Chapter 15 – coral reef systems and the complexity of hazards

Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards

Coral Reef Systems and

the Complexity of Hazards

Paul S Kench and Susan D Owen

School of Environment, The University of Auckland, Auckland, New Zealand

ABSTRACT

Coral reefs are unique coastal systems as they represent the balance between ecologicaland physical processes Known for their high biological diversity, both the ecology andgeomorphic structure of reef systems support a range of ecosystem services Thischapter explores the complexities of hazards in reef systems, underpinned by anunderstanding of the dynamic interplay between ecological and physical processes Keydrivers of impact in reef systems are examined, which include extreme events andslow-onset changes in the environmental boundary conditions of reefs Natural hazards,incremental environmental change, and anthropogenic stresses can each drivesignificant impacts on reefs Case studies indicate that the degree of impact istemporally and spatially variable dependent on the antecedent condition of reefs.Impacts include the catastrophic loss of living reef cover, erosion of adjacent coastlines,the formation of extensive rubble deposits on reefs, and slow deterioration in reefhealth, leading to structural collapse of reef systems However, coral reef systems areresilient to natural and anthropogenic perturbations, and the recovery period of reefs to

a range of impacts is highlighted This chapter also discusses how the resilience of reefscan be compromised through the compounding effect of natural and anthropogenicstresses on reefs that can force major changes in reef health and structure over decadaltimescales A reef system model is used to highlight this complexity and allows forconsideration of factors such as impact and recovery timescales in reef systems

15.1 INTRODUCTION

Coral reefs are the most biologically diverse marine ecosystems in the world(Wilkinson, 1999) Less well recognized, coral reef systems are also complexthree-dimensional geological structures that support the living veneer ofbiological diversity, which in turn contributes to the ongoing development

of reef structure Situated across tropical to temperate latitudes (largelybounded 28
north and south of the Equator), coral reefs provide a suite of

Coastal and Marine Hazards, Risks, and Disasters http://dx.doi.org/10.1016/B978-0-12-396483-0.00015-7

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ecosystem services to coastal communities that include biological and foodresources, the physical substrate for island accumulation and humanhabitation, aggregates for construction, and protection from incident oceanicwave energy Indeed, coral reefs provide the foundation for a number ofmidocean atoll nations and along continental coastlines millions of peoplelive in close association with reef systems However, at the global scale, coralreefs are considered to be in serious ecological decline as a consequence ofanthropogenic impacts, natural stresses, and climate change (e.g., Hughes

et al., 2003; Buddemeier et al., 2004) Broad-scale assessments (e.g.,Wilkinson, 2004) have argued that 20 percent of the world’s coral reefs havebeen destroyed and that 25 percent of reefs are under an imminent orlong-term risk of collapse

Reef systems are subject to a range of natural perturbations fromshort-term “pulse” events, such as cyclones and tsunamis, to longer-termpressures and shifts in environmental controls on reef state, such assea-level change and changing ocean water chemistry Added to these naturalperturbations are a range of anthropogenic stressors that impact reef systems

at event through to long timescales, and include dredging and constructionactivities, and exploitation of biological resources Natural and anthropo-genic stresses on reefs can be categorized into three types, based on thegeographic relationship between the stressor and reef system Some impactsare local and direct (e.g., cyclone impact or coral blasting), some are

“proximal” (e.g., resort development, harbor construction), and others aredistant but are then translated to the reef (e.g., sediment release fromcatchments) These differences influence the time lag and duration of stress

on the reef system

The effects of perturbations on reef systems, whether natural or pogenic, have the potential to alter the natural functioning of the biophysicalsystem These changes can lead to deteriorations in reef health and reefstructure that will compromise the ecosystem services provided by coral reefsand promote exposure to hazards for reef-associated communities Thehazards faced by reef systems and communities are frequently a tangle ofinterconnected stressors, including single events and sustained pressures Themagnitude and persistence of these stressors influence the ability of reefsystems to respond and recover

anthro-This chapter presents an overview of the major hazards affecting coralreefs and associated human communities An outline of the unique charac-teristics of reef systems, as a balance between ecological and physicalprocesses and the ecosystem services afforded by reefs, is first presented as abasis to explore perturbations and impacts to reef systems The chapter thenexamines the range of hazards affecting reefs with a focus on the influencebetween temporal (pulse versus slow onset) and spatial (local to distal)perturbations A broad definition of hazards is adopted that encompasses anynatural or anthropogenic process that can fundamentally alter the functioning

