Parrotfish predation on massive Porites on the Great Barrier Reef

Parrotfish grazing scars on coral colonies were quantified across four reef zones at Lizard Island, Northern Great Barrier Reef (GBR). The abundance of parrotfish grazing scars was highest on reef flat and crest, with massive Porites spp. colonies having more parrotfish grazing scars than all other coral species combined. Massive Porites was the only coral type positively selected for grazing by parrotfishes in all four reef zones. The density of parrotfish grazing scars on massive Porites spp., and the rate of new scar formation, was highest on the reef crest and flat, reflecting the lower massive Porites cover and higher parrotfish abundance in these habitats. Overall, it appears that parrotfish predation pressure on corals could affect the abundance of preferred coral species, especially massive Porites spp, across the reef gradient. Parrotfish predation on corals may have a more important role on the GBR reefs than previously thought.     

Declining coral calcification in massive Porites in two nearshore regions of the northern Great Barrier Reef

Temporal and spatial variation in the growth parameters skeletal density, linear extension and calcification rate in massive Porites from two nearshore regions of the northern Great Barrier Reef (GBR) were examined over a 16‐year study period. Calcification rates in massive Porites have declined by approximately 21% in two regions on the GBR ～450 km apart. This is a function primarily of a decrease in linear extension (～16%) with a smaller decline in skeletal density (～6%) and contrasts with previous studies on the environmental controls on growth of massive Porites on the GBR. Changes in the growth parameters were linear over time. Averaged across colonies, skeletal density declined over time from 1.32 g cm^?3 (SE = 0.017) in 1988 to 1.25 g cm^?3 (0.013) in 2003, equivalent to 0.36% yr^?1 (0.13). Annual extension declined from 1.52 cm yr^?1 (0.035) to 1.28 cm yr^?1 (0.026), equivalent to 1.02% yr^?1 (0.39). Calcification rates (the product of skeletal density and annual extension) declined from 1.96 g cm^?2 yr^?1 (0.049) to 1.59 g cm^?2 yr^?1 (0.041), equivalent to 1.29% yr^?1 (0.30). Mean annual seawater temperatures had no effect on skeletal density, but a modal effect on annual extension and calcification with maxima at ～26.7 °C. There were minor differences in the growth parameters between regions. A decline in coral calcification of this magnitude with increasing seawater temperatures is unprecedented in recent centuries based on analysis of growth records from long cores of massive Porites. We discuss the decline in calcification within the context of known environmental controls on coral growth. Although our findings are consistent with studies of the synergistic effect of elevated seawater temperatures and pCO₂ on coral calcification, we conclude that further data on seawater chemistry of the GBR are required to better understand the links between environmental change and effects on coral growth.     

Spatial and temporal variation in coral predation by parrotfishes on the GBR: evidence from an inshore reef

There have been few studies of coral predation by fishes on the Great Barrier Reef (GBR). However, these studies have indicated that it is an important factor that may shape coral demographics. Here, for the first time, we document the spatial and temporal variation in coral predation by parrotfishes on an inshore reef on the GBR. The densities of parrotfish feeding scars on massive Porites spp. were compared within core and non-core areas of three Chlorurus microrhinos home ranges. The density of parrotfish feeding scars on massive Porites is among the highest recorded on the GBR and elsewhere with a higher abundance of excavating feeding scars within core areas, reflecting the higher occupancy of these areas by C. microrhinos. Furthermore, excavating scars were more abundant in October than in April. This may be related to the higher nutritional quality of coral colonies in October, as coral spawning usually occurs in November at this study location. No spatial or temporal variation was noted in the abundance of feeding scars from scraping parrotfishes. The lack of temporal differences may be a result of the shallow scraping scars which would not be able to reach the gonads within coral polyps. The frequency of parrotfish predation on Porites and the spatial and temporal variation recorded herein highlight the potential importance of parrotfish corallivory on the GBR.     

