Fringing and Nearshore Coral Reefs of the Great Barrier ... · Fringing and nearshore reefs, the...

Journal of Coastal Research 22 1 175–187 West Palm Beach, Florida January 2006

Fringing and Nearshore Coral Reefs of the Great Barrier Reef:Episodic Holocene Development and Future ProspectsS.G. Smithers, D. Hopley, and K.E. Parnell

School of Tropical Environment Studies and GeographyJames Cook UniversityTownsville, Queensland 4811, [email protected]

ABSTRACT

SMITHERS, S.G.; HOPLEY, D., and PARNELL, K.E., 2006. Fringing and nearshore coral reefs of the Great BarrierReef: episodic Holocene development and future prospects. Journal of Coastal Research, 22(1), 175–187. West PalmBeach (Florida), ISSN 0749-0208.

The Holocene growth of fringing and nearshore reefs on the GBR is examined. A review of data from 21 reefs indicatesthat most grow upon Pleistocene reef, boulder, and gravel, or sand and clay substrates, with no cored examplesgrowing directly over rocky headlands or shores. Dated microatolls and material from shallow reef-flat cores indicatethat fringing and nearshore reefs have experienced several critical growth phases since the mid-Holocene: (1) frominitiation to 5500 YBP, optimum conditions for reef and reef-flat growth prevailed; (2) from 5500–4800 YBP, reef-flatprogradation stalls in almost 50% of the reefs examined; (3) of reefs prograding post-4800 YBP, approximately halfceased active progradation around 3000–2500 YBP; (4) reefs prograding to present do so at rates well below mid-Holocene rates; (5) a group of nearshore reefs has established since 3000 YBP, in conditions traditionally consideredpoor for reef establishment and growth. Importantly, many of the reefs that appear to have grown little for severalmillennia are veneered by well-developed coral communities. Although local conditions no doubt exert some influenceover these growth patterns, the apparent synchronicity of these growth and quiescent phases over wide geographicalareas suggests the involvement of broader scale influences, such as climate and sea-level change. Recognition andunderstanding these phases of active and moribund reef growth provides a useful longer term context in which toevaluate reported current declines in fringing and nearshore reef condition.

ADDITIONAL INDEX WORDS: Coral reef geomorphology, Holocene reef growth, sea-level change, turbidity, microatoll.

INTRODUCTION

The Great Barrier Reef (GBR) extends almost 2000 kmdown the northeast Australian coast and covers approxi-mately 250,000 km2. It is the largest contiguous coral reefecosystem in the world and arguably the best protected andmanaged. Despite this, many reef scientists believe that theGBR is not isolated from the alarming decline affecting reefsglobally (HUGHES et al., 2003). Indirect anthropogenic im-pacts such as rising sea-surface temperatures due to green-house gas emissions (HOEGH-GULDBERG, 1999) and changesin ecosystem structure due to overfishing (JACKSON et al.,2001; PANDOLFI et al., 2003) have been inferred to be signif-icant drivers of deteriorating reef condition. However, themost critical direct pressures affecting GBR reefs are gener-ally associated with terrestrial anthropogenic activities, suchas increased sediment influx (FABRICIUS and WOLANSKY,2000; MCCULLOCH et al., 2003) and deteriorating water qual-ity (HAYNES and MICHALEK-WAGNER, 2000).

Fringing and nearshore reefs, the former comprising shore-attached reefs and the latter including reefs located veryclose to the shore or separated from it by back reef areasshallower than 10 m (LARCOMBE, COSTEN, and WOOLFE,2001) are particularly vulnerable to land-based threats. Eco-logical surveys of fringing and nearshore reefs on the GBR

DOI:10.2112/05A-0013.1 received and accepted 10 May 2005.

document reduced biodiversity and coral cover, reduced re-cruitment, and increased bleaching and other disturbancesover recent decades, raising concerns that these systems faceimminent collapse (BELLWOOD et al., 2004).

Many reef scientists believe that the long-term prospectsof all fringing and nearshore reefs on the GBR are poor, andinvariably anthropogenic activities are implicated in their de-mise. In this article, we wish to provide a longer term per-spective on the growth of these reefs with which the signifi-cance of contemporary ecological changes can be assessed. Wehave examined the Holocene growth histories available forfringing and nearshore reefs on the GBR, including severalfrom Torres Strait. These data suggest that the reef-buildingcapacity of many fringing and nearshore reefs in this regiondramatically declined around two distinct periods after themid-Holocene. We believe that the identification of such per-turbations provides an important context for assessments ofreported recent declines in inshore reef health.

DISTRIBUTION AND PREVIOUS WORK

HOPLEY, PARNELL, and ISDALE (1989) identified 758 fring-ing reefs in the Great Barrier Reef Marine Park using aerialphotographs and satellite imagery. This total includes 545reefs with recognisable reef flats and 213 incipient fringingreefs defined as shore-attached reefs lacking reef flats. Re-cent spatial analysis and modelling yields similar results

176 Smithers, Hopley, and Parnell

Journal of Coastal Research, Vol. 22, No. 1, 2006

Figure 1. Distribution of fringing and nearshore reefs for which Holocene growth data are available.

(LEWIS, 2001). Although they are relatively abundant, mostGBR fringing reefs are small (mean: 1 km2). They cover ;350km2 of the inner continental shelf and comprise just 1.8% ofthe GBR’s total reefal area (HOPLEY, PARNELL, and ISDALE,1989). Fringing reefs are common around many inshore highislands but are rare along the mainland coast. Mainlandfringing reefs are most prevalent north of Cairns and alongthe Whitsunday Coast. Inshore reef growth in the southernGBR (south of 218 S) is generally poor, except around theKeppel Islands, where substantive reef flats have developed(Figure 1). Diminished inshore reef growth in the southern

GBR appears principally related to turbidity controls on com-munity composition and demography and on carbonate pro-ductivity, preservation, and accumulation (KLEYPAS, 1996;VAN WOESIK and DONE, 1997). Reduced reef growth towardthe Southern GBR has also been associated with larger scalelatitudinal gradients in parameters such as water tempera-ture and available light (HARRIOTT and BANKS, 2002).

