Competition between macroalgae and corals:effects of herbivore exclusion and increasedalgal biomass on coral survivorship and growth

7/27/2019 Competition between macroalgae and corals:effects of herbivore exclusion and increasedalgal biomass on coral survivorship and growth

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R E P O R T

D. Lirman

Competition between macroalgae and corals:effects of herbivore exclusion and increasedalgal biomass on coral survivorship and growth

Accepted: 2 May 2000 / Published online: 19 December 2000 Springer-Verlag 2000

Abstract Recent declines in coral abundance accompa-nied by increases in macroalgal cover on Florida reefs

highlight the importance of competition for space be-tween these groups. This paper documents the frequencyof coralalgal interactions on the Northern Florida ReefTract and evaluates the eects of grazer exclusions andexperimental algal addition on growth and tissue mor-tality of three coral species, Siderastrea siderea, Poritesastreoides, and Montastraea faveolata. The frequency ofinteractions between corals and macroalgae was high asmore than 50% of the basal perimeter of colonies was incontact with macroalgae; turf forms, Halimeda spp., andDictyota spp. were the most common groups in contactwith corals. Decreased grazing pressure resulted in sig-nicant increases in algal biomass within cages, and

caged corals showed species-specic susceptibility toincreased algal biomass. While no eects were detectedfor S. siderea, signicant decreases in growth rates weredocumented for caged P. astreoides which had growthrates three to four times lower than uncaged colonies.When an algal addition treatment was included to du-plicate maximum algal biomass levels documented forreefs in the area, colonies of P. astreoides in the algaladdition treatment had growth rates up to ten timeslower than uncaged colonies. High susceptibility to algalovergrowth was also found for the reef-building coralM. faveolata, which experienced signicant tissue mor-tality under both uncaged (5.2% decrease in live tissue

area per month) and caged (10.2% per month) condi-tions. The documented eects of increased algal biomasson coral growth and tissue mortality suggest a potentialthreat for the long-term survivorship and growth of

corals in the Florida Reef Tract if present rates of algalgrowth and space utilization are maintained.

Key words Coralalgal competition Caging Grazerexclusion Algal overgrowth Florida Reef Tract

Introduction

In the recent past, reports of rapidly declining reef healthand localized phase shifts towards macroalgal-domi-nated systems have highlighted the importance of com-petition for space between corals and macroalgae, theoutcome of which can have signicant implications forthe long-term survivorship and growth of corals (e.g.,

Smith et al. 1981; Lessios 1988; Hughes 1989, 1994;Done 1992; Knowlton 1992; Lapointe et al. 1997;McClanahan and Muthiga 1998). The main objective ofthe present study was to document the intensity of cor-alalgal interactions on the Northern Florida Reef Tractand to evaluate the eects of reduced herbivory andalgal competition on coral growth and tissue mortality.

Several studies have reported decreases in coral coverand diversity along the Florida Reef Tract in the recentpast (Dustan and Halas 1987; Jaap et al. 1988). Just as inmany other places in the Caribbean, declines in coralcover in Florida have been accompanied by corre-sponding increases in the percent cover of macroalgae

(Porter and Meier 1992). Although the actual causes ofcoral decline are being debated, changes in water qualityand trophic structure linked to human activities as wellas natural stressors such as elevated temperatures,storms, and epizootic diseases have all been proposed aspotential causal agents (Hughes 1994; Lapointe 1997;Lirman and Fong 1997; Hughes et al. 1999; Porter et al.1999).

Even though macroalgae are important componentsof Florida reef communities, only limited information isavailable on their abundance, distribution, and com-munity dynamics (Chiappone and Sullivan 1997; Lapo-inte 1997). In the Northern Florida Reef Tract, the study

Coral Reefs (2001) 19: 392399DOI 10.1007/s003380000125

D. LirmanCenter for Marine and Environmental Analyses,Rosenstiel School of Marine and Atmospheric Science,University of Miami,4600 Rickenbacker Causeway,Miami, Florida 33149, USAE-mail: [email protected].: +1-305-361-4168; Fax: +1-305-361-4077


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area for this project, macroalgal cover uctuates sea-sonally, reaching up to 56% during the summer (Lirmanand Biber 2000). During the time of peak algal abun-dance, small coral colonies can become completelycovered by dense macroalgal mats. Similarly, largemassive colonies can experience intense competitionfrom algae growing along colony margins or on deadportions of colonies.