of reef systems By viewing hazard events in isolation, the connectivity of reefsystems is quickly obscured While individual hazard events can shock a reefsystem, this chapter explores how the interaction of multiple hazards and thecumulative effects of such events can impact reef system resilience (ability torecover) Understanding the complex humaneecosystem dynamics of coralreefs and the implications these relationships have for ecosystem resilience is

of high importance The chapter explores such interconnected and complexprocesses from the perspective of cumulative impacts, thresholds of collapse,and potential for recovery

15.2 STRUCTURE AND FUNCTION OF CORAL REEFS

Coral reefs are unique coastal systems formed from the interaction betweenecological processes, responsible for the growth of coral and other calciumcarbonate producing organisms, and physical processes (waves, currents, andsea-level change) that modulate ecological processes and redistributecarbonate material within reef systems (Figure 15.1;Perry et al., 2012; Kench,2013; Yap, 2013) Without the presence of carbonate-producing organisms,reef systems would not exist

Coral reefs are three-dimensional structures, consisting of veneers of livingcoral and reef-associated organisms that overlie vast sequences of previouslydeposited calcium carbonate that can extend thousands of meters beneathmidocean reef platforms These structures evolve over geological (millennial)timescales and produce a number of characteristic landform types includingatolls, barrier reefs, fringing reefs, and reef platforms (Kench, 2013) Thesereef structures vary in size from<1 km2, in the case of smaller patch reefs, to

>100 km2 in extent, with some reef networks forming barrier complexes

>2400 km in length, such as the Great Barrier Reef

Coral reef surfaces also support a range of sedimentary landforms that arecoherent accumulations of sediment deposited by wave and current processes

on, or adjacent to, a coral reef structure (Kench, 2013) Of interest to theanalysis of hazards is the formation of subaerial deposits, such as islands andcoastal plains, which are geomorphically important at the human timescale asthey form the foundation for coastal communities and provide the onlyhabitable land in a number of midocean atoll nations, such as Tuvalu, Kiribati,and the Maldives While the physical structure of reefs and sedimentarylandforms vary (Kench, 2013), they serve similar habitat functions However,their exposure to hazards can vary as a function of their structure andproximity to threats

15.2.1 The Building Blocks of Reef Systems

To understand the effect of hazard events, it is necessary to highlight keyecological processes and relationships that underpin reef system health and

FIGURE 15.1 Conceptual diagram of the coral reef system and interaction between ecological and physical processes (central box), the processes

driving change in the system, ecosystem services provided by reefs and anthropogenic impacts.

complexity (seeKench (2013)for an in-depth review) Central to the tion of reef structure and sedimentary landforms is the generation of calciumcarbonate (CaCO3) resulting from ecological processes The major carbonateproducers on reefs are typically divided into three groups First, corals areconsidered the principal building blocks of coral reefs These hermatypiccorals are characterized by a symbiotic relationship between a coral animaland single-celled algae, zooxanthellae, which live within coral tissue Thisrelationship enables corals to secrete a rigid skeleton of calcium carbonatethrough a process known as calcification, a biologically mediated process thatconverts calcium and carbonate ions in supersaturated seawater to CaCO3(Kinzie and Buddemeier, 1996) Second, a range of encrusting organisms, such

forma-as calcareous algae, forma-assists in the structural development of reefs and also bindloose sediment into the reef framework These first two producers are known

as primary producers as their growth can contribute directly to coral reefdevelopment The third set of carbonate producers are benthic organisms thatdwell on and within the reef Such producers include molluscs, calcareousalgae, foraminifera, bryozoans, and echinoderms These organisms are known

as secondary producers as they do not contribute directly to reef growth.However, once they die, their skeletal remains contribute to the detritalsediment reservoir

In terms of the structural development of coral reefs, the most importantconsequence of reef metabolic processes is calcification Rates of calcification

on reefs are temporally and spatially variable and are dependent on a range offactors that include the density and growth rates of organisms across reefs.Typical rates of carbonate production range from 10 kg m2year1 onproductive (coral rich) forereef zones to<0.8 kg m2year1 in lagoons and

rubble substrates (Kinsey, 1983)