Environmental Records from Great Barrier Reef Corals: Inshore versus Offshore Drivers

The biogenic structures of stationary organisms can be effective recorders of environmental fluctuations. These proxy records of environmental change are preserved as geochemical signals in the carbonate skeletons of scleractinian corals and are useful for reconstructions of temporal and spatial fluctuations in the physical and chemical environments of coral reef ecosystems, including The Great Barrier Reef (GBR). We compared multi-year monitoring of water temperature and dissolved elements with analyses of chemical proxies recorded in Porites coral skeletons to identify the divergent mechanisms driving environmental variation at inshore versus offshore reefs. At inshore reefs, water Ba/Ca increased with the onset of monsoonal rains each year, indicating a dominant control of flooding on inshore ambient chemistry. Inshore multi-decadal records of coral Ba/Ca were also highly periodic in response to flood-driven pulses of terrigenous material. In contrast, an offshore reef at the edge of the continental shelf was subject to annual upwelling of waters that were presumed to be richer in Ba during summer months. Regular pulses of deep cold water were delivered to the reef as indicated by in situ temperature loggers and coral Ba/Ca. Our results indicate that although much of the GBR is subject to periodic environmental fluctuations, the mechanisms driving variation depend on proximity to the coast. Inshore reefs are primarily influenced by variable freshwater delivery and terrigenous erosion of catchments, while offshore reefs are dominated by seasonal and inter-annual variations in oceanographic conditions that influence the propensity for upwelling. The careful choice of sites can help distinguish between the various factors that promote Ba uptake in corals and therefore increase the utility of corals as monitors of spatial and temporal variation in environmental conditions.     

Congruent patterns of connectivity can inform management for broadcast spawning corals on the Great Barrier Reef

Connectivity underpins the persistence and recovery of marine ecosystems. The Great Barrier Reef (GBR) is the world's largest coral reef ecosystem and managed by an extensive network of no‐take zones; however, information about connectivity was not available to optimize the network's configuration. We use multivariate analyses, Bayesian clustering algorithms and assignment tests of the largest population genetic data set for any organism on the GBR to date (Acropora tenuis, >2500 colonies; >50 reefs, genotyped for ten microsatellite loci) to demonstrate highly congruent patterns of connectivity between this common broadcast spawning reef‐building coral and its congener Acropora millepora (~950 colonies; 20 reefs, genotyped for 12 microsatellite loci). For both species, there is a genetic divide at around 19°S latitude, most probably reflecting allopatric differentiation during the Pleistocene. GBR reefs north of 19°S are essentially panmictic whereas southern reefs are genetically distinct with higher levels of genetic diversity and population structure, most notably genetic subdivision between inshore and offshore reefs south of 19°S. These broadly congruent patterns of higher genetic diversities found on southern GBR reefs most likely represent the accumulation of alleles via the southward flowing East Australia Current. In addition, signatures of genetic admixture between the Coral Sea and outer‐shelf reefs in the northern, central and southern GBR provide evidence of recent gene flow. Our connectivity results are consistent with predictions from recently published larval dispersal models for broadcast spawning corals on the GBR, thereby providing robust connectivity information about the dominant reef‐building genus Acropora for coral reef managers.     

Demographic aspects of the soft coral Sinularia flexibilis leading to local dominance on coral reefs

We evaluated the role that demography may play in the formation of local aggregations of Sinularia flexibilis (Quoy & Gaimard, 1833), a soft coral that commonly dominates inshore coral reefs of the Great Barrier Reef (GBR), Australia. Two populations on inshore reefs of the Palm Islands were censused once a year for 3 years, starting 10 mo after the extensive bleaching mortality in early 1998. Larger colonies became more prevalent over time; mean colony size increasing by 35%, from 276 cm² in 1998 to 373 cm² in 2000. Growth rates were size dependent, with smaller colonies growing proportionally faster than larger colonies. Change in size relative to initial size indicated an expected mean annual growth of 128 cm² for a 50-cm² colony. Zero growth was predicted at 532±21cm², with colonies larger than this likely to undergo fission or shrink. Forty-three percent of colonies were undergoing fission at any time at both localities. Most new colonies were produced by fission (70%, n=285), with the remainder produced by the recruitment of sexually produced larvae (19%) or by colony translocation (11%). The sexual and asexual recruitment rates were 0.24 and 1.0 recruits m- ² year−¹, respectively. Opportunistic recruitment and rapid growth following disturbances are commonly assumed to be the mechanisms leading soft corals to dominate locally. In this study, these mechanisms operated more slowly than expected, with no net change in population size.     

