Macroalgal Extracts Induce Bacterial Assemblage Shifts and Sublethal Tissue Stress in Caribbean Corals

Benthic macroalgae can be abundant on present-day coral reefs, especially where rates of herbivory are low and/or dissolved nutrients are high. This study investigated the impact of macroalgal extracts on both coral-associated bacterial assemblages and sublethal stress response of corals. Crude extracts and live algal thalli from common Caribbean macroalgae were applied onto the surface of Montastraea faveolata and Porites astreoides corals on reefs in both Florida and Belize. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene amplicons was used to examine changes in the surface mucus layer (SML) bacteria in both coral species. Some of the extracts and live algae induced detectable shifts in coral-associated bacterial assemblages. However, one aqueous extract caused the bacterial assemblages to shift to an entirely new state (Lobophora variegata), whereas other organic extracts had little to no impact (e.g. Dictyota sp.). Macroalgal extracts more frequently induced sublethal stress responses in M. faveolata than in P. astreoides corals, suggesting that cellular integrity can be negatively impacted in selected corals when comparing co-occurring species. As modern reefs experience phase-shifts to a higher abundance of macroalgae with potent chemical defenses, these macroalgae are likely impacting the composition of microbial assemblages associated with corals and affecting overall reef health in unpredicted and unprecedented ways.     

Herbivory facilitates growth of a key reef‐building Caribbean coral

The decline of reef‐building corals in conjunction with shifts to short‐lived opportunistic species has prompted concerns that Caribbean reef framework‐building capacity has substantially diminished. Restoring herbivore populations may be a potential driver of coral recovery; however, the impact of herbivores on coral calcification has been little studied. We performed an exclusion experiment to evaluate the impact of herbivory on Orbicella faveolata coral growth over 14 months. The experiment consisted of three treatments: full exclusion cages; half cage procedural controls; and uncaged control plates, each with small O. faveolata colonies. We found that herbivorous fish exclusion had a substantial impact on both macroalgal cover and coral growth. Fleshy macroalgae reached 50% cover within some exclusion cages, but were almost absent from uncaged control plates. Critically, O. faveolata calcification rates were suppressed by almost half within exclusion cages, with monthly coral growth negatively related to overgrowth by fleshy macroalgae. These findings highlight the importance of herbivorous fishes for coral growth and the detrimental impact of macroalgal proliferation in the Caribbean. Policy makers and local managers should consider measures to protect herbivorous fishes and reduce macroalgal proliferation to enable coral communities to continue to grow and function.     

Caribbean corals exhibit species-specific differences in competitive abilities with an aggressive encrusting alga,Ramicrusta textilis

Caribbean coral cover has decreased substantially in recent decades, with much of the live coral being replaced by macroalgae. Encrusting red algae in the genus Ramicrusta have become abundant throughout the region and have demonstrated widespread harm to corals by overgrowing living tissue, causing colony mortality, and impairing coral recruitment. In this research, Ramicrusta textilis was identified by morpho-anatomy and DNA sequencing from nine sites around St. Thomas, US Virgin Islands, and 3D photogrammetry was used to measure the rate of algal growth on stony corals. 3D models of individual coral colonies (five species plus controls, N = 72) competing with R. textilis revealed differential competitive abilities among taxa, with Siderastrea siderea being the only species capable of inhibiting overgrowth by the alga (mean linear algal growth − 1.1 mm yr⁻¹). Important reef building coral species such as Orbicella annularis and Orbicella faveolata were poor competitors (mean linear algal growth + 15 mm yr⁻¹ and + 7.7 mm yr⁻¹, respectively), indicating that the emergence of the alga could have significant impacts on Caribbean coral reef species diversity, community composition, and structural complexity.

