P.S.Z.N. I: Marine Ecology, 17 (1-3): 399-410 (1996)@ 1996 Blackwell Wissenschafts-Verlag, BerlinISSN 0173-9565

Accepted: May 1, 1995

BERNHARD RlEGL & ANDREA RlEGL

Institut fur Palaontologie, Geozentmm, AlthanstraBe 14, A-I091 Wien, Austria.(fonnerly: Coastal Ecology Unit, Zoology Department, University of Cape Town, Ronde-bosch 7700, South Africa)

With 8 figures and I table

Key-words: Episodic disturbance, coral community structure, coral reef, South Africa,Scleractinia, Alcyonacea, fragmentation, regeneration.

Abstract. Africa's southernmost coral reefs are situated in Natal Province, South Africa. The Natal coastis exposed to open ocean swells and episodic storm swell conditions. Benthic communities on these reefsdifferentiated into three community types: shallow reefs (8-18 m) were dominated by alcyonacean coralsand low-growing, massive Scleractinia; intermediate reefs (18-25 m) were dominated chiefly bybranching and tabular Scleractinia of the genus Acropora (A. austera, A. clathrata); deep reefs were notdominated by corals but by sponges. Breakage and recovery experiments indicated that the differencein Acropora dominance between shallow and intermediate sites was caused by breakage in high swellconditions. Survival of experimentally produced A. austera fragments was significantly higher inintermediate than in shallow sites, where higher surge made re-attachment and regeneration unlikely.Also, colony morphology was adapted to differential surge conditions: colonies on the shallow reefswere smaller with shorter branches, while on intermediate reefs they were much bigger with long, widelyspreading branches. EpisodiQ breakage and low fragment survival due to high water-motion thusexcluded branching corals from shallow reef sites.

Problem

Coral reefs can react very sensitively to disturbances. Severe disturbances, usuallyepisodic events occurring at long, irregular time intervals, can completely changethe entire coral community by killing all or a large proportion of its corals (HAR-MELIN-VIVIEN & LABOUTE, 1986; DOLLAR & TRIBBLE, 1993). Subsequent recoverycan take place from surviving fragments (HIGHSMITH, 1982) or by colonization byspecies present in the larval pool.

Smaller-scale disturbances, also episodic but more frequent, are only likely todamage a portion of the resident corals, without necessarily disadvantaging theother community members (PORTER & MEIER, 1992; ROBERTS et ai., 1994): Onlyindividual colonies or species will be affected by mortality (GLADFELTER, 1982;CURRAN et ai., 1994) and no community turn-over as after a large-scale disturbancewill be observed. These smaller-scale disturbances can nevertheless also profound-ly influence community structure.

U. S. Copyright Clearance Center Code Statement: 0173-9565/96/1701 -0399 $

.50/0

400 RlEGL & RlEGL

Wave energy has long been known as one of the most important factors control-ling reef growth. High wave-energy can do considerable damage to coral reefs(DOLLAR, 1982; ARONSON et al., 1994; ROGERS, 1994):

On South African reefs, branching corals are particularly at risk of being dam-aged by high wave-energy. However, breakage in branching corals needs notnecessarily be a disadvantage, as many species are able to reproduce by fragmen-tation (HIGHSMrrn, 1982). For this strategy to be successful, fragmentation must notoccur too frequently and wave-action has to be low enough to allow fragments toremain stable until they can re-attach. Otherwise, this episodic disturbance wouldgravely disadvantage the species.

This paper investigates the coral community structure of reefs in different depthsand therefore in different wave-exposure regimes (DENNY, 1988) in South Africaand the effects of frequently recurring small-scale disturbances by high wave-ener-gy. The impact of high wave-energy on the common, open-arborescently-growing,branching coral Acropora austera (DANA, 1846) and the importance of episodichigh wave-energy events on this species' role in community structure were evalu-ated.

