The dynamics of benthic microbial communities at Davies Reef,central Great Barrier Reef

The dynamics of benthic microbial communities were examined within different functional zones (reef crest, reef flat, lagoon) of Davies Reef, central Great Barrier Reef, in winter. Bacterial numbers did not change significantly across the reef with a mean abundance of 1.3 (±0.6) x 10⁹ cells g^-1 DW of sediment. Bacterial production, measured as thymidine incorporation into DNA, ranged from 1.2 (±0.2) to 11.6 (±1.5) mg C m^-2h^-1 across the reef and was significantly lower in a reef crest basin than in the other zones. Bacterial growth rates () across the reef (0.05 to 0.33 g^-1) correlated only with sediment organic carbon and nitrogen. Protozoan and meiofaunal densities varied by an order of magnitude across the reef and correlated with one or more sediment variables but not with bacterial numbers or growth rates. Nutrient flux rates were similar to those found at other reefs in the central and southern Great Barrier Reef and are significantly lower than rates measured in temperate sand communities. In the front lagoon, bioturbation and feeding acitivity by thalassinid shrimps (Callianassa spp.) negatively influenced microbial and meiofaunal communities with a net import of organic matter necessary to support the estimated rates of bacterial productivity. In lagoonal areas not colonized by shrimps, primary productivity (400–1100 mg C m^-2d^-1) from algal mats was sufficient to support bacterial growth. It is suggested that deposit-feeding macrobenthos such as thalassinid crustaceans play a major role in the tructuring and functioning of lower trophic groups (bacteria, microalgae, protozoa, meiofauna) in coral reef sedments, particularly in laggons.     

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.     

Spatiotemporal patterns of coral disease prevalence on Heron Island,Great Barrier Reef,Australia

Despite increasing research effort on coral diseases, little is known about factors driving disease dynamics on the Great Barrier Reef (GBR). This is the first study to investigate the temporal patterns of coral disease prevalence and potential drivers of disease around Heron Island, in the southern Capricorn Bunker sector of the GBR. Surveys were conducted in two austral summers and three winters between November 2007 and August 2009 on six sites around the island. Six diseases were detected: brown band syndrome (BrB), growth anomalies (GA), ulcerative white spots (UWS), white syndrome (WS), skeletal eroding band disease (SEB) and black band disease (BBD). The lowest overall mean disease prevalence was 1.87 ± 0.75% (mean ± SE) in November 2007 and the highest 4.22 ± 1.72% in August 2008. There was evidence of seasonality for two diseases: BrB and UWS. This is the first study to report a higher prevalence of BrB in the winter. BrB had a prevalence of 3.29 ± 0.58% in August 2008 and 1.53 ± 0.28% in August 2009, while UWS was the most common syndrome in the summer with a prevalence of 1.12 ± 0.31% in November 2007 and 2.67 ± 0.52% prevalence in January 2008. The prevalence of GAs and SEB did not depend on the season, although the prevalence of GAs increased throughout the study period. WS had a slightly higher prevalence in the summer, but its overall prevalence was low (<0.5%). Sites with high abundance of staghorn Acropora and Montipora were characterised by the highest disease prevalence (12% of Acropora and 3.3% of Montipora species were diseased respectively). These results highlight the correlations between coral disease prevalence, seasonally varying environmental parameters and coral community composition. Given that diseases are likely to reduce the resilience of corals, seasonal patterns in disease prevalence deserve further research.     

Topography, substratum and benthic macrofaunal relationships on a tropical mesophotic shelf margin, central Great Barrier Reef, Australia

Habitats and ecological communities occurring in the mesophotic region of the central Great Barrier Reef (GBR), Australia, were investigated using autonomous underwater vehicle (AUV) from 51 to 145 m. High-resolution multibeam bathymetry of the outer-shelf at Hydrographers Passage in the central GBR revealed submerged linear reefs with tops at 50, 55, 80, 90, 100 and 130 m separated by flat, sandy inter-reefal areas punctuated by limestone pinnacles. Cluster analysis of AUV images yielded five distinct site groups based on their benthic macrofauna, with rugosity and the presence of limestone reef identified as the most significant abiotic factors explaining the distribution of macrofaunal communities. Reef-associated macrofaunal communities occurred in three distinct depth zones: (1) a shallow (<60 m) community dominated by photosynthetic taxa, notably scleractinian corals, zooxanthellate octocorals and photosynthetic sponges; (2) a transitional community (60–75 m) comprising both zooxanthellate taxa and azooxanthellate taxa (notably gorgonians and antipatharians); and (3) an entirely azooxanthellate community (>75 m). The effects of depth and microhabitat topography on irradiance most likely play a critical role in controlling vertical zonation on reef substrates. The lower depth limits of zooxanthellate corals are significantly shallower than that observed in many other mesophotic coral ecosystems. This may be a result of resuspension of sediments from the sand sheets by strong currents and/or a consequence of cold water upwelling.     