Measured coral luminescence as a freshwater proxy: comparison with visual indices and a potential age artefact

Illuminating slices from massive coral skeletons under ultraviolet light can reveal bright luminescent lines in nearshore corals affected by freshwater river flows. The occurrence and intensity of these lines in long-lived corals can then be used to reconstruct past river flow and rainfall, extending the instrumental records of past tropical climate variability considerably. Earlier studies from the Great Barrier Reef, Australia, have used visual assessments of luminescent line intensity to develop semi-quantitative (though potentially subjective) indices of spatial and temporal variations in freshwater flows. Annual visual assessments and relative coral luminescence intensity (measured by fluorescence spectroscopy) and growth variables are first compared for 89 Porites coral colonies from 30 reefs throughout the length and breadth of the GBR. This demonstrates that simple visual assessments can provide useful information, in the absence of quantitative measurements, of this proxy freshwater tracer. The annual range of measured luminescence between the preceding winter minimum and summer maximum, rather than annual average or annual maximum luminescence is shown to be the most robust measure of freshwater flow. Second, from analyses of the coral colony data and over 40-century-long or longer coral core records, attention is drawn to a potential age artefact in annual average and annual maximum measured coral luminescence. These variables show a significant decline through time, similar to the observed decline in average skeletal density. Although the reasons for this decline are unknown, it could compromise interpretation of long-term variations in freshwater flows and subsequent climatic inferences. This artefact does not appear to affect the annual luminescence range which, it is concluded, is a robust proxy for inter-annual variations in river flow and rainfall.     

Long-term records of coral calcification across the central Great Barrier Reef: assessing the impacts of river runoff and climate change

Calcification rates are reported for 41 long-lived Porites corals from 7 reefs, in an inshore to offshore transect across the central Great Barrier Reef (GBR). Over multi-decadal timescales, corals in the mid-shelf (1947–2008) and outer reef (1952–2004) regions of the GBR exhibit a significant increase in calcification of 10.9 ± 1.1 % (1.4 ± 0.2 % per decade; ±1 SE) and 11.1 ± 3.9 % (2.1 ± 0.8 % per decade), respectively, while inner-shelf (1930–2008), reefs show a decline of 4.6 ± 1.3 % (0.6 ± 0.2 % per decade). This long-term decline in calcification for the inner GBR is attributed to the persistent ongoing effects of high sediment/nutrients loads from wet season river discharges, compounded by the effects of thermal stress, especially during the 1998 bleaching event. For the recent period (1990–2008), our data show recovery from the 1998 bleaching event, with no significant trend in the rates of calcification (1.1 ± 2.0 %) for the inner reefs, while corals from the mid-shelf central GBR show a decline of 3.3 ± 0.9 %. These results are in marked contrast to the extreme reef-wide declines of 14.2 % reported by De’ath et al. (2009) for the period of 1990–2005. The De’ath et al. (2009) results are, however, found to be compromised by the inclusion of incomplete final years, duplicated records, together with a bias toward inshore reefs strongly affected by the 1998 bleaching. Our new findings nevertheless continue to raise concerns, with the inner-shelf reefs continuing to show long-term declines in calcification consistent with increased disturbance from land-based effects. In contrast, the more ‘pristine’ mid- and outer-shelf reefs appear to be undergoing a transition from increasing to decreasing rates of calcification, possibly reflecting the effects of CO₂-driven climate change. Our study highlights the importance of properly undertaken, regular assessments of coral calcification that are representative of the distinctive cross-shelf environments and discriminate between local disturbances and the global impacts of climate change and ocean acidification.     

Local variation in herbivore feeding activity on an inshore reef of the Great Barrier Reef

Threats to coral reefs may be manifested through an increase in macroalgae. Across the globe, phase-shifts from coral to macroalgal dominance have been reported from the Caribbean, Indian and Pacific Oceans. While the Great Barrier Reef (GBR) is in relatively good condition, inshore reefs may exhibit over 50% macroalgal cover. However, our understanding of the processes preventing the macroalgal expansion remains uncertain. Using a remote video bioassay approach, this study quantified herbivory in three bays along the leeward margin of Orpheus Island. Despite significant with-in bay variation in herbivory there was no detectable statistical difference in the rates of herbivory among bays. Furthermore, of the 45 herbivore species recorded from the island, only three played a significant role in bioassay removal, Siganus canaliculatus, Siganus javus and Kyphosus vaigiensis, with only one species predominating in each bay. Reefs of the GBR may therefore be more vulnerable than previously thought, with the removal of macroalgae depending on just a few species, which exhibit considerable spatial variability in their feeding behaviour.     