Material and Methods

1 .Study area

The study area was situated in the Maputaland reef system in northern Natal, South Africa (Fig. 1). Thegeomorphology of these reefs deviates from that found on typical coral reefs (RAMSEY & MASON, 1991;RlEGL et al., 1995). They generally do not reach the surface (minimum depth 6-8 m) and therefore lacka typical reef crest, do not enclose a lagoon, and have no pronounced reef slope (mostly sloping at lessthan 10°). Major topographical features are gullies and associated drop-offs of up to 5 m, dissecting the

reefs in irregular intervals and orientation.Two types of reef, which developed on two different types of underlying topography, occur: deep,

flat outcrops between 18 and 24 m depth (4-Mile Reef, Kosi Mouth Reef) and typical fossil dunes orshallow sandstone outcrops, typically reaching from 8 to about 34 m depth (2-Mile Reef, 9-Mile Reef,Red Sands Reef).

The Maputaland coastline is influenced by the headwaters of the developing Agulhas current,flowing north-south. Maximum current speed in the area is around 1.5 m .s-l. Surface waters are amixture of tropical water from the Mozambique channel and subtropical water from the east (SCHUMANN

& ORREN, 1980).The area is characterized by high swells, predominantly from the south (SCHUMANN & ORREN, 1980).

Swells generated in the Southern Ocean reach the reefs. These usually have a long period (around 10 s)and deepwater wave heights around 2 m, although they may be considerably larger. Swells generatedby cyclones in the Mozambique channel can also reach the area. Strong coastal lows propagating in anorth easterly direction up the coast can create high wind seas and local currents. Superimposed onocean swells, wave heights are sufficient to cause shoaling on the shallow reefs (8 m depth). Suchconditions occurred twice in a three-year observational period (1991-1993). Water velocities createdby surge in 20 m depth ranged from 0.07 m .s-1 in calm to over 1 m. s-1 in medium swell conditions

(unpubl. data).

2. Community structure analysis

Quantitative surveys were carried out using the line-transect method with continuous recording of theintercepts of all organisms and geological features underlying the transect line (LoYA, 1978). Idealtransect length was established by means of a species-per-area curve and was found to be at 10 m.

401How episodic coral breakage can detennine community structure

r'-\,.../"'-~~~L~

MOZAMBIQUE

27"00' 5

T

, 9-Mile Reef

/8 4-Mile Reef1 2-Mile Reef

SOUTH AFRICA

Red Sands Reef

I

JN10km

Fig. 1. The study sites on the Maputaland reef complexes in northern Natal, South Africa. Only the namesof reefs sampled for this study are given.

402 RlEGL & RIEGL

Therefore, 10-m-1ong line transects were recorded. Series of at least 10 transects, following the depthcontour with one meter spacing between them, were repeatedly recorded at randomly chosen sites. Thiswas necessary due to the low topographical differentiation of the reefs. Transect depths varied between8 and 34 m, and 5-7 sample sites were surveyed per reef.

Phototransects, covering areas of 4x10 m each, were taken for control purposes. These providedfurther line-transect information. The photograph's scale was determined by using markings on thetransect line, which was visible in each photograph. Coral intercepts on the transect line were measuredusing a ruler; the data were then multiplied by the scale of the photographs to estimate the actualintercept distances.

The intercepts of corals, all other major invertebrate groups such as sponges and ascidians, as wellas sand and unoccupied rock were recorded. Unoccupied rock was defined as lacking macroalgae or

invertebrates.A total of 171 transects was recorded on five reefs (Fig. 1), and two data sets were produced: one

with quantitative information, indicating the proportional surface cover of each species, and one withqualitative information, giving only presence/absence data. Quantitative data were standardized bystandard deviation in order to achieve scale independence (DIGBY & KEMPTON, 1987) and then subjectedto hierarchical, agglomerative cluster analysis using W ARD'S method of linkage. In a second step, thetransects of each locality were pooled and localities were compared. Squared Euclidian distance or theCorrelation Similarity Coefficient was used as distance measure for quantitative pooled data. Forqualitative data, the Binary LANCE-WILUAMS Dissimilarity Coefficient was used (DIGBY & KEMPTON,

1987).Species diversity was expressed using SHANNON'S Diversity Index (PiELOU, 1975). Prior to statistical

analysis data were tested using KOLMOGOROF-SMIRNOV tests. Data fulfilled the assumption of normaldistribution; therefore, parametric statistics were used.

