Strategies for Gardening Denuded Coral Reef Areas: The Applicability of Using Different Types of Coral Material for Reef Restoration

Recreational and other human activities degrade coral reefs worldwide to a point where efficient restoration techniques are needed. Here we tested several strategies for gardening denuded reefs. The gardening concept consists of in situ or ex situ mariculture of coral recruits, followed by their transplantation into degraded reef sites. In situ nurseries were established in Eilat's (Northern Red Sea) shallow waters, sheltering three types of coral materials taken from the branching species Stylophora pistillata (small colonies, branch fragments, and spat) that were monitored for up to two years. Pruning more than 10% of donor colonies' branches increased mortality, and surviving colonies displayed reduced reproductive activity. Maricultured isolated branches, however, exceeded donor colony life span and reproductive activity and added 0.5–45% skeletal mass per year. Forty‐four percent of the small colonies survived after 1.5‐year mariculture, revealing average yearly growth of 75 ± 32%. Three months ex situ maintenance of coral spat (sexual recruits) prior to the in situ nursery phase increased survivorship. Within the next 1.5 years, they developed into colonies of 3–4 cm diameter. Nursery periods of 2 years, 4–5 years, and more than> 5 years have been estimated for small colonies, spat, and isolated branches, respectively. These and other results, including the possible use of nubbins (minute fragments the size of a single or few polyps), are discussed, revealing benefits and drawbacks for each material. In situ coral mariculture is an improved practice to the common but potentially harmful protocol of direct coral transplantation. It is suggested that reef gardening may be used as a key management tool in conservation and restoration of denuded reef areas. The gardening concept may be applicable for coral reefs worldwide through site‐specific considerations and the use of different local coral species.     

Testing the first phase of the ‘gardening concept’ as an applicable tool in restoring denuded reefs in Tanzania

Studies on coral reef restoration through a two-step coral gardening protocol have lately proved it to be a viable solution for future reef restoration. This involves a first step of gardening small colonies in mid-water nurseries and a second step, their transplantation, upon reaching suitable size, onto the pre-surveyed damaged areas. We established in September 2007 two mid-water nurseries, each holding 10,000 fragments measuring 2 cm average initial size, at 4 m depths (high tide) in Zanzibar and Mafia Islands, Tanzania. Each nursery comprised six species, each of which was represented by three genotypes. During 9 months, we followed developments by analyzing and comparing survivorship and growth rates of fragments between the different nurseries, species and genotypes. A significant difference between species survival and growth rates was observed in acroporid species, in Pocillopora verrucosa and Millepora sp., which showed better success than Porites cylindrica. In both sites, Millepora suffered no mortality and other species exhibited low mortality, ranging (per coral genotype) between 3% and 24% in Zanzibar (most cases below 10%) and between 13% and 44% (mostly below 25%) in Mafia Island. Most of fragments’ mortality occurred during the first two nursery months. Coral species in Zanzibar nursery also performed better in growth rates than those in Mafia, but in both sites, farmed corals were ready for transplantation just 9 months after the nursery was set up. Economic evaluations involved in the overall nursery set-up and the results indicated that the coral gardening approach could be used in Tanzania to generate large quantities of coral colonies for the restoration of damaged reefs at relatively low cost.     

Ecological impacts of coral gardening outplanting in the Maldives

As coral reefs continue to degrade at an alarming rate, coral restoration efforts are increasing worldwide in an attempt to keep up with the global challenge of preserving these iconic ecosystems and the many services they provide. Coral gardening, the farming and outplanting of coral fragments, is a commonly applied practice; however, regional validation is required before upscaling can be considered. This study follows up from the successful farming of fragments in mid-water rope nurseries, by reporting on the successive outplanting of these corals. Specifically, 60 Pocillopora verrucosa colonies were outplanted to a degraded reef at different depths (1–12 m), applying three arrangement patterns (equal, clustered, random). After 1 year, 72% were considered successfully outplanted (alive and still attached), with detachment being the main challenge at wave-impacted shallow depths, while loose coral rubble caused more partial mortality at depth. Outplanting stress was observed at 1–6 m depth, but had no impact on survival or growth. Drupella sp. predation was most common at 3 m and 79% of colonies hosted mutualistic fauna after 1 year. Outplanting significantly benefitted the reef environment with a higher fish abundance and diversity along with a higher increase in natural coral cover (H = 2.7; 6.2% increase) in comparison with the control sites. These are promising results, considering that the restoration site has shown little natural recovery in the last few years (coral cover <4%). We hope that our findings provide useful initial insights and help to guide effective restoration practices in the Maldives.     