A limited number of studies have examined experi-mentally the eects of coralalgal competition on coralgrowth and survivorship. These studies have shownconsistently that coral colonies in direct contact withmacroalgae can experience reduced growth and fecun-dity, as well as increased tissue mortality (Potts 1977;Lewis 1986; Hughes 1989; Coyer et al. 1993; Tanner1995; Miller and Hay 1996, 1998). The present studyquanties the frequency of interactions between coralsand macroalgae at the peak time of algal abundance anddetermines experimentally the eects of reduced herbi-vory and increased algal biomass on the growth and

tissue mortality of three common coral species on

Florida reefs, Siderastrea siderea, Porites astreoides, andMontastraea faveolata.

Methods

Coralalgal contact interaction surveys

Photographic surveys were performed on reefs within BiscayneNational Park, Florida (Fig. 1) to document the frequency ofcontact interactions between corals and macroalgae. These surveyswere carried out on two inshore patch reefs, Bache Shoals(2529.187N, 8008.880W, depth=4 m) and Elkhorn Reef(2521.782N, 8009.841W, depth=4 m), and two oshore bankreefs, Triumph Reef (2528.333N, 8006.704W, depth=6 m) andPacic Reef (2522.186N, 8008.360W, depth=6 m) during JulyAugust 1998, at the seasonal peak of algal abundance when meanpercent cover of macroalgae on these reefs was 56.7% (Lirman andBiber 2000). A belt transect (25 2 m) was haphazardly placed ateach site, and all coral colonies (550 cm in diameter) found withinthe transect were photographed. The resulting images were scannedand the percentage of each colony's basal perimeter in direct con-tact with macroalgae was determined using NIH Image. Smallercolonies were not detected by this photographic method.

Fig. 1 Map of South Floridaand the study area includinglocation of study sites

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Algae were grouped by genus if abundant (e.g., Halimeda andDictyota) or by functional group (Steneck and Dethier 1994). Thefunctional groups used here are: laments (e.g., Polysiphonia spp.,Cladophora spp.), corticated terete algae (CTA, e.g., Laurenciaspp., Acanthophora spp.), articulated calcareous algae (ACA, e.g.,Amphiroa spp., Jania spp.), and crustose coralline algae (CCA).

Herbivore exclusion experiments

Porites astreoides and Siderastrea siderea

In 1998, a caging study was carried out on two reefs, Triumph Reef(oshore bank reef, depth=6 m) and East Bache Shoals (inshorepatch reef, 2529.002N, 8008.388W, depth=5 m) from 1 July20September (Fig. 1) to test the eects of herbivore exclusion on coralsurvivorship and growth. On each reef, colonies (515 cm in di-ameter) of Porites astreoides and Siderastrea siderea were labeledwith metal tags and assigned to one of four treatments: caged (n=8per reef), caged-open top controls (n = 4), caged-open side con-trols (n = 4), and uncaged controls (n = 8). Cages built of gal-vanized steel mesh (22 cm in diameter, 15 cm in height, 1 1 cmopening size, Fig. 2A) were placed over colonies and secured tonails hammered onto the bottom. Cages were brushed free of col-

onizing organisms at bi-weekly intervals. Photographs of the coralcolonies were taken at the start and at the end of the study with aNikonos V camera equipped with a 28-mm lens and close-up lensoutt. The resulting prints were scanned and analyzed with NIHImage. Growth was estimated as the percent change in projectedsurface area of live coral tissue. At the end of the experiment, allalgal tissue was collected by hand from caged and uncaged plots,identied, and dry weights compared among treatments.