Spatial and temporal variations in turbidity on two inshore turbid reefs on the Great Barrier Reef, Australia

This study describes the natural turbidity regimes at two inshore turbid reefs on the central Great Barrier Reef where wind-driven waves are the main agent of sediment resuspension. Many corals on inshore turbid reefs have adapted to high and fluctuating turbidity, however, anthropogenic activities such as dredging are speculated to produce larger and more prolonged turbidity events that may exceed the environmental tolerance and adaptive capacity of corals on these reefs. Natural turbidity regimes must be described and understood to determine whether and when coral communities on inshore turbid reefs are at risk from anthropogenically elevated turbidity, but at present few baseline studies exist. Here, we present turbidity data from (a) Middle Reef, a semi-protected reef located between Magnetic Island and Townsville and (b) Paluma Shoals, a reef exposed to higher energy wind and waves located in Halifax Bay. Instruments were deployed on both reefs for 16 days to measure spatial and temporal variations in turbidity and its driving forces (waves, currents, tides). Locally driven wind waves were the key driver of turbidity, but the strength of the relationship was dependent on wave exposure. Turbidity regimes thus vary markedly over individual reefs and this is reflected in community assemblage distributions, with a high abundance of heterotrophic corals (e.g. Goniopora) in reef habitats subjected to large fluctuations in turbidity (>100 NTU). A turbidity model developed using local wind speed data explained up to 75 % and up to 46 % of the variance in turbidity at Paluma Shoals and Middle Reef, respectively. Although the model was based on a brief two-week observational period, it reliably predicted variations in 24-h averaged turbidity and identified periods when turbidity rose above ambient baseline levels, offering reef managers insights into turbidity responses to modified climate and coastal sediment delivery regimes.     

Patterns in the use of space by benthic communities on two coral reefs of the Great Barrier Reef

A rapid benthic line-transect survey method for use by non-specialist observers is described. At both Davies Reef (mid-continental shelf) and Myrmidon Reef (outer-continental shelf) in the central Great Barrier Reef a set of 6 sites of varying depths on the reef flat, crest and slope were sampled using this method. At least 10 contiguous 10 m transects were made at each site. Benthic organisms were recorded as life forms with categories based on both high level taxa and morphologies, and including scleractinian corals, alcyonarians, sponges, algae and others. Percentage cover data for 19 benthic categories are presented for all sites. Coral cover on both reefs is high on the crest and slope but low on the reef flat. At all sites the cover of soft corals and sponges is much less than cover of hard corals and algae. Abundances of soft corals and sponges increase with depth. Analysis of gaps between hard corals show that many colonies grow close to each other (<1 cm)even when total coral cover is low.     

Patterns in the structural typology of benthic communities on two coral reefs of the central Great Barrier Reef

The morphological life-forms, that is to say, physiognomic-structural attributes, of two coral reef communities were used in a numerical analysis to determine the power of these attributes in recovering the underlying community structure. We used 17 attributes from the benthic communities at 6 reef slope sites on each of a midshelf and off-shore reef of the central Great Barrier Reef. These reefs had been previously well studied by traditional species-level means for several major taxonomic groups such as corals, fish and soft corals. Our multivariate analyses were able to recover broad patterns of between-reef affinity and discrete within-reef zonation patterns similar to those found in earlier studies, and in broad accord with the prevailing model of reef community structure, but with far greater efficacy. But perhaps more importantly, by placing all the benthos within the same context for the first time, our analyses were able to recover new patterns of community structure independent of the ones described earlier. This suggests that single-model explantations for the complex phenomena of coral reefs are likely to be inadequate.     