     

Catastrophic mortality on inshore coral reefs of the Florida Keys due to severe low‐temperature stress

Coral reefs of the Florida Keys typically experience seasonal temperatures of 20–31 °C. Deviation outside this range causes physiological impairment of reef‐building corals, potentially leading to coral colony death. In January and February 2010, two closely spaced cold fronts, possibly driven by an unusually extreme Arctic Oscillation, caused sudden and severe seawater temperature declines in the Florida Keys. Inshore coral reefs [e.g., Admiral Reef (ADM)] experienced lower sustained temperatures (i.e., < 12 °C) than those further offshore [e.g., Little Grecian Reef (LG), minimum temperature = 17.2 °C]. During February and March 2010, we surveyed ADM and observed a mass die‐off of reef‐building corals, whereas 12 km away LG did not exhibit coral mortality. We subsequently measured the physiological effects of low‐temperature stress on three common reef‐building corals (i.e., Montastraea faveolata, Porites astreoides, and Siderastrea siderea) over a range of temperatures that replicated the inshore cold‐water anomaly (i.e., from 20 to 16 to 12 °C and back to 20 °C). Throughout the temperature modulations, coral respiration as well as endosymbiont gross photosynthesis and maximum quantum efficiency of photosystem II were measured. In addition, Symbiodinium genotypic identity, cell densities, and chlorophyll a content were determined at the beginning and conclusion of the experiment. All corals were significantly affected at 12 °C, but species‐specific physiological responses were found indicating different coral and/or Symbiodinium cold tolerances. Montastraea faveolata and P. astreoides appeared to be most negatively impacted because, upon return to 20 °C, significant reductions in gross photosynthesis and dark respiration persisted. Siderastrea siderea, however, readily recovered to pre‐treatment rates of dark respiration and gross photosynthesis. Visual surveys of inshore reefs corroborated these results, with S. siderea being minimally affected by the cold‐water anomaly, whereas M. faveolata and P. astreoides exhibited nearly 100% mortality. This study highlights the importance of understanding the physiological attributes of genotypically distinct coral‐Symbiodinium symbioses that contribute to tolerance, recovery, and consequences to an environmental perturbation. These data also document effects of a rarely studied environmental stressor, possibly initiated by remote global climate events, on coral‐Symbiodinium symbioses and coral reef communities.     

Fecundity and sexual maturity of the coral Siderastrea siderea at high latitude along the Florida Reef Tract,USA

Siderastrea siderea is one of the most abundant corals at high latitude shallow sites along the Florida Reef Tract (25°–27°N). This species is able to tolerate wide seawater temperature fluctuations and sedimentation stress, but its reproductive status at high latitudes and under marginal environmental conditions is poorly understood. The objectives of this study were to evaluate the reproductive potential of S. siderea along a latitudinal gradient (25°–27°N) and to determine if sexual maturity occurs in small (<12.0 cm) S. siderea colonies. Samples of coral tissue were collected in 2007, 2008, and 2009 at three sites along the latitudinal gradient and were processed for histological analysis. Oocyte size, volume, and abundance were used to calculate fecundity. Results showed that fecundity decreased with increasing latitude and that oocyte volume was the major contributing factor to this variation. Mature oocytes were observed in S. siderea colonies at sizes as small as 1.1 cm in diameter. The ability of S. siderea to reach fertility at high latitude areas suggests this species is able to reproduce under marginal environmental conditions; however, reduction in oocyte size could increase local retention of larvae. The presence of mature oocytes in small colonies suggests that stress can reduce somatic growth and shift sexual maturity to smaller colony sizes.     

