New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef,Australia

Halimeda bioherms occur as extensive geological structures on the northern Great Barrier Reef (GBR), Australia. We present the most complete, high-resolution spatial mapping of the northern GBR Halimeda bioherms, based on new airborne lidar and multibeam echosounder bathymetry data. Our analysis reveals that bioherm morphology does not conform to the previous model of parallel ridges and troughs, but is far more complex than previously thought. We define and describe three morphological sub-types: reticulate, annulate, and undulate, which are distributed in a cross-shelf pattern of reduced complexity from east to west. The northern GBR bioherms cover an area of 6095 km², three times larger than the original estimate, exceeding the area and volume of calcium carbonate in the adjacent modern shelf-edge barrier reefs. We have mapped a 1740 km² bioherm complex north of Raine Island in the Cape York region not previously recorded, extending the northern limit by more than 1° of latitude. Bioherm formation and distribution are controlled by a complex interaction of outer-shelf geometry, regional and local currents, coupled with the morphology and depth of continental slope submarine canyons determining the delivery of cool, nutrient-rich water upwelling through inter-reef passages. Distribution and mapping of Halimeda bioherms in relation to Great Barrier Reef Marine Park Authority bioregion classifications and management zones are inconsistent and currently poorly defined due to a lack of high-resolution data not available until now. These new estimates of bioherm spatial distribution and morphology have implications for understanding the role these geological features play as structurally complex and productive inter-reef habitats, and as calcium carbonate sinks which record a complete history of the Holocene post-glacial marine transgression in the northern GBR.

     

Environmental factors associated with the spatial distribution of crustose coralline algae on the Great Barrier Reef

Crustose coralline algae (CCA) fulfill two key functional roles in coral reef ecosystems: they contribute significantly to reef calcification, and they induce larval settlement of many benthic organisms. Percentage cover of CCA, and environmental conditions, were visually estimated on 144 reefs of the Great Barrier Reef between 10 and 24° latitude S. Reefs were located across the shelf and ranged from turbid near-shore reefs close to rivers to clean-water reefs hundreds of kilometers from coastal influences. On each reef, two sites were surveyed between 0.5 and 18 m depth. Strong cross-shelf trends occurred in cover of CCA, amount of sediment deposited, water clarity, and slope angle. Relative distance across the shelf and sedimentation jointly explained 84% of variation in CCA cover. Three regions running parallel to the shore were identified, with a mean CCA cover of <1% on the inner third of the shelf, and >20% cover on the outer half of the shelf, with a narrow transition region between the two. Within each region, the cover of CCA was unrelated to distance across the shelf, but was related to the sedimentary environment, being relatively higher on reefs with low sediment deposits. On the inner third of the shelf, the most sediment-exposed reefs were unsuitable habitats for CCA. The inverse relationship between CCA and sediment has implications for the recruitment of CCA-specialised organisms, and for rates of reef calcification.     

Biomass and distribution of herbivorous blennies in the southern Great Barrier Reef

At the small scale (<1 m²), a correlation between microhabitat availability and abundance of comb–toothed blennies was apparent at Heron Island. Investigation of medium scale (>20 m2) differences, showed that community structure and biomass of herbivorous blennies differed among the 14 geomorphological zones identified. Large scale differences (>10 km²), detected using comparisons between Heron, Lady Elliot and North West reefs, revealed that species composition and biomass, but not density, differed significantly. At Heron Reef, density and biomass of comb–toothed blennies exceeded published estimates for most groups of conspicuous herbivorous fish in equivalent zones. The influence of scale on interpretation of patterns of fish distribution and the role of inconspicuous grazers such as comb–toothed blennies, deserves closer attention.     

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.     

Temporal and spatial variation in fatty acid composition in Acropora tenuis corals along water quality gradients on the Great Barrier Reef,Australia

Coral Reefs - Fatty acids (FA) play a vital role in coral physiology, metabolism and stress resistance. Optimal health requires a balance of fatty acids, and more specifically essential...     

Sponge distribution across Davies Reef,Great Barrier Reef,relative to location,depth, and water movement

Sponge populations were surveyed at different depths in three zones of Davies Reef, a large platform reef of the central Great Barrier Reef. Depth is the major discriminatory factor as few sponges are found within the first 10 m depth and maximal populations occur between 15 m and 30 m on fore-reef, lagoon and back-reef slopes. Reef location is another major factor, with the lagoon containing a significantly different sponge population to either the fore-reef or the back-reef slopes. Physical factors are considered to be the major influences behind these patterns. Physical turbulence is strongest within the first 10 m and apparently limits sponge growth within these shallow zones. Insufficient photosynthetic radiation limits the growth of the sponge population below 30 m depth as many of the species are phototrophic with a dependence on cyanobacterial symbionts for nutrition. Sponge populations on the outer (fore- and back-) reef slopes are comparable with each other but different from those on lagoon slopes where currents are reduced and fine sediment loads are higher. The largest populations occur on the back-reef slope where currents are stronger and there are possibly higher concentrations of organic nutrients originating from the more productive shallow parts of the reef. While there are correlations between sponge populations and environmental parameters, data are insufficient to enable more definitive conclusions to be drawn. Most sponge species are distributed widely over the reef, however, some are restricted to a few habitats and, hence, may be used to characterize those habitats.     