Spatial variation in the effects of grazing on epilithic algal turfs on the Great Barrier Reef, Australia

Of all benthic components on tropical reefs, algal turfs are the most widespread and the main source of primary productivity. We compared the importance of grazing by herbivores on algal turfs on two zones with marked differences in terms of benthic composition, herbivore biomass and grazing pressure, the inner flat and crest, of an inshore reef on the Great Barrier Reef, Australia. A combination of herbivore exclusion cages and transplants of coral rubble covered by algal turfs between reef zones was used to examine changes in algal turfs over a 4-day experimental period. In situ crest turfs had lower algal height, sediment loads and particulate content than reef flat turfs. Caged samples on the crest exhibited an increase in all three variables. In contrast, in situ and caged treatments on the flat presented algal turfs with similar values for the three analysed variables, with high algal height and heavy particulate and sediment loads. In the absence of cages, reef flat turfs transplanted to the crest had decreased algal height, total particulate material and particulate inorganic content, while the opposite was found in crest turf samples transplanted to the flat. Our results highlight the dynamic nature of algal turfs and the clear differences in the relative importance of herbivory in shaping turf length and sediment load between the reef crest and inner flat.     

Predicting the Location and Spatial Extent of Submerged Coral Reef Habitat in the Great Barrier Reef World Heritage Area,Australia

Aim

Coral reef communities occurring in deeper waters have received little research effort compared to their shallow-water counterparts, and even such basic information as their location and extent are currently unknown throughout most of the world. Using the Great Barrier Reef as a case study, habitat suitability modelling is used to predict the distribution of deep-water coral reef communities on the Great Barrier Reef, Australia. We test the effectiveness of a range of geophysical and environmental variables for predicting the location of deep-water coral reef communities on the Great Barrier Reef.

Location

Great Barrier Reef, Australia.

Methods

Maximum entropy modelling is used to identify the spatial extent of two broad communities of habitat-forming megabenthos phototrophs and heterotrophs. Models were generated using combinations of geophysical substrate properties derived from multibeam bathymetry and environmental data derived from Bio-ORACLE, combined with georeferenced occurrence records of mesophotic coral communities from autonomous underwater vehicle, remotely operated vehicle and SCUBA surveys. Model results are used to estimate the total amount of mesophotic coral reef habitat on the GBR.

Results

Our models predict extensive but previously undocumented coral communities occurring both along the continental shelf-edge of the Great Barrier Reef and also on submerged reefs inside the lagoon. Habitat suitability for phototrophs is highest on submerged reefs along the outer-shelf and the deeper flanks of emergent reefs inside the GBR lagoon, while suitability for heterotrophs is highest in the deep waters along the shelf-edge. Models using only geophysical variables consistently outperformed models incorporating environmental data for both phototrophs and heterotrophs.

Main Conclusion

Extensive submerged coral reef communities that are currently undocumented are likely to occur throughout the Great Barrier Reef. High-quality bathymetry data can be used to identify these reefs, which may play an important role in resilience of the GBR ecosystem to climate change.     

Coral Reef Community Composition in the Context of Disturbance History on the Great Barrier Reef,Australia

Much research on coral reefs has documented differential declines in coral and associated organisms. In order to contextualise this general degradation, research on community composition is necessary in the context of varied disturbance histories and the biological processes and physical features thought to retard or promote recovery. We conducted a spatial assessment of coral reef communities across five reefs of the central Great Barrier Reef, Australia, with known disturbance histories, and assessed patterns of coral cover and community composition related to a range of other variables thought to be important for reef dynamics. Two of the reefs had not been extensively disturbed for at least 15 years prior to the surveys. Three of the reefs had been severely impacted by crown-of-thorns starfish outbreaks and coral bleaching approximately a decade before the surveys, from which only one of them was showing signs of recovery based on independent surveys. We incorporated wave exposure (sheltered and exposed) and reef zone (slope, crest and flat) into our design, providing a comprehensive assessment of the spatial patterns in community composition on these reefs. Categorising corals into life history groupings, we document major coral community differences in the unrecovered reefs, compared to the composition and covers found on the undisturbed reefs. The recovered reef, despite having similar coral cover, had a different community composition from the undisturbed reefs, which may indicate slow successional processes, or a different natural community dominance pattern due to hydrology and other oceanographic factors. The variables that best correlated with patterns in the coral community among sites included the density of juvenile corals, herbivore fish biomass, fish species richness and the cover of macroalgae. Given increasing impacts to the Great Barrier Reef, efforts to mitigate local stressors will be imperative to encouraging coral communities to persist into the future.     