In 1999, a second caging experiment was carried out withP. astreoides on Marker 3 Reef (2522.408N, 8009.667W,depth=23 m, Fig. 1) from 14 May10 September. Althoughgrazer exclusion resulted in increased biomass within cages in 1998(Fig. 3), algal biomass did not reach maximum levels observed forother reefs in the Northern Florida Reef Tract where mean bio-mass of Dictyota exceeded 20 g m2 and small colonies ofP. astreoides and other species were totally covered by macroalgae

(Lirman and Biber 2000). Therefore, an algal addition treatmentwas added to duplicate observed maximum algal biomass levels.Caged corals were divided into two groups, and algal tissue was

added as loose clumps to one group at bi-weekly intervals. Clumpsof Dictyota spp. (1.52.5 g dry weight) were collected and intro-duced into the cages. The treatments used in this experiment werecaged (n=10), caged with algal addition (n=10), caged-open topcontrols (n=10), caged-open side controls (n=10), and uncagedcontrols (n=10). The amount of algal biomass introduced intocages approximated the maximum macroalgal biomass values re-corded on reefs of the Northern Florida Reef Tract (Lirman and

Biber 2000).

Montastraea faveolata

Rectangular cages were placed on large M. faveolata colonies (di-ameter >1 m) on Elkhorn Reef (depth=6 m, Fig. 1) from 19 May10 September 1999. Cages (15 cm in length, 10 cm in width, 6 cm inheight, 1 1 cm opening size, Fig. 2B) were placed so that roughlyone half of the surface area enclosed was occupied by live coraltissue and the other half by macroalgae growing on dead coralskeleton. Cages were also placed over live coral tissue to documentcoral survivorship in the absence of direct algal competition and todetermine whether algae could colonize live coral surfaces in theabsence of grazing. Uncaged plots were established on coralalgalinterfaces and live coral tissue without algal interaction. Each M.

faveolata colony (n=10) received one set of treatments (caged in-terface, caged live coral tissue without algal interaction, uncagedinterface, and uncaged live coral tissue). Cages were brushed free ofcolonizing organisms at bi-weekly intervals and changes in percentlive tissue were determined from digitized photographs of the ex-perimental plots. In addition, the retreat rate of the live coral tissuemargin was estimated by tracing the initial and nal positions ofthe coral margin within each plot and estimating the distance be-tween them. Distance estimates between the margins were mea-sured at 2-cm intervals and averaged within plots. All algal tissuewas collected from each plot at the end of the experiment, identi-ed, and dry weights compared among treatments.

Statistics

The data were examined for conformity to the assumptions of eachstatistical test prior to analysis. Normality was tested with theShapiro-Wilk test and homoscedasticity was tested with the Bart-lett's test. As the normality assumptions were not met for percentgrowth of P. astreoides and S. siderea, an arcsine transformation(arcsine x) was successfully applied prior to analysis. To evaluatepotential caging artifacts, algal biomass and coral growth werecompared for caged open-top controls, cage open-side controls anduncaged controls within reefs using ANOVA. Since no signicantdierences or patterns in algal biomass or coral growth were found

Fig. 2 Two types of herbivore exclusion cages used in this study: Acircular cages (22 cm in diameter, 15 cm in height, 1 1 cmopening size) were placed over colonies of Siderastrea siderea andPorites astreoides and B rectangular cages (15 10 6 cm,1 1 cm opening size) were placed over coralalgal interfaces onMontastraea faveolata colonies

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(ANOVA, p > 0.05 for all experiments), data from these plotswere pooled into a single control group for each coral species (P.astreoides and S. siderea) from each reef (Triumph Reef, EastBache, and Marker 3 Reef). Data from the 1998 caging experimentwith P. astreoides and S. siderea were analyzed using a two-wayANOVA with grazing (ambient grazinggrazer exclusion cages)and reef location (inshoreoshore) as factors.

