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Page 1: Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

Coral Reefs (1998) 17 :71—81 ( Springer-Verlag 1998

REPORT

P. K. Dunstan · C. R. Johnson

Spatio-temporal variation in coral recruitment at different scales onHeron Reef, southern Great Barrier Reef

Accepted: 1 September 1997

Abstract Recruitment of scleractinian corals on settle-ment plates at Heron Island, Great Barrier Reef, wasexamined over four years (September 1991—September1995) to quantify spatio-temporal patterns at differentscales and to assess post settlement mortality. Recruit-ment was dominated by pocilloporid corals whichaccounted for 80.1% of the 8627 spat counted, whereasnon-isoporan acroporids represented only 16.4%.Poritids, faviids and isoporan acroporids rarely recruit-ed to the plates (3.5%), despite their obvious abund-ance as adults on the reef. Recruitment patterns on theplates indicate strong space-time interactions as evid-enced by patchy recruitment of both pocilloporid andacroporid spat. Interactions were found between space(on the scale of 102 m, i.e. sites within zones, and 101 m,i.e. racks within sites) and time (on the scale of years) forpocilloporids and between space (on the scale of 103 m,i.e. zones, and 102 m) and time (on the scale of years) foracroporids. Post-recruitment mortality of acroporidspat in the period 3—10 months after their major spawn-ing was dependent on their initial recruitment density,but pocilloporid mortality was either independent ofinitial recruitment density or, more likely, obscured byadditional recruitment of pocilloporids to plates be-tween late February and September. High rates ofrecruitment and growth by other sessile organisms,particularly bryozoans and oysters, appear to result inincreased post-recruitment mortality and limit recruit-ment of scleractinian corals on settlement plates. Thework reinforces an emerging picture that coral recruit-ment patterns are determined by mechanisms thatmanifest over a large range of spatial scales.

P. K. Dunstan ( ) ) C. R. JohnsonDepartment of Zoology, University of Queensland, QLD 4072,Australia

Present address:1Department of Zoology, University of Tasmania, GPO Box252—05, Hobart, TAS 7001, Australia

Key words: Coral recruitment ) Space-timevariation ) Scale ) Competition

Introduction

Recruitment can play an important role in shaping theoverall dynamics and spatio-temporal patchiness inassemblages of species. Also, the pattern and magni-tude of recruitment strongly influences options for con-servation and management (e.g. Brock and Kam 1994;Punt 1995). In tropical marine systems, spatio-temp-oral variation in the structure of populations (Dohertyand Fowler 1994) and assemblages (Sale et al. 1994) ofsite-attached fishes inhabiting small patch reefs is ex-plained largely by variability in recruitment. However,the magnitude of variation in recruitment of corals atdifferent spatio-temporal scales and the influence ofthis variation on the dynamics of assemblages of adultcorals is poorly documented (but see Connell et al.1997). This is in marked contrast to temperate systems,where it is well established that recruitment patterns(Keough 1983; Caffey 1985; Gaines and Roughgarden1985; Raimondi 1990; Lively et al. 1993; Todd andTurner 1993) and early post-settlement mortality (De-nley and Underwood 1979; Keough and Downes 1982;Connell 1985; Keough and Chernoff 1987; Hurlbut1991) strongly affect the structure and dynamics ofbenthic communities. These studies have usually detec-ted strong spatio-temporal interactions, reflecting a re-cruitment signal that is patchy in time and space acrossa variety of scales and which may result from eitherpre-settlement (Keough 1983; Caffey 1985; Lively et al.1993; Turner and Todd 1993) or post-settlement factors(Connell 1985; Osman et al. 1989; Raimondi 1990;Lively et al. 1993).

Several aspects of recruitment patterns of sclerac-tinian corals have been studied on the Great BarrierReef (GBR) for over a decade. However, most studies

Page 2: Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

have emphasised either variation in space (Harriott1985; Babcock 1988; Harriott and Fisk 1988; Fisk andHarriott 1990; Sammarco 1991; Harriott 1992), or vari-ation in time (Wallace 1985a, b). Moreover, the role ofimmediate post settlement processes, which have beendemonstrated as very important in the development oftemperate benthic communities, have been addressedexplicitly in only a few cases (Rylaarsdam 1983;Sammarco 1991). Since the majority of corals on theGBR are broadcast spawners and release their gametesannually in a mass spawning event (Harrison et al.1984; Willis et al. 1985; Babcock et al. 1986), studies oftemporal variability should span several years. How-ever, any study over this time scale should also considerthe intra-annual recruitment signal from pocilloporidand other corals that brood larvae and are capable ofproducing larvae and recruiting throughout the year(Harriott 1983; Kojis and Quinn 1984; Stoddart andBlack 1985; Kojis 1986; Tanner 1996). It is desirablethat studies encompass several scales to enable identi-fication of scales at which variation is greatest andallow comparison of scale-dependent variabilityamong coral families that manifest dissimilar life his-tories and/or dispersal patterns.