Effectiveness of benthic foraminiferal and coral assemblages as water quality indicators on inshore reefs of the Great Barrier Reef, Australia

Although the debate about coral reef decline focuses on global disturbances (e.g., increasing temperatures and acidification), local stressors (nutrient runoff and overfishing) continue to affect reef health and resilience. The effectiveness of foraminiferal and hard-coral assemblages as indicators of changes in water quality was assessed on 27 inshore reefs along the Great Barrier Reef. Environmental variables (i.e., several water quality and sediment parameters) and the composition of both benthic foraminiferal and hard-coral assemblages differed significantly between four regions (Whitsunday, Burdekin, Fitzroy, and the Wet Tropics). Grain size and organic carbon and nitrogen content of sediments, and a composite water column parameter (based on turbidity and concentrations of particulate matter) explained a significant amount of variation in the data (tested by redundancy analyses) in both assemblages. Heterotrophic species of foraminifera were dominant in sediments with high organic content and in localities with low light availability, whereas symbiont-bearing mixotrophic species were dominant elsewhere. A similar suite of parameters explained 89% of the variation in the FORAM index (a Caribbean coral reef health indicator) and 61% in foraminiferal species richness. Coral richness was not related to environmental setting. Coral assemblages varied in response to environmental variables, but were strongly shaped by acute disturbances (e.g., cyclones, Acanthaster planci outbreaks, and bleaching), thus different coral assemblages may be found at sites with the same environmental conditions. Disturbances also affect foraminiferal assemblages, but they appeared to recover more rapidly than corals. Foraminiferal assemblages are effective bioindicators of turbidity/light regimes and organic enrichment of sediments on coral reefs.     

Coral communities and reef growth in the southern Great Barrier Reef

 Fringing reef development is limited around 22° S along the inner Great Barrier Reef, although there is substantial development north and south of this latitude. This study examined the relationships among coral communities and the extent of reef development. Reefs were examined to determine coral composition, colony abundance, colony size and growth form between the latitudes 20°S and 23°S. Major reef framework builders (scler- actinian genus Acropora and families Faviidae and Poritidae) dominated reefs north and south of 22°S, but declined significantly at 22°S where foliose and encrusting corals (Turbinaria and Montipora spp.) were most common. Porites spp. were present at 22° S but had encrusting morphologies. Consistently high turbidity at this latitude, caused by a 10 m tidal range and strong tidal flows, resuspends silts from the shallow shelf, and appears to have precluded reef development throughout the Holocene, by limiting the abundance, stunting the growth, and shortening the life expectancies of reef framework corals. The distinctions between ‘natural’ and ‘human-induced’ degradation may be interpreted on the basis of the relationship between Holocene development and current benthic community longevity. A mismatch between substantial past reef building capacity (a broad and/or thick reef) and non-existent or limited present reef-building capacity could signify anything from a long-period, natural cycle to an unprecedented deterioration in ecosystem function caused by human influence. Accepted: 29 July 1996     

Abundance patterns of cubozoans on and near the Great Barrier Reef

The ecology of cubozoans is poorly understood and there are few quantitative studies on their distribution patterns. Sampling was designed to test first for variation in abundance with distance across the continental shelf in waters of the Great Barrier Reef, Australia. Second, we tested for the possible influence of islands versus submerged reefs on the abundances of cubozoan jellyfishes. Jellyfishes were collected after attraction to tethered night lights. Additional sampling focused on turbid near-shore waters. Carybdeid jellyfishes were found at mainland, inner, and mid-shelf reefs during summers between 2007 and 2010. No cubozoan medusae were found at outer reef sites. Copula sivickisi and Carukia barnesi were more abundant near reefs with islands than at fully submerged reefs. There was no evidence of lunar periodicity in abundance for these cubozoan taxa. Chironex fleckeri medusae were only found close to shore near the mainland, but they were rarely observed when riverine runoff was high. All taxa were characterized by high spatial and temporal variation and there was some evidence for small populations at spatial scales of less than one kilometer. “Blooms” and related intensity of predation and risk to humans are most likely at small spatial scales.     