The exact number of nearshore reefs is unknown due todifficulties of classification and recognition. However, an es-timate exceeding 100 appears reasonable based on recentlyreleased maps (GBRMPA, 2004). Field observations indicate

177Fringing and Nearshore Reefs of the GBR


Table 1. Fringing reef inundation and colonisation by substrate.

Foundation Type Reef NameDepth of

Foundation Age of InitiationYears afterInundation* Reference

Pleistocene reef Yam IslandHayman IslandCockermouth IslandPenrith IslandHigh Peak IslandHammond IslandScawfell Island

5.5159.98.1

106.8

.17

7460932078807430765063408070

600100700

10001250850

.1000

Woodroffe et al. (2000)Hopley et al. (1978)Kleypas (1996)Kleypas (1996)Kleypas (1996)Woodroffe et al. (2000)Kleypas (1996)

Clay/sand Pioneer Bay, Orpheus Island 10 6610 2050 Hopley et al. (1983)Fantome IslandRattlesnake IslandPaluma Shoals

6105

591070101657

220016006300†

Johnson and Risk (1987)Hopley (1982)Smithers and Larcombe (2003)

Gravel/boulders Emmagen ReefRykers ReefMyall Reef

4.0567.9

628077807330

1500350

1100

Partain and Hopley (1989)Partain and Hopley (1989)Partain and Hopley (1989)

Iris Point, Orpheus Island 6‡ 7320 750 Hopley and Barnes (1985)

* Derived by comparison with sea-level curve established by Thom (1983).† Substrates of equivalent depth would be inundated; however, the substrate at Paluma Shoals has been exposed more recently by coastal erosion.‡ Shallower depth close to shore, but 6 m is the general depth to basement over most of the reef.

that many shoals exhibit moderate to high coral cover, andalthough most are topographically simple (e.g., ShoalwaterBay; see AYLING et al., 1998), others develop bathymetric re-lief (e.g., Paluma Shoals; see SMITHERS and LARCOMBE,2003). In this regard, they may be synonymous with incipientfringing reefs (HOPLEY, PARNELL, and ISDALE, 1989), whichmay include reefs at various growth stages (KLEYPAS, 1996).For example, they may be catch-up reefs destined to formtrue fringing reefs with time, they may be reefs in which ver-tical accretion rates have slowed, or they may be coral com-munities incapable of accumulating carbonate and producinga coral reef under present conditions. Numerous definitionsof coral reefs, coral-reef communities, coral communities, etc.,exist in the literature, and will not be replicated here (e.g.,BUDDEMEIER and HOPLEY, 1988; KLEYPAS, BUDDEMEIER,and GATTUSO, 2001). We recognise that shoals may not meetthe definition of a coral reef preferred by some researchers,but we consider it appropriate given the longer term viewpresented in this article and the possibility that shoals mayturn-on (BUDDEMEIER and HOPLEY, 1988) and develop moresubstantially in the future.

Nearshore reefs are more evenly distributed along the GBRcoast than fringing reefs but appear particularly common inHalifax Bay north of Townsville, and south of 218 S (Figure1) (see KLEYPAS, 1996; Table 1). Many occur in turbid waterand adjacent to sedimentary coastlines, both conditions usu-ally considered marginal for reef growth (PERRY, 2003).

Holocene growth histories have been established for 21fringing reefs in the GBR (not including Torres Strait), rep-resenting just 3% of the total. All these have well-developedreef flats. The Holocene growth history of only one nearshorereef has been published (SMITHERS and LARCOMBE, 2003).Furthermore, although conceptual models of incipient fring-ing reef growth exist (CHAPPELL et al., 1983), they are basedon chronostratigraphic investigations of reefs with reef flats,and their application to incipient fringing reefs remains un-verified. The regrettably small sample available for our anal-ysis means that it is unlikely that the diversity of morphol-

ogy, setting, structure, and Holocene growth history possiblefor fringing reefs was fully captured. Current and plannedresearch will develop a more complete understanding ofmainland fringing reefs in the Whitsunday region, nearshoreturbid zone reefs in the Central GBR, and fringing reefgrowth in the Keppel Islands in the southern GBR.

COASTAL SETTINGS AND REEF-FLATMORPHOLOGY

KENNEDY and WOODROFFE (2002) recently reviewed thegeomorphology and development of fringing reefs fromaround the world, including those of the GBR. Most fringingand nearshore reefs on the GBR can be broadly classified asone of four main types based on their coastal setting: (1)headland, (2) bayhead, (3) narrow beach-base, (4) nearshoreshoals. This simple scheme requires little explanation, but itis important to recognise that any particular fringing reef, orsection of it, may display characteristics transitional betweenseveral classes, both spatially and through time.

Detailed descriptions of the surface morphologies of variousGBR fringing reefs are accessible in BIRD (1971), CHAPPELL

et al. (1983), HOPLEY (1982), HOPLEY and BARNES (1985),HOPLEY et al. (1978, 1983), JOHNSON and RISK (1987), KAN,NAKASHIMA, and HOPLEY (1997), KLEYPAS (1996), and VAN

WOESIK and DONE (1997). Reef flats on these fringing reefsusually develop a simple surface morphology, comprising aforereef, reef crest, and backreef. The compositional complex-ity of each zone, however, can differ over a single reef andbetween reefs. For example, morphologically complex back-reefs can evolve where marine and terrestrial processes com-bine to produce terrigenous sand aprons, ponds moated byalgal rims, and fossil microatoll fields. A late-Holocene sea-level fall of around 1 m (CHAPPELL, 1983) has also influencedthe backreef morphological complexity of many fringing reefflats on the GBR. HOPLEY and BARNES (1985) identifiedeight zones on the reef flat at Iris Point, Orpheus Island,including seven over the backreef. After examining published



Figure 2. Fringing and nearshore reef initiation ages by substrate.

data for GBR fringing reefs, KENNEDY and WOODROFFE

(2002) concluded that wider and more conspicuously zonedreef flats usually develop in windward settings and awayfrom the mainland coast or river mouths. Whilst wide reefflats do occur in some windward locations (e.g., Penrith Is-land), many of the widest reef flats occur in leeward, mod-erately recessed bayheads, such as Pioneer Bay and HazardBay, Orpheus Island.