The aims of the present work were to (1) quantify thevariability in recruitment of coral spat at several spatialscales (of the order of 100—104 m) across several con-secutive years; (2) examine post-settlement mortality ofrecruits by comparing recruitment patterns about3 months and 10 months after settlement of broadcastspawning species; (3) compare patterns of recruitmentof broadcast spawning species with those of broodingspecies; and (4) to examine the relationship betweendensity of coral spat and utilisation of space by poten-tial competitors.

Materials and methods

Field work was conducted at Heron Reef (23°27@S 151°151@E) at thesouthern end of the Great Barrier Reef over the four year periodSeptember 1991—September 1995. Recruitment of scleractiniancorals, bryozoans and oysters was examined on the unglazed under-surfaces of ceramic plates (200*200 mm bathroom tiles) at 9 sites onthe reef. In September of each year from 1991 to 1994, 10 metalracks, each supporting a pair of juxtaposed plates (i.e. not sand-wiched), were deployed in the same position at each of three sitesapproximately 400—600 m apart within each of three distinct zones(northern and southern reef slopes and lagoonal coral bommies).Racks at sites within both reef slope zones were positioned at depths9—12 m, while racks at sites within the lagoon were located at 2—3 m.The plates were fixed in the horizontal plane at a distance about200 mm above the substratum. Previous work has demonstrated thesuitability of unglazed undersurfaces of bathroom tiles for assessingrecruitment of scleractinian corals (Harriott and Fisk 1987; Maida etal. 1994).

Plates were collected on two occasions in each year: 90 plates(5 of the 10 racks at each site) were collected in February-March andthe remaining 90 plates in September. These tiles represent 3 and 10months respectively after the annual mass spawning event of acrop-orid corals, and 3 and 10 months after the beginning of the summer

peak of pocilloporid larval production (Tanner 1996). Plates werebleached in a chlorine solution to remove organic matter beforemicroscopic examination of their undersurfaces for settled corals.Coral colonies were identified to family (or subgenus in the case ofisoporid acroporids) with the aid of extensive SEM photomicro-graphs (R. Babcock, unpublished), and the size of each colony wasrecorded in terms of the number of polyps. Unidentifiable recruits(i.e. those that had suffered high levels of erosion and those thatotherwise lacked distinguishable features) were designated as’others’. The area covered by cheilostome bryozoans and oysters,which were the dominant space occupants on the plates, was deter-mined for one plate from each rack (i.e. for a total of 180 plates)deployed during 1991 and 1992. Area was determined from digitisedtracings of colony perimeters on acetate sheets using AutoCadsoftware (release 11).

Statistical analyses were conducted using the SAS software pack-age. Spatio-temporal variation in recruitment of scleractinian coralsand cover of bryozoans and oysters on the plates raised 3 and 10months after the mass spawning were analysed using Model IInested ANOVA. Since the focus of the study was to estimate thecontribution to variance of different spatial scales (as distinct fromthe magnitude of differences between particular zones), the effects ofzone, site within zone, and rack within site were random. In unbal-anced designs (as a result of loss of some plates), degrees of freedomand sums of squares are adjusted using the Satterthwaite approxi-mation (SAS Institute 1988). Data were log-transformed to stabilisethe variances. The relationship between density of coral recruits andcover of bryozoans and oysters was examined using Pearson’s cor-relation coefficient.

Results

Total abundances of corals

A total of 8627 coral spat were counted on all settle-ment plates over four years (Table 1). Recruitment ofspat was dominated by the family Pocilloporidae whichaccounted for 80.1% (6921 colonies) of all recruitsrecorded, while the Acroporidae (excluding sub-genusIsopora) represented only 16.4% (1405 colonies). Rep-resentatives of the families (or subgenus) Poritidae,Faviidae and isoporan acroporids amounted to only1.7% (143 colonies), 0.0% (1 colony) and 0.1% (14colonies) respectively. Colonies lacking distinguishablefeatures (referred to as ’others’) totalled 1.7% (143colonies) of all recruits observed.

