Competition between corals and algal turfs along a gradient of terrestrial influence in the nearshore central Great Barrier Reef

Competition between benthic algae and corals is a key process in the community ecology of reefs, especially during reef degradation. However, there have been very few experimental tests for competition between corals and benthic algae, despite widespread assumptions that algae are generally superior competitors, especially in eutrophic conditions. This study tested for competition for space between the massive coral Porites lobata and algal filamentous turfs on three reefs along a cross-shelf gradient of terrestrial influence, by experimentally removing or damaging either corals or algae. The corals and algae were competing for space, but, significantly, the algae appeared to have little effect on coral growth. In contrast, corals significantly inhibited algal growth, suggesting Porites was the competitive superior. Importantly, coral growth was generally positive, even on the reef with the greatest terrestrial influence. Competitive outcomes did not support the argument that algae are more successful competitors in more eutrophic conditions.     

Restricted terrestrial carbon input to the continental shelf during Cyclone Winifred: implications for terrestrial runoff to the Great Barrier Reef Province

Tropical Cyclone Winifred (1 February 1986) provided an ideal opportunity to examine the fate of high river discharge in the Central Great Barrier Reef by producing near-record floods between Townsville and Cairns. Comparison of the carbon isotope ratio of organic matter in shelf sediment collected immediately before and after the cyclone showed that the bulk of terrestrial plant detritus from the Johnstone River was deposited within 2 km of the rivermouth and none moved more than 15 km offshore. By comparing the magnitude of the Johnstone River flow to the maximum recorded flows on other rivers in the Great Barrier Reef Province, we conclude that terrestrial runoff has not reached the Reef in historical times except, perhaps, during rare Burdekin River floods. Terrestrial detritus initially deposited near chore, however, is resuspended during tropical cyclones and may eventually be transported to the Reef.     

Shifts in coralline algae,macroalgae, and coral juveniles in the Great Barrier Reef associated with present‐day ocean acidification

Seawater acidification from increasing CO₂ is often enhanced in coastal waters due to elevated nutrients and sedimentation. Our understanding of the effects of ocean and coastal acidification on present‐day ecosystems is limited. Here we use data from three independent large‐scale reef monitoring programs to assess coral reef responses associated with changes in mean aragonite saturation state (Ω_ar) in the Great Barrier Reef World Heritage Area (GBR). Spatial declines in mean Ω_ar are associated with monotonic declines in crustose coralline algae (up to 3.1‐fold) and coral juvenile densities (1.3‐fold), while non‐calcifying macroalgae greatly increase (up to 3.2‐fold), additionally to their natural changes across and along the GBR. These three key groups of organisms are important proxies for coral reef health. Our data suggest a tipping point at Ω_ar 3.5–3.6 for these coral reef health indicators. Suspended sediments acted as an additive stressor. The latter suggests that effective water quality management to reduce suspended sediments might locally and temporarily reduce the pressure from ocean acidification on these organisms.     

Bacterial consumption of total and dissolved organic carbon in the Great Barrier Reef

Heterotrophic bacteria typically take up directly dissolved organic matter due to the small molecular size, although both particulate and dissolved organic matter have labile (easily consumed) compounds. Tropical coastal waters are important ecosystems because of their high productivity. However, few studies have determined bacterial cycling (i.e. carbon uptake by bacteria and allocation for bacterial biomass and respiration) of dissolved organic carbon in coastal tropical waters, and none has determined bacterial cycling of total and dissolved organic carbon simultaneously. In this study we followed bacterial biomass and production, and organic carbon changes over short-term (12 days) dark incubations with (total organic carbon, TOC) and without particulate organic carbon additions (dissolved organic carbon, DOC). The study was performed at three sites along the middle stretch of the Great Barrier Reef (GBR) during the dry and wet seasons. Our results show that the bacterial growth efficiency is low (0.1–11.5%) compared to other coastal tropical systems, and there were no differences in the carbon cycling between organic matter sources, seasons or locations. Nonetheless, more carbon was consumed in the TOC compared to the DOC incubations, although the proportion allocated to biomass and respiration was similar. This suggests that having more bioavailable substrate in the particulate form did not benefit bacteria. Overall, our study indicates that when comparing the obtained respiration rates with previously measured primary production rates, the GBR is a heterotrophic system. More detailed studies are required to fully explore the mechanisms used by bacteria to cycle TOC and DOC in tropical coastal waters.