Holocene coral reef accretion in Hawaii: a function of wave exposure and sea level history

 In the high Hawaiian Islands, significant accretion due to coral reef growth is limited by wave exposure and sea level. Holocene coral growth and reef accretion was measured at four stations off Oahu, Hawaii, chosen along a gradient in wave energy from minimum to maximum exposures. The results show that coral growth of living colonies (linear extension) at optimal depths is comparable at all stations (7.7–10.1 mm/y), but significant reef accretion occurs only at wave sheltered stations. At wave sheltered stations in Hanauma Bay and Kaneohe Bay, rates of long term reef accretion are about 2.0 mm/y. At wave exposed stations, off Mamala Bay and Sunset Beach, reef accretion rates are virtually zero in both shallow (1 m) and deeper (optimal) depths (12 m). At wave sheltered stations, such as Kaneohe Bay and Hanauma Bay, Holocene reef accretion is on the order of 10–15 m thick. At wave exposed stations, Holocene accretion is represented by only a thin veneer of living corals resting on antecedent Pleistocene limestone foundations. Modern coral communities in wave exposed environments undergo constant turnover associated with mortality and recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. Breakage, scour, and abrasion of living corals during high wave events appears to be the major source of mortality and ultimately limits accretion to wave sheltered environments. Depth is particularly important as a modulator of wave energy. The lack of coral reef accretion along shallow open ocean coastlines may explain the absence of mature barrier reefs in the high Hawaiian Islands. Accepted: 14 May 1998     

Facilitation in Caribbean coral reefs: high densities of staghorn coral foster greater coral condition and reef fish composition

Recovery of the threatened staghorn coral (Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species.     

Ontogenetic changes in the capacity of the coral Pocillopora damicornis to originate branches

Most colonial corals vary intraspecifically in growth forms, and the diversity in branching morphology is especially striking. While the effects of environmental factors on growth forms have been studied, the genetic control of coral branching patterns has received little attention. The discovery of ontogenetic changes in the capacity to originate branching would set the stage for studies of how branch formation is genetically controlled. During experiments investigating contact reactions in the coral Pocillopora damicornis, we observed that young colonies derived from settled planulae and colonies regenerated from adult branch tips assumed different growth forms. Young colonies formed at least one branch from the central region of the colony, while colonies regenerated from adult branch tips (3-5 mm long) did not form branches during the 9-month observation period. This pattern was invariable, regardless of the types and outcomes of the contact experiments or the orientation of the branch tips. However, some fragments taken from 1- or 2-year-old colonies formed branches. This suggests that the rate of branch formation in P. damicornis colonies decreases with age. These findings will facilitate investigations of the mechanism of coral branch formation at the molecular level.     

Location‐Specific Metrics for Rapidly Estimating the Abundance and Condition of the Threatened Coral Acropora cervicornis

We developed a method for quantifying the abundance of the threatened staghorn coral (Acropora cervicornis) and evaluated the accuracy of commonly used methods to assess colony condition. For small‐ to medium‐sized colonies, we show that colony ellipsoid volume estimated from simple colony dimensions serves as a reliable and efficient proxy for the more time‐consuming, conventional measure of colony total linear extension, and that this predictive relationship varies significantly among extant populations in the Caribbean. We also determined that visual estimates of colony partial mortality closely approximate to true values for colonies with <25% mortality, with in situ estimates outperforming estimates from digital images. These results provide coral reef managers and restoration practitioners with guidance for assessing partial mortality and location‐specific regression models to estimate “amount” of staghorn coral in both extant and restored staghorn populations in Belize, the United States Virgin Islands, and the Dry Tortugas National Park, Florida, U.S.A. As staghorn coral monitoring and restoration efforts continue to expand in the Caribbean, these methods for quickly determining staghorn abundance and condition will directly aid resource managers tasked with monitoring wild populations and tracking restoration success over time.     