3. Morphological characteristics of fragile species

The most common branching species (Acropora austera), which appeared to be susceptible to breakageand dislocation in high wave-energy conditions, was examined separately on three reefs in differentdepths and therefore wave-exposure (2-Mile, 4-Mile, 9-Mile Reefs).

Colony size was expressed as colony volume and was calculated by considering the colonies tooccupy a rectangular space, which could be calculated by width times length times height. Also, averagebranch length and branch diameter at the base of branches was measured (averaged from 10 measure-ments per colony). The number of unattached fragments within 50 cm of each colony (this being adistance within which the fragments could easily be allocated to their colony of origin) was recorded.

4. Field experiment to determine fragment survival indifferent reef zones

Five sites were chosen to test the survival of fragments in localities at different depth and thereforewave-exposure. The sites represented all major environments on the investigated reefs and differed insubstratum type, morphology, and coral cover. One shallow, exposed site (12 m) was situated on 2-MileReef, where A. austera is rare. It was the dominant community member on 4-Mile Reef, where siteswere at 18 m on a gentle slope and at 24 m near a ledge on a flat sand plain and in a shallow depressionon a coral carpet. These sites represented environments of different wave exposure, as water movementdue to wave action decreases with depth (DENNY, 1988).

A total of 170 fragments was generated by cutting or chiselling branches off previously unfrag-mented colonies. The fragments were then placed near each other on the sea floor within five markedareas (characterized above), where they could be recovered. Only healthy branches covered entirely byliving tissues were cut off. Initially, therefore, only the areas of breakage were tissue-free. After onemonth the fragments were collected and analyzed. The percentage of fragments recovered from eachsite was recorded. Surviving tissues were easily identified by their cream to yellow colour, and thepercentage of surviving tissue was estimated.

How episodic coral breakage can detennine community structure

Results

403

1. Community structure

Benthic communities were dominated by corals (94.6% of total living coverage).Scleractinia occupied slightly more space than Alcyonacea (52.1 % versus 42.5%);other organisms like sponges and ascidians only covered 5.4% of the total intercept.

Among the Scleractinia, the Acroporidae had the highest proportional coverage(16.2% of total scleractinian cover) followed by the Faviidae (13.8%). AmongAlcyonacea, the leathery, low-growing forms in the genera Lobophytum (20.6%)and Sinularia (16.3%) dominated.

Classification of quantitative data of all transects (unpooled, all sites; Fig. 2)showed differentiation into two major community types, one dominated by Scler-actinia and another dominated by Alcyonacea and other taxa. Within the majorclusters (A and B, Fig. 2), the transects from different reefs did not separate intodifferent subclusters.

Cluster A was made up to the greatest part by transects from 4-Mile Reef. Almostall transects in this cluster were from depths greater than 15 m, mostly from 18 to25 m. Two distinct coral communities were represented in this cluster. One groupof transects from gullies was mostly dominated by Montipora species and had lowliving coverage. A second group of transects was from flat parts of the reefs andrepresented a distinct Acropora-dominated community. The predominant specieswere the open arborescent A. austera and the vasiform A. clathrata.

Cluster B was made up largely by transects from 2-Mile, 9-Mile, and Red SandsReefs, most transects being shallower than 15 m, except a group of sponge-domi-nated transects from 25-34 m. The shallow transects were characterized by Alcyo-nacea of the genera Sinularia and Lobophytum. The most common Scleractinia inthis cluster were various Faviidae and the vasiform A. clathrata.

Acropora-dominated (mainly A. Busters)(18-25 m)

Fig. 2. Classification of all transects. Clusters correspond to community types within defined depth zones.