Ubiquitous associations and a peak fall prevalence between apicomplexan symbionts and reef corals in Florida and the Bahamas

Although apicomplexans are a widely recognized and important parasitic group, little is known about those associated with invertebrates, such as reef-building scleractinian corals. To resolve the potential impact of apicomplexans on coral health, it is first necessary to further describe this group of putative parasites and determine their prevalence among host species. Here, it was hypothesized that apicomplexan prevalence would vary seasonally, similar to what occurs in other marine apicomplexans as well as some coral symbionts. To test this, Caribbean scleractinian species Porites astreoides, Montastraea (=Orbicella) annularis, M. (=O.) faveolata, and Siderastrea siderea were sampled seasonally from two reefs each in the Florida Keys and the Bahamas for 9- and 5.5-year periods, respectively. Utilizing a PCR-based screening assay, apicomplexan DNA was detected from most Floridian (80.1 %: n = 555/693) and Bahamian (90.7 %: n = 311/343) coral tissue samples collected over these multi-year periods. Furthermore, apicomplexan DNA was detected from nearly all (98.7 %: n = 78/79) single polyps sampled at multiple locations within six M. faveolata colonies, indicating little to no intracolonial variation in the screening assay. Mixed-model logistic regression was utilized to determine the effects of season, host species, and reef on apicomplexan prevalence. The model identified a significant seasonal effect, with the highest apicomplexan prevalence occurring during fall. There also was a large effect of host species, with apicomplexan prevalence significantly lower among S. siderea colonies relative to the other species. While reef did not have a significant effect in the full model, there was a significant difference in apicomplexan prevalence between Floridian and Bahamian reefs for S. siderea, implying regional differences in this host species. Despite seasonal and species-specific differences in prevalence, apicomplexans are ubiquitous constituents of these particular scleractinian coral species from Florida and the Bahamas.     

Latitudinal variation in coral communities in eastern Australia: a qualitative biophysical model of factors regulating coral reefs

The processes underlying the distributional limits of both corals and coral reefs can be elucidated by examining coral communities at high latitudes. Coral-dominated communities in eastern Australia cover a latitudinal range of >2,500 km, from the northern Great Barrier Reef (11°S) to South West Rocks (31.5°S). Patterns of coral species richness from 11 locations showed a clear separation between the Great Barrier Reef and subtropical sites, with a further abrupt change at around 31°S. Differences in community structure between the Great Barrier Reef and more southern sites were mainly attributable to higher cover of massive corals, branching Acropora, dead coral and coralline algae on the Great Barrier Reef, and higher cover of macroalgae and bare rock at more southern sites. The absence of some major reef-building taxa (i.e., staghorn Acropora and massive Porites) from most subtropical sites coincided with the loss of reef accretion capacity. Despite high cover of hard corals in communities at up to 31°S, only Lord Howe Island contained areas of reef accretion south of the Great Barrier Reef. Factors that have been hypothesized to account for latitudinal changes in coral community structure include water temperature, aragonite saturation, light availability, currents and larval dispersal, competition between corals and other biota including macroalgae, reduced coral growth rates, and failure of coral reproduction or recruitment. These factors do not operate independently of each other, and they interact in complex ways.     

Coral mortality and interaction with algae in relation to sedimentation

The impact of sedimentation on coral–algal interactions was studied by monitoring tissue mortality and radial growth in two coral species, Colpophyllia natans and Siderastrea siderea, over a continuum of sediment input intensities. This study sets out to investigate (1) whether sedimentation can facilitate algal overgrowth of corals and (2) whether this was a significant cause of coral mortality. Over a 15-month period, 198 coral colonies were tagged and photographed at six sites along two replicate gradients of sediment input, spanning high inputs near river mouths to low inputs at exposed headlands. Photographs were taken so that they covered the interface between colonies and algae. Radial growth was measured along colony edges in contact with algae and unaffected by tissue loss from causes other than competition with algae. To establish whether algal overgrowth was a significant cause of coral mortality, tissue mortality on the colony surface area visible in the photographs was related to different causes, including sediment smothering, disease, and algal overgrowth. Radial growth became negative with increasing proximity to river mouths in C. natans and remained negative or close to zero throughout the gradients in S. siderea, overall suggesting that sedimentation can facilitate algal overgrowth on corals. However, the analysis of tissue mortality revealed that algal overgrowth was a relatively minor cause of tissue loss. In contrast, the most important cause of coral mortality in relation to sedimentation was from sediment smothering, probably during intense episodes of deposition associated with heavy rainfall. We conclude that sedimentation may lead to reef degradation by causing coral mortality through sediment smothering and burial, and then by suppressing the regrowth of surviving adult colonies through increased competition with algae.     