     

Locomotion in labrid fishes: implications for habitat use and cross-shelf biogeography on the Great Barrier Reef

Coral reefs exhibit marked zonation patterns within single reefs and across continental shelves. For sessile organisms these zones are often related to wave exposure. We examined the extent to which wave exposure may shape the distribution patterns of fishes. We documented the distribution of 98 species of wrasses and parrotfishes at 33 sites across the Great Barrier Reef. The greatest difference between labrid assemblages was at the habitat level, with exposed reef flats and crests on mid- and outer reefs possessing a distinct faunal assemblage. These exposed sites were dominated by individuals with high pectoral fin aspect ratios, i.e. fishes believed to be capable of lift-based swimming which often achieve high speeds. Overall, there was a strong correlation between estimated swimming performance, as indicated by fin aspect ratio, and degree of water movement. We propose that swimming performance in fishes limits access to high-energy locations and may be a significant factor influencing habitat use and regional biogeography of reef fishes.     

Linkages between coral assemblages and coral proxies of terrestrial exposure along a cross-shelf gradient on the southern Great Barrier Reef

Coral core records, combined with measurements of coral community structure, were used to assess the long-term impact of multiple environmental stressors on reef assemblages along an environmental gradient. Multiple proxies (luminescent lines, Ba/Ca, δ¹⁵N) that reflect different environmental conditions (freshwater discharge, sediment delivery to the nearshore, nutrient availability and transformations) were measured in Porites coral cores collected from nearshore reefs at increasing distance from the intensively agricultural region of Mackay (Queensland, Australia). The corals provide a record (1968–2002) of the frequency and intensity of exposure to terrestrial runoff and fertilizer-derived nitrogen and were used to assess how the present-day coral community composition may have been influenced by flood-related disturbance. Reefs closest to the mainland (5–32 km offshore) were characterized by low hard coral cover (≤10%), with no significant differences among locations. Distinct annual luminescent lines and elevated Ba/Ca values (4.98 ± 0.63 μmol mol⁻¹; mean ± SD) in the most inshore corals (Round Top Island; 5 km offshore) indicated chronic, sub-annual exposure to freshwater and resuspended terrestrial sediment that may have historically prevented reef formation. By contrast, corals from Keswick Island (32 km offshore) indicated episodic, high-magnitude exposure to Pioneer River discharge during extreme flood events (e.g., 1974, 1991), with strongly luminescent lines and substantially enriched coral skeletal δ¹⁵N (12–14‰). The reef assemblages at Keswick and St. Bees islands were categorically different from all other locations, with high fleshy macroalgal cover (80.1 ± 7.2% and 62.7 ± 7.1%, respective mean ± SE) overgrowing dead reef matrix. Coral records from Scawfell Island (51 km offshore) indicated little exposure to Pioneer catchment influence: all locations from Scawfell and further offshore had total hard and soft coral cover comparable to largely undisturbed nearshore to middle shelf reefs of the southern Great Barrier Reef.     