404 RIEGL & RmGL

Kosi Bay ReefA

qualitative= presence/absence Red Sands Reef

9 -Mile Reef

L[4 -Mile Reef

2 .Mile Reef

0 5 10 lS 20 25

BKosi Bay Reef

4 -Mile Reef

9 -Mile Reef= intercept

2 -Mile Reef

Red Sands ReefFig. 3. Classification of values pooled for each site. A) Qualitative data set giving only presence/absenceinformation on species; B) Quantitative data set giving total coverage information for each species oneach reef.

Therefore, four different communities could be differentiated: a gully communi-ty dominated by Scleractinia (Montipora, Faviidae) in all depths; a communityoutside the gullies dominated by Alcyonacea (Lobophytum spp. and Sinularia spp.)in depths less than 18 m; a Scleractinia-dominated community (A. austera, A. cla-thrata) between 18 and 25 m; a very deep community between 25 and 34 mpredominated by sponges (20-70% of total living cover).

The comparison of the pooled datasets of quantitative as well as qualitative datashowed similar trends (Fig. 3). The pooled values of the medium-deep, flat reefsbetween 18 and 25 m (4- Mile and Kosi Mouth Reefs) separated from those on theshallow and very deep reefs (2-Mile, 9-Mile, Red Sands Reefs), reflecting the highAcropora dominance exclusive to medium depth areas.

Pooled qualitative data (presence/absence of species) showed the species com-position of mo.st reefs to be very similar. Only Kosi Mouth Reef fell outside onecluster with high similarity, which was due to the exclusive occurrence of twoAcropora (A. horrida, A. palifera).

No significant differences existed between coral species diversity and coralcoverage between the depth groupings 8 to 18 m and 18 to 25 m, but these depthgroupings did differ significantly from that at 25 to 34 m (ANOV A, F = 15.9, df:2,20; P < 0.001; TuKEY HSD test; Table 1).

How episodic coral breakage can determine community structure 405

Table 1. Diversity (SHANNON'S Index, H') in different depth zones, Pooled values from all surveyed reefs.

depth n transects diversity (H')8-18 m 89 2.09:t 0.30

18-25 m 70 2.23:t 0.3625-34 m 13 1.69:t 0.36

The alcyonacean species which dominated in shallow water were also commonon the deeper reefs. The branching A. austera, however, which dominated wideareas of the deeper reefs was uncommon in the shallow areas. Based on the abovefindings, we assumed that this species was excluded from shallow areas by thehigher wave-energy experienced there, which can cause breakage and high mortal-ity. Therefore, this species was separately evaluated in shallow and deep areas andthe survival of fragments was tested.

2. Morphological characteristics of fragile species

Apart from occupying more space in the community, individual colonies of branch-ing A. austera were bigger (t = -2.12, P < 0.05), more upward oriented, and hadlonger branches on the deep 4-Mile Reef than on the shallow 2-Mile and 9-MileReefs (ANOV A, F = 22.1, df: 2, 20; P < 0.01).

On the shallow reefs, this species was generally rare and where present, colonieswere smaller, low-growing, and had shorter branches (average colony volume on4-Mile Reef: 562.7 :t 1075.3 (SO) litres, on 2- and 9-Mile Reefs: 3.5 :t 4.1 (SO)litres; Figs. 4 & 5). There was a significant positive correlation between branchdepth and colony volume (r2 = 0.60, P < 0.05; Fig. 6); average branch length andmaximal branch width also increased linearly with depth (r2 = 0.62, P < 0.01; r2 =0.79, P < 0.01; Figs. 7 & 8). The number of unattached fragments within 50 cmradius of colonies also differed significantly between the depths (ANOV A: F =6.32; df: 2, 20; P < 0.05), increasing linearly with depth (r2 = 0.84, P < 0.05; Fig. 9).