The effect of species and colony size on the bleaching response of reef-building corals in the Florida Keys during the 2005 mass bleaching event

Understanding the variation in coral bleaching response is necessary for making accurate predictions of population changes and the future state of reefs in a climate of increasing thermal stress events. Individual coral colonies, belonging to inshore patch reef communities of the Florida Keys, were followed through the 2005 mass bleaching event. Overall, coral bleaching patterns followed an index of accumulated thermal stress more closely than in situ temperature measurements. Eight coral species (Colpophyllia natans, Diploria strigosa, Montastraea cavernosa, M. faveolata, Porites astreoides, P. porites, Siderastrea siderea, and Stephanocoenia intersepta), representing >90% of the coral colonies studied, experienced intense levels of bleaching, but responses varied. Bleaching differed significantly among species: Colpophyllia natans and Diploria strigosa were most susceptible to thermal stress, while Stephanocoenia intersepta was the most tolerant. For colonies of C. natans, M. faveolata, and S. siderea, larger colonies experienced more extensive bleaching than smaller colonies. The inshore patch reef communities of the Florida Keys have historically been dominated by large colonies of Montastraea sp. and Colpophyllia natans. These results provide evidence that colony-level differences can affect bleaching susceptibility in this habitat and suggest that the impact of future thermal stress events may be biased toward larger colonies of dominant reef-building species. Predicted increases in the frequency of mass bleaching and subsequent mortality may therefore result in significant structural shifts of these ecologically important communities.     

Macroalgae Inhibits Larval Settlement and Increases Recruit Mortality at Ningaloo Reef,Western Australia

Globally, many coral reefs are degraded and demonstrate reduced coral cover and increased macroalgal abundance. While negative correlations between macroalgae and coral recruitment have commonly been documented, the mechanisms by which macroalgae affects recruitment have received little attention. Here we examined the effect of macroalgae on larval settlement and the growth and survival of coral recruits, in a field experiment over nine months. Exclusion treatments were used to manipulate herbivory and macroalgal biomass, while settlement tiles measured coral settlement and survival. After nine months the volume of macroalgae was up to 40 times greater in the caged treatments than in controls and the settlement of coral larvae on the undersides of tiles within caged plots was 93% lower than in the uncaged treatments. The growth and survival of coral recruits was also severely reduced in the presence of macroalgae: survival was 79% lower in caged treatments and corals were up to 58% smaller with 75% fewer polyps. These data indicate that macroalgae has an additive effect on coral recruitment by reducing larval settlement and increasing recruit mortality. This research demonstrates that macroalgae can not only inhibit coral recruitment, but also potentially maintain dominance through a positive feedback system.     

Effects of herbivore exclusion and nutrient enrichment on coral reef macroalgae and cyanobacteria

Although phase shifts on coral reefs from coral-dominated to algal-dominated communities have been attributed to the effects of increased nutrient availability due to eutrophication and reduced herbivore abundance due to overfishing and disease, these factors have rarely been manipulated simultaneously. In addition, few studies have considered the effects of these factors on benthic, filamentous cyanobacteria (blue-green algae) as well as macroalgae. We used a combination of herbivore-exclusion cages and nutrient enrichment to manipulate herbivore abundance and nutrient availability, and measured the impacts of these treatments on macroalgal and cyanobacterial community structure. In the absence of cages, surface cover of the cyanobacterium Tolypothrix sp. decreased, while surface cover of the cyanobacteria Oscillatoria spp. increased. Cyanobacterial cover decreased in partial cages, and Tolypothrix sp. cover decreased further in full cages. Lower cyanobacterial cover and biomass were correlated with higher macroalgal cover and biomass. Dictyota bartayresiana dominated the partial cages, while Padina tenuis and Tolypiocladia glomerulata recruited into the full cages. Palatability assays demonstrated that herbivore-exclusion shifted macroalgal species composition from relatively unpalatable to relatively palatable species. Nutrient enrichment interacted with herbivore exclusion to increase the change in cover of D. bartayresiana in the uncaged and fully caged plots, but did not affect the final biomass of D. bartayresiana among treatments. Nutrient enrichment did not significantly affect the cover or biomass of any other taxa. These results stress the critical role of herbivory in determining coral reef community structure and suggest that the relative palatabilities of dominant algae, as well as algal growth responses to nutrient enrichment, will determine the potential for phase shifts to algal-dominated communities.     