Spatio-temporal patterns in recruitment of broadcast-ing and brooding corals

Pocilloporidae

Recruitment of pocilloporid corals varied significantlyin space and time, and in every case, error variance (i.e.plates within racks at a scale of 10~1 m) accounted formost of the total variance. Recruitment assessed inFebruary-March, after the summer peak in larvalrelease (see Tanner 1996), varied significantly betweenyears, zones and sites (Table 2), but there were complexinteractions among these factors (interactions betweenyear*site within zone and year*rack within site were

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Page 3: Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

Table 1 Total abundances of corals settled on plates by family. Data describe the total number of recruits for each time period. Deploymentof plates was in September each year

Year Months of Number of Pocilloporidae Acroporidae Acroporidae Poritidae Faviidae Other Totaldeployment plates removed (non-isoporan) (Isopora)

91/92 5 months 90 1071 334 0 20 1 42 146891/92 12 months 83 220 17 0 1 0 3 24192/93 5 months 90 663 279 2 13 0 33 99092/93 12 months 89 493 100 0 7 0 6 60693/94 5 months 90 837 252 1 13 0 14 111793/94 12 months 75 827 88 9 14 0 15 95394/95 5 months 90 1715 241 2 58 0 12 202894/95 12 months 90 1095 94 0 17 0 18 1224

TOTAL: 6921 1405 14 143 1 143 8627

Table 2 Analysis of spatio-temporal variation inrecruitment of pocilloporidcorals 5 months and 12 monthsafter deployment of plates (i.e.3 and 10 months respectivelyafter the commencement of theirsummer peak in larval release(Model II nested ANOVA of logtransformed data)

Source of variation df F P % VarianceExplained

A Recruitment 5 months after deployment of platesYear"Y 3,12 3.58 0.0469* 6.3Zone"Z 2,4 17.05 0.0110* 26.1Sites within Zones"S(Z) 4,12 5.35 0.0104* 3.5Racks within sites"R(S) 12,274 0.98 0.4680 1.3

Y * Z 6,12 1.18 0.3785 2.4Y * S(Z) 12,36 3.68 0.0012** 18.6Y * R(S) 36,274 1.71 0.0091** 3.7Error 274 38.1

B Recruitment 12 months after deployment of platesYear"Y 2,12 9.76 0.0030** 17.6Zone"Z 2,4 5.28 0.0755 19.3Sites within zones"S(Z) 4,24 8.78 0.0002*** 6.3Racks within sites"R(S) 24,255 1.24 0.2106 0

Y * Z 6,12 0.60 0.7287 1.6Y * S(Z) 12,24 2.81 0.0150* 8.9Y * R(S) 24,255 1.15 0.2862 0.7Error 255 45.6

Significance levels are *0.05'P50.01, **0.01'P50.001, ***P(0.001.

significant) indicating that patterns among sites withinzones, and among racks within sites, were not consis-tent from year to year. The most important sourcesof variation in the model (i.e. excluding the errorterm), were zone and the interaction of year*site,which accounted for 26.1% and 18.6% of thetotal variation respectively. Thus, variability at spatialscales of the order of 103—104 m (among zones)was temporally consistent and exceeded variability atscales of the order of 102—103 m (among sites). Recruit-ment of pocilloporids was consistently higher on thesouthern reef slope than within either of the other zones(Fig. 1).

In the model, the effect of zone also accounted for thesingle largest component (19.3%) of the variation inrecruitment on plates recovered 12 months after their

deployment (i.e. 10 months after the commencement ofthe summer peak in release of pocilloporid larvae),although the effect was significant only at the a"0.1level (Table 2). Inter-annual differences were the nexthighest contributor to variance (17.6%), but the effectof year was dependent on site, i.e. differences amongsites within zones were not consistent across years(Table 2).