Patterns in the distribution of the crinoid community at Davies Reef on the central Great Barrier Reef

The crinoid community of Davies Reef, a midshelf reef in the central Great Barrier Reef, was systematically sampled in all major crinoid habitats. A total of 294 individuals of 27 species-level taxa was found in 25 sites across the reef. Of these 27 taxa, 20 were confidently assigned to known species. The 25 sitesx27 taxa matrix was subjected to an array of pattern extraction and diagnostic techniques — numerical classification, ordination and minimum spanning trees — to elucidate the structure of the community. These analyses revealed a consistent structure characterized by a species-rich ensemble around the periphery of the reef which was attenuated towards the inside of the reef. This structure contrasts strongly with the patterns seen in other major reef communities, such as hard and soft corals, fish or sponges. In these communities, different parts of the reef are characterized by distinctive sets of species, a depthbased zonation of the communities is evident, and the fore-reef slope typically supports a different ensemble from the back-reef slope. We conclude that the crinoid community offers a significant opportunity to observe the coral reef ecosystem from a different perspective.     

Temporal clustering of tropical cyclones on the Great Barrier Reef and its ecological importance

Tropical cyclones have been a major cause of reef coral decline during recent decades, including on the Great Barrier Reef (GBR). While cyclones are a natural element of the disturbance regime of coral reefs, the role of temporal clustering has previously been overlooked. Here, we examine the consequences of different types of cyclone temporal distributions (clustered, stochastic or regular) on reef ecosystems. We subdivided the GBR into 14 adjoining regions, each spanning roughly 300 km, and quantified both the rate and clustering of cyclones using dispersion statistics. To interpret the consequences of such cyclone variability for coral reef health, we used a model of observed coral population dynamics. Results showed that clustering occurs on the margins of the cyclone belt, being strongest in the southern reefs and the far northern GBR, which also has the lowest cyclone rate. In the central GBR, where rates were greatest, cyclones had a relatively regular temporal pattern. Modelled dynamics of the dominant coral genus, Acropora, suggest that the long-term average cover might be more than 13 % greater (in absolute cover units) under a clustered cyclone regime compared to stochastic or regular regimes. Thus, not only does cyclone clustering vary significantly along the GBR but such clustering is predicted to have a marked, and management-relevant, impact on the status of coral populations. Additionally, we use our regional clustering and rate results to sample from a library of over 7000 synthetic cyclone tracks for the GBR. This allowed us to provide robust reef-scale maps of annual cyclone frequency and cyclone impacts on Acropora. We conclude that assessments of coral reef vulnerability need to account for both spatial and temporal cyclone distributions.

     

Patterns in the distribution of soft corals across the central Great Barrier Reef

Distribution patterns of soft coral genera were examined at 11 reefs situated in a broad transect from inshore to the Coral Sea in the central region of the Great Barrier Reef. Twenty-five genera representing the Orders Alcyonacea and Stolonifera were recorded, and the survey also included one genus of the Order Gorgonacea. Total living soft coral cover is greatest on outershelf reef slopes, and is often less than and inversely related to the cover by stony corals. Soft coral diversity is generally low on reef flats, where soft coral cover is low or nil except in protected, inshore areas. The most diverse assemblages occur on reef slopes in midshelf and outershelf areas, where Efflatounaria and nephtheid genera predominate, and widely distributed alcyoniid genera are common. These richer assemblages are less well represented in the Coral Sea, while innershelf reefs support a less diverse fauna of somewhat different generic composition. Distribution patterns of soft corals across the transect broadly match similar variations in the distributions of stony corals and fishes, inshore reefs being generally depauperate. Such variations across the continental shelf are closely associated with changes in prevailing environmental conditions, but further research will be required to elucidate the effects of environmental parameters on benthic community structure.     

Patterns in the distribution of fish communities across the Central Great Barrier Reef

Summary Changes in the structure of fish communities along a transect from the Australian mainland to the Coral Sea, in the Central region of the Great Barrier Reef, were examined. Visual censuses of fish were made on the outer reef slopes (0 to 13 m deep) of two inshore reefs, approximately 10 km offshore, three reefs on the mid-shelf, 50 km offshore, three reefs on the outer shelf, 100 km offshore, and three reefs in the Coral Sea approximately 200 km offshore. The Pomacentridae, Chaetodontidae, Acanthuridae and Scaridae were examined in detail—the Labridac, Siganidae and the lutjanid genus Caesio in less detail. Major changes in the composition of fish communities occurred along the transect (Fig. 3). There were differences in the composition of assemblages among replicate censuses within individual reefs and also differences between reefs at the same location on the transect but these differences were small compared to those among locations. The nature of the distribution of species across the transect differed between families (Figs. 4–6). Pomacentrid and chaetodontid species were significantly more restricted in distribution than acanthurids, scarids or labrids.     