Nearshore reefs occur in a range of settings and at variousstages of development. Descriptions of their morphology arerare, but at Paluma Shoals, a series of broad (;400-m-wide)coral rubble armoured reef flats occur near lowest astronom-ical tide (LAT) (SMITHERS and LARCOMBE, 2003). The sea-ward reef slope is steep but short, rising from the sea bed ;5m below LAT. A narrow (20–50 m) shingle and rubble-cov-ered zone borders the reef crest, behind which another zonedominated by Goniastrea sp. microatolls occupies most of thereef flat. The locally indistinct and narrow leeward zone con-sists almost exclusively of Galaxea fascicularis. Structuressimilar to Paluma Shoals occur elsewhere along the coast, butfew have developed large reef flats (e.g., Ollera Shoals (178589S, 1468059 E), and Lugger Bay (188599 S, 1468229 E). AYLING,AYLING, and BERKELMANS (1998) reported that shoals inShoalwater Bay lacked true reef development, with mostcomprising shallow rubble banks with low topographic com-plexity, biodiversity, and coral cover. The reasons for suchdifferences are unclear but may reflect different ages, growthmodes, and disturbance regimes.

INTERNAL STRUCTURE AND FABRIC

Several decades of drilling and radiometric dating of fring-ing reefs around the world has dispelled the earlier view thatthey are simple structures (STEERS and STODDART, 1977).CHAPPELL et al. (1983) developed a basic structural classifi-cation for GBR fringing reefs that differentiates betweenthose that build reef flats by prograding out from shore andthose that form reef flats when isolated patch reefs coalesceover a broad nearshore area. The spatial pattern of reef-flatage, usually established by dating coral microatolls or ma-terial in shallow reef-flat cores, is diagnostic in this classifi-cation. HOPLEY and PARTAIN (1987) proposed a more com-plex structural classification for fringing reefs that considersthe nature of reefal foundations and the relative proportionsand positions of framework, detrital, and terrigenous sedi-ments within reef structure. The availability of a stable sub-strate and/or one elevated above mobile sea-floor sedimentsis a commonly cited requirement for colonisation and subse-quent reef growth (VERON, 1995). Suitable surfaces on theGBR vary from hard rock on high island headlands to coastaldeposits, such as spits and bars. Rock surfaces are widelyconsidered excellent colonisation substrates due to their sta-bility and elevation, but very few fringing reefs on the GBRare headland attached (despite widespread availability). In-deed, most of the GBR’s largest fringing reefs have grownover sediments deposited within well-defined coastal embay-ments during the postglacial marine transgression (PMT).Available data indicate that Pleistocene reef substrates weregenerally the earliest colonised during the transgression

(even at shallower depths, and thus sooner after inundation),with gravel and boulder substrates and clay and sand sub-strates typically colonised later (Figure 2). This lag probablyrelates to the interval between sea-level inundation of thesesurfaces and improved water quality and substrate stability(HOPLEY, 1994).

HOPLEY and PARTAIN’s (1987) structural classification ispresented in Figure 3, augmented with additional classesidentified or inferred from later stratigraphic investigationsfrom the GBR and Torres Strait. The major structural char-acteristics of each main class are described below:

(a) Simple fringing reefs formed on inundated rocky fore-shores during the PMT (Figure 3a). These reefs are rare onthe GBR. As far as we are aware, no reefs of this type havehad their Holocene growth history determined by coring anddating. Narrow fringing reefs on the windward coast of GreatPalm Island are a possible example.

(b) Fringing reefs developed over gently sloping substratesthat become outflanked by transgressionary shoreline retreatand are often dominated by detrital backfill (Figure 3b).Shoreline backstepping during the PMT leaves a formerfringing reef offshore. Once sea level stabilises and the reefreaches sea level, both backfilling and seaward progradationcan occur. Hayman Island is an example of this type of fring-ing reef (HOPLEY et al., 1978).

(c) Fringing reefs developed over pre-existing positive sed-imentary structures, including Pleistocene alluvial fans (e.g.,Paluma Shoals; SMITHERS and LARCOMBE, 2003) (Figure3c[i]), transgressionary sediment accumulations trapped inbays (e.g., Pioneer Bay, Orpheus Island; HOPLEY et al., 1983)



Figure 3. Fringing reef-flat structural classification (modified from Hopley and Partain, 1987). Isochrons are indicative only.

(Figure 3c[ii]), leeside island spits (e.g., Rattlesnake Island;HOPLEY, 1982) (Figure 3c[iii]), and deltaic gravels (e.g., MyallReef, Cape Tribulation; PARTAIN and HOPLEY, 1989) (Figure3c[iv]).

(d) Fringing reefs developed by episodically building a newreef structure parallel to the existing reef front and thenbackfilling the intervening space predominantly with uncon-solidated reef sediments (Figure 3d). This mode of fringingreef development has been established for Yam Island, TorresStrait (WOODROFFE et al., 2000).

(e) Fringing reefs developed offshore and the terrigenous

shoreline has prograded seaward, connecting the reef to thecoast and/or provided a foundation over which the reef hasbuilt shoreward (Figure 3e). Possible examples of this type offringing reef include Yule Point (BIRD, 1971) and King Reef(GRAHAM, 1993).

Holocene growth fabrics developed by GBR fringing reefsand revealed by drilling provide important insights into theirdevelopment and dynamics. First, the variety of substrateson which fringing reefs form is clearly demonstrated (Table1). This characteristic distinguishes them from shelf reefs,which exclusively develop over Pleistocene reefal founda-



tions. Second, the diversity of fringing reef growth fabric andconstruction is revealed. Third, the importance of reefal pro-duction, preservation as framework or detrital facies, inter-actions with terrigenous sediments, and ultimately the rela-tive contributions of each component to reef growth are re-corded. Fourth, the taxonomic, demographic, morphologic,taphonomic, and chemical characteristics of reefal compo-nents can provide insights into time-transgressive interac-tions between environmental conditions and reef building,reef destruction, and reef recovery for the interval over whichthe reef has accreted. Clearly, longer-term changes in fring-ing reef morphology and condition archived in reef structuresare an important data source against which to evaluate con-temporary change.