Acroporidae (non-isoporan)

Recruitment of non-isoporan acroporids on plates col-lected 3 months after their mass spawning was alsohighly variable in space and time. The error variancealso accounted for the largest portion of the total

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Page 4: Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

Fig. 1 Mean density ($1 SE) of coral spat on settlement plates inthree zones at Heron Island. Plates collected three months aftermass spawning of broadcast spawning corals (hatched) are distin-guished from those raised 10 months after the mass spawning event(open). Note that the scale for 1995 differs to that for other years

variance, although the overall pattern was dissimilar tothat for pocilloporid recruits (Table 3, Fig. 1). Interac-tions of year*zone and year*site were both significant,and were the greatest sources of variation in the model(21.3% and 16.4% respectively). Thus, there was signifi-cant spatial variation in recruitment density at scales ofthe order of 103—104 m (zones) and 102—103 m (siteswithin zones), but the nature of this variation variedsignificantly among years. For example, recruitment ofacroporids was significantly higher to the reefs slopesthan to the lagoon in 1992, while recruitment washigher in the lagoon than on the slopes in 1994, and thepatterns in 1993 and 1995 were different again (Fig. 1).Variation at the scale of 100—101 m (rack within site)was not significant.

Recruits remaining 10 months after the mass spawn-ing event (i.e. 12 months after deployment of plates)

showed a strong effect of year (8.4% of variance ex-plained), but post-recruitment mortality had reducedrecruitment to relatively uniform levels at most spatialscales (Fig. 1 and Table 3, B), so that added compo-nents of variance at the different spatial scales (zone,site within zone, rack within site) were not significant.However, the interaction of year*zone contributed10.5% of the total variation in the model and wassignificant at the level of a"0.1 (Table 3). Errorvariance was high (76.8%), indicating that greatestvariation in the model was attributable to differencesbetween plates within racks.

Mortality and the size structure of coral recruits

Small recruits in the size classes 1—5 and 6—10 polypsdominated all recruitment on settlement plates in allyears irrespective of whether plates were collected 3 or10 months after the mass spawning of acroporids(Fig. 2). The growth rate of acroporids (sensu rate ofaddition of new polyps) was much less than for pocil-loporid recruits; 10 months after the mass spawning,3—16% of acroporids exceeded 15 polyps in size whilea similar proportion (7—12%) of pocilloporids exceeded30 polyps per colony (Fig. 2). A small number of pocil-loporids exceeded 150 polyps per colony ((0.1%). Inaddition, even 10 months after the mass spawning,during which time further settlement of acroporids washighly unlikely, a large proportion ('50% in mostyears) of acroporid recruits did not develop and/orsurvive beyond a size of 5 polyps per colony. In con-trast, most ('60% in all years) pocilloporid colonieson the plates after a similar time interval exceeded5 polyps (Fig. 2), even though recruitment to thesmallest size class was likely to occur throughout theyear (Tanner 1996).

Both pocilloporid and acroporid recruits were char-acterised by high levels of mortality in the seven monthperiod between March and September. Mortality wasestimated by comparing the mean density of recruits ateach site three months after mass spawning of acrop-orids (i.e. five months after deployment of plates) withthe density of live recruits ten months after the massspawning (i.e. 12 months after deployment of plates).Mortality rates of acroporid recruits during winterwere particularly high and this reduced numbers to lowlevels by September, particularly in 1992 (Figs. 1,2;note: the small differences in the numbers of platesretrieved in March and September are insufficient toaccount for the absolute differences in numbers of spatcounted at the two times). Mortality of pocilloporidrecruits was highly variable over the four years, al-though most sites in most years exhibited significantreductions in density over the observation period(Fig. 1). The level of mortality in acroporid recruits tenmonths after their mass spawning was dependent oninitial recruitment density assessed three months after

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Page 5: Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

Table 3 Analysis of spatio-temporal variation inrecruitment of acroporid (non-isoporan) corals 3 months and10 months after spawning(Model II nested ANOVA of logtransformed data). Significancelevels are as for Table 2

Source of variation df F P % Varianceexplained

A 3 months after mass spawningYear"Y 3,12 1.54 0.2547 0Zone"Z 2,4 1.83 0.2723 0Sites within zones"S(Z) 4,12 4.95 0.0137* 1.0Racks within sites"R(S) 12,274 0.97 0.4819 0.1

Y * Z 6,12 3.37 0.0349* 21.3Y * S(Z) 12,36 4.54 0.0002*** 16.4Y * R(S) 36,274 0.95 0.5547 0Error 274 61.2

B 10 months after mass spawningYear"Y 2,12 11.77 0.0015** 8.4Zone"Z 2,4 3.37 0.1385 0Sites within zones"S(Z) 4,24 1.48 0.2395 1.9Racks within sites"R(S) 24,255 0.87 0.6398 0

