Coral reefs• What is a Reef?• Types of Reefs• Reef Formation• Reef Distribution• Coral Biology• Coral Reef Ecology• Reef ConservationML2007S. Norton
What is a reef?"...a rigid structure composed of calcareousskeletons of various organisms, interlockedor cemented together by growth, and ofdetrital material derived from the break upof such skeletons, and the structuremaintains its upper surface at or near thelevel of the sea." (MacNiel, 1954)
Coral reefsLimestone and calcareous sediments built byliving organisms.Coral colonies are short-lived (years to decades).They are the largest biogenic structures on theface of the earth.www.abc.net.au/science/news/enviro/EnviroRepublish_1003558.htmCoral reefs have been around for millions of years.
Reef Forming Organisms• Worms (Polychaeta, etc.)• Calcareous Algae• Sponges (Porifera)• Bivalves (and other membersof Mollusca)• Other minor phyla (Bryozoa,Brachiopoda, etc.)• Corals (Cnidaria)
Three major types of coral reefs• Fringing• Barrier• Atoll• Proposed by Darwin (1842)“On the Structure andDistribution of Coral Reefs”
Three major types of coral reefs• Fringing• Barrier• Atoll• ...cuspate,mesh,patch,platform...• Proposed by Darwin (1842)“On the Structure andDistribution of Coral Reefs”• Morphology based on “size,shape and nearby land ifany”
Fringing ReefNearshore, well-lighted marine watersNarrow zone if submarine slope is steepWide zone if submarine slope is gentleYoung volcanic islands in tropics ideal:•Lots of oxygen and nutrients•Very low FW input, so high S‰ & low silt
Fringing ReefsCoral Rock of Fringing Reefs is relatively“thin” (usually 25-50 m), except wherelandmass is subsidingLongest reef in world is Red Sea = 4000 km ifstraightened out.
Fringe ReefsCastro & Huber Fig. 13.14Reef regions include the flat, crest, and slope.Mangroves, seagrass or sandy beaches may line the shore;fringing reefs typically lack a lagoon.
Fringing Reefs grade into Barrier ReefsCastro & Huber Fig. 13.17www.tropichorizon.com/belize.htmlBarrier reefs typically have a deeper water lagoonor channel separating the reef crest from the shore.
Barrier ReefsLinear structures separated by a lagoon (up to 100 km)100-200 m thickness of coral rockAnnular around islandReefs rise from terrace or platform
Great Barrier Reef• 2000 km long• 5-75 km wide• Long, complex geographichistory
AtollsName originally from Darwin (1842).Maldives
AtollsCastro & Huber Fig. 13.22Found primarily in the Pacific, atolls develop on the tops of seamounts.
Formation of AtollsRaven et al. 6.21Subsidence Theory:Atolls begin as fringingreefs around volcanicislands that have emergedabove sea level.
Formation of AtollsRaven et al. 6.21Subsidence Theory:As magma sources drop, theseamount subsides and thereef becomes a barrier reef.
Formation of AtollsRaven et al. 6.21Subsidence Theory:As the seamount continuesits subsidence below thesurface, corals grow on topof the seamount, keepingcorals in the photic zone.
Atoll ExamplesDeep sea atolls from sea floor (e.g.,Palau).Shelf atolls from continentalshelf (e.g., Belize).Eniwetok Atoll is 1.25 km ofshallow-water coral limestones ontop of a 3.25 km high volcano
A Map of Earths Coral Reefs• The pink regions represent the primary reef areas.• The dark gray lines are major ocean currents.• The lighter gray lines represent the 20 degree isotherm latitude. It is above orbelow this location that the water is too cold to sustain coral polyps.
Global distribution of coral reefsPurves et al. Fig. 54.28Reefs are absent where coastal waters are too cold or rivers bring inlow-salinity water and high sediment loads.