HOLOCENE REEF GROWTH

Most fringing reefs on the GBR were initiated very soonafter the PMT flooded their foundations, with substrate typepossibly affecting take-off (Table 1). The fringing reef at Hay-man Island has the earliest known initiation; an in situ coralabove the Pleistocene reef contact returned a conventionalradiocarbon age of 9320 YBP (HOPLEY et al., 1978). The youn-gest radiometric date immediately above a fringing reef sub-strate is from Percy Island (3720 conventional radiocarbonYBP) (KLEYPAS and HOPLEY, 1992), but the oldest basal datefrom Paluma Shoals, a nearshore reef, is even younger atonly 1657 YBP (SMITHERS and LARCOMBE, 2003). Excludingthese outliers, most fringing reefs initiated within a relative-ly narrow period, with an average age of initiation around7100 YBP.

Most fringing and nearshore reefs on the GBR have accu-mulated modest Holocene thicknesses, generally extending5–10 m above their foundations. Shallow inshore depths con-strain accommodation space, limiting the thickness of manyreefs. At the extreme end of the spectrum, the Holocene reefat Digby Island is limited to a thin living veneer overlying aPleistocene reef that is patchily exposed at the surface (KLEY-PAS and HOPLEY, 1992). More impressive Holocene reef ac-cumulations do occur; for example, the Holocene reef at Scaw-fell Island rises more than 18 m from an undetermined sub-strate (KLEYPAS, 1996). Following initiation, the verticalgrowth curve for most fringing reefs was sigmoidal (DAVIES

and MARSHALL, 1979). Early growth rates were slow but ac-celerated as water depth over the reef increased during thePMT, improving conditions for framework construction. Fa-vourable conditions appear to have persisted until the peakof the transgression, after which reefs grew into the stillstandwave base and vertical accretion rates slowed. A summary ofvertical accretion rates established for many GBR reefs fromdrill-core data is provided by HOPLEY, SMITHERS, and PAR-NELL (unpublished manuscript). The modal framework ac-cumulation rate for the entire dataset is ;7.5 mm/y. Thisfigure represents a net annual accretion rate for material ac-cumulated over centuries, thus filtering out distortions dueto short-term variability. PARTAIN and HOPLEY (1989) estab-lished that vertical accumulation rates for fringing reefs out-stripped midshelf and outer reefs at various paleo-depths andthat rapid rates of vertical accretion (.5.5 mm/y) for fringing

reefs occur within a narrow depth window of between 4 and8 m below palaeo-sea-level. High vertical accretion rates aresustained to depths approaching 16 m at outer and midshelfreefs (HOPLEY, 1989), but near-surface accretion rates areubiquitously slow over the entire GBR (DAVIES and HOPLEY,1983).

The majority of reefs on the GBR for which radiometricallydated core material is available accumulated most of theirbulk between 8000 and 5000 YBP, but the trend is particu-larly pronounced on fringing reefs (DAVIES and HOPLEY,1983; HOPLEY, 1989). Discussions of reef growth typically fo-cus on net rates of vertical accretion (e.g., DAVIES and HO-PLEY, 1983; DAVIES, MARSHALL, and HOPLEY, 1985). Ratesof lateral growth, particularly framework growth to seaward,have received comparatively little attention (MASSE andMONTAGGIONI, 2001, is an exception). CHAPPELL et al. (1983)provide the most detailed consideration of lateral fringingreef growth on the GBR, reporting the ages of fossil micro-atolls across numerous Central GBR fringing reef flats. Reef-flat progradation rates are affected by a number of factorsthat are independent of net carbonate productivity rates, in-cluding substrate depth and shape, terrigenous influx, sea-level history, and accommodation space. These factors arediscussed later. However, a simple plot of reef-flat age (es-tablished from radiometrically dated microatolls or core ma-terial ,1 m below surface) against relative position on reefflat suggests fringing reefs on the GBR have experienced sev-eral critical phases since the mid-Holocene that have affectedlateral reef-flat growth (Figure 4). These phases are outlinedbelow.

Initiation to 5500 YBP: Optimum Reef Growth andReef-Flat Extension

Prior to 5500 YBP, conditions for both coral calcificationand inshore reef growth on the GBR were good. Vertical reefaccretion rates on most fringing reefs were close to maximum(HOPLEY, 1989), probably encouraged by continuously risingsea level, which created accommodation space at optimaldepths for carbonate production and vertical accretion. In wa-ter depths between 4 and 8 m, corals lie below the prevailingwave base, allowing a more fragile and open framework todevelop, but are shallow enough for light-enhanced calcifi-cation to continue in turbid conditions. Radiocarbon datesfrom microatolls and near-surface corals indicate that manyfringing reefs reached sea level near the start of the stillstand;5500 YBP, and some even before (e.g., a reef-top coral atNorth Iris Point was dated at 6260 6 120 YBP (HOPLEY andBARNES, 1985)). As a result, many have grown up to a risingsea-level position for most of their history and thus experi-enced optimal conditions for vertical accretion for an extend-ed period.