Y * Z 6,12 2.99 0.0501 10.5Y * S(Z) 12,24 1.34 0.2620 0.8Y * R(S) 24,255 0.94 0.5486 1.6Error 255 76.8

spawning in that higher densities of initial recruitmentalways suffered high levels of mortality whereas lowlevels of initial recruitment suffered either low or highlevels of mortality (Fig. 3; note that this ‘triangulardistribution’ or ‘factor-ceiling distribution’ is not wellrepresented by standard correlation analysis; seeThompson et al. 1996). Pocilloporid recruits did notexhibit this pattern, possibly because of additionalsettlement after the plates were examined in March (seeTanner 1996).

Cover of bryozoans and oysters

Cover of bryozoan species on the settlement platesvaried between 0% and 60% depending on zone(Fig. 4). In general, plates collected 12 months aftersubmergence (i.e. ten months after the annual massspawning of corals) had a higher cover of bryozoansthan plates collected seven months earlier in March(Fig. 4). The exception were the plates from the northslope in 1993 which showed about 20% cover ofbryozoans after submergence of either 5 or 12 months.Plates from the lagoon showed virtually no fouling bybryozoans, whereas bryozoans recruited readily toplates on the northern and southern slopes (Fig. 4),yielding a significant added component of variance dueto variation among zones (Table 4). Overall, differencesin the cover of bryozoans on plates from the north andsouth side of Heron Reef were not significant. Variationin colonisation of bryozoans among sites within zonesand between the two years was not significant.

Compared to bryozoans, cover of oysters was rela-tively evenly distributed across years, zones and sites

(Table 5, Fig. 4). The only significant variation detectedwas in initial recruitment (5 months after submergingplates) at the level of sites within zones, but cover didnot vary significantly at this scale after a further seven-months of growth had occurred (Table 5). For bothbryozoans and oysters, most of the variance measuredwas at the scale of 10~1 m, or that attributable to plateswithin racks ("error).

Relationship between coral spat density and othersessile taxa

There was a significant negative correlation betweencoral spat and total cover of bryozoans and oysters forplates raised in 1992. While there were no other signifi-cant linear correlations between coral spat density andcover of other sessile taxa at the level of a"0.05(Fig. 5), standard correlation analysis is not well suitedto ‘triangular distributions’ or ‘factor ceiling distribu-tions’ of this form (see Thompson et al. 1996). Theimportant point is that while low numbers of coral spatco-occurred with bryozoans and oysters irrespective ofthe cover of bryozoans and oysters, with very fewexceptions, high coral spat density occurred only whencover of bryozoans and oysters was low.

Discussion

Composition of coral recruits

Pocilloporid corals clearly dominated recruitment onsettlement plates at Heron Island after 5 and 12 months

75

Page 6: Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

Fig. 2 Size distribution of coral spat of families Pocilloporidae andAcroporidae. Plates collected 3 months after mass spawning ofbroadcast spawning corals (hatched) are distinguished from thoseraised 10 months after the mass spawning event (open)

deployment (Table 1). This pattern is in marked con-trast to the results of similar studies on more northernmidshelf reefs in the GBR system where acroporidsdominate recruitment to plates (Harriott 1985; Wallace1985a, b; Babcock 1988; Harriott and Fisk 1988; Fiskand Harriott 1990), but is similar to observations ofrecruitment on settlement plates at other higher lati-tude tropical and sub-tropical reefs in eastern Australia(Harriott 1992, 1995; Harriott and Banks 1995). Theemerging picture is that there is a transition from domi-nance of recruitment on settlement plates by acrop-orids in central and northern regions of the GBR todominance by pocilloporids at the southern extremitiesof the GBR and on subtropical reefs to the south of the

Fig. 3 Relationship between rates of estimated post-settlement mor-tality of coral spat between 5 and 12 months after deployment andrecruitment density of coral spat at 5 months post deployment. Dataare means for each site. Since mortality is estimated as the differencein mean spat density between 3 and 10 months after peak settlementin summer, mortality rates for pocilloporid corals are likely to beunderestimated because of continual settlement during this period.Zero mortality indicates mean densities 12 months after deploymentof plates that were greater than mean densities 5 months afterdeployment. Pearson’s correlation coefficients: for Acroporidae,r"0.493, df"35, P"0.002; for Pocilloporidae, r"0.272, df"35,P"0.108