Palaeoecological evidence of a historical collapse of corals at Pelorus Island,inshore Great Barrier Reef,following European settlement

The inshore reefs of the Great Barrier Reef (GBR) have undergone significant declines in water quality following European settlement (approx. 1870 AD). However, direct evidence of impacts on coral assemblages is limited by a lack of historical baselines prior to the onset of modern monitoring programmes in the early 1980s. Through palaeoecological reconstructions, we report a previously undocumented historical collapse of Acropora assemblages at Pelorus Island (central GBR). High-precision U-series dating of dead Acropora fragments indicates that this collapse occurred between 1920 and 1955, with few dates obtained after 1980. Prior to this event, our results indicate remarkable long-term stability in coral community structure over centennial scales. We suggest that chronic increases in sediment flux and nutrient loading following European settlement acted as the ultimate cause for the lack of recovery of Acropora assemblages following a series of acute disturbance events (SST anomalies, cyclones and flood events). Evidence for major degradation in reef condition owing to human impacts prior to modern ecological surveys indicates that current monitoring of inshore reefs on the GBR may be predicated on a significantly shifted baseline.     

Large-scale bleaching of corals on the Great Barrier Reef

 The Great Barrier Reef (GBR) experienced its most intensive and extensive coral bleaching event on record in early 1998. Mild bleaching commenced in late January and intensified by late February/early March 1998. Broad-scale aerial surveys conducted of 654 reefs (∼23% of reefs on the GBR) in March and April 1998, showed that 87% of inshore reefs were bleached at least to some extent (>1% of coral cover) compared to 28% of offshore (mid- and outer-shelf) reefs. Of inshore reefs 67% had high levels of bleaching (>10% of coral) and 25% of inshore reefs had extreme levels of bleaching (>60% of coral). Fewer offshore reefs (14%) showed high levels of bleaching while none showed extreme levels of bleaching. Ground-truth surveys of 23 reefs, which experienced bleaching in intensities ranging from none to extreme, showed that the aerial survey data are likely to be underestimates of the true extent and intensity of bleaching on the GBR. The primary cause of this bleaching event is likely to be elevated sea temperature and solar radiation, exacerbated by lowered salinity on inshore and some offshore reefs in the central GBR. Accepted: 30 July 1998     

Coral growth on three reefs: development of recovery benchmarks using a space for time approach

This 14-year study (1989–2003) develops recovery benchmarks based on a period of very strong coral recovery in Acropora-dominated assemblages on the Great Barrier Reef (GBR) following major setbacks from the predatory sea-star Acanthaster planci in the early 1980s. A space for time approach was used in developing the benchmarks, made possible by the choice of three study reefs (Green Island, Feather Reef and Rib Reef), spread along 3 degrees of latitude (300 km) of the GBR. The sea-star outbreaks progressed north to south, causing death of corals that reached maximum levels in the years 1980 (Green), 1982 (Feather) and 1984 (Rib). The reefs were initially surveyed in 1989, 1990, 1993 and 1994, which represent recovery years 5–14 in the space for time protocol. Benchmark trajectories for coral abundance, colony sizes, coral cover and diversity were plotted against nominal recovery time (years 5–14) and defined as non-linear functions. A single survey of the same three reefs was conducted in 2003, when the reefs were nominally 1, 3 and 5 years into a second recovery period, following further Acanthaster impacts and coincident coral bleaching events around the turn of the century. The 2003 coral cover was marginally above the benchmark trajectory, but colony density (colonies.m⁻²) was an order of magnitude lower than the benchmark, and size structure was biased toward larger colonies that survived the turn of the century disturbances. The under-representation of small size classes in 2003 suggests that mass recruitment of corals had been suppressed, reflecting low regional coral abundance and depression of coral fecundity by recent bleaching events. The marginally higher cover and large colonies of 2003 were thus indicative of a depleted and aging assemblage not yet rejuvenated by a strong cohort of recruits.     

Coral growth in subtropical eastern Australia

 Extension rates of corals at two sites in subtropical eastern Australia (Solitary Islands and Lord Howe Island) were measured to determine whether growth was low relative to tropical locations. Growth was measured using alizarin staining of skeletons and X-radiographic analysis, and was compared between colonies, species, and sites. Linear extension of individual Pocillopora damicornis colonies averaged 12.4 to 16.1 mm per year at Solitary Islands and Lord Howe Island respectively, which is 50% to 80% of published values for this species at tropical sites. Similarly, average extension of most massive faviid species examined at these sites was between 2.6 mm and 4.6 mm per year, considerably lower than most values reported from lower latitudes (generally 6 mm to 10 mm per year). However, growth rates of Acropora yongei, Turbinaria frondens, and Porites heronensis were close to those of closely-related taxa from the tropics. Causal links between latitude, growth rates of coral colonies, and the potential for reef accretion remain unclear. Accepted: 22 April 1999     