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.     

Submerged reefs and terraces on the shelf edge of the Great Barrier Reef,Australia

The morphology and distribution of seabed features on the shelf edge and upper slope adjacent to the Great Barrier Reef, Australia, has been examined using shallow seismic profiling, side-scan sonar and precision echo sounding data, supplemented by submersible investigations. The data reveal a submerged barrier reef system at different locations between 15° 45 S and 21° 00 S. At two locations, an extensive offshore platform rising above the 50 m isobath and extending for over 20 km parallel to the shelf edge is backed by a relict lagoon at an average depth of 75 m. In addition, outer shelf and upper slope terraces are found at many depths; however, only some occur consistently throughout the region while most others occur only locally. Frequency distributions indicate the greatest occurrence of features at depths of 44–46, 60–66, 72–78, 80–84, 102–106 and 146–148 m. Caution should be exercised when interpreting these features with respect to specific lower sea level stands.     

New and noteworthy plants from Great Barrier Reef sand cays,Australia

Observations are presented on new and critical plants from the northern sand cays on the Great Barrier Reef, Queensland, Australia, based mainly on recent collections made by David R. Stoddart and Ralf Buckley. New species and varieties are described:Lepturus stoddartii (Poaceae),Boerhavia fistulosa var.fistulosa and var.puberuliflora (Nyctaginaceae),Boerhavia albiflora var.heronensis, Spermacoce everistiana (Rubiaceae), andSpermacoce buckleyi; a new combination is made:Diospyros ferrea var.compacta (R. Br.) (Ebenaceae); and additional taxonomic notes are given onBoerhavia, Euphorbia (Euphorbiaceae), andAbutilon (Malvaceae).     

Benthic diatom community composition in three regions of the Great Barrier Reef,Australia

Despite their ecological importance, very little is known about the taxonomy and ecology of benthic diatoms in coral-reef ecosystems. Diatom densities and community compositions were investigated in three distinct regions of the Great Barrier Reef (GBR): (a) Wet Tropics (WT), (b) Princess Charlotte Bay (PCB), and (c) the Outer Shelf (OS). About 209 taxa were observed in the GBR sediments studied, with an average abundance of 2.55 × 10⁶ cells ml⁻¹ in the upper 1 cm of sediment. Total diatom abundances were about twice as high in inshore reefs of PCB and WT compared with OS reefs. A redundancy analysis (RDA) of diatom composition clearly grouped the three regions separately but showed little influence of grain size, nitrogen and organic carbon content of the sediments. The only distinct correlates were inorganic carbon and the distance to the mainland associated with OS communities. Analysis of similarity (ANOSIM) of diatom community composition revealed significant differences between all three regions. Indicator values showed that most highly abundant taxa occurred in all regions. However, several taxa were clearly identified as characteristic of particular regions. It is hypothesised that variations in nutrient and light availability are the most likely explanation for the observed differences in community composition.     

Role of plankton in the turnover of organic matter on the Great Barrier Reef,Australia

Number, biomass and production of phytoplankton, bacteria, micro- and mesozooplankton and turnover of labile and stable organic matter were measured in waters over some Capricornia round reefs, and over the reefs of Lizard Island. Primary production was 10 to 40 mg C m^–3 d^–1 but was lower over the living reefs. Microbial wet biomass in reef waters varied from 100 to 500 mg m^–3, and production from 4 to 68 mg C m^–3 d^–1, which was commensurable with primary production. The biomass of microzooplankton (ciliates, zooflagellates and larvae) in waters of Lizard Island reefs reached 100–300 mg m^–3. Mesozooplankton biomass at night in reef waters of Heron Island varied from 200 to 800 mg m^–3. Its composition depended upon the tide phase. PB coefficients in bacterioplankton were 0.3 to 1.2 per day. The food demand of bacterioplankton in waters over the reefs was 5 to 20 times higher than the primary phytoplankton production. Labile organic matter (LOM) doubled in waters after it stayed over living reef for several hours. The turnover time of LOM in reef waters was as short as 1–2 weeks.     

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