Restoration of coral populations in light of genetic diversity estimates

Due to the importance of preserving the genetic integrity of populations, strategies to restore damaged coral reefs should attempt to retain the allelic diversity of the disturbed population; however, genetic diversity estimates are not available for most coral populations. To provide a generalized estimate of genetic diversity (in terms of allelic richness) of scleractinian coral populations, the literature was surveyed for studies describing the genetic structure of coral populations using microsatellites. The mean number of alleles per locus across 72 surveyed scleractinian coral populations was 8.27 (±0.75 SE). In addition, population genetic datasets from four species (Acropora palmata, Montastraea cavernosa, Montastraea faveolata and Pocillopora damicornis) were analyzed to assess the minimum number of donor colonies required to retain specific proportions of the genetic diversity of the population. Rarefaction analysis of the population genetic datasets indicated that using 10 donor colonies randomly sampled from the original population would retain >50% of the allelic diversity, while 35 colonies would retain >90% of the original diversity. In general, scleractinian coral populations are genetically diverse and restoration methods utilizing few clonal genotypes to re-populate a reef will diminish the genetic integrity of the population. Coral restoration strategies using 10–35 randomly selected local donor colonies will retain at least 50–90% of the genetic diversity of the original population. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Model of fringing reef development in response to progressive sea level fall over the last 7000 years – (Kikai-jima, Ryukyu Islands, Japan)

 Kikai-jima in the central Ryukyu Islands of Japan is fringed by exposed terraces of Holocene reefs, which formed as a result of periodic local tectonic uplift associated with subduction/collision. The terraces form four topographically distinct features (TI-IV) around the island and represent reefs that grew to sea level at 9000–6065 y BP, 6065–3390 y BP, 3790–2630 y BP, and 2870 to 1550 y BP. The modern reef terrace has been growing since approximately 1550 y BP. The reef terraces were uplifted sequentially around 6050 y BP (4 m), 3390–3790 y BP (2.5 m), 2630–2870 y BP (1 m) and 1550 y BP (2.5 m). Five sites were studied to define reef development in response to periodic relative sea level fall and different stillstand recovery periods. Thirty coral genera and 70 species were recorded from four distinct shallow reef flat to upper reef slope and one deeper reef slope coral assemblage. Significant lateral variations in total coral abundance, genera number, diversity, and the coverage density of Acropora spp. and Faviids occur both within and between the terraces. Stratigraphically, drill core and outcrop data recorded shallowing upward sequences characterised by tabulate Acropora spp. overlying massive Porites sp. and Faviids. The biological variations may represent growth strategies responding to initial colonisation, episodic perturbation (relative sea level fall) and differing recovery times during stillstands, and indicate a reef ecosystem stable and strong enough to recover after substantial perturbations. However, this study suggests that relatively small geological changes have had substantial biological effects, and modelling indicates that such changes would have been more profound had a third factor, such as substrate angle, varied more dramatically. In such a case, the drowning growth strategy exhibited in the drill core transect may have been more prevalent, and reefs would be struggling to grow around Kikai-jima today. Accepted: 27 May 1998     

Occupation Dynamics and Impacts of Damselfish Territoriality on Recovering Populations of the Threatened Staghorn Coral,Acropora cervicornis

Large-scale coral reef restoration is needed to help recover structure and function of degraded coral reef ecosystems and mitigate continued coral declines. In situ coral propagation and reef restoration efforts have scaled up significantly in past decades, particularly for the threatened Caribbean staghorn coral, Acropora cervicornis, but little is known about the role that native competitors and predators, such as farming damselfishes, have on the success of restoration. Steep declines in A. cervicornis abundance may have concentrated the negative impacts of damselfish algal farming on a much lower number of coral prey/colonies, thus creating a significant threat to the persistence and recovery of depleted coral populations. This is the first study to document the prevalence of resident damselfishes and negative effects of algal lawns on A. cervicornis along the Florida Reef Tract (FRT). Impacts of damselfish lawns on A. cervicornis colonies were more prevalent (21.6% of colonies) than those of other sources of mortality (i.e., disease (1.6%), algal/sponge overgrowth (5.6%), and corallivore predation (7.9%)), and damselfish activities caused the highest levels of tissue mortality (34.6%) among all coral stressors evaluated. The probability of damselfish occupation increased as coral colony size and complexity increased and coral growth rates were significantly lower in colonies with damselfish lawns (15.4 vs. 29.6 cm per year). Reduced growth and mortality of existing A. cervicornis populations may have a significant effect on population dynamics by potentially reducing important genetic diversity and the reproductive potential of depleted populations. On a positive note, however, the presence of resident damselfishes decreased predation by other corallivores, such as Coralliophila and Hermodice, and may offset some negative impacts caused by algal farming. While most negative impacts of damselfishes identified in this study affected large individual colonies and <50% of the A. cervicornis population along the FRT, the remaining wild staghorn population, along with the rapidly increasing restored populations, continue to fulfill important functional roles on coral reefs by providing essential habitat and refuge to other reef organisms. Although the effects of damselfish predation are, and will continue to be, pervasive, successful restoration efforts and strategic coral transplantation designs may help overcome damselfish damage by rapidly increasing A. cervicornis cover and abundance while also providing important information to educate future conservation and management decisions.     

