Spatio-temporal variation in coral recruitment at different scales on Heron Reef, southern Great Barrier Reef

 Recruitment of scleractinian corals on settlement plates at Heron Island, Great Barrier Reef, was examined over four years (September 1991–September 1995) to quantify spatio-temporal patterns at different scales and to assess post settlement mortality. Recruitment was dominated by pocilloporid corals which accounted for 80.1% of the 8627 spat counted, whereas non-isoporan acroporids represented only 16.4%. Poritids, faviids and isoporan acroporids rarely recruited to the plates (3.5%), despite their obvious abundance as adults on the reef. Recruitment patterns on the plates indicate strong space-time interactions as evidenced by patchy recruitment of both pocilloporid and acroporid spat. Interactions were found between space (on the scale of 10² m, i.e. sites within zones, and 10¹ m, i.e. racks within sites) and time (on the scale of years) for pocilloporids and between space (on the scale of 10³ m, i.e. zones, and 10² m) and time (on the scale of years) for acroporids. Post-recruitment mortality of acroporid spat in the period 3–10 months after their major spawning was dependent on their initial recruitment density, but pocilloporid mortality was either independent of initial recruitment density or, more likely, obscured by additional recruitment of pocilloporids to plates between late February and September. High rates of recruitment and growth by other sessile organisms, particularly bryozoans and oysters, appear to result in increased post-recruitment mortality and limit recruitment of scleractinian corals on settlement plates. The work reinforces an emerging picture that coral recruitment patterns are determined by mechanisms that manifest over a large range of spatial scales. Accepted: 1 September 1997     

Spatio-temporal persistence of scleractinian coral species at Lizard Island,Great Barrier Reef

Coral Reefs - The Great Barrier Reef is a diverse ecosystem of outstanding universal value that is under an increasing level of cumulative threat. The status of biodiversity, particularly that of...     

Sensitivity of coral cays to climatic variations,southern Great Barrier Reef,Australia

Analysis of available wind data for the years 1962–80 from Heron Island which is located within the southern Great Barrier Reef indicates that the annual wind energy vector has oscillated within a 45 degree arc from the SSE in the early 1960's to ESE in the late 1970's. Such changes in wind direction influence the direction of propagation of the waves which mould the shape of coral sand cays in this region. Documentation is provided which shows that the variability of the shoreline positions on Erskine Island, an uninhabited vegetated sand cay reflects this change. The implication is that contemporary shoreline erosion on Heron Island is not caused by the development associated with the tourist resort there. It is a symptom of the change in the propagation direction of the wind-induced waves which is related to long-term climatic change.     

Benthic microalgae in coral reef sediments of the southern Great Barrier Reef,Australia

The abundance and productivity of benthic microalgae in coral reef sediments are poorly known compared with other, more conspicuous (e.g. coral zooxanthellae, macroalgae) primary producers of coral reef habitats. A survey of the distribution, biomass, and productivity of benthic microalgae on a platform reef flat and in a cross-shelf transect in the southern Great Barrier Reef indicated that benthic microalgae are ubiquitous, abundant (up to 995.0 mg chlorophyll (chl) a m^–2), and productive (up to 110 mg O₂ m^–2 h^–1) components of the reef ecosystem. Concentrations of benthic microalgae, expressed as chlorophyll a per surface area, were approximately 100-fold greater than the integrated water column concentrations of microalgae throughout the region. Benthic microalgal biomass was greater on the shallow water platform reef than in the deeper waters of the cross-shelf transect. In both areas the benthic microalgal communities had a similar composition, dominated by pennate diatoms, dinoflagellates, and cyanobacteria. Benthic microalgal populations were potentially nutrient-limited, based on responses to nitrogen and phosphorus enrichments in short-term (7-day) microcosm experiments. Benthic microalgal productivity, measured by O₂ evolution, indicated productive communities responsive to light and nutrient availability. The benthic microalgal concentrations observed (92–995 mg chl a m^–2) were high relative to other reports, particularly compared with temperate regions. This abundance of productive plants in both reef and shelf sediments in the southern Great Barrier Reef suggests that benthic microalgae are key components of coral reef ecosystems.Communicated by Environmental Editor, B.C. Hatcher     