Global diversity of coralsAt the species level (??), there are over 700 species of corals in theIndo-Pacific versus approximately 60 in the Atlantic.Purves et al. Fig. 54.28The center of biodiversity of corals is the Indo-Pacific: Australia,Micronesia, Indonesia, Philippines, etc.Genera of corals
Biodiversity of coral reefsCoral reefs harbor more examples of evolutionary diversity (phyla,classes, etc.) than any place on Earth.The Great Barrier Reef~350 spp. of hermatypic corals>4000 spp. of molluscs1500 spp. of fish240 spp. of seabirds.Nybakken & Bertness Table 9.2
Clines in diversityClines in species richness have been recorded for marine organismsas well, including:fishcoralssnailslobstersRoberts et al. 2002. Science 295:1280-1284
General Conditions for Coral Reefs• 20-30°C• 32‰ S or above• Adequate Surface Substrate• High Light Levels• 25 m or shallower (light)• Low Turbidity (feeding, light, substrate)
Necessary conditions for coral reefs:Low turbidity and sedimentationSedimentation and turbidity harmcoral nutrition.www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.html
Suspended particles absorb/scattersunlight.www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.htmlNecessary conditions for coral reefs:Low turbidity and sedimentation
Sediments smother small coralsand/or inhibit feeding.www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.htmlNecessary conditions for coral reefs:Low turbidity and sedimentation
Some corals produce largequantities of mucus to trap andremove sediment.www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/overview/part_02.htmlNecessary conditions for coral reefs:Low turbidity and sedimentation
Necessary conditions for coral reefs: Full salinity watersCorals are stenohalinephotography.nationalgeographic.com
Necessary conditions for coral reefs: Subtidal watersThe upper limit of coralcolonies is the lowerpart of the intertidal.Castro&HuberFig.13.16
Necessary conditions for coral reefs: Low [Nutrients]Grow well under low[nutrient].Under higher[nutrient], fleshyalgae flourish.knowledge.allianz.com
Necessary conditions for coral reefs: High wave energyAt least moderate amounts of wave or current
Necessary conditions for coral reefs: High wave energyWaves and longshore currentsbring nutrients andphytoplankton.dnr.louisiana.gov
Necessary conditions for coral reefs: High wave energyStorms destroy coral calciumcarbonate skeletons.
Castro & Huber 13.7Branching corals:fast growth ratesvulnerable to storm damageMorphological diversity among corals
Castro & Huber 13.7Massive corals:grow more slowlybut are very stableMorphological diversity among corals
Castro & Huber 13.7Free-living corals:single polypnot cemented to the surfaceMorphological diversity among corals
Mechanisms for reef growthCastro & Huber Fig. 13.8Corals provide a large framework forreef development.Encrusting coralline algae, sponges, andbryozoans bind the CaCO3 sedimentstogether.They build a 3-D framework.Calcareous sediments from algae andalgal rubble collect in gaps.Further geological processes turn thiscomplex into limestone.
Importance of calcareous algaeOn the Great Barrier Reef, 17-40% of surface sediment are derivedfrom calcareous red algae and 10-30% from Halimeda.coexploration.org/bbsr/coral/assets/images/halimeda.jpgHalimeda and Porolithon, have 95% or moreof their body mass as calcium carbonate.www.coralreefnetwork.com/marlife/stepath/rhodophyta2.html
Distribution of corals along a reefNybakken & Bertness Fig. 9.23The inner lagoon/reef flat.
Distribution of corals along a reefNybakken & Bertness Fig. 9.23At the reef crest:-Pacific: crustose coraline algae, heavybranching corals-Atlantic: just branching corals.
Distribution of corals along a reefMassive corals dominate themiddle depths of the reefslope.
Patterns of coral recruitmentBoth sexual reproduction (brooding and broadcast) and asexualreproduction (fragmentation) contribute to the recruitment of new corals.
Patterns of coral recruitmentLevinton 2001 Fig. 5.12Planulae settle close to theirparents.Both sexual reproduction (brooding and broadcast) and asexualreproduction (fragmentation) contribute to the recruitment of new corals.