GBR system. Possible explanations of this gradientthat warrant critical examination include latitudinalchanges in fecundity, fertilisation success, larval-devel-opment rates, settlement cues, and immediate post-settlement mortality. Alternatively, this pattern mayreflect the fact that effective distances for larval trans-port between reefs (or groups of reefs) is greater for theCapricorn-Bunker group (which includes Heron Reefand is separated from the main body of the GBR by'100 km) and reefs further south than for more

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Table 4 Analysis of spatio-temporal variation in thepercentage cover of cheilostomebryozoans (all taxa combined)on settlement plates assessed5 months and 12 months afterdeployment (Model II nestedANOVA of log transformeddata). Significance levels are asfor Table 2

Source of variation df F P % Varianceexplained

A 5 months after deploymentYear"Y 1,6 0.17 0.6971 0Zone"Z 2,6 11.40 0.0090** 22.8Sites within zones"S(Z) 6,162 1.83 0.0971 4

Y * Z 2,6 1.22 0.3600 0.4Y * S(Z) 6,162 0.73 0.6300 0Error 162 72.8

B 12 months after deploymentYear"Y 1,6 2.16 0.1919 0.9Zone"Z 2,6 16.75 0.0035** 22.5Sites within zones"S(Z) 6,162 1.12 0.3511 1.6

Y * Z 2,6 0.54 0.6078 0Y * S(Z) 6,162 0.69 0.6558 0Error 162 75

Fig. 4a–d Mean percentage cover ($1 SE) on settlement plates byzone and year of bryozoans and oysters 5 months (hatched) and 12months (open) after deployment of plates

northern reefs. Given that the competency period of6—10 days post fertilisation for non-isoporan acroporidlarvae (R. Babcock, personal communication) is no-tably longer than the usual competency of (1—2 dayspost release for brooded larvae, brooding species havea greater likelihood of self-seeding of their natal reefs(Fadlallah 1983; Harrison and Wallace 1990). Thus,

with increasing distances between reefs, rates of recruit-ment of broadcast spawning acroporids are likely todecline relative to recruitment of brooding pocilloporidcorals (Preece and Johnson 1993). Harriott (1992, 1995)has also suggested increased rates of self-seeding bybrooding species relative to spawners as a mechanismgenerating overall domination by pocilloporid corals(and by isoporan acroporids at one site) on reefs atLord Howe Is. (at latitude approximately 32 °S).

Despite their notable abundance as adult colonies onHeron Reef, recruitment by isoporan-acroporids,poritids and particularly faviids was disproportionatelysmall. These families (and sub-genus) are also relativelyrare on plates collected from other parts of the GBR(Harriott and Fisk 1988; Fisk and Harriott 1990),although high numbers of poritid recruits have beenreported on settlement plates from the Solitary Is.,south of the GBR system (Harriott and Banks 1995).Given that many marine invertebrates (e.g. Johnson etal. 1991; Johnson and Sutton 1994) including somecorals (Morse et al. 1988), require specific cues to in-duce metamorphosis, the low rates of recruitment toplates of faviids, isoporan-acroporids, poritids andother families that occur commonly on reefs may reflectan absence of appropriate cues for metamorphosis ofmany broadcast spawning species. The alternative, thatsettlement onto plates reflects availability of larvae inthe water column and/or is a reliable indicator of rela-tive recruitment onto natural substrata, seems unlikelygiven the high fecundity and abundances (C. Johnson,personal observation) of established colonies on HeronReef of the families that are rare or absent from plates.

Spatial and temporal variation

Spatial variation in recruitment has been recordedalong depth gradients (Birkeland 1977; Bak and Engel

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Table 5 Analysis of spatio-temporal variation in thepercentage cover of oysters onsettlement plates assessed5 months and 12 months afterdeployment (Model II nestedANOVA of log transformeddata). Significance levels are asfor Table 2

Source of variation df F P % Varianceexplained

A 5 months after deploymentYear"Y 1,6 5.05 0.0658 1.6Zone"Z 2,6 0.30 0.7545 0Sites within zones"S(Z) 6,162 3.45 0.0031** 11.6

Y * Z 2,6 2.50 0.1621 2.9Y * S(Z) 6,162 0.68 0.6648 0Error 162 83.9

B 12 months after deploymentYear"Y 1,6 0.01 0.9078 0Zone"Z 2,6 0.80 0.4933 0Sites within zones"S(Z) 6,162 0.74 0.6170 0.6