Trematodes of the Great Barrier Reef,Australia: emerging patterns of diversity and richness in coral reef fishes

The Great Barrier Reef holds the richest array of marine life found anywhere in Australia, including a diverse and fascinating parasite fauna. Members of one group, the trematodes, occur as sexually mature adult worms in almost all Great Barrier Reef bony fish species. Although the first reports of these parasites were made 100 years ago, the fauna has been studied systematically for only the last 25 years. When the fauna was last reviewed in 1994 there were 94 species known from the Great Barrier Reef and it was predicted that there might be 2,270 in total. There are now 326 species reported for the region, suggesting that we are in a much improved position to make an accurate prediction of true trematode richness. Here we review the current state of knowledge of the fauna and the ways in which our understanding of this fascinating group is changing. Our best estimate of the true richness is now a range, 1,100–1,800 species. However there remains considerable scope for even these figures to be incorrect given that fewer than one-third of the fish species of the region have been examined for trematodes. Our goal is a comprehensive characterisation of this fauna, and we outline what work needs to be done to achieve this and discuss whether this goal is practically achievable or philosophically justifiable.     

Microbiological assessment of a disease outbreak on corals from Magnetic Island (Great Barrier Reef, Australia)

Unusual disease lesions were observed in Montipora corals on the fringing reef of Magnetic Island (Great Barrier Reef, Australia) following a period of high water temperature in early January 2002. Tissue death in Montipora spp. appeared as a black layer that spread rapidly across the colony surface, though this appeared as the final phase of disease progression (with three previous disease phases now identified, S. Anthony, unpublished). Culture and molecular-based microbial analysis of this layer did not identify a likely microbial pathogen. Despite this, DNA sequencing of microbial 16S rDNA indicated a shift in the bacterial population associated with affected coral tissue. A clone library of the healthy coral sample predominantly contained sequences within the -Proteobacteria. A disease coral sample representing the margin of the black lesion and healthy coral tissue was dominated by sequences, which demonstrated low sequence identity to a range of -Proteobacteria, -Proteobacteria and cyanobacteria. The microbes identified in the diseased Montipora spp. samples are likely to be opportunistic rather than the causative agent of the observed lesion. Studies are in progress to further characterise the ecology of this disease and describe the potential microbial pathogen(s).     

Retention of phytoplankton and planktonic microbes on coral reefs within the Great Barrier Reef,Australia

Concentrations of phytoplankton (coccoid cychobacteria and total chlorophyll) and planktonic microrial communities (heterotropic bacteria, nanoflagellates and ciliates) were lower over leeward reef flats than over open water or reef faces, around Davies Reef and Myrmidon Reef in the central Great Barrier Reef, Australia. Concentrations of cyanobacteria, which accounted for approximately 15–50% of the carbon biomass of phytoplankton in open water, decreased from the reef face towards the leeward reef flat. Concentrations of ciliates were consistently lower at the leeward reef flat than at the reef face. For Davies Reef, the retention rates of phytoplankton and planktonic microbial communities were estimated to reach 253 gC d^-1 per 1 m strip of the reef or about 0.09 gC m^-2 d^-1. This value is virtually equal to estimates of net community production (0.1 gC m^-2 d^-1). This allocthonous organic subsidy may help maintain spositive carbon balance on both Davies and Myrmidon Reefs on the Great Barrier Reef.     

Epibenthie fish larvae in the Great Barrier Reef Lagoon near Lizard Island,Australia

Epibenthic fish larvae near Lizard Island in the Great Barrier Reef Lagoon were sampled with a plankton sled during daylight in November 1981 and January–February 1982. Abundance in the epibenthos was highly variable, and although many types of larvae were present, few were concentrated there relative to the water column. Among those taxa concentrated in the epibenthos, abundances were low and variances were high. Larvae of bregmacerotids, callionymids, clupeids, monacanthids, pinguipedids, platycephalids, pseudochromids, and especially schindleriids, leiognathids and terapontids were concentrated in the epibenthos. Few reef fish larvae were epibenthic. There was some evidence of diel and ontogenetic movements into and out of the epibenthos. Our limited sampling indicates that conventional midwater plankton sampling is adequate for most fish larvae found in the Lizard Island area, but for the larvae of the above ten families, this could produce large underestimates of abundance.     