Interspecific interactions on coral reefsAmong primary space occupiers,competition is a critically importantphenomenon.
Interspecific interactions on coral reefsExploitative CompetitionHeron Island, Joe Connell in Nybakken & Bertness Fig. 9.2719631965Faster growers may extend overand shade slower-growing forms.
Interference Competition-Mesenterial filamentsInterspecific interactions on coral reefs
Importance of Invertebrate-Alga SymbiosisCoral ReefsHigh productivity in low [nutrient] water (over 2000g C m-2y-1)Some octocorals have no nematocysts, reduceddigestive tracts and show no behavioural changes inthe presence of µzooplankton preySome die without light, but can live and grow in FSWInsufficient zooplankton populations to sustain reefgrowth (~10%)
Autotrophy vs. heterotrophy in coralsIn general, shallow water corals rely moreon their symbionts for nutrition.Nybakken & Bertness Fig. 9.8Deeper water corals are more predaceous.Nybakken & Bertness Fig. 9.26This pattern can even be seenintraspecifically in these skeletonsof Montastrea cavernosa fromshallow water (left) vs. deep water(right) in the Caribbean Sea .
Importance of ReefsWhile they comprise only 0.2% of the ocean’s surface, coral reefs arehome to a third of the ocean’s fish species.
Importance of ReefsCoral reefs provide food for up toone billion people in Asia alone.wwf.org
Importance of ReefsTourism in the Caribbean is worth over $10 billion.noaa.gov
Threats• Coral bleaching• Climate Change• Sea Level Rise• Ocean Acidification• Temperature Increases• Crown of Thorns Starfish• Diadema antillarum losses• Harvesting for coral and foraquarium trade• Tourism
a-sul.blogspot.comImportance of ReefsTourism in the Caribbean is worth over $10 billion.
Coral BleachingExposed reef flat in Hawai’i with white patches of coralthat have expulsed their algal symbionts.
• Archipelago is 823 km long and 130 km wide at its greatest width.• ~90,000 square km.• 26 natural Atolls• About 200 islands are inhabited.Sea level rise and the Maldives
• Maldives islands are on average1.5 meters above sea level• Flooding will be a problem• Sea level may rise so quickly thatit will erode the coral islands.Sea level rise and the Maldives
Ocean AcidificationIncreased Atmospheric[CO2] results indecrease in pH of theoceans.Corals and otherorganisms with calciumcarbonate skeletonscan’t lay down CaCO3and skeletons dissolve.
Tales of two echinodermsDiadema antillarum -long-spined urchinWhile not described as such, both species play major roles in thehealth of coral reefs - keystone species??Acanthaster planci -Crown of thorns starfish
Diadema’s storysv.wikipedia.org/wiki/TagghudingarBefore 1983, D. antillarum was alarge, abundant herbivore found onCaribbean coral reefs, eelgrass beds,mangrove roots, and sand flats.Noctural foraging by D. antillarumin seagrass beds brought nutrientsinto the patch reefs that it used forshelter.Levinton 2001, CD
Lessios 1988. Fig. 1In January 1983, sick D. antillarum appeared at Punta Galeta,Panama.Spatial and temporal patterns of Diadema mortalityWithin a year, thedisease spread2000 km east.At a wide range of sites, D. antillarum populations plummeted bymore than 93% from pre-outbreak levels, up to 98% reductions.
What was responsible?The fast spread of the disease andspread of disease to lab animalsliving in running seawater indicateda water-borne agent.www.health.qld.gov.au/EndoscopeReprocessing/images/page_images/137_clostridium.jpgTwo species of Clostridium bacteria,collected from infected animals,were lethal when injected intohealthy animals.
The delayed recoveryRecruitment of D. antillarum failed across the Caribbean within 5-7 months after theepidemic.Over a decade later, recruitment of D. antillarum was still sparse.Hughes (1994. Science 265:1547-1551)documented changes in Diademapopulations and community impacts inJamaica.