The possibility that high accretion rates were promoted bycoincident rapid calcification prior to 5500 YBP is suggestedby wide annual band widths in massive Porites interceptedin cores through fringing reefs at Cape Tribulation (PARTAIN

and HOPLEY, 1989). Sea-surface temperature is strongly andpositively correlated with calcification and linear extensionin modern massive Porites (LOUGH and BARNES, 2000), and



Figure 4. Reef-flat progradation rates established from published sources. Three broad growth trajectories can be distinguished: (1) Sprinters that hadbuilt around 90% of their reef flats by 4.8 ka BP, (2) strugglers that had built most of their reef flats by around 2.5 ka BP, and (3) stayers that havecontinued to grow to present but at relatively slow rates during the Late Holocene.

inner GBR sea temperatures were ;18C warmer than pres-ent at this time (GAGAN et al., 1998). In an early review,BUDDEMEIER and KINZIE (1976) concluded that coral growthrates and reef accretion were not directly linked, but othersargue that lower coral growth rates impede reef accretion athigh latitudes (HARRIOTT, 1999; LOUGH and BARNES, 2000).It is possible that temporal shifts in environmental parame-ters such as sea temperature may similarly modulate coralcalcification and reef accretion.

Carbonate productivity and/or reef growth prior to 5500YBP may also have been enhanced by a less variable andintense rainfall regime as indicated by pollen cores (KER-SHAW and NANSON, 1993), fluorescence, and stable isotopeand trace element records contained within coral skeletons(GAGAN et al., 1996). A more benign climate would have sev-eral benefits. Most particularly, extreme floods and droughtswould be rarer and good vegetation cover would limit sedi-ment runoff to the GBR, where reef growth may be affected(MCCULLOCH et al., 2003). More benign inshore water-qual-ity conditions may also have existed at that time due to great-er nearshore flushing and mixing of terrestrial runoff by oce-anic swell and waves under open mid-Holocene high-energy-window conditions (HOPLEY, 1984). Also, the nearshore mudwedge, a muddy sediment body that extends from near theshoreline to ;15 m depth along most of the coast (JOHNSON

and CARTER, 1987), was not yet significantly developed. Themud wedge is shore attached in sheltered embayments, pre-venting coral and reef growth. However, it is shore detached

on exposed coasts where more energetic waves in coastalshallows (to ;25 m LAT) keep fine sediments suspended(WOOLFE and LARCOMBE, 1998) and turbidity levels high(LARCOMBE, COSTEN, and WOOLFE, 2001).

5500–4800 YBP: Reef-Flat Progradation Stalls

In almost half of our sample, reef-flat growth and extensionslowed abruptly between approximately 5500 and 4800 YBP.On these reefs, ;90% of reef-flat progradation occurred inthe millennium preceding 5500 YBP and only ;10% since(Figure 4). Despite this marked decline in reef-flat progra-dation, seemingly healthy coral communities occupy the con-temporary reef fronts of many. Examples include the north-ern part of the reef flat at Iris Point (HOPLEY and BARNES,1985) and the Cape Tribulation fringing reefs (Rykers, Em-magen, Myall) (PARTAIN and HOPLEY, 1989).

The most straightforward explanation for this stall in pro-gradation is that these reefs had simply outgrown their foun-dations. The widespread synchrony in Holocene reef initia-tion around 7000 YBP (DAVIES and HOPLEY, 1983) suggeststhat suitable substrate for successful colonisation was abun-dant at this time, and rapid vertical accretion may havequickly exhausted both vertical and lateral accommodationspace, completely covering optimum foundations and leavingfew opportunities for sustained lateral growth. Structuraldata suggest that this scenario may apply to fringing reefs atCape Tribulation (Rykers, Emmagen, Myall), which grow



Figure 5. Average growth rates for GBR fringing reefs over the last 8000years based on radiometrically dated cores.

over fan gravels (PARTAIN and HOPLEY, 1989); the fringingreef at North Iris Point, which grows over a boulder beach(HOPLEY and BARNES, 1985); and possibly the fringing reefsat Cockermouth and Penrith Islands, which have Pleistocenereef and aeolionite foundations (KLEYPAS and HOPLEY,1992). All substantially covered their present foundations by5500 YBP. Fringing reefs at Dunk Island and Great Palmextend over low gradient clays and sands and are more dif-ficult to explain in this way. Significantly, on Hayman Island,the initial reef crest was set back around 100 m from theseaward edge of the Pleistocene reef foundation, leaving un-occupied substrate to seaward, and reef-flat progradationcontinued there after 5500 YBP. Where original foundationsare completely occupied, reef-front talus provides a self-sourced substrate for continued progradation on some reefs(e.g., Pioneer Bay, Orpheus Island; HOPLEY et al., 1983). How-ever, progradation rates would generally be expected to beslower over these relatively disturbance-prone and unstablesubstrates.

Reduced reef-flat progradation around 5500–4800 YBPclosely coincides with the PMT peak, thus implicating directand indirect consequences of sea-level stabilisation and sub-sequent relative sea-level fall. Relative sea levels in the innerGBR around 5500 YBP were around 1 m above present andhave fallen gradually since (CHAPPELL et al., 1982; HOPLEY

and THOM, 1983). Assuming a constant decline (CHAPPELL,1983), sea levels would have dropped 0.10–0.15 m by 4800YBP, possibly enough to turn off production over many reefflats and restrict healthy reef growth to reef-edge environ-ments. Falling sea levels gradually exposed reef flats con-structed during higher mid-Holocene sea levels. Prolongedemergence has obvious impacts on most reef-flat communi-ties but may also affect those that remain submerged on thereef crest and front. Fringing reef flats increasingly exposedby relative sea-level fall suffered increased mortality andmany changed from intertidal coral communities to algal-and sediment-dominated habitats. Reef-flat hydrology andhydrodynamics were altered, and bioerosion rates on, andsediment yields off these reef flats probably increased. Causallinks between modified backreef conditions, especially water-quality deterioration and reduced reef-front community vig-our, have been argued elsewhere (NEUMANN and MAC-INTYRE, 1985; SCHLAGER, 1981). An analogous but smallerscale situation may have developed on GBR fringing reefs asthey became exposed by falling sea levels, reducing reef pro-ductivity and progradation potential. The emergence of vastareas of intertidal reef rapidly contracted local and possiblyregional sources for larval recruits, potentially slowing post-disturbance recolonisation and longer-term reef growth. Ha-waiian reefs also collapsed around 5000 YBP, possibly due toan El Nino Southern Oscillation (ENSO)-related shift towarda more energetic wave climate at that time, which alteredthe disturbance regime and reduced recovery potential (ROO-NEY et al., 2004). Although we have primarily focused on lat-eral progradation in this article, vertical accretion rates forGBR fringing reefs calculated from radiometrically datedcores also document a rapid decline in reef growth around5000 YBP (Figure 5). The average vertical accretion rate forthe period 5000–3000 years ago (;2 m/ka) is approximately

half that experienced between 7000 and 5000 years ago (4–5m/ka). Closer to present, the average vertical accretion ratefor fringing reefs over the last millenium is just 0.75 m/ka,demonstrating a dramatic and progressive decline in reefgrowth capacity since the mid-Holocene.