The behaviour of Abudefduf zonatus (Pisces,pomacentridae) at Heron Island,Great Barrier Reef

Social organization and reproductive behaviour of Abudefduf zonatus were studied at Heron Island, Great Barrier Reef. This species is closely associated with the sand and coral substrate of the inner reef flat zone. Territorial, clustering, nest-digging, breeding and parental care behaviours are described. Mature males maintain territories for several weeks or longer around empty clam shells in which periodic spawning occurs. Females and sub-adult males hold territories interspersed among those of adult males. No pair bonds are formed, and eggs are guarded only by males. Breeding is roughly synchronized among fish of one area. Clustering behaviour may contribute to breeding syndrony in local populations.     

Surface alkaline phosphatase activities of macroalgae on coral reefs of the central Great Barrier Reef, Australia

Inshore reefs of the Great Barrier Reef (GBR) are subject to episodic nutrient supply, mainly by flood events, whereas midshelf reefs have a more consistent low nutrient availability. Alkaline phosphatase activity (APA) enables macroalgae to increase their phosphorus (P) supply by using organic P. APA was high (~4.0 to 15.5 µmol PO₄^3- g DW^-1 h^-1) in species colonising predominantly inshore reefs and low (<2 µmol PO₄^3- g DW^-1 h^-1) in species with a cross-shelf distribution. However, APA values of GBR algae in this study were much lower than data reported from other coral reef systems. In experiments with two Sargassum species tissue P levels were correlated negatively, and N:P ratios were positively correlated with APA. High APA can compensate for a relative P-limitation of macroalgae in coral reef systems that are subject to significant N-inputs, such as the GBR inshore reefs. APA and other mechanisms to acquire a range of nutrient species allow inshore species to thrive in habitats with episodic nutrient supply. These species also are likely to benefit from an increased nutrient supply caused by human activity, which currently is a global problem.     

Bioerosion experiments at Lizard Island,Great Barrier Reef

The rates at which dead coral substrates are modified by bioerosional processes were determined by exposing recently killed corals for up to four years in a variety of reef environments at Lizard Island (northern Great Barrier Reef). Grazers were the major croding agents of these coral substrates and exhibited differences between sites that varied between sampling periods. Subtidal reef slopes and lagoon environments of water depths < 20 m were subjected to higher average rates of grazing erosion (0.30–1.96 kg/m²/y) than shallow depths less than 1 m (0.07–0.26 kg/m²/y). A deep site at 20 m experienced low average rates of grazing (0.08–0.29 kg/m²/y). Boring rates by worms (polychaetes and sipunculans), sponges and molluscs were relatively low and varied between sites, but increased with length of sampling period as larger borers succeeded the initial colonizing small polychaete worms. We hypothesize from these experiments that the extent of boring in reef substrates will be influenced by the interaction between the succession of the boring community and the rate at which the substrate is destroyed by grazing. We suggest that the level of grazing modifies the successional pattern of borers by removing the surface substrate and continually exposing bare substrate that can be colonized by early boring colonists. Thus, constant high levels of grazing may maintain the boring community at an early successional stage and prevent the development of a mature boring community. In order to establish large borer populations, reef substrates must be protected from extensive grazing bioerosion. This interaction of grazing and boring has important implications for the way dead coral is preserved in different reef environments.     

Distribution, host range and large-scale spatial variability in black band disease prevalence on the Great Barrier Reef, Australia

The prevalence and host range of black band disease (BBD) was determined from surveys of 19 reefs within the Great Barrier Reef Marine Park, Australia. Prevalence of BBD was compared among reefs distributed across large-scale cross-shelf and long-shelf gradients of terrestrial or anthropogenic influence. We found that BBD was widespread throughout the Great Barrier Reef (GBR) and was present on 73.7% of the 19 reefs surveyed in 3 latitudinal sectors and 3 cross-shelf positions in the summer of 2004. Although BBD occurred on all mid-shelf reefs and all but one outer-shelf reefs, overall prevalence was low, infecting on average 0.09% of sessile cnidarians and 0.1% of scleractinian corals surveyed. BBD affected approximately 7% of scleractinian taxa (25 of approximately 350 GBR hard coral species) and 1 soft coral family, although most cases of BBD were recorded on branching Acropora species. Prevalence of BBD did not correlate with distance from terrestrial influences, being highest on mid-shelf reefs and lowest on inshore reefs (absent from 66%, n = 6, of these reefs). BBD prevalence was consistently higher in all shelf positions in the northern (Cooktown/Lizard Island) sector, which is adjacent to relatively pristine catchments compared to the central (Townsville) sector, which is adjacent to a more developed catchment. BBD cases were clustered within reefs and transects, which was consistent with local dispersal of pathogens via currents, although the spread of BBD was not dependent on the density or cover of any of the coral taxa examined. In combination, these results suggest that BBD is part of the natural ecology of coral assemblages of the GBR, and its prevalence is relatively unaffected by terrestrial influences on the scales characteristic of cross-shelf gradients.     