Pleistocene morphology and Holocene emergence of Christmas (Kiritimati) Island, Pacific Ocean

 Christmas (Kiritimati) Island is an unusually large coral atoll, of which a large proportion of the surface is presently subaerial. Extensive outcrops of in situ branching Acropora corals, together with Porites microatolls, Tridacna, and other shallow marine biota, indicate that the present low-lying area of interconnecting lakes in the island interior formed as a reticulate lagoon. Radiocarbon dating indicates that these lagoonal reefs flourished between 4500 and 1500 radiocarbon years BP, and surveying confirms that sea level was 0.5–1.0 m above present at that time, with subaerial exposure resulting from Late Holocene emergence. Boreholes undertaken for a water resources survey of the island penetrated near-surface Pleistocene limestones on the northern, southern, and eastern sides of the island. These are highly weathered and fractured, and although aragonitic clasts are preserved, U-series dating indicates a Middle Pleistocene or older age. At one location flanking the Bay of Wrecks, an outcrop of limestone, with an erosional notch, 1–2 m above present sea level, yielded a U-series age of 130 ka, and is interpreted as Last Interglacial in age. In contrast to previous interpretations which have suggested that Christmas Island comprised an atoll superstructure that is entirely Holocene, or the layer-cake interpretation appropriate for many mid-ocean atolls, Christmas Island appears to have had a form similar to its present in the Middle Pleistocene or earlier. It has undergone karstification during lowstands. Interglacials, particularly the Last Interglacial and the Holocene, appear to have resulted in only a minor veneer of coral over older limestone surfaces. Christmas Island is considered characteristic of an atoll that has not experienced significant subsidence through the Late Quaternary. Accepted: 15 May 1998     

Symbiodinium (Dinophyceae) community patterns in invertebrate hosts from inshore marginal reefs of the southern Great Barrier Reef,Australia

The broad range in physiological variation displayed by Symbiodinium spp. has proven imperative during periods of environmental change and contribute to the survival of their coral host. Characterizing how host and Symbiodinium community assemblages differ across environmentally distinct habitats provides useful information to predict how corals will respond to major environmental change. Despite the extensive characterizations of Symbiodinium diversity found amongst reef cnidarians on the Great Barrier Reef (GBR) substantial biogeographic gaps exist, especially across inshore habitats. Here, we investigate Symbiodinium community patterns in invertebrates from inshore and mid‐shelf reefs on the southern GBR, Australia. Dominant Symbiodinium types were characterized using denaturing gradient gel electrophoresis fingerprinting and sequencing of the ITS2 region of the ribosomal DNA. Twenty one genetically distinct Symbiodinium types including four novel types were identified from 321 reef‐invertebrate samples comprising three sub‐generic clades (A, C, and D). A range of host genera harbored C22a, which is normally rare or absent from inshore or low latitude reefs in the GBR. Multivariate analysis showed that host identity and sea surface temperature best explained the variation in symbiont communities across sites. Patterns of changes in Symbiodinium community assemblage over small geographic distances (100s of kilometers or less) indicate the likelihood that shifts in Symbiodinium distributions and associated host populations, may occur in response to future climate change impacting the GBR.     

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     

Environmental controls on growth of the massive coral Porites

Annual density banding provided growth characteristics for 245 similar-sized, massive colonies of Porites from similar locations on 29 reefs from across the length and breadth of the Great Barrier Reef (GBR), Australia. Values obtained were density, extension rate, and calcification rate. Tissue thickness, the depth to which skeletons were occupied by tissue at the time of collection, was also measured. Extension rate, calcification rate, and tissue thickness were significantly greater at the top of colonies than at the sides. Extension rate and calcification rate decreased from north to south along the GBR (latitudinal range of approximately 9 degrees ) and were significantly and directly related to annual average sea surface temperature (SST; range approximately 25-27 degrees C). For each 1 degrees C rise in SST, average annual calcification increased by 0.39 g cm(-2) year(-1) and average annual extension increased by 3.1 mm year(-1) (c.f. average values of 1.63 g cm(-2) year(-1) and 12.9 mm year(-1), respectively). Density was inversely correlated with extension rate and increased with distance offshore. Data for massive Porites colonies from the GBR were extended though 20 degrees of latitude and an average annual SST range of 23-29 degrees C using published data for the Hawaiian Archipelago (Grigg, R.W., 1981. Coral reef development at high latitudes in Hawaii. Proc. 4th Int. Coral Reef Symp., Manila, Vol. 1, pp. 687-693; Grigg, R.W., 1997. Paleoceanography of coral reefs in the Hawaiian-Emperor Chain - revisited. Coral Reefs 16, S33-S38) and Phuket, Thailand (Scoffin. T.P., Tudhope. A.W., Brown. B.E., Chansang. H., Cheeney. R.F., 1992. Patterns and possible environmental controls of skeletogenesis of Porites lutea, South Thailand. Coral Reefs 11, 1-11). The response of calcification rate to temperature remained linear. Variation in annual average SST accounted for 84% of the variance. For each 1 degrees C rise in SST, average annual calcification increased by 0.33 g cm(-2) year(-1) and average annual extension increased by 3.1 mm year(-1) (c.f. average values of 1.50 g cm(-2) year(-1) and 11.6 mm year(-1), respectively). The sensitivity of calcification rate in Porites to SST, combined with observed 20th Century increases in SSTs, suggests that calcification rates may have already significantly increased along the GBR in response to global climate change.     