In Jamaica, coral% cover declinedfrom an average of52% in 1980 to3% in the 1990’s.Hughes 1984. Fig. 5Within months of the mass mortality of Diadema, populations of fleshy andfilamentous algae exploded across the Caribbean.Consequences of Diadema’s absence
The bottom lineIn Jamaica, overfishing of herbivorous fishes increased thevulnerability of the coral reef to algal competition, but the fleshy algaewere kept in check by Diadema.Hughes 1984. Fig. 6Reefs dominated by coralsin the 1970’s weredominated by algae in the1990’s.
The NEW bottom lineIdjadi1, Haring, Precht, 2011 MEPS
The story of Acanthaster planciAcanthaster planci ranges from theIndian Ocean, through the Indo-Pacificto the Pacific Coast of central America.In general, this species prefersfeeding on hermatypic corals,especially the dominant coral inthe Indo-Pacific, Acropora spp.
The story of A. planciAcanthaster planci feeds on corals byeverting its stomach over the coralsurface.www.reef.crc.org.au/discover/plantsanimals/cots/On the Great Barrier Reef (GBR), a healthy coral reef (40-50% coralcover) can support 20-30 individuals per hectare.
The story of AcanthasterOutbreaks have been noted sincethe 1960’s.
Recovery after Acanthaster outbreaksEven on heavilydamaged reefs, somecorals remain aliveand begin recovery.Reefs with poorer larval supply may not recover after 15 years or more.Levinton 2001. CD
What causes outbreaks?2) They are the result of natural fluctuations caused by variation in the number ofnew recruits.1) Under conditions of food stress (e.g, after a storm), adult Acanthaster living indeep water will use chemical cues to form aggregations in shallow water.
Milne 1995. Fig. 15.8Examining rubble onreefs does reveal periodswith high concentrationsof Acanthaster spines asfar back as 3,355 yearsago.What causesoutbreaks?
www.culture-bx1.u-bordeaux.fr/Ateliers/Internet/Etudiants/recifs/stress.htmlThe giant triton, Charonia tritonus, cutsopen an adult Acanthaster with its radulaand scapes up gonads and viscera.What causes outbreaks?3) Overharvesting of predators leads to highersurvival of Acanthaster juveniles and adults.This species has been collected for its shell and numbers areseverely reduced.Its effectiveness, even before its overharvest, is questioned becauseeach triton typically eats only one adult starfish per week.
What causes outbreaks?If predation is important, why would one see sudden, simultaneousoutbreaks in multiple places in the same years?In the Red Sea, pufferfishes and triggerfishes kill up to 4,000individuals/ha/yr, leading to low abundance of A. planci there.www.bio.davidson.edu/people/midorcas/animalphysiology/websites/1999/Rice/Aaron.htmIn the Indo-Pacific, the humpheadwrasse Cheilinus undulatus) is an activepredator on Acanthaster.While adults are large (up to 200 kg),they are not known to occur at highdensities.
What causes outbreaks?4) Anthopogenic changes in water quality, especially after high rainfallon land, have been proposed (Birkeland 1982) to drive outbreaks.A) Low salinities and high temperatures from terrestrial runoff mayimprove larval survival.B) Increases in nutrients in runoff stimulates phytoplanktonproduction and this improves larval survival.
Strategies for Acanthaster controlAcanthaster outbreaks at economically-valuable sites (e.g.,tourist sites) may be controlled by injecting sodiumbisulfate into each starfish.Efforts of control the latest outbreak at just a few sites costthe Australian government $1 million/yr.
Acknowledgements (Incomplete)• Reef map courtesy of Jeremy Staford-Deitsch from his book entitled Reef.• University of Hawaii, http://www.hawaii.edu/HIMB• http://earthobservatory.nasa.gov• Chuck Fisher• Steve Norton