Predicting outbreaks of a climate-driven coral disease in the Great Barrier Reef

Links between anomalously high sea temperatures and outbreaks of coral diseases known as White Syndromes (WS) represent a threat to Indo-Pacific reefs that is expected to escalate in a changing climate. Further advances in understanding disease aetiologies, determining the relative importance of potential risk factors for outbreaks and in trialing management actions are hampered by not knowing where or when outbreaks will occur. Here, we develop a tool to target research and monitoring of WS outbreaks in the Great Barrier Reef (GBR). The tool is based on an empirical regression model and takes the form of user-friendly interactive ~1.5-km resolution maps. The maps denote locations where long-term monitoring suggests that coral cover exceeds 26% and summer temperature stress (measured by a temperature metric termed the mean positive summer anomaly) is equal to or exceeds that experienced at sites in 2002 where the only severe WS outbreaks documented on the GBR to date were observed. No WS outbreaks were subsequently documented at 45 routinely surveyed sites from 2003 to 2008, and model hindcasts for this period indicate that outbreak likelihood was never high. In 2009, the model indicated that outbreak likelihood was high at north-central GBR sites. The results of the regression model and targeted surveys in 2009 revealed that the threshold host density for an outbreak decreases as thermal stress increases, suggesting that bleaching could be a more important precursor to WS outbreaks than previously anticipated, given that bleaching was severe at outbreak sites in 2002 but not at any of the surveyed sites in 2009. The iterative approach used here has led to an improved understanding of disease causation, will facilitate management responses and can be applied to other coral diseases and/or other regions.     

Diel pCO2 variation among coral reefs and microhabitats at Lizard Island,Great Barrier Reef

Most laboratory experiments examining the effect of ocean acidification on marine organisms use stable pH/pCO₂ treatments based on average projections for the open ocean. However, pH/pCO₂ levels vary spatially and temporally in marine environments, and this variation can affect organism responses to pH/pCO₂. On coral reefs, diel pH/pCO₂ variability at the individual reef scale has been reported in a few studies, but variation among microhabitats within a reef remains poorly understood. This study determined the pH/pCO₂ variability of three different reefs, and three contrasting coral reef microhabitats (dominated by hard coral, soft coral, or open substrate) within each reef. Three SeaFET pH loggers were deployed simultaneously at the three microhabitats within a reef over a 9-day period. This was repeated at three different reefs around the Lizard Island lagoon. The loggers recorded pH_T and temperature every 5 min. Water samples were collected from each microhabitat during four points of the tidal cycle (high, low, rising, and falling) and analysed for total alkalinity and dissolved inorganic carbon. The data show a clear diel pCO₂ cycle, increasing overnight and decreasing during the day, in association with photosynthesis and respiration cycles. Diel pCO₂ differed more between reefs than between microhabitats within reefs. Variation between reefs was most likely influenced by water flow, with the more protected (low flow) reefs experiencing a greater range in pCO₂ (Δ 250 μatm) than the exposed (high flow) reefs (Δ 116 μatm). These results add to a growing body of the literature on the diel variation of pCO₂ of shallow, nearshore environments and suggest that when projecting future pCO₂ levels, it is important to consider reef metabolism as well as physical and hydrodynamic factors.