Banding,age and growth in the calcareous hydrozoan Millepora complanata Lamarck

Prominent bands or lines were present on colonies of Millepora complanata on coral reefs at Barbados, West Indies. These bands appeared as regular, dark and light horizontal zones or stripes on vertically growing plates and blades making up the colonies. Bands are formed by wave-like undulations or currugations of the skeletal surface. The number of bands correlates with colony height and mean band width equals the mean annual vertical growth of measured colonies. Variation in annual growth and differences in growth rates of colonies between different reefs were determined from band width measurements. Bands provide a rapid and non-destructive means for measuring growth and ageing colonies of M. complanata.     

Latitudinal variation in growth and survival of juvenile corals in the West and South Pacific

Reef-building corals are found across > 30° of latitude from tropical to temperate regions, where they occupy habitats greatly differing in seawater temperature and light regimes. It remains largely unknown, however, how the demography of corals differs across this gradient of environmental conditions. Variation in coral growth is especially important to coral populations, because aspects of coral demography are dependent on colony size, with both fecundity and survivorship increasing with larger colonies. Here we tested for latitudinal variation in annual growth rate and survival of juvenile corals, using 11 study locations extending from 17° S to 33° N in the West and South Pacific. Regression analyses revealed a significant decline in annual growth rates with increasing latitude, whereas no significant latitudinal pattern was detected in annual survival. Seawater temperature showed a significant and positive association with annual growth rates. Growth rates varied among the four common genera, allowing them to be ranked Acropora > Pocillopora > Porites > Dipsastraea. Acropora and Pocillopora showed more variation in growth rates across latitudes than Porites and Dipsastraea. Although the present data have limitations with regard to difference in depths, survey periods, and replication among locations, they provide evidence that a higher capacity for growth of individual colonies may facilitate population growth, and hence population recovery following disturbances, at lower latitudes. These trends are likely to be best developed in Acropora and Pocillopora, which have high rates of colony growth.

     

Distribution of recent outbreaks of the crown-of-thorns starfish (Acanthaster planci) along the Great Barrier Reef: 1985–1986

A large survey program was conducted during 1985/1986 to determine the extent of activity of the crown-of-thorns starfish, Acanthaster planci, and its broad effects on the coral communities of the Great Barrier Reef (GBR). The perimeters of 228 reefs (about 9% of reefs in the GBR system) were surveyed within 1 year using rapid survey, manta tow techniques. These reefs encompassed the broad latitudinal and longitudinal gradients within the GBR. Approximately 27% (62 reefs) of the reefs surveyed had recently experienced (18%), or were experiencing (9%), an outbreak of the crown-of-thorns starfish. These outbreaks were mainly confined to reefs in the central third of the GBR (between Lizard Island and Townsville) and had affected, to varying degrees, approximately 65% of the reefs surveyed within this region. A greater proportion of mid-shelf reefs had experienced outbreaks than outer-shelf reefs, although this difference was not statistically significant. Of the small number of inner-shelf reefs surveyed, none had been recently affected by an outbreak. Large active outbreaks of starfish were reported on many of the reefs located off Townsville while much smaller outbreaks were found on several reefs at the southern end of the GBR, in the Swain Reef complex. Almost 86% of reefs currently experiencing an outbreak had moderate to high coral mortality over at least a third of their perimeters. Only 10% of reefs with active outbreaks had high coral mortality over most of their windward and leeward margins. A similar proportion of reefs had low to moderate coral mortality over less than a third of their perimeters.