While the geomorphic development of a reef and its associated mentary structures is dependent on the growth of carbonate-producingorganisms, a suite of additional chemical, physical, and ecologicalprocesses also play a critical role in cycling carbonate sediment within reefs.These processes can aid the construction of reef landforms, others convertframework to detrital sediment, and some can destroy the reef framework(Figure 15.1) Constructive processes include sediment production bycalcium carbonate-secreting organisms and precipitation of cements that bindand stabilize sediments (Scoffin, 1992) Destructive processes includebioerosion, the action of organisms in destroying reef framework throughmechanical boring, etching and chemical dissolution (Perry and Hepburn,2008), and physical processes, in which waves mechanically break theskeletal structure of carbonate material (Scoffin, 1993) Physical processesthat erode, transport, and deposit carbonate sediment also control thedistribution, structure, and morphology of reefs and sedimentary landformssuch as islands and shorelines (Kench, 2013)

sedi-15.2.2 Environmental Limits on Coral and Reef Growth

Corals thrive in a relatively defined range of environmental limits standing these limits is essential for contextualizing a range of hazard impacts

Under-on reef systems The principle envirUnder-onmental cUnder-ontrols Under-on tropical CaCO3production are sea-surface temperature, light penetration, and the calciumcarbonate saturation state of seawater (Table 15.1) Regional variations inthese parameters govern the global distribution of coral reefs and thecomposition of carbonate sediment-producing biota Coral reefs characteris-tically occur in shallow tropical and subtropical marine settings (between

28
N and 28
S) where sea-surface temperature rarely drops below 17e18
C,

or exceeds 33e34
C, for prolonged periods

Coral growth and reef development are also restricted by a number of otherenvironmental thresholds (Kleypas et al., 1999; Perry and Larcombe, 2003).Corals depend on light for photosynthetic energy, but light levels reducemarkedly with depth Consequently, reef-building corals are limited to the

“photic zone,” the lower boundary of which is the depth of water at whichsurface light level is reduced to 1 percent The depth of the photic zone alsovaries depending on turbidity levels and can range from>90 to <5 m in highlyturbid environments Salinity levels also pattern the distribution of hermatypiccorals While corals can endure a range of salinity levels (Table 15.1), reef

TABLE 15.1 Environmental Parameters Governing the Distribution ofReef-Building (Hermatypic) Corals and Tropical Coral Reef Development

“Optimal” Values for Coral Growth Are Shown as Well as Recorded Upperand Lower Environmental Limits

Environmental Parameter

“Optimal”

Levels

Environmental Limits

Overall averages (1972e1978).

Source: Kench et al (2008)

development is constrained where salinities are low, such as river entrances,

or zones of intense evaporation that elevate salinity Nutrient levels of reefalwaters are an additional regulator of coral growth While corals thrive in lownutrient conditions, their growth can be inhibited where nutrient levels areelevated and in extreme cases may lead to replacement of coral communitieswith macroalgae The constrained range of environmental parametersconducive to coral growth provides intrinsic thresholds against whichdeteriorations in coral health and hazards can be assessed

15.2.3 The Ecomorphodynamic Framework

The concept of ecomorphodynamics provides a framework to conceptualizethe dynamic balance between constructive and destructive processes thatgovern the structure and state of coral reefs and associated landforms(Figure 15.1;Kench, 2011a) The framework also provides a powerful lens tohighlight how hazards can perturb the system leading to fundamental shifts inreef state The framework highlights a number of key aspects of reef systeminterrelationship, which are critical to informing hazard analysis First, thecycling of calcium carbonate is essential to supply the building blocks (corals,sands, and gravels) for reef and landform construction Sediment transfer is atime-dependent process controlling morphological change in coastalmorphodynamics (Cowell and Thom, 1994) As noted above, unique to coralreef systems, sediment is primarily produced by ecological processes and therate of sediment production varies spatially between coral reef settings.Therefore, the “carbonate sediment factory” is a highly space- andtime-dependent coupling mechanism This time dependency emerges from thetime lags in redistribution of sediment (as occurs in other coastal settings), andfrom the time lags related with organism growth, mortality, and conversion tosediment (Perry et al., 2008) Perturbations that influence the sediment factorymay exacerbate hazards in the medium-term (decades)