Results

Coralalgal interaction surveys

The frequency of interactions, estimated as the per-centage of the basal perimeter of colonies (n=273) thatwas in direct contact with macroalgae, was high, ex-ceeding 50% on all four sites surveyed, reaching up to85% on Pacic Reef (Fig. 4). Although the relativepercentage of interactions varied for algal groups amongsites, turf forms, Halimeda spp., and Dictyota spp. werethe groups most commonly in contact with coral colo-

nies. Cross-shelf patterns in the frequency of interactionswere detected, with Halimeda spp. being more com-monly in contact with corals at East Bache Shoals, andDictyota spp. on oshore reefs (Fig. 3).

Herbivore exclusion experiments

Porites astreoides and Siderastrea siderea

The exclusion of grazers had signicant eects on mac-roalgal biomass at Triumph Reef, East Bache Shoals,and Marker 3 Reef (Fig. 3). Although all groups accu-

mulated higher biomass within cages, signicant dier-ences were not detected for all of these. Biomasses ofDictyota spp. and articulated calcareous algae (ACA)were signicantly higher within caged plots (n=16 forTriumph Reef and East Bache Shoals and 10 for Marker3 Reef) compared to control plots (n=32 for TriumphReef and East Bache Shoals and 30 for Marker 3 Reef;t-tests, p


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those corals in the algal addition treatment (n=10) hadgrowth rates signicantly slower than the other treat-ments. No net tissue losses were recorded for any of the

corals, but photographic surveys showed that colonies inthe algal addition treatment had retracted polyps ontheir upper surfaces starting 1 week after the onset of theexperiment, and that this behavior was observedthroughout the study.

Montastraea faveolata

Macrograzer exclusion inuenced macroalgal biomassalong coralalgal interfaces on M. faveolata colonies.Although biomass of CTA and lamentous algae washigher within caged interface plots (n=10) compared to

uncaged interface plots (n=10), statistically signicantdierences were only found for Dictyota spp., the mostabundant group along coralalgal interfaces (t-test,p


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growth rates three to four times lower than uncagedcolonies. Similarly, caged coralalgal interfaces onMontastraea faveolata colonies had tissue mortality ratesthat were double those of uncaged interfaces.

Other studies have documented the negative eects ofalgal competition. Potts (1977) reported reduced coralgrowth within damselsh territories with high algalcover, and Tanner (1995) found decreased growth ratesfor two acroporid species in competition with macroal-gae. In addition, Miller and Hay (1996) showed throughmanipulative experiments that algal competition caninhibit the growth of the temperate coral Oculina

arbuscula. However, exceptions to these patterns werenoted. In the present study, no eects of increased algalcompetition were detected for Siderastrea siderea. Also,Tanner (1995) noted no eects of macroalgal competi-tion on growth rates of Pocillopora damicornis.

Even further decreases in growth rates were docu-mented for P. astreoides when additional algal tissue wasintroduced into caged treatments to duplicate maximumlevels of algal biomass documented for reefs of theNorthern Florida Reef Tract (Lirman and Biber 2000).Colonies of P. astreoides in the algal addition treatmentexhibited retracted polyps and had growth rates up to

Fig. 5 Mean percent growthrates (1 SEM) of caged (n=8)and control (n=16) Siderastreasiderea and Porites astreoidesduring caging studies at EastBache Shoals and TriumphReef. Study ran from 1 July20Sept 1998. Control plots com-

bine data for caged open-topcontrols, caged open-sidecontrols, and uncaged controls.P values from two-factorANOVA

Fig. 6 Mean percent growthrates (1 SEM) of caged(n=10), caged + algae (n=10),and uncaged (n=30) Poritesastreoides during caging studiesat Marker 3 Reef (14 May10Sept 1999). Control plots com-bine data for caged open-topcontrols, caged open-side con-

trols, and uncaged controls.Additional algal tissue (1.52.5gdry wt of Dictyota) was addedto algal addition group at bi-weekly intervals. Bars with dif-ferent letters are statisticallydierent (ANOVA, Tukeypairwise comparisons)

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ten times lower than uncaged colonies. It is important tonote, however, that polyp retraction was only observedin corals in the algal addition treatments and may be aresult of the potential scouring action of the clumps ofalgae added, which remained unattached within cagesfor several days. Polyp retraction and reduced growth

in corals in contact with macroalgae were previouslydocumented by Coyer et al. (1993) on a temperaterocky reef.