Y * Z 2,6 0.94 0.4430 0Y * S(Z) 6,162 0.62 0.7128 0Error 162 99.4

1979; Birkeland et al. 1981; Rogers et. al . 1984), amongzones within reefs (Harriott 1985; Babcock 1988), andbetween reefs (Harriott and Fisk 1988; Fisk and Har-riott 1990; Sammarco 1991; Harriott and Banks 1995),and temporal variation has been observed both within(Harriott 1985; Babcock 1988; Harriott and Banks1995) and between (Wallace 1985a, b; Harriott andBanks 1995) years. Our results likewise indicate recruit-ment by pocilloporid and acroporid corals is highlyvariable in space and time, and that distinct spatialpatterns in any one year are unlikely to be consistentamong years. Importantly, the exact nature of variationin recruitment depended on the length of time aftersettlement that recruits were censused (3 versus 10months after mass spawning).

The range of scales of significant spatial variation inrecruitment of acroporid corals was less than that ofpocilloporids irrespective of the time of census. In allyears pocilloporids demonstrated significant spatialvariation among zones (103 —104 m) three months afterthe annual mass spawning and in some years they alsodemonstrated significant spatial variation at scales ofsites within zones (102 m) and racks within sites(100—101 m). In contrast, recruitment of acroporidsthree months after their spawning varied significantlyonly at scales of 103—104 m (among zones) and 102 m(among sites within zones), and this occurred only insome years. Furthermore, the magnitude of spatialvariability at scales of 100—104 m (in terms of the per-centage of total variance explained) in recruitment ofpocilloporids was notably greater than that of acrop-orids (see Tables 2, 3). Ten months after spawning thedensity of acroporids was more uniform, so that spatialvariation was not significant (at a"0.05) at any scalein the range 100—104 m (although in some years differ-ences among zones, as indicated by the zone*year inter-action, were significant at a"0.06). However, on the

same plates, pocilloporid corals showed highly signifi-cant variation among sites within zones in some years.Again, the magnitude of spatial variation (as indicatedby the percent of variance explained by the model)among pocilloporid recruits was greater than that foracroporid recruits at scales of 100—104 m. Our result ofsignificant inter-annual variability parallels the find-ings of Wallace (1985a) and Harriott and Banks (1995)who also detected strong inter-annual (as well as sea-sonal) signals in recruitment of acroporids.

In line with our results, Babcock (1988) also foundthat recruitment of brooding corals on settlementplates was more spatially variable than recruitment ofspawning species, which he interpreted as reflectinglocalised settlement in the vicinity of parent coloniesgiven the short larval competency period of mostbrooded larvae. In this context, Harriott (1985, 1995)also offers some, albeit limited, evidence that spatialpatterns of recruitment of brooding species, with larvaeable to settle soon after their release (Fadlallah 1983;Harrison and Wallace 1990), are influenced by adultdistributions. Temporal variability in abundances ofadult colonies or production of larvae is also likely torealise greater temporal variability in recruitmentin brooding corals with a short competency periodthan in free-spawning corals (see Tables 2 and 3)where larvae spend more time in the water column andtherefore be more likely to experience hydrodynamicmixing.

Post-settlement mortality

Patterns at scales of 100—104 m

Early post-settlement mortality has frequently beenproffered as a mechanism shaping recruitment patterns

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Fig. 5a–f Relationship between density of coral spat (all families)and the cover of bryozoans and a, b oysters c, d separately, and thecombined cover of bryozoans and oysters e, f Data points are foreach plate examined; filled circles indicate plates collected at3 months post mass-spawning, and open circles those collected at 10months post spawning; linear correlations are for both sets of platescombined

of corals (e.g. Harriott 1985; Sammarco 1991). However,studies are seldom designed to estimate early mortality(but see Babcock 1991). Our results for non-isoporanacroporid corals indicated that density-dependentpost-recruitment mortality had the effect of ‘smoothing’initial spatial differences in recruitment so that tenmonths after mass spawning, variation in recruitmentdensity did not vary significantly (at a"0.05) overscales of 100—104 m. Density dependence was of theform where initially high levels of recruitment alwayssuffered high rates of mortality, whereas initially lowdensities experienced both low and high rates ofmortality. There was no evidence of density dependent

mortality among pocilloporid recruits. However,patterns of mortality are likely to have been masked byadditional recruits between March and September eachyear (Tanner 1996).