Second, changes in boundary controls will force alterations in the healthand physical state of the coral reef system Such changes in boundaryconditions can occur as a consequence of extrinsic factors (e.g., as sea-levelrise, ocean temperature, and chemistry variations) or intrinsic factors such

as anthropogenic impacts (e.g., coastal construction or resource exploitation).Third, the geomorphic sensitivity (magnitude, style, and timescales ofgeomorphic change) of reef systems varies between different geomorphicunits For example, the development of coral reef platforms is modulated bysea-level oscillations at millennial timescales (Montaggioni, 2005) Incontrast, the dynamics of reef island shorelines occur at event to decadaltimescales in response to changes in wave energy input (Maragos et al., 1973;Kench and Brander, 2006a)

Fourth, there are feedbacks in the system that can be temporally specific orcascade across timescales For example, while sea-level oscillations govern the

pattern of reef growth at millennial timescales, at shorter timescales, the reefstructure modulates wave and current processes (Gourlay, 1988; Kench andBrander, 2006b) The characteristics of incident energy in turn control thestructure of ecological communities (Chappell, 1980), reef morphology,sedimentation processes, and short-term geomorphic change of beach andisland shorelines (Sheppard et al., 2005).

Fifth, feedbacks may be nonlinear and can be associated with significanttime lags For instance, changes in ecological condition of a reef may occur inresponse to short-term stresses, such as storms, human impact, or disease.However, depending on the magnitude and temporal scale of ecologicalchange (severity, persistence, or ephemeral transition), alterations in thecarbonate budget may, or may not, propagate through the system to yielddetectable changes in the geomorphic system at decadal to centennialtimeframes An understanding of the dynamics of these feedbacks is necessary

to exploring the linkages in changes to reef health, the propagation of hazardevents, and their impacts and the longer-term resilience of reef systems

15.2.4 Ecosystem Services

It has long been recognized that coral reefs provide a raft of ecosystem goodsand services Indeed, there has been considerable interest in quantifying thevalue of such services to neighboring communities to estimate the economiclosses that would be sustained with catastrophic loss of reef function.Best andBornbusch (2005)estimated that globally the goods and services provided byreefs exceed US$375 billion per annum However, such a value is likely anunderestimate in many reef regions, such as the central Pacific, wherenonmonetary values, customary and subsistence life styles prevail (Laurans

a South Pacific context, reflecting lower population concentrations andinfrastructure at risk Fisheries and aggregate extraction for construction aretwo prevalent activities that also derive significant economic benefit from reefsystems In the Maldives, fishing accounts for 99 percent of all exports and is

worth approximately US$170 million (16 percent of GDP) Perhaps equallyimportant are the extractive practices of communities that harvest reef fish,shellfish, and aggregates for subsistence or community-scale trade, which are acritical source of food and income, but which is often not captured in nationalscale assessments of income (Laurens et al., 2013; Jaleel, 2013) Less wellknown, and more difficult to quantify, are the intrinsic values of coastal andmarine biodiversity that emerge through the diversity of species and habitats.Least recognized are the range of goods and services afforded by thethree-dimensional structure of coral reefs (Moberg and Folke, 1999) First,reefs provide structural support for the living veneer of the reef system.Second, carbonate material is a source of aggregate for construction activities.Indeed, reef-derived sand and gravel for land reclamation and cement are theonly viable source of aggregate in many low-lying atoll nations Third, thephysical structure of coral reefs also acts as a buffer to incident ocean swell,thereby affording protection for reefal habitat and adjacent shorelines(Sheppard et al., 2005; Kennedy et al., 2013).

Despite debates over actual “values,” there is little doubt that the goods andservices outlined above exist because of the existence of coral reef systems(Figure 15.1) Consequently, a shift in the health or physical state of anindividual reef may impact negatively on the diversity and abundance ofservices provided At one end of the spectrum, healthy reef systems are mostlikely to maintain goods and services, and such services are likely to beimpacted in unhealthy or stressed reefs Further, as the system is perturbed bynatural or anthropogenic stresses the reef system will alter, which may beexpressed as a change in the level of ecosystem services provided by reefs(Figure 15.1) However, the time lag for such changes to propagate throughreef systems and be expressed in altered ecosystem services will varydepending on the magnitude and persistence of the stress event and whethermultiple stressors are acting in concert