3. Field experiment to determine fragment survival indifferent reef zones

The amount of fragments recovered after one month differed depending on the depthof the experimental sites. All fragments were lost at the shallow (12 m) stations, 53%were found at the middle (18 m) station, and 82% at the deep (24 m) station.

Overall loss of fragments was clearly depth-dependent. Most fragments did notremain in the same locality where they were initially deposited but were moved bywater motion. None remained in their initial position at the shallow site (12 m), 75%at the middle site (18 m), and 90% at the deep site (24 m, Fig. 8).

Also, survival rates varied with depth. In the deep (24 m) stations, 56% of allfragments lost up to half of their living tissues within one month; 30% lost all theirtissues. If lost fragments were counted as dead, 48% of all fragments had died. Inthe middle (18 m) station, 41 % of fragments lost up to 50% of their tissues, while18% were found dead. Again, if lost specimens were counted as dead, the total losswould have been 58%. In the shallow (12 m) station, no fragments were recovered,

RIEGL & RIEGL

Acropora-dominatedcommunity

406

E§..c-C)c.!.cucm..

,Q

depth (m)

Fig. 4. Mean differences in average branch length (:t: SO) on unbroken colonies between depth zonespooled from all reefs.

as they were washed off the reef by wave action. We suspect that all lost fragmentshad died and therefore the total failure to recover any fragment led to assumed100% mortality.

Discussion

The community study showed a clear differentiation of coral communities into fourbasic types: a shallow (8-18 m) Alcyonacea- (Lobophytum spp., Sinularia spp.)dominated community, a medium-depth (18-25 m) Scleractinia- (Acropora austera,A. clathrata) dominated community, and a deep (25-34 m) sponge-dominated com-munity. A distinct gully community (dominated by Scleractinia, Montipora spp.,Faviidae) alternated with the shallow and the medium-depth communities.

The factor causing the differentiation into gully- and non-gully community isbelieved to be high sedimentation in the gullies due to resuspended local sand(RIEGL et al., 1995). The differentiation between the coral-dominated shallow andmedium-depth communities and the sponge-dominated deep community was mostlikely caused by different light availability (PORTER et al., 1985; JOKIEL, 1988).Corals did not appear to receive sufficient light below 28 m depth.

The differentiation between the shallow and medium-depth community is unlike-ly to be caused by light availability. Coral species diversity and coral coverage inthe medium depth community is as high as in the shallow community. Speciescomposition is also similar, only branching Acropora, such as A. austera, are rare

407How episodic coral breakage can detennine cornrnunity structure

Acropora-dominatedcommunity

depth (m)

Fig. 5. Mean differences in colony volume (:t SD) between depth zones pooled from all reefs.

Acropora--dominatedcommunity30

Alcyonacea-dominatedcommunity I"" A ~"""E

.§.iiE..U~uc..~

A """,

10

11 15 18 22

depth (m)

Fig. 6. Mean differences in average maximum branch diameter (:f: SD) on unbroken colonies betweendepth zones pooled from all reefs.

13

in the shallows. It is therefore assumed that small-scale episodic disturbances whichonly disadvantage one dominant species are responsible for this community differ-entiation.

A. austera is an aggressive and efficient space utilizer, achieving dominance inthe coral community by rapid linear extension of the skeleton. This leads to rapidcolony growth and overtopping of competitors. Furthermore, this species is capableof reproducing by fragmentation (HIGHSMITH, 1982; WALLACE, 1985). -

RlEOL & RIEGL408Acropora -dominated

community

I" A..:-"",\

>-c0"0t)

iQ.

Alcyonacea-dominatedcommunity

IIICQ)

ECI

~'CQ).c(.)ca::cac~'0..Q)

.QE~c

4-'

2-

11 13 15 18 22

depth (m)

Fig. 7. Mean number of unattached branches (:t SO) found within 50 cm radius of colonies betweendepth zones pooled from all reefs.

100

EJ fragment mortality~ fragments retrieved

80

~~iUt:0E

C.,Eg)

~

60

;-(Q3II~1Z

~fII~m;:~~

30~SO

~.!.