Competition with other sessile organisms

While density dependent post-settlement mortalityacted to reduce variability in recruitment of acroporidsat scales of 100—104 m, there was evidence that spatialvariability increased at smaller scales. Variabilityamong plates within racks at a scale of 10~1 m (i.e. thevariance attributable to error in the nested ANOVA)was high for acroporid corals, and increased from 61%to 77% of the total variance (determined from thevariance not accounted for by the terms specified in thesaturated nested model) on plates collected three andten months after the mass spawning respectively. Errorvariance (i.e. attributable to among plates within racks)for pocilloporid corals (38% at 3 months and 46% at10 months after the mass spawning) was considerablyless than that for acroporids, and was little affected bythe length of exposure of plates.

Two questions arise: first, what processes act to gen-erate high variability in coral spat abundance betweenplate pairs within about three months of settlement,and second, what mechanisms act to increase thisvariability between 3 and 10 months post-settlement?While differences in initial settlement densities mayaccount for some of the recruitment variability esti-mated three months after settlement, differential post-settlement mortality is also likely to play a role, parti-cularly in affecting variance among plates within racksbetween 3 and 10 months post-settlement when therewas presumably no further settlement of acroporids.

There is evidence that competition with other sessileorganisms on the plates was an important componentof post-settlement mortality of corals at this scale.A wide variety of organisms other than corals recruitedto settlement plates; the principle space occupants werebryozoans, oysters and non-geniculate coralline algae(NCA), but ascidians and sponges also occurred. Thecombined cover of bryozoans and oysters on the platesoften exceeded 30—40% and was as high as 80%(Fig. 5). Moreover, bryozoans and oysters were fre-quently observed overgrowing coral spat, whereascoral spat were rarely observed growing on the oystersand only a single spat was found growing ona bryozoan. Two lines of (correlative) evidence suggestthat competition with other fouling organisms limitedcoral recruitment and influenced coral recruitment pat-terns at small scales. First, coral recruitment densitywas uniformly low when cover of bryozoans and oy-sters was high, and occurred at high levels only whencover of these competitors was low (Fig. 5). Second,variability in coral recruitment density and cover ofbryozoans and oysters were correlated at small scales.

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Whereas patterns of cover of bryozoans and oysterswere remarkably consistent at larger scales (103 m forbryozoans, i.e. sites within zones, and 104 m for oysters,i.e. zones; Tables 4,5; Fig. 4), variability at small scalesof 100—101 m (i.e. between racks within sites) was muchgreater and by far the greatest source of variation (theerror variance) in the ANOVA models (Tables 4,5).Given the large variability at the scale of racks withinsites (100—101 m) in the cover of bryozoans (72.8%and 75% of the total variance in cover on plates sub-merged for about 5 and 12 months respectively) andoysters (83.9% and 99.4% of the total variance in coveron plates submerged for about 5 and 12 monthsrespectively), mortality of corals through competitionfor space with these organisms is an obvious candidateto explain differential mortality of coral spat at thisscale. It is also likely to account for a significant portionof variation in coral recruitment we observed at scalesof 10~1 m, i.e. plates within racks.

Given our observations of overgrowth of coral spatand correlations between recruitment of coral spat andcover of other space occupants, and experimental evid-ence of competition among small sessile organisms oncoral reefs (Buss and Jackson 1979; Jackson and Win-ston 1982; Jackson 1984), it is likely that competitionfor space with coral spat can greatly influence patternsof recruitment of larger juveniles, particularly sincebare space on mid- and outer-shelf GBR reefs is rare ornon-existent. In this context it is noteworthy that inseveral temperate systems, recruitment plays an impor-tant role in determining community structure if spaceoccupancy is low, but as space becomes occupied, therelative importance of the recruitment signal decreases,and post-settlement mortality becomes increasingly im-portant as a determinant of community structure (Con-nell 1985; Gaines and Roughgarden 1985).

Acknowledgements This project was supported by University ofQueensland grants to CRJ. We thank the many students of theCoral Reef Ecology Course for their assistance with the work.Thanks are also due to Annie Preece for introducing the joys ofAutoCad, Simone Dunstan for years of patience and work, numer-ous others who helped in the field, and Russ Babcock for providingphotomicrographs of coral spat.

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