15.3 IDENTIFYING HAZARDS AND KEY STRESSES

ON CORAL REEF SYSTEMS

The major stresses on reef systems and the range of impacts such eventsgenerate are summarized inTable 15.2 When evaluating the impact of naturalhazards and other stressors on reef systems, it is important to highlight

a number of additional considerations First, the concept of hazard oftenevokes natural climatic or tectonic events negatively impacting upon reefenvironments However, studies suggest that more prevalent hazards on reefsystems are anthropogenically or biologically induced Second, the impacts ofdiscrete perturbations on reef systems are temporally and spatially highlyvariable Third, the magnitude of impact of stress events on reef systems canvary depending on whether the stress event is a short-lived “pulse” event or aslow-onset change in environmental conditions Fourth, hazards may not affect

TABLE 15.2 Summary of Natural and Anthropogenic Stressors on CoralReef Systems and the Range of Reef Impacts

Stressor

Evidence of Stressor Impacts

on Reef Structure and Function

Changes to reef structure and stability, uplift or subsidence

of reefs and islands ( Baird et al., 2005; Aronson et al., 2012 ) Tsunami (earthquake/

landslide generated);

storms; significant weather events (cyclones, typhoons, hurricanes); king tides.

Coral mortality, generation

of rubble, loss of reef structure, erosion of shorelines, creation

of new land, or coastal erosion ( Maragos et al., 1973; Hagan

et al., 2007 ; Kench et al., 2006; Kench, 2011b; Richmond et al.,

2011 ), localized flooding and salination of crops and freshwater lens, terrestrial runoff, disruption of infrastructure ( White et al., 2013 ).

in ocean acidification

Calcifying organisms reduced growth rates and increasingly fragile skeletons.

Increasing sea-surface temperatures

Thermal stressors resulting in coral bleaching ( Hughes et al.,

2003 ) Predicted to increase in extent ( van Hooidonk et al.,

2013 ) Impact species specific and influenced by the level of anthropogenic disturbance ( Polidoro and Carpenter, 2013 ) Sea-level rise Increased water depth, enhanced

wave energy across reefs, and impacting shorelines Increased instability of islands Coastal inundation, erosion, saltwater intrusion in aquifers ( Mumby and Steneck, 2008 ).

Biological disruption;

invasive species, predator dominance, disease (also triggered by anthropogenic activity)

Increased diseases and changes

in ecological state of reefs, reduced species diversity, and abundance, including phase shifts and collapse of corals.

all components of the reef system equally For example, a particular event maypromote severe disruption to local communities, but may have more limitedimpact on the surrounding coral reef system The transition of an event oractivity in a reef system from a discrete, short-lived impact to an impact thatcan be deemed hazardous and disruptive to the wider reef system is highlycontext specific Fifth, the dominant metric for reef impact has been a shift inreef health, as commonly measured by changes in the proportion of livingcoral Such a metric has limited value in assessing other important servicesprovided by reefs Consequently, this chapter adopts a more encompassing

TABLE 15.2 Summary of Natural and Anthropogenic Stressors on CoralReef Systems and the Range of Reef Impactsdcont’d

Stressor

Evidence of Stressor Impacts

on Reef Structure and Function

Anthropogenic

stressors

Increases in suspended sediment in receiving waters as a result of runoff from mining, deforestation, agriculture.

Shading and smothering of corals ( Erftemeijer et al., 2012 );

increased macroalgal growth ( Yap, 2013 ); reduced live coral ( Fabricius, 2005 ); reduced species diversity ( Jaleel, 2013 ) Diffuse and direct source

inorganic and organic pollution resulting in nutrient enrichment/

Fishery exploitation (overfishing of resource and destructive fishing methodsdblast fishing, poisoning).

Overfishing results in loss of predator and/or herbivores and changes to trophic chain, changed biodiversity function ( Jaleel, 2013; Hughes, 1994; Ruppert et al., 2013 ).

Coral and coral sand extraction (harvesting and mining for aggregate extraction);

infrastructure development, reclamation, navigation channels, and port/

harbor expansion;

military activities.