In Situ Coral Nurseries Serve as Genetic Repositories for Coral Reef Restoration after an Extreme Cold‐Water Event

During an unusual cold‐water event in January 2010, reefs along the Florida Reef Tract suffered extensive coral mortality, especially in shallow reef habitats in close proximity to shore and with connections to coastal bays. The threatened staghorn coral, Acropora cervicornis, is the focus of propagation and restoration activities in Florida and one of the species that exhibited high susceptibility to low temperatures. Complete mortality of wild staghorn colonies was documented at 42.9% of donor sites surveyed after the cold event. Remarkably, 72.7% of sites with complete A. cervicornis mortality had fragments surviving within in situ coral nurseries. Thus, coral nurseries served as repositories for genetic material that would have otherwise been completely lost from donor sites. The location of the coral nurseries at deeper habitats and distanced from shallow nearshore habitats that experienced extreme temperature conditions buffered the impacts of the cold‐water event and preserved essential local genotypes for future Acropora restoration activities.     

Influence of water flow on skeletal isotopic composition in the coral Pocillopora damicornis

Fragments of branching Pocillopora damicornis coral colonies were grown in experimental flumes under two water flow regimes. Colony size and buoyant weight increased most rapidly in the fast-flow regime. Branch tips from the upper and outer parts of the colonies showed the lowest and most consistent skeletal oxygen isotope ratios. Flow regime had little influence on the lowest oxygen isotope ratios, which were at least 3.5‰ lighter than the calculated oxygen isotopic equilibrium. These “kinetic” isotope effects are comparable to those observed in Porites corals. Relatively more branch tips showed extreme ¹⁸O depletions under low-flow conditions, and among small coral colonies. Isotopic variability was greater among branch tips from the lower and inner parts of the colonies and at high flow. Skeletal oxygen and carbon isotope ratios generally showed positive correlations. Despite the particularly large offsets from isotopic equilibrium, the isotopically lightest branches showed the greatest isotopic consistency and therefore would make the best isotopic thermometers. Isotopic variability within the colony may provide an indication of flow regime.     

Growth of Mediterranean reef of<Emphasis Type="Italic"> Cladocora caespitosa</Emphasis> (L.) in the Late Quaternary and climate inferences

A sclerochronological analysis was performed on Cladocora caespitosa corals from Late Pleistocene terraces near Taranto (Apulia, Italy) to reconstruct the main palaeoenvironmental conditions at the time of their growth. The fossil corallites were sampled in the Santa Teresiola uplifted bank or ‘open frame reef’ attributed to the Last Interglacial Period. The typical, annual growth pattern of the temperate coral with two alternate high- and low-density bands allowed the reconstruction of two multidecadal growth curves of 61 and 95 years. Trend analysis showed oscillations in annual growth rates similar to those observed in recent, living colonies sampled along a north–south latitudinal transect around the Italian and Croatian coasts as far as Tunisia. The mean growth rate of the fossil reef (4.2 ± 2 mm year⁻¹) is comparable to those measured on colonies living in the coldest part of the Mediterranean Sea. The comparison with data from living Croatian banks shows how fossil C. caespitosa lived in a semi-enclosed environment characterized by seasonal inputs of fresh, cold water. The greatest variations in decadal growth rates of the fossil colonies support the hypothesis of larger amplitude of the seasonal cycles in the past. The death of the fossil bank was probably due to a sudden alluvial input that suffocated the reef with a great amount of mud. Another possible cause of the death of the bank was a prolonged increase in summer temperatures that caused colony mortality and enhanced algal colonization.     