It is significant that apparently healthy coral communitiesveneer the fronts of many fringing reefs today, but have notsubstantially contributed to reef construction over the last5500 years. Detailed data on contemporary community attri-butes are available for several fringing and nearshore reefs(e.g., AYLING and AYLING, 1995; AYLING, AYLING, and BER-KELMANS, 1998; DONE, 1982; VAN WOESIK and DONE, 1997;VAN WOESIK, TOMASCIK, and BLAKE, 1999; VERON, 1987).VAN WOESIK and DONE (1997) skilfully demonstrated rela-tionships between a range of coral community attributes andreef accretion, noting that the coral communities lacking ma-jor reef-building taxa (massive Porites and branching Acro-pora) and dominated by short-lived individuals and/or thosewith skeletons with high surface-to-volume ratio prone to dis-integration and/or dispersal, were unlikely to construct reefs.HARRIOTT and BANKS (2002) similarly concluded that lati-tudinal shifts in the abundance and vigour of key reef-build-ing taxa are a major control of reef construction off easternAustralia.

The predominance of massive Porites and Acropora shingleas structural components in drill cores extracted from GBRfringing reefs verifies their importance as reef builders andrecords their longstanding contributions to reef growth. VAN



WOESIK, TOMASCIK and BLAKE (1999) proposed that the oc-currence of non–reef-building communities on existing reefstructures signals a reduced reef-building capacity, possiblydriven by natural cycles or anthropogenic stress. Cores fromGBR fringing reefs have not been systematically examined toassess whether changes in community composition, demog-raphy, calcification rate, resilience, or taphonomy can be de-tected and correlated with Holocene reef growth, but suchshifts could clearly affect fringing reef development. The in-tegrative character of core records and the variable ways inwhich reefs respond to a single or series of causative eventsmakes the recognition of such signals difficult (BUDDEMEIER

and HOPLEY, 1988), but PANDOLFI (2002) has presented auseful review of how skeletal material preserved in reefstructure can improve our understanding of ecosystem dy-namics.

The 3000–2500 YBP Reef-Flat Quiescence

Of those reefs that continued to prograde post-5500 YBP,a subgroup representing ;20% of the sample ceased activeprogradation ;3000–2500 YBP. Some of this cohort mayhave been slowly shutting down since ;4500 YBP. Goold Is-land (CHAPPELL et al., 1983), Hayman Island (HOPLEY et al.,1978), and Yam and Hammond Islands in Torres Strait(WOODROFFE et al., 2000) are included in this group. It ispossible that these reefs simply took longer to collapse in re-sponse to the same critical conditions that caused the demiseof many around 5500 YBP. Reefs with this history are widelydistributed, extending from Torres Strait to the Whitsun-days. This suggests the possible influence of broader scalefactors, such as continued sea-level fall or late-Holocene cli-mate change. Sea temperatures cooler than at the PMT peakmay have lowered calcification rates (LOUGH and BARNES,2000). Rainfall became increasingly irregular through thisperiod (GAGAN et al., 1996), increasing drought and high-magnitude flood frequencies and also possibly sedimentyields to the GBR. Elevated sediment influx since Europeansettlement of the Queensland coast is argued to reduce reefgrowth (MCCULLOCH et al., 2003), and it may have had sim-ilar impacts then.

Cyclone frequency in the Central GBR has changed littlesince the mid-Holocene (HAYNE and CHAPPELL, 2001), butcyclone impacts may have changed as the coastal environ-ment has evolved. By 3000 YBP, the inshore mud wedgewould have accumulated a significant volume of easily resus-pended sediment, and the Holocene high-energy window hadclosed. Each of these conditions might increase ambient in-shore turbidity, possibly producing chronic stress for somereef corals and dampening resilience. It is also possible thataccess to the mud wedge and the generation of turbid plumesthat are less efficiently dispersed in lagoonal rather thanopen-ocean conditions might worsen cyclone impacts on reefgrowth.

Local factors, either alone or together with broader scaleinfluences, may also have imposed significant constraints oncontinued reef-flat expansion on particular reefs. Chronostra-tigraphic evidence suggests that fringing reefs at Yam andHammond Islands outgrew their foundations between 3000

and 2000 YBP, arresting their lateral development (WOOD-ROFFE et al., 2000). This scenario may also apply at Haymanand Goold Islands, where reef-flat progradation also slowedduring this period. In most of these cases, reef-flat progra-dation had previously progressed over shallow substrates, of-ten of unconsolidated sediments, at less than 8 m depth. Thereef fronts of many of these reefs now terminate in deeperwater, with obvious volumetric consequences for reef con-struction and implications for rates of lateral reef-flat expan-sion. Variations in vertical accretion rate and paleo-depth es-tablished from reef cores suggest that, where the depth tosubstrate exceeds 8 m, the rate of vertical accretion may rap-idly decline, further reducing the rate at which the reef couldlaterally grow. It may be that the bottom fell out of the ac-cretion potential for these reefs once they outgrew founda-tions within the 4- to 8-m optimum depth window.