     

Effect of colony size and surrounding substrate on corals experiencing a mild bleaching event on Heron Island reef flat (southern Great Barrier Reef,Australia)

In January–May 2006, Heron Island in the Great Barrier Reef experienced a mild bleaching event. The effect of colony size, morphology and surrounding substrate on the extent of bleaching was explored. In contrast with previous studies, colony size did not influence bleaching sensitivity, suggesting that there may be a threshold of light and temperature stress beyond which size plays a role. Also contrasting with previous studies, massive corals were more affected by bleaching than branching corals. Massive corals surrounded by sand were more affected than the ones surrounded by rubble or dead coral. It is hypothesized that light reflectance from sand increases stress levels experienced by the colonies. This effect is maximized in massive corals as opposed to branching corals that form dense thickets on Heron Island. These results emphasize the importance of the ecological dynamics of coral communities experiencing low, moderate and high levels of bleaching for the understanding of how coral communities may change under the stress of climate change.     

Picoplankton dynamics in Davies Reef lagoon, the Great Barrier Reef, Australia

The diel variations in abundance and frequency of dividing cells(FDC) of coccoid cyanobacteria in a coral reef lagoon were investigatedin June, September and December 1989, and April 1990. Cyanobacteriaand picoplanktonic eukaryotes (<3 µm) were sampledmonthly from January to December 1990. The average abundancesof cyanobacteria and eukaryotes ranged between 1.17–10.0610⁴cells ml^–1 and 0.16–2.4110⁴ cells ml^–1, respectively,with abundances of both being higher in summer (November-April)than in winter (May-October). The ratio of cyanobacteria toeukaryotes fluctuated from 1.93 to 8.67, independent of theseasonal variation in their abundances. The instantaneous growthrate of cyanobacteria, which was estimated from the daytimeabundance increment, ranged between 0.430 and 3.144 day^–1The estimated daily specific growth rate of cyanobacteria bythe FDC method ranged between 0.231 and 0.966 day^–1. InApril, despite the high specific growth rate and low flushingconditions. cyanobacterial abundance showed a cyclic diel pattern,suggesting a strong grazing impact on their population.     

Copepoda associated with the coral Gardineroseris planulata (Dana) on the Great Barrier Reef,northeastern Australia

Three new lichomolgid copepods associated with the scleractinian Gardineroseris planulata (Dana) are described: Sociellus torus, n. gen., n. sp., characterized by a 2-segmented endopod in leg 3 and the lack of an inner seta on the first segment of the endopod in legs 1–3; Odontomolgus pumilus, n. sp., of very small size and having a much elongated free segment in leg 5 of the female; and Paramolgus ampullaceus, n. sp., distinguished by the bottle-shaped genital segment of the female. In addition, the harpacticoid Alteuthellopsis corallina Humes, 1981, is reported from this coral. With the inclusion of 3 species of the poecilostomatoid genus Xarifia already described (Humes, 1985) there are now 7 species of copepods known to be associated with Gardineroseris planulata on the Great Barrier Reef.     

Long-term coral community records from Lugger Shoal on the terrigenous inner-shelf of the central Great Barrier Reef,Australia

Long-term (millennial timescale) records of coral community structure can be developed from the analysis of corals preserved in radiometrically dated reef cores. Here, we present such a record (based on six cores) from Lugger Shoal, a turbid zone, nearshore reef on the inner-shelf of the central Great Barrier Reef. Lugger Shoal initiated growth ~800 cal yBP. It is constructed of large in situ Porites bommies, between which a framework of coral rubble (dominated by Acropora pulchra, Montipora mollis, Galaxea fascicularis and Cyphastrea serailia) has accumulated. Reef accretion occurred under conditions of net long-term fine-grained, terrigenous sediment accumulation, and with a coral community dominated throughout by a consistent, but low diversity, suite of coral taxa. This dataset supports recent suggestions that nearshore coral communities that establish themselves under conditions that are already close to the thresholds for coral survival may be resilient to water quality deteriorations associated with human activities.     

Newly characterized distinct phases of the coral disease 'atramentous necrosis' on the Great Barrier Reef

Previously undetected earlier phases of the coral disease 'atramentous necrosis' are documented and described. New observations relating to the occurrence of initial stages and progression of the disease are reported, and potential cause(s) are examined. In direct contrast to earlier published findings, temperature data indicated that occurrence of early bleached stages of atramentous necrosis is not correlated with warmer water temperatures; however, the relationship between temperature and disease prevalence is still unclear.