Herbivory by crabs and the control of algal epibionts on Caribbean host corals

Summary A short-term experiment was conducted to examine the relationships among the branching coral Porites porites, algal epibionts, and a facultative crab associate Mithrax sculptus in Belize, Central America. Initial field observations suggested that coral colonies supporting resident crabs generally had lower algal cover than colonies without crabs. The hypothesis was tested that Mithrax significantly depresses host coral algal cover and thereby indirectly affects host survivorship and growth. Crab accessibility to an array of coral colonies, similarly covered with algal epibionts, was manipulated in three treatments. Results strongly support the hypothesis, with significant differences in algal cover (primarily Dictyota spp.) noted among treatments after only one month. Caged heads with crabs included and uncaged natural controls allowing crabs free access averaged less than 10% cover, whereas mean algal cover exceeded 75% where crabs were excluded. The uncaged treatment, in which crabs were allowed free access to Porites heads was not significantly different from the crab inclusion treatment. Collectively, these results demonstrate that under natural conditions, crabs can have pronounced effects on host corals by reducing fouling algal epibionts. Furthermore, these facultative coral associates may have more important, albeit localized effects on Caribbean corals than has been suggested previously.     

Siderastrea siderea spawning and oocyte resorption at high latitude

Abstract

At high latitudes (>25°), sexual reproduction and the maintenance of coral populations can be impaired by marginal environmental conditions. However, little is known about sexual reproduction of many coral species at high latitude on the northern-most extension of the Florida Reef Tract. This study aimed to histologically characterize the reproductive ecology of Siderastrea siderea, near Fort Lauderdale, Florida (26°N). Tissue samples of S. siderea were collected semi-monthly to multi-weekly from August to November in 2007 and 2008. Spawning was inferred from gametogenesis and oocyte resorption was observed in detail. Environmental variables including temperature and lunar cycle were examined for relationship with potential spawning times. Based on the histological evidence, we infer that spawning likely occurred primarily in October. Gametogenesis in this species is likely mediated by seasonal temperature variation, whereas lunar cycle could act as finer scale environmental cue for coordination of spawning. Our findings highlight that S. siderea spawning occurs later in the year compared to other populations of this species throughout the Caribbean and to other coral species near Fort Lauderdale. For the first time, oocyte resorption stages are described and constitute a baseline for future projects that aim to understand this process in corals.     

Recovery of the coral Montastrea annularis in the Florida Keys after the 1987 Caribbean “bleaching event”

Many reef-building corals and other cnidarians lost photosynthetic pigments and symbiotic algae (zooxanthellae) during the coral bleaching event in the Caribbean in 1987. The Florida Reef Tract included some of the first documented cases, with widespread bleaching of the massive coral Montastrea annularis beginning in late August. Phototransects at Carysfort Reef showed discoloration of >90% of colonies of this species in March 1988 compared to 0% in July 1986; however no mortality was observed between 1986 and 1988. Samples of corals collected in February and June 1988 had zooxanthellae densities ranging from 0.1 in the most lightly colored corals, to 1.6x10⁶ cells/cm² in the darker corals. Minimum densities increased to 0.5x10⁶ cells/cm² by August 1989. Chlorophyll-a content of zooxanthellae and zooxanthellar mitotic indices were significantly higher in corals with lower densities of zooxanthellae, suggesting that zooxanthellar at low densities may be more nutrientsufficient than those in unbleached corals. Ash-free dry weight of coral tissue was positively correlated with zooxanthellae density at all sample times and was significantly lower in June 1988 compared to August 1989. Proteins and lipids per cm² were significantly higher in August 1989 than in February or June, 1988. Although recovery of zooxanthellae density and coral pigmentation to normal levels may occur in less than one year, regrowth of tissue biomass and energy stores lost during the period of low symbiont densities may take significantly longer.     