Coastal erosion due to loss of sedimentary buffer Loss of reef structure and habitat ( Kench

et al., 2008 ) Modification of currents, wave energy, and processes across the reef platform Influences water circulation and flushing of lagoon reef flat.

definition of reef health that includes the structural integrity of reefs and itsability to maintain a positive carbonate budget state (e.g.,Perry et al., 2008).Sixth, when identifying hazards to reef systems, it is necessary to also considerthe relaxation period for reefs to recover or evaluate recovery is possible Thefollowing sections provide an overview of the key types of natural andanthropogenic stressors that impact the functioning of reef systems and theecosystem services they provide.

15.3.1 Environmental Stresses on Reef Systems

Changes in boundary environmental processes can pose severe stresses forcoral reefs (Figure 15.1) Such changes can occur as extreme high magnitudeand short-lived pulse events or occur more gradually through time and buildthe pressure on reef systems

15.3.1.1 Extreme Events

There are multiple geological stresses on reef systems that can causecatastrophic change in reef systems In tectonically active zones, verticaldisplacement of reefs can be caused by earthquakes (Baird et al., 2005) Forexample, the earthquake that generated the Sumatran tsunami in 2004promoted differential uplift and subsidence along >1,000 km of reef at theplate boundary between the Andaman and Nicobar islands (Bilham, 2005,Figure 15.2(a)) One of the most dramatic effects on reef systems was the uplift

of some fringing reefs by 1.6e2.0 m on the west coast of the Andaman Islands(Sieh, 2005; Searle, 2006) Uplift of this magnitude caused extensive mortality

of corals due to subaerial exposure (Hagan et al., 2007) and shifted previouslysubtidal reef communities into the intertidal zone, with likely medium-termconsequence for survival and adaption of these communities (Hagan et al.,2007) Development of cracks in the reef structure can also occur throughuplift, which reduces the structural integrity of the reef system (Bahuguna

et al., 2008) In contrast, other tracts of reef subsided by up to 2.5e3.0 m(Searle, 2006; Hagan et al., 2007), displacing reef communities to deeper reefzones While this subsidence has created new accommodation space for coralgrowth on reef tops, there have also been changes in hydrodynamics and watertemperatures that may affect species’ survival and reef community structure inthe medium term Aronson et al (2012) recorded the impact of the 2009earthquake in the Caribbean on previously monitored sites on the Belize reef.They note that reef slope destruction accounted for the loss of habitat in 10 (of21) sites, rendering any speculation about resilience to past environmentalchange pointless

Tsunami, generated by tectonic processes, undersea slumps and bolideimpacts can also cause major impacts on coral systems On human timeframes,tsunamis are relatively rare, but their impacts can be significant More than1,040 tsunamis have been recorded during 1914e2014 (Scheffers and Kelletat,

2003) although the majority of these events have been low in magnitude withnegligible impacts Catastrophic tsunami, having flow depths at the shoreline

>10 m, comprise <2 percent of the centennial tsunami record However, ongeological timescales, tsunamis are regular occurrences, there having been

>2,000 events during the past 4,000 years (NGDC, 2009) Of relevance to anassessment of reef vulnerability to tsunami is that coral reefs and their

associated sedimentary landforms occur throughout the Caribbean Sea andtropical Indian and Pacific Oceans, surrounded by tsunamigenic seismic zonesand have been the subject of multiple tsunami during the Holocene TheSumatran tsunami in 2004 was the worst tsunami disaster in recorded historyand provided the first megatsunami with which scientists could documentimpacts on coral reefs, and against which they could calibrate historicalreconstructions of tsunami impact (Kench et al., 2007).

Tsunami impacts on reefs can be expressed in different ways including(1) mechanical damage to corals such as the breaking and fracturing of corals(Baird et al., 2005,Figure 15.2(b)); (2) physical overturning and transport ofcorals from deeper reef fronts to reef flats (Figure 15.2(c),(d),(f)) For example,Goto et al (2007) identified>1,000 one-meter size coral blocks transportedfrom the reef front to the reef flat surface at Pakanang Cape, Thailand, as aconsequence of the 2004 Sumatran tsunami; (3) sedimentation throughdeposition of remobilized deeper marine sediments smothering corals andcausing mortality (Baird et al., 2005; Kelletat et al., 2007,Figure 15.2(e)) Asynthesis of posttsunami surveys indicates a number of preconditioning factorsthat make reefs susceptible to tsunami damage These factors include relativeexposure of reefs to wave impact; reef slope and nearshore bathymetry thatcontrol wave shoaling and breaking potential; the water depth across the reefsurface; the ecological composition of reef communities; and the substratetype upon which corals are attached Due to this combination of factors,damage to reef communities is highly variable and difficult to predict (Baird

et al., 2005; Kench, 2011b; Worachananant et al., 2007)