Neogene reef coral assemblages of the Bocas del Toro region, Panama: the rise of Acropora palmata

Temporal patterns are evaluated in Neogene reef coral assemblages from the Bocas del Toro Basin of Panama in order to understand how reef ecosystems respond to long-term environmental change. Analyses are based on a total of 1,702 zooxanthellate coral specimens collected from six coral-bearing units ranging in age from the earliest Late Miocene to the Early Pleistocene: (1) Valiente Formation (12–11 Ma), (2) Fish Hole Member of the Old Bank Formation (5.8–5.6 Ma), (3) La Gruta Member of the Isla Colon Formation (2.2–1.4 Ma), (4) Ground Creek Member of the Isla Colon Formation (2.2–1.4 Ma), (5) Mimitimbi Member of the Urracá Formation (1.2–0.8 Ma), and (6) Hill Point Member of the Urracá Formation (1.2–0.8 Ma). Over 100 coral species occur in the six units, with faunal assemblages ranging from less than 10% extant taxa (Valiente Formation) to over 85% extant taxa (Ground Creek Member). The collections provide new temporal constraints on the emergence of modern Caribbean reefs, with the La Gruta Member containing the earliest occurrence of large monospecific stands of the dominant Caribbean reef coral Acropora palmata, and the Urracá Formation containing the last fossil occurrences of 15 regionally extinct taxa. Canonical correspondence analysis of 41 Late Miocene to Recent reef coral assemblages from the Caribbean region suggests changes in community structure coincident with effective oceanic closure of the Central American Seaway (~3.5 Ma). These changes, including increased Acropora dominance, may have contributed to a protracted period of elevated extinction debt prior to the major peak in regional coral extinctions (~2–1 Ma).     

Evaluating Methods for Transplanting Endangered Elkhorn Corals in the Virgin Islands

Restoration of rare corals is desirable and restoration projects are fairly common, but scientific evaluation of this approach is limited. We tested several techniques for transplant and restabilization of Acropora palmata (the elkhorn coral), an ecologically important Caribbean coral whose populations have suffered severe declines. In rough weather, fragments break‐off colonies of branching corals like A. palmata as a normal form of asexual reproduction. We transplanted naturally produced coral fragments from remnant populations to nearby restoration sites. Untouched control fragments at the donor site died faster and grew slower than fragments attached to the reef, so attaching fragments to the reef is beneficial. Transplanted fragments grew and died at a rate similar to fragments left at the donor site (both groups were attached to the reef), so there were no effects of moving fragments or differences in habitat quality between donor and restoration sites. Growth and survival were similar using four methods of attaching fragments to the reef: cable ties, two types of epoxy resin, and hydrostatic cement. Corals sometimes compete with the macroalgae that dominate degraded reefs, and clearing surrounding algae improved the growth of fragments. After 4 years, transplanted fragments grew to 1,450 cm² in area and so were potentially sexually active. Because the methods tested are simple and cheap, they could be used by volunteer recreational divers to restore locally extirpated A. palmata populations or to enhance reefs where natural recovery is slow.     

Critical Evaluation of Branch Polarity and Apical Dominance as Dictators of Colony Astogeny in a Branching Coral

The high morphological resemblance between branching corals and trees, can lead to comparative studies on pattern formation traits, best exemplified in plants and in some cnidarians. Here, 81 branches of similar size of the hermatypic coral Stylophora pistillata were lopped of three different genets, their skeletons marked with alizarin red-S, and divided haphazardly into three morphometric treatment groups: (I) upright position; (II) horizontal position, intact tip; and (III) horizontal position, cut tip. After 1 y of in-situ growth, the 45 surviving ramets were brought to the laboratory, their tissues removed and their architectures analyzed by 22 morphological parameters (MPs). We found that within 1 y, isolated branches developed into small coral colonies by growing new branches from all branch termini, in all directions. No architectural dissimilarity was assigned among the three studied genets of treatment I colonies. However, a major architectural disparity between treatment I colonies and colonies of treatments II and III was documented as the development of mirror structures from both sides of treatments II and III settings as compared to tip-borne architectures in treatment I colonies. We did not observe apical dominance since fragments grew equally from all branch sides without documented dominant polarity along branch axis. In treatment II colonies, no MP for new branches originating either from tips or from branch bases differed significantly. In treatment III colonies, growth from the cut tip areas was significantly lower compared to the base, again, suggesting lack of apical dominance in this species. Changes in branch polarity revealed genet associated plasticity, which in one of the studied genets, led to enhanced growth. Different genets exhibited canalization flexibility of growth patterns towards either lateral growth, or branch axis extension (skeletal weight and not porosity was measured). This study revealed that colony astogeny in S. pistillata is a regulated process expressed through programmed events and not directly related to simple energy trade-off principles or to environmental conditions, and that branch polarity and apical dominance do not dictate colony astogeny. Therefore, plasticity and astogenic disparities encompass a diversity of genetic (fixed and flexible) induced responses.     