Context-dependent corallivory by parrotfishes in a Caribbean reef ecosystem

This study assesses the patterns of corallivory by parrotfishes across reefs of the Florida Keys, USA. These reefs represent a relatively unique combination within the wider Caribbean of low coral cover and high parrotfish abundance suggesting that predation pressure could be intense. Surveys across eight shallow forereefs documented the abundance of corals, corallivorous parrotfishes, and predation scars on corals. The corals Porites porites and Porites astreoides were preyed on most frequently with the rates of predation an order of magnitude greater than has been documented for other areas of the Caribbean. In fact, parrotfish bite density on these preferred corals was up to 34 times greater than reported for corals on other reefs worldwide. On reefs where coral cover was low and corals such as Montastraea faveolata, often preferred prey for parrotfishes, were rare, predation rates on P. porites and P. astreoides, and other less common corals, intensified further. The intensity of parrotfish predation increased significantly as coral cover decreased. However, parrotfish abundance showed only a marginal positive relationship with predation pressure on corals, likely because corallivorous parrotfish were abundant across all reefs. Parrotfishes often have significant positive impacts on coral cover by facilitating coral recruitment, survival, and growth via their grazing of algae. However, abundant corallivorous parrotfishes combined with low coral cover may result in higher predation on corals and intensify the negative impact that parrotfishes have on remaining corals.     

Algal contact as a trigger for coral disease

Diseases are causing alarming declines in reef‐building coral species, the foundation blocks of coral reefs. The emergence of these diseases has occurred simultaneously with large increases in the abundance of benthic macroalgae. Here, we show that physical contact with the macroalga Halimeda opuntia can trigger a virulent disease known as white plague type II that has caused widespread mortality in most Caribbean coral species. Colonies of the dominant coral Montastraea faveolata exposed to algal transplants developed the disease whereas unexposed colonies did not. The bacterium Aurantimonas coralicida, causative agent of the disease, was present on H. opuntia sampled close to, and away from diseased corals, indicating that the alga serves as a reservoir for this pathogen. Our results suggest that the spread of macroalgae on coral reefs could account for the elevated incidence of coral diseases over past decades and that reduction of macroalgal abundance could help control coral epizootics.     

Nearshore coral growth declining on the Mesoamerican Barrier Reef System

Anthropogenic global change and local stressors are impacting coral growth and survival worldwide, altering the structure and function of coral reef ecosystems. Here, we show that skeletal extension rates of nearshore colonies of two abundant and widespread Caribbean corals (Siderastrea siderea, Pseudodiploria strigosa) declined across the Belize Mesoamerican Barrier Reef System (MBRS) over the past century, while offshore coral conspecifics exhibited relatively stable extension rates over the same temporal interval. This decline has caused nearshore coral extension rates to converge with those of their historically slower growing offshore coral counterparts. For both species, individual mass coral bleaching events were correlated with low rates of skeletal extension within specific reef environments, but no single bleaching event was correlated with low skeletal extension rates across all reef environments. We postulate that the decline in skeletal extension rates for nearshore corals is driven primarily by the combined effects of long‐term ocean warming and increasing exposure to higher levels of land‐based anthropogenic stressors, with acute thermally induced bleaching events playing a lesser role. If these declining trends in skeletal growth of nearshore S. siderea and P. strigosa continue into the future, the structure and function of these critical nearshore MBRS coral reef systems is likely to be severely impaired.     