The Present: Decline in Fringing Reef Condition

Holocene growth histories show that many fringing reefson the GBR are now merely surviving rather than activelygrowing. For many, this has been so for a long time (Figure4), even where apparently healthy coral communities occupyreef fronts and slopes. Less than half of the sample have con-tinued to prograde up to the present, most more slowly thanin the mid-Holocene. Examples include Pioneer Bay, OrpheusIsland (HOPLEY et al., 1983), Fantome Island (JOHNSON andRISK, 1987), and Magnetic Island (CHAPPELL et al., 1983).Surprisingly, reefs established over terrigenous clay or sandsubstrates (e.g., Fantome Island; Pioneer Bay, Orpheus Is-land) dominate those that have continued to prograde. Thislonger term survival may reflect the preferential selection ofmore resilient taxonomic assemblages and/or individuals inthese less-than-ideal settings, an hypothesis worth testingbut beyond the scope of this article.

Current environmental stresses that may equal or exceedthose that accompanied previous declines in fringing reef pro-gradation earlier in the Holocene include

● further natural declines in water quality. As the inshoremud wedge has continued to accumulate, and north-facingbays, which trap a significant proportion of these sedi-ments, are filled (ORPIN and RIDD, 1996), potential trap-ping of fine sediments in natural depocentres may be re-duced and natural turbidity levels increased;

● the Holocene high-energy window is now closed, and flush-ing by waves and wave generated currents has diminishedrelative to that experienced in the mid-Holocene (HOPLEY,1984);

● sediment and contaminant delivery to the inshore GBR la-goon has increased due to landuse changes in adjacentcatchments (MCCULLOCH et al., 2003);

● greenhouse gas emissions have raised sea temperatures,increased the frequency of bleaching, and reduced coralcover and calcification rates (HOEGH-GULDBERG, 1999;KLEYPAS et al., 1999).

It also seems probable that optimal substrates for coloni-sation and reef growth are becoming increasingly rare (asmost are already occupied), and this may be a significant lim-



itation as conditions for start-up and survival become tougherdue to natural and anthropogenically forced declines in in-shore water quality.

Reefs Initiated Since 3000 YBP

The nearshore reef at Paluma Shoals is less than 2000years old, with basal coral near the reef front dated at 1657YBP (SMITHERS and LARCOMBE, 2003). This shoal lies overstiff Pleistocene clay 5 m below LAT, although it is possiblethat growth initiated on an alluvial gravel fan in a similarfashion to fringing reefs at Cape Tribulation (PARTAIN andHOPLEY, 1989). Paluma Shoals has established in the narrowcorridor between the inner edge of the sediment wedge andan erosionary sandy coastline. Importantly, the substrateover which it has developed has only become available forcolonisation following recent coastal retreat, indicated by theexposure of extensive late-Holocene mangrove sedimentsalong this coast (SMITHERS and LARCOMBE, 2003). It appearsthat Paluma Shoals, and possibly other nearshore reefs likeit, have been able to recently establish and grow in conditionstraditionally considered marginal for reef growth. Turbidityat Paluma Shoals exceeds 40 nephelometer turbidity units(NTU) more than 30% of the time (LARCOMBE, COSTEN, andWOOLFE, 2001), but the reef initiated and grew because, closeto the shore, waves kept sediments in suspension and appro-priate substrate was available in depths shallow enough forlight-enhanced calcification to occur in turbid water. The in-shore zone between the sediment wedge and shore includessubstrate at depths within the 4- to 8-m optimal accretionwindow; at Paluma Shoals, the mean tide depth to the initi-ation substrate is approximately 6 m.

These reefs are difficult to observe due to turbid water butare reasonably common along similar coasts. Significantly,stratigraphic cores indicate that, although the reef flats canbe dominated by live coral, the internal structure is domi-nated by detrital carbonate clasts, often deposited in distinctunits and interpreted as signifying the importance of episodicaccretion events such as cyclones (SMITHERS and LARCOMBE,2003). Rigidity is mostly provided by a capping of detritalcarbonate material, which forms the substrate for the con-temporary reef-flat assemblages, which include abundantPorites and Goniastrea microatolls. The internal structure isnot significantly bound by biological or chemical means andthe long-term persistence of individual reefs of this type isunknown. Clearly, they are vulnerable to shifts in the posi-tion of the shoreline and inner limit of the nearshore sedi-ment wedge, either of which might shut down production.Their internal structure confirms that cyclonic events canerode and redistribute their sedimentary components. Thelargely unbound internal construction of these nearshorereefs and their limited thickness due to depth constraintsrender them particularly vulnerable to disintegration anddispersal if accretion rates stall. Unlike the fringing reefsthat accumulated most of their bulk in the mid-Holocene, itis possible that these reefs would leave little fossil trace otherthan a temporary shingle strandline on a sandy beach. PERRY

(2003) has described reefs in a similar turbidity and sedi-mentary setting in Mozambique, which historical accounts in-

dicate are ephemeral. We note that species diversity at boththe inshore reefs of the GBR (VERON, 1995) and the Moz-ambique fringing reefs described by PERRY is relatively high,emphasising the point that community diversity and reef-building capacity need not be correlated (PERRY and LAR-COMBE, 2003). Paluma Shoals is unusual compared withmost nearshore reefs on the GBR in that it has verticallyaccreted to sea level and has formed a well-developed reefflat. Middle Reef in Cleveland Bay has similarly grown to sealevel. However, other reefs in Halifax Bay and possibly else-where along the GBR coast are not presently sea-level con-strained. These differences may reflect a later start-up, slow-er accretion, or possibly the impacts of disturbance and de-struction.

PATTERNS IN THE PAST AND LESSONS FORTHE FUTURE?

An examination of the available data on past patterns offringing and nearshore reef growth on the GBR reveals sev-eral main points.

(1) There have been very active phases of fringing reefgrowth, the most significant being between around 7500 and5500 YBP. We attribute this to the availability of good-qual-ity substrate and increasing accommodation space as thePMT progressed. These conditions are also argued to explainthe rapid growth of reefs elsewhere on the GBR and othertropical coasts. The most likely reasons for the turnoff expe-rienced on some reefs at the end of this phase include theexhaustion of available accommodation space over suitablesubstrates and stresses associated with sea-level stabilisationand slight fall. Climate changes, including changes in the in-tensity and frequency of strong ENSO conditions, may havealso influenced the reduction in reef accretion at this time.