Coral colony morphology may have an importantinuence on the outcome of coralalgal competition andmay partly explain the species-specic results reportedhere. Tanner (1995) and Hughes (1989) found encrustingcolonies to be more aected by algal encroachment thanbranching or massive morphotypes based on the higherperimeter-to-area ratio of encrusting colonies comparedto colonies with higher three-dimensionality. In agree-ment with these reports, growth rates of encrustingPorites astreoides colonies in this study were negatively

aected by increased algal competition, whereas growthrates of massive Siderastrea siderea colonies were notaected. However, colonies of the massive Montastraeafaveolata were found to be highly susceptible to algalovergrowth as caged tissue plots lost an average of 10%of their live tissue area per month.

For massive corals that commonly maintain livetissue cover over their whole colony surface (e.g. Sid-erastrea spp., Diploria spp., Colpophyllia spp.), algalcompetition would be concentrated along the colonyperimeter, as few dead areas would be available foralgal colonization. As colonies of these species grow, asmaller percentage of their surface area would be in

direct contact with macroalgae, reducing the negativeeects of competitive interactions. However, for coralspecies that do not normally maintain live tissue coverover their whole colony surface, dead patches providesuitable substrate where macroalgae can attach andgrow. In these cases, coralalgal interfaces would befound not only along the colony base, but also at nu-merous points within the colony surface. Such is thecase of Montastraea annularis where live tissue is foundonly at the tops of columns (Knowlton et al. 1992) orlarge colonies of M. faveolata like those surveyed in thisstudy, which have experienced partial mortality due

to the establishment of damselsh territories or otherdisturbances.

The present study showed that large, massive colonieslike those of M. faveolata can experience rapid tissuelosses at coralalgal interface margins. These resultsemphasize the importance of chronic disturbance agentssuch as corallivores, sedimentation, diver and boat im-pacts, elevated temperatures, and storms that rarelycause whole-colony mortality, but can cause numerousdead patches within coral colonies that can be subse-quently colonized by algae (e.g., Bythell et al. 1993;Meesters et al. 1996; Lirman 2000). Cages placed overlive tissue areas without any direct competition from

algal margins experienced no tissue mortality, reinforc-ing the need for dead patches to be created before algaecan colonize, even in the absence of grazing. Once thesedead patches are colonized, the subsequent reductions incoral growth and enhanced tissue losses documentedhere may take place, compromising the long-termsurvivorship of the aected colonies.

Competition for primary space has always beenconsidered a major structuring force within reef com-munities (Lang and Chornesky 1990). Increases inmacroalgal cover and biomass on Caribbean reefs inresponse to reduced herbivory, damselsh territorialbehavior, and eutrophication can result in increased

Fig. 7 A Mean percent tissue mortality rates (1 SEM) and Bmean retreat rate (1 SEM) of coral margin of M. faveolata withincaged (n=10) and uncaged (n=10) coralalgal interface plots atElkhorn Reef (19 May10 Sept 1999). * Signicant dierencesbetween groups (t-test, p0.05)

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rates of coralalgal interactions. Moreover, results fromthe present as well as previous studies show that theseinteractions can have signicant, species-specic, nega-tive eects on coral growth and survivorship. The sus-ceptibility of reef-building corals like Montastraeafaveolata to algal overgrowth under present levels ofgrazing suggest long-term negative eects on reef de-velopment in the area if present rates of macroalgalspace utilization are maintained.

Acknowledgements This research was funded by NOAA CoastalOcean Program award #NA37RJ0149 to the University of Miami.Biscayne National Park provided eld support. Field assistance byB. Orlando, S. Macia , T. Jones, L. Kaufman, and S. Tosini isgreatly appreciated. This manuscript beneted from comments byM. Miller, W. Cropper Jr., and three anonymous reviewers.

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Competition between macroalgae and corals:effects of herbivore exclusion and increasedalgal biomass on coral survivorship and growth