40

20

12 18 21

depth (m)

Fig. 8. Fragment retrieval and fragment mortality between three experimental sites at different depths.

Wave energy is a limiting factor in A. austera distribution on the reefs. Whiledisadvantaging this coral in shallow water, however, episodic disturbances maybenefit this species in deeper water. On the deeper zones of all surveyed reefs(18-25 m), A. austera is dominant or among the dominant species. In this area itexhibits all traits typical for an aggressive, well-adapted species. Competitivestrategies include fast growth, aggressive interactions, and frequent propagation byfragmentation. All colonies surveyed on 4-Mi1e Reef had produced unattachedfragments, and numerous colonies appeared to have resulted from reattached frag-ments. Sexual recruits can be distinguished from asexually produced colonies by adifferent basal attachment to the substratum (HIGHSMITH, 1982).

16,

14-

12-

10-

81

6~

How episodic coral breakage can determine community structure 409

In this depth zone, severe stonns could advantage fragmenting corals such asA. austera. Drag induced by unusually high water motion is likely to break manyof the long, upward-growing coral branches (DENNY, 1988). The breakage experi-ment showed high survival chances of fragments (between 50 and 80% in the firstmonth). The high number of unattached fragments and colonies supports this pointof view. Therefore, periodic high wave-energy events do not disadvantage thebranching Acropora between 18 and 25 m, but aid asexual reproduction.

In the shallow areas, however, colonies are small with short, low-growingbranches close to the substratum. Coral size is a poor indicator for coral age(HUGHES & JACKSON, 1980) and it is therefore difficult to evaluate whether thesmall corals are young or simply old corals unable to grow to a larger size. Bothpossible cases indicate that only small corals can survive on shallow reefs, whilelarger colonies, which experience more drag during high wave-action, becomeeliminated by episodic disturbances.

No signs of asexual reproduction by fragmentation were observed. None of thecolonies had produced fragments, which is not surprising given the small colonysizes and short branch lengths, which add to mechanical stability. Furthennore, thebreakage experiment showed survival chances of fragments to be extremely low.Already during the first month of the experiment all fragments were washed off thereef.

Episodic high wave-energy events therefore effectively bar the branching A. aus-tera from dominating shallow reef parts in two ways: colonies cannot grow as largeas on the deep reef, since the longer branches are more easily broken; once broken,fragments have a low survival chance and asexual reproduction by fragmentationis not possible.

Therefore, A. austera cannot utilize the same life history strategies in shallowwater which allows it to dominate deeper reef zones. Breakage and fragmentdislocation during recurrent high wave-energy events severely disadvantage thisspecies in shallow water. Alcyonacea and massive Scleractinia are not affected bythese disturbances due to their growth fonn. This led to the dominance of theseslower-growing species in reef areas where fast-growing, branching Acroporacannot prevail.

Summary

The community structure of South African coral reefs was influenced by episodicdisturbances due to high wave-energy events. These caused breakage and dislodge-ment of Acropora austera, a dominant competitor for space with a branchingmorphology. A. austera dominated reefs between 18 and 24 m depth, where colonieswere less frequently broken than in shallow areas, and where fragments survivedand could serve as asexual propagules. In shallow water (8-18 m), A. austeracolonies were broken during high wave-energy events; fragments were washed offthe reefs and died. Such colonies therefore remained small and were largely unableto reproduce asexually by fragmentation. This allowed other, competitively inferiorspecies to dominate shallow communities.

410 RlEGL & RlEGL

Acknowledgements

Financial support from the South African Foundation for Research Development, Department forNational Education, Association for Marine Biological Research and the University of Cape Town aswell as logistic support by the Natal Parks Board and the Oceanographic Research Institute areacknowledged. Special thanks are due to Dr. M. H. SCHLEYER for his support during field work. Thispaper is a result of Natal Parks Board/University of Cape Town Research Project SM 6/1/14.

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