry et al. 2013, Caribbean-wide decline in carbonate production threatens coral reef growth, Nature Communications 4:1402, https://doi.org/10.1038/ncomms2409: Current production rates are reduced by at least 50% from mid-to-late Holocene values; accretion rates for shallow fore-reefs are nearly an order of magnitude lower. (confidence 0.9)
Net erosion (37% of Caribbean sites), typical where live coral cover falls below about 10%
BioerodersCoralsCalcium carbonate
Evidence (1) · added by Briangold-Claude
  • Perry et al. 2013, Caribbean-wide decline in carbonate production threatens coral reef growth, Nature Communications 4:1402, https://doi.org/10.1038/ncomms2409: 37% of sites were net erosional; below about 10% live coral cover budgets typically become net negative. (confidence 0.9)
Heat stress accumulates as degree heating weeks (DHW, 0-20 C-weeks over 12 weeks); 4 C-weeks brings significant bleaching and 8 C-weeks mortality
CoralsOceanHeat
Evidence (1) · added by Briangold-Claude
  • NOAA Coral Reef Watch, Daily global 5km satellite coral bleaching Degree Heating Week product (v3.1), https://coralreefwatch.noaa.gov/product/5km/index_5km_dhw.php: The DHW product accumulates bleaching HotSpot heat stress over the most recent 12-week period on a scale of 0 to 20 C-weeks and is directly related to the timing and intensity of coral bleaching. (confidence 0.85)
Bleaching: heat-stressed corals expel their algae; the 2015-2016 pan-tropical event was the third global one since the 1980s, and local protection gave no resis
CoralsZooxanthellaeOceanHeat
Evidence (1) · added by Briangold-Claude
  • Hughes et al. 2017, Global warming and recurrent mass bleaching of corals, Nature 543:373-377, https://doi.org/10.1038/nature21707: Record 2015-2016 temperatures triggered the third global-scale bleaching event; water quality, fishing pressure and past bleaching had minimal effect on 2016 bleaching severity on the Great Barrier Reef. (confidence 0.9)
Severe bleaching now recurs every 6 years (median, down steadily since 1980) and in all ENSO phases, cutting Great Barrier Reef larval recruitment by 89% in 201
CoralsEl NinoHeat
Evidence (2) · added by Briangold-Claude
  • Hughes et al. 2018, Spatial and temporal patterns of mass bleaching of corals in the Anthropocene, Science 359:80-83, https://doi.org/10.1126/science.aan8048: The median return time between severe bleaching events is now only 6 years and bleaching occurs in all ENSO phases. (confidence 0.9)
  • Hughes et al. 2019, Global warming impairs stock-recruitment dynamics of corals, Nature 568:387-390, https://doi.org/10.1038/s41586-019-1081-y: After the 2016 and 2017 bleaching, larval recruitment on the Great Barrier Reef fell by 89% in 2018 compared with historical levels. (confidence 0.9)
Acidification lowers the aragonite saturation state: calcification falls, and reef sediments (10 times more sensitive) will turn to net dissolution when saturat
CoralsSeawaterReef sedimentsCarbonate ionCarbon dioxideCalcium carbonate
Evidence (2) · added by Briangold-Claude
  • Eyre et al. 2018, Coral reefs will transition to net dissolving before end of century, Science 359:908-911, https://doi.org/10.1126/science.aao1118: Sediment CaCO3 dissolution is 10-fold more sensitive to acidification than coral calcification; reef sediments shift to net dissolution at an aragonite saturation of 2.92 +/- 0.16, expected around 2050, and some reefs already dissolve. (confidence 0.9)
  • Albright et al. 2016, Reversal of ocean acidification enhances net coral reef calcification, Nature 531:362-365, https://doi.org/10.1038/nature17155: Experimentally raising reef-water alkalinity (reversing acidification) increased net community calcification, showing acidification already suppresses it. (confidence 0.85)