While the ecological impacts of tsunami can be benign in some reef areas,adjacent coastal landforms can experience catastrophic wave forces thatphysically destabilize and alter landforms and cause major losses ofinfrastructure and life Shoreline erosion (net loss of land) is a commonly citedeffect of tsunami wave interaction with reef sedimentary landforms Forinstance, along a 9.2-km stretch of coastline at Lhok Nga Bay, Banda Aceh, themean rate of shoreline displacement was approximately 60 m and involvedreworking of 276,000 m3of sediment from the 2004 Sumatran tsunami (Paris

et al., 2009) However, in other reef sites, erosion was less prevalent Likeecological impacts, the magnitude of reported erosion is spatially variable Forexample, based on a comparison of pre-tsunami and post-tsunami surveys ofreef islands, Kench et al (2006) found that erosion of the vegetated core ofislands in the Maldives ranged from 1 percent to 9 percent of island area Thereare a number of explanations for this disparity in erosion response that includeproximity (exposure) to tsunami source; elevation of the seaward margin; dif-ferences in the way tsunami interact with the shoreline; the volume of sedimentpresent at shorelines to act as a buffer to tsunami impact; and the presence orabsence of vegetation On Maldivian reef islands, where plentiful volumes

of sediment were stored in the beaches, this material was able to absorb impact

of the tsunami and shoreline erosion was minimal (Kench et al., 2008)

A number of studies have also shown that an additional geomorphicoutcome of tsunami is the creation and aggradation of coastal landformsthrough overwash sedimentation Landward deposition of sand sheets has beenobserved on numerous reef-fringed shorelines and low-lying atoll islands,where sediment rich coasts have undergone net erosion Boulder fields areperhaps the most commonly cited product of the landward transfer ofnearshore material under tsunami flow Paleo studies have identified boulders(reef framework and coral bommies) up to an 8-m diameter and weighing

>260 tonnes (Scheffers et al., 2009,Figure 15.2(c))

Cyclones/hurricanes (storms) are an additional extreme “pulse” event thatcan impart swathes of destruction to coral reefs with significant ecologicaland geological outcomes Much attention has been focused on the role ofextreme storms and cyclones in promoting change in coral reef ecosystems(Woodley et al., 1981; Done, 1992; Woodley, 1992) and geomorphic structure

of reef systems (e.g., Stoddart, 1963; Maragos et al., 1973; Bayliss-Smith,1988; Scoffin, 1993; Blanchon et al., 1997) While some of these impactsare similar to those of tsunami, there are significant differences in the nature

of extreme weather impacts During cyclone/hurricane events, the waves thatare generated differ markedly to tsunami In particular, waves have shorterperiods (12e20 s) and larger heights, which can exceed 10 m During anindividual storm event, reefs are assaulted by thousands of these large waves,which shoal and break across the shallow forereef and reef edge, deliveringextreme force and energy on the reef This impact differs from tsunamievents, which are characterized by a smaller number of waves (w1e20) in anindividual episode In addition, low atmospheric pressure conditions thataccompany extreme events can superelevate water levels across reef systems,resulting in destructive wave energy propagating across the reef flat

to adjacent coastlines Such effects include wave overtopping leading toflooding, and remobilization of shoreline sediments (erosion and accretion).Forbes et al (2013)make reference to the significant damage to Niue, a raisedatoll, arising as a result of cyclones Ofa and Heta They argue that despite theelevation of infrastructure the narrow reef width offered limited protectionfrom high wave energy

At the extreme end of the impact spectrum, storms can decimate the livingcoral veneer of reefs to wave base water depths (the water depth of physicaleffect of waves), effectively resetting the physical substrate ready forrecolonization In such instances, the time lag for recovery may be manydecades and the resultant diversity and abundance of coral cover may shift to anew ecological state (White et al., 2013) There is growing evidence that, liketsunami, the effects of such extreme storms can be highly localized Forexample,Perry et al (2014)examined the ecological and geomorphic impacts

of Tropical Cyclone Yasi, a 700-km-wide category 5 cyclone, on sections ofthe Great Barrier Reef that were directly impacted They found impacts of TCYasi to be site specific and spatially heterogeneous, strongly influenced by the

orientation of the reef to the storm path At the most affected sites, small-scaletaxa-specific impacts were documented (breaking of branching Acroporacorals), but other sites remained unaffected However, geomorphic changeswere minor and ecological impacts highly variable between sites, with noobserved evidence of major reef structural change.