Employing a highly fragmented,weedy coral species in reef restoration

A major determinant in reef restoration programs is the choice of species employed. In this paper, we concentrate on the potential use of Montipora digitata (Dana, 1846), a highly fragmented and weedy coral species, for reef restoration and for recreation of reef physiognomy in Bolinao, The Philippines, under the ‘gardening concept’. Coral ramets (n = 1960) were reared in a shallow lagoon nursery for 1 year and then, the resulting nursery-grown colonies were transplanted onto denuded reef knolls at two different sites and according to three transplantation designs (grid design with high or low density and patch design). The nursery grown ramets, which had initially exhibited a 99% survivorship, were reduced to 87% by a super typhoon. Low detachment rates, low mortality (<1% both) and low bleaching (<3% over most months), have been indicating good nursery conditions for corals. Monitoring transplanted colonies for over 15 months showed no significant differences between the sites or among the three different transplantation designs. The major events of mortality and detachment were documented during the first 3 months post-transplantation and in the wake of a bleaching event. M. digitata transplants exhibited rapid growth (a 384% increase in ecological volume) and frequent fragmentation. Some fragments remained by their colonies of origin entangled in the attached branches, while other fragments fell onto the surrounding sandy substrate, forming M. digitata ‘beds’ around the knolls. In both, nursery and transplanted corals, creation of complex 3D structures was followed by recruitment of juvenile fishes into aggregations. Above results reveal that employing fast growing, weedy and easily fragmenting branching species, which are also considered as ‘modifier species’, in reef restoration may result in a fast regeneration of reef physiognomy. Instead of ‘copying’ natural processes, reef managers should therefore concentrate on recovering the reef's ability for self-maintenance.     

Coral recruitment and juvenile mortality as structuring factors for reef benthic communities in Biscayne National Park, USA

Coral communities of Biscayne National Park (BNP) on offshore linear bank-barrier reefs are depauperate of reef corals and have little topographic relief, while those on lagoonal patch reefs have greater coral cover and species richness despite presumably more stressful environmental regimes closer to shore. We hypothesized that differences in rates of coral recruitment and/or of coral survivorship were responsible for these differences in community structure. These processes were investigated by measuring: (1) juvenile and adult coral densities, and (2) size-frequency distributions of smaller coral size classes, at three pairs of bank- and patch-reefs distributed along the north-south range of coral reefs within the Park. In addition, small quadrats (0.25 m²) were censused for colonies <2 cm in size on three reefs (one offshore and one patch reef in the central park, and one intermediate reef at the southern end), and re-surveyed after 1 year. Density and size frequency data confirmed that large coral colonies were virtually absent from the offshore reefs, but showed that juvenile corals were common and had similar densities to those of adjacent bank and patch reefs. Large coral colonies were more common on inshore patch reefs, suggesting lower survivorship (higher mortality) of small and intermediate sized colonies on the offshore reefs. The more limited small-quadrat data showed similar survivorship rates and initial and final juvenile densities at all three sites, but a higher influx of new recruits to the patch reef site during the single annual study period. We consider the size-frequency data to be a better indicator of juvenile coral dynamics, since it is a more time-integrated measurement and was replicated at more sites. We conclude that lack of recruitment does not appear to explain the impoverished coral communities on offshore bank reefs in BNP. Instead, higher juvenile coral mortality appears to be a dominant factor structuring these communities. Accepted: 9 September 1999