Patterns in the distribution of sponge populations across the central Great Barrier Reef

Coral reef sponge populations were surveyed at two spatial scales: different depths and different reef locations across the continental shelf of the central Great Barrier Reef. The surveys were conducted on the forereef slopes of 12 reefs from land-influenced, inner-shelf reefs to those in the oligotrophic waters of the Coral Sea. Few sponges occur in shallow waters and the largest populations are found between 10 and 30 m depth. Sponges are apparently excluded from shallow waters because of excessive turbulence and possibly by high levels of damaging light. Sponge biomass is highest on the innershelf reefs and decreases away from the coast, whereas abundance is generally higher on middle-shelf reefs. There are considerable overlaps in the species composition on middle-, outer-shelf and Coral Sea reefs, but those on inner-shelf reefs are significantly different. The nature and size of sponge populations reflect environmental conditions across the continental shelf. The larger inner-shelf populations probably reflect higher levels of organic and inorganic nutrients and reduced amounts of physical turbulence, whereas sponges on reefs further from shore may be able to resist greater turbulence but appear more sensitive to the effects of fine sediments. These latter populations are smaller, reflecting the reduced availability of organic matter, however, many of these sponges rely on cyanobacterial symbionts to augment nutrition in these clearer, more oligotrophic waters.Contribution no. 487 from the Australian Institute of Marine Science     

Zooplankton community structure and copepod egg production in coastal waters of the central Great Barrier Reef lagoon

We describe zooplankton community structure and copepod eggproduction in the vicinity of the coastal boundary zone of theGreat Barrier Reef lagoon, Australia. The abundance and eggproduction rate of constituents of the zooplankton assemblagecharacteristic of the coastal zone rapidly increase subsequentto events such as flooding and upwelling. Our sampling spannedtwo summer monsoonal seasons, the first of which (1990-91) wasvery wet. The second monsoonal season (1991-92) was very dryand was characterized by intrusive upwelling events from theCoral Sea. Chlorophyll a concentrations did not rise in thewet year, probably because of light limitation, but did riseas a result of upwelling. Terrestrial run-off in the wet yearhad a greater apparent effect on zooplankton abundance patternsthan did upwelling in the dry year, except where coastal trappingallowed sufficient time for increases in zooplankton abundanceto occur. Egg production rates by the copepods Acrocalanus gibberand Acrocalanus gracilis showed haphazard spatial differences.Nitrogen-specific egg production ranged between 0.03 and 0.21day^–1 for A.gibber, and between 0.13 and 0.41 day^–1for A.gracilis. The egg production rate by A.gibber was foodlimited for most of the year and showed a poor correlation withtemperature. ³Present address: Department of Biological Sciences, FloridaTech, 150 W University Boulevard, Melbourne, FL 32901, USA     

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.     

Modelling wind driven circulation One Tree Reef,Southern Great Barrier Reef

A vertically integrated numerical model of wind-driven circulation at One Tree Reef is presented. The model is the numerical model SURGE developed originally to study tropical cyclone surge. Current data collected in the reef lagoon and over the reef flats is used to test the model's applicability. The reef topography has been modelled explicitly, rather than using an assumed reef shape, with a grid spacing of 150 m. The model corresponds well to the measured current behaviour. The greatest drawback to use of the model is that, at low tide, currents reverse with depth due to lagoon enclosure and a depth integrated model cannot produce water velocity depth profiles. The model can be used to predict current behaviour in strong wind conditions, such as during a cyclone, and to estimate net flows into and out of the lagoon.     

Halimeda bioherms of the northern Great Barrier Reef

The reefless tract directly behind the ribbon reefs on the outer shelf off Cooktown supports a luxuriant growth of Halimeda that, during the Holocene, has developed into bioherms. These mounded biodies of unconsolidated sediment have formed banks that vary in height between 2 and 20 m. Combined shallow, high-resolution seismic reflection profiles and side-scan sonar have diferentiated three areas of biohermal complexes behind the ribbon reefs of Cooktown. Observations by SCUBA and submersible plus the sedimentology of the bioherms indicate that they are in situ accumulations. Evidence from dating of cores suggests that the Halimeda bioherms began to grow about 10 000 years B.P. and their growth has continued to the present time, even though their tops are presently restricted to a depth of -20 m. It is suggested that the origin and morphology of the bioherms are related to a specific hydrodynamic phenomenon, involving jets of nutrient-rich, upwelled oceanic water intruding onto the outer shelf via the narrow passes between the ribbon reefs, and forming eddies behind the ribbons.