While storm damage is potentially disruptive to coral structures and cannegatively impact on the reef systems (in the short to medium term), there isevidence of storms playing a significant role in the geological development ofreefs Geomorphically, storms can have both destructive and constructiveeffects on reef landforms Storms can mechanically erode coral material,converting it to sand, gravel, and boulders, which can be deposited onto reefsurfaces and islands (Scoffin, 1993,Figure 15.3) For example, Hurricane Bebe

in 1972 deposited 1.4 106m3of storm rubble (dead coral) on the windwardreef flat of Funafuti atoll, Tuvalu (Maragos et al., 1973,Figure 15.3) Post-storm reworking of this rubble has increased the area of reef islands by>10percent (Baines and McLean, 1976) In another example,Chivas et al (1986)and Hayne and Chappell (2001) show that Lady Elliot Island in the GreatBarrier Reef was formed through the sequential deposition of storm rubble

FIGURE 15.3 Examples of cyclone-driven changes on coral systems: (a) storm blocks and rubble tract, southwest reef flat, Funafuti atoll (b) Cyclone deposited boulders on island surface, Funamanu, Funafuti atoll, Tuvalu (c) and (d) Remnant rubble ridges deposited following cyclone Bebe (1972) on the eastern reef flat of Funafuti atoll, Tuvalu Source of all photographs, author.

banks Sequential storm deposition has also been highlighted as a key agent inthe vertical development of coral reefs structures (Blanchon et al., 1997).Extreme events can impart both erosional and constructional responses onreef islands and therefore are of primary concern for coastal communities Thelikely geomorphic response is dependent upon the physical size of materialcomprising land and the frequency and intensity of storms (Bayliss-Smith,1988) Where storm frequency is low, landforms are generally composed ofsand-size sediments (e.g., sand cays), which are susceptible to erosion duringstorms For example,Stoddart (1963)documented mass destruction of somereef islands in the Belize Barrier Reef as a consequence of Hurricane Hattie.The recovery time for such landforms to reform may be decades In contrast,

in storm dominated reef settings, islands are commonly composed of coarserubble on their exposed reef margins while islands on leeward reefs aretypically composed of sand As noted above, in these settings, storms mayprovide episodic additions of material allowing landforms to expand

15.3.1.2 Slow-Onset Environmental Changes

The environmental boundary controls on reef ecosystem function and state,such as ocean water chemistry and sea level (Figure 15.1), are expected tochange over the coming centuries forcing transitions in reef ecological andphysical state, such as the change from healthy to degraded reef condition andassociated loss of reef structure (e.g., Figure 15.4) Rising sea levels willinundate reef surfaces and create additional accommodation space for verticalreef growth Sea-level rise per se poses no specific threats to reef ecologicalsystems Increasing water depth may promote shifts in habitat zonation andstimulate recolonization of coral reef surface Considerable debate exists con-cerning the capacity of reefs to be able to accrete vertically with sea-level rise,which is likely dependent on the health of a particular reef system Geologicalevidence suggests that healthy reefs may be able to respond to current pro-jections of sea-level change (Kench et al., 2008) However, impacted reef sitesmay have less capacity to respond leading to submergent reef surfaces.Increasing water depths across reef surfaces, as a consequence of sea-level rise,poses more significant consequences for reef-associated landforms such as reefislands Reefs act as an effective buffer to incident wave energy, as their shallowdepth forces waves to shoal and break releasing their energy at the reef edge.However, as water depth increases, greater wave energy can propagate acrossreefs and impact shorelines promoting flooding and coastal change andthreatening coastal infrastructure and communities (Sheppard et al., 2005)

15.3.1.2.1 Ocean Temperatures and Coral Bleaching

Sea-surface temperature has a major role in modulating the distribution andgrowth of coral systems There are some predictions that warming of the oceansmight extend the region of reef growth into areas that are currently too cool to