(2) There have been periods since the mid-Holocene whenfringing reefs that had been actively expanding become mor-ibund. Scarcity of accommodation space is also likely to be asignificant control of this condition, with the availability ofsubstrates for colonisation and reef growth within 8 m of thesurface decreasing with time. However, as there seems tohave been some synchronicity over large geographical areas,broader scale influences, such as climatic changes, are alsosuggested. Climatic changes that reduce calcification andpossibly increase disturbance can reduce coral and reefgrowth, especially when superimposed on a more general de-cline in inshore water quality (compared with that experi-enced between 7500 and 5500 YBP) caused by the progressiveaccumulation of the inshore mud wedge and closure of theHolocene high-energy window. Healthy but non–reef-buildingcoral communities occupy the slopes of many of these mori-bund reefs but have not significantly added to their structure,in many cases for several thousand years.

(3) Fringing reefs that have continued to prograde to thepresent since ca. 2500 years usually do so at slower ratesthan occurred during the mid-Holocene.

(4) Nearshore reefs have established and grown, usuallyin conditions traditionally considered marginal for reefgrowth, as growth rates for those reefs established in themid-Holocene have declined.



This history provides some challenging questions for thoseinterested in understanding reef growth and health. The con-cepts and issues of reef turnons and turnoffs have been wellcovered (BUDDEMEIER and HOPLEY, 1988) and provide animportant context for the changes documented here. Gener-ally, the transgressionary start-up was ubiquitous, but sub-sequent growth trajectories are more diverse, probably re-flecting the fact that combinations of impacts can produceturnoffs, with the effects varying with time and/or location.As discussed, we believe that some of the variation can bebroadly explained in terms of time-transgressive changes inthe acuteness of stresses imposed by the geomorphologicaldevelopment of the shelf environment and the reefs them-selves, in many ways similar to the evolutionary scheme pro-posed by HOPLEY (1982) for shelf reefs, where rapidly grow-ing juvenile reefs eventually become senile.

We have inferred that fringing reef-flat progradationslowed as accommodation space became scarce and changingenvironmental conditions introduced productivity con-straints. We note, however, that accommodation space aloneis unlikely to be the driver of the synchronous stalls in reefgrowth indicated by the data; accommodation space avail-ability and infill should be independent between sites. Moredetailed investigations are required to (1) establish if carbon-ate productivity of the standing community has declined, re-ducing accumulation rate; or (2) whether destruction of pro-duced carbonate was increased at various times lowering netproduction; or (3) whether export rates have intensified, pro-ducing similar effect (KLEYPAS, BUDDEMEIER, and GATTUSO,2001). Changes in community composition and demographyoccur over environmental gradients (HARRIOTT and BANKS,2002). It is possible that, as conditions around fringing reefshave changed since the mid-Holocene, less effective reef-building communities have increasingly dominated manyreefs. VAN WOESIK, TOMASCIK, and BLAKE (1999) arguedthat the presence of non–reef-building communities on theseaward slopes of fringing reefs in the southern Whitsundaysreflects a shift from reef building to moribundity driven byanthropogenic influence. We do not refute this possibility, butwe caution the acceptance of this conclusion without knowl-edge of longer term reef growth history. Clearly, our resultsdemonstrate that many reefs ceased prograding long ago andcannot simply be attributed to anthropogenic influence.

The occurrence of healthy but essentially non–reef-buildingcommunities on reef structures mostly constructed in the dis-tant past presents problems for workers who stridently definea healthy reef as one that is actively building a reef structure.The debate whether reef building is necessary for healthyreef growth is well summarised elsewhere (KLEYPAS, BUD-DEMEIER, and GATTUSO, 2001), but as in the case of VAN

WOESIK, TOMASCIK, and BLAKE (1999), a shift to nonbuild-ing is often viewed as indicative of a degraded condition. Formany fringing reefs, this shift appears to have occurred in-dependently of anthropogenic impacts. Probable future re-ductions in reef-building capacity due to climate change(GUINNOTTE, BUDDEMEIER, and KLEYPAS, 2003; KLEYPAS etal., 1999) may critically influence many of the reefs and coralcommunities that remain, but nearshore reefs that have ini-tiated in the late Holocene give cause for limited optimism.

These reefs have developed as many of those that flourishedin the mid-Holocene become conspicuously less vigorous andexploit new environments traditionally considered marginal.These new reefs do not fit nicely into definitions developedbased on reefs that grew in the mid-Holocene. However, theymay be the reefs of the future, whereas many of the reefsbuilt during the final phases of the Holocene transgressionare increasingly reefs of the past.

CONCLUSION

An examination of Holocene reef growth histories availablefor fringing and nearshore reefs of the GBR clearly illustratesthat most experienced a period of active growth near the endof the transgression and that many have struggled since. Thereef flats on the majority of these reefs have expanded by lessthan 10% over the last 4000 years. Several periods can beidentified when reef-flat progradation apparently stalled,suggesting the influence of regional scale factors, such as cli-mate change and the evolution of the shelf reefs and sedimentbodies. On those reefs where significant reef-flat growthceased thousands of years ago, European landuse clearly can-not be involved.

In the 7000 years since most fringing reef growth on theGBR began, the availability of the optimum substrates forfringing reef growth and expansion has diminished. It is pos-sible that an important focus of recent and potentially futureinshore reef growth will be the relatively young substratesmade available for colonisation by shoreline retreat. Thesereefs are clearly different in many ways (physically, ecologi-cally) from the fringing and nearshore reefs developed earlierin the Holocene, and we should perhaps be prepared to acceptthat their permanence and stability will also challenge exist-ing norms. Understanding and accepting any differences willobviously be important for effective management of these sys-tems, particularly with regard to identifying where scant re-sources should be directed.

ACKNOWLEDGMENTS

We would like to acknowledge the previous work carriedout by postgraduate students and colleagues whose work hasbeen included in this article. We thank Adella Edwards fordrafting the figures. Thoughtful reviews by Paul Kench, AndyShort, and Bruce Thom greatly improved the manuscript.

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