The tissue composition of<Emphasis Type="Italic"> Montastraea franksi</Emphasis> during a natural bleaching event in the Florida Keys

In this study, we used a correlative approach to (1) test for an association between bleaching and host tissue composition during a natural bleaching event, and (2) assess whether bleaching susceptibility varies between years. In August 1997, Montastraea franksi at 15-m depth on Conch Reef, Florida, bleached and the severity of the response varied among individuals. Seventy-five randomly selected colonies were quantified for bleaching using both an ordinal scale, assigned by eye, and a continuous scale, assessed using red, green, and blue (RGB) spectral analysis of photographs. Zooxanthella density and chlorophyll a content were evaluated as measures of bleaching, and coral tissue was analyzed for glycerol, free amino acids (FAA), protein, and mycosporine-like amino acids (MAAs); collectively, these are described as the "tissue composition." In 1998, most of the same coral colonies were analyzed for color and zooxanthella density to determine whether colony color in 1997 was correlated with color in 1998. In 1997, colonies of M. franksi that were ranked by color differed significantly in RGB brightness, zooxanthella density, and chlorophyll a content, but not in tissue composition. Similarly, a multivariate test for a linear relationship between color and tissue composition did not reveal a significant association. Analyses of corals in 1997 and 1998 revealed a significant positive relationship between color in both years (i.e., the same colonies were similarly colored in each year). These results are discussed in the context of the temporal scale of the sampling regime, the nature of the measured traits, and the adaptive bleaching hypothesis.     

Competition between corals and algal turfs along a gradient of terrestrial influence in the nearshore central Great Barrier Reef

Competition between benthic algae and corals is a key process in the community ecology of reefs, especially during reef degradation. However, there have been very few experimental tests for competition between corals and benthic algae, despite widespread assumptions that algae are generally superior competitors, especially in eutrophic conditions. This study tested for competition for space between the massive coral Porites lobata and algal filamentous turfs on three reefs along a cross-shelf gradient of terrestrial influence, by experimentally removing or damaging either corals or algae. The corals and algae were competing for space, but, significantly, the algae appeared to have little effect on coral growth. In contrast, corals significantly inhibited algal growth, suggesting Porites was the competitive superior. Importantly, coral growth was generally positive, even on the reef with the greatest terrestrial influence. Competitive outcomes did not support the argument that algae are more successful competitors in more eutrophic conditions.     

Competition between scleractinian corals and macroalgae: An experimental investigation of coral growth, survival and reproduction

Macroalgae are a major component of many coral reef flat communities, and are potentially major competitors with corals. The influence of macroalgae on several demographic parameters of four species of scleractinian coral by means of an algal clearance experiment was examined to determine specifically if macroalgae are affecting coral cover, growth, fecundity, fission, survivorship and recruitment. Also investigated were patterns of natural encounters between corals and algae.

Algal cover at the study site ranged from 41 to 56%, and coral cover from 8 to 10%. In total, 92 ± 4 ( )% of coral colonies were in contact with one or more species of macroalgae. Changes in coral cover were significantly affected by the presence of macroalgae, with cover of Acropora species increasing faster in areas from which algae had been cleared compared to control areas where algae had not been removed, although this pattern did not occur for Pocillopora damicornis (Linnaeus). Similarly, growth of individual colonies was faster when macroalgae were absent for three Acropora species but not for P. damicornis. There were no differences detected in rates of fission or survivorship of corals between algal clearance and control treatments, although there were high levels of variability in both of these parameters. Fecundity of Acropora palifera (Lamarck), the only species examined, was approximately double in colonies in cleared plots compared to those in control plots with macroalgae present. As no recruitment occurred throughout the 2-yr study, it remains to be determined how macroalgae effect the settlement of coral larvae. The results show that macroalgae can have a major influence on the demography of scleractinian corals.