Bioerosion in Acropora across the continental shelf of the Great Barrier Reef

The degree of internal bioerosion was examined in the dead basal portions of live branches of the scleractinian coral Acropora formosa collected from six reefs across the continental shelf in the central region of the Great Barrier Reef, Australia. The bioeroders included the sponges Cliona spp. and Cliothosa spp., the boring bivalve Lithophaga sp., and sipunculid and polychaete worms. Total internal bioerosion exhibited higher means and variances inshore and at the mid-shelf than the outer shelf specimens, which were characterized by low means and low variances. Bioerosion by Cliothosa and all sponges combined declined slightly across the shelf. Bivalves accounted for a small proportion of the internal bioerosion in A. formosa. The bioerosion pattern exhibited by worms (polychaetes and sipunculids) was similar in pattern to that of the sponges. All groups exhibited lowest levels of bioerosion at the outer shelf. Highest variance in the data was observed at the intra-branch/intra-colony and the intercolony levels. Inter-site variance was high in worms and vivalves. Boring sponges generally dominated the bioeroder community. The relative abundance of Cliona declined on the outer shelf while the relative abundance of worms increased. Percent bioerosion in Acropora formosa was 2–3 times higher than in Porites lobata in this region. The low level of bioerosion at the outer shelf versus the inner- and mid-shelf areas may be partially due to lower levels of productivity and lower concentrations of terrestrially derived organic matter. Other potential factors may include higher fish grazing/predation activity on the outer shelf.     

Effects of ocean acidification on microbial community composition of, and oxygen fluxes through, biofilms from the Great Barrier Reef

Rising anthropogenic CO(2) emissions acidify the oceans, and cause changes to seawater carbon chemistry. Bacterial biofilm communities reflect environmental disturbances and may rapidly respond to ocean acidification. This study investigates community composition and activity responses to experimental ocean acidification in biofilms from the Australian Great Barrier Reef. Natural biofilms grown on glass slides were exposed for 11 d to four controlled pCO(2) concentrations representing the following scenarios: A) pre-industrial (～300 ppm), B) present-day (～400 ppm), C) mid century (～560 ppm) and D) late century (～1140 ppm). Terminal restriction fragment length polymorphism and clone library analyses of 16S rRNA genes revealed CO(2) -correlated bacterial community shifts between treatments A, B and D. Observed bacterial community shifts were driven by decreases in the relative abundance of Alphaproteobacteria and increases of Flavobacteriales (Bacteroidetes) at increased CO(2) concentrations, indicating pH sensitivity of specific bacterial groups. Elevated pCO(2) (C + D) shifted biofilm algal communities and significantly increased C and N contents, yet O(2) fluxes, measured using in light and dark incubations, remained unchanged. Our findings suggest that bacterial biofilm communities rapidly adapt and reorganize in response to high pCO(2) to maintain activity such as oxygen production.     

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     

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 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.     

Microbial diversity in marine biofilms along a water quality gradient on the Great Barrier Reef

Microbial communities are potential indicators for water quality as they respond rapidly to environmental changes. In the Whitsunday Islands, Australia, microbial biofilm communities from two offshore islands were compared to those from two inshore islands subjected to poor water quality. Biofilm community composition was characterized using three culture-independent molecular techniques. The clone libraries indicated high genetic diversity, with somewhat higher scores in the offshore sites (57%) compared to the inshore sites (41%). The majority of microbes in the biofilms were related to Alphaproteobacteria (39.8%), Gammaproteobacteria (14.1%), Bacteroidetes (13.2%), diatoms (8.3%) and Cyanobacteria (3.9%). Redundancy analysis (RDA) for the CARD-FISH data showed distinct microbial assemblages between offshore and inshore communities. Additionally, 5 out of 13 water quality parameters (DIN, Chla, POP, TSS and POC) explained a significant amount of variation in the microbial communities and high values of these were associated with inshore communities. Analysis of variance (ANOVA) indicated that Cyanobacteria (p = 0.01), Bacteroidetes (p = 0.04) and to some extent Alphaproteobacteria (p = 0.07), were significantly more abundant in the offshore biofilm communities. Principal Component Analysis (PCA) of DGGE data showed clear grouping of cyanobacterial communities into inshore and offshore communities. Reasons for community shifts in the bacterial lineages are currently not resolved. One possible causative factor may be that autotrophic primary producers are more dominant in offshore sites due to the higher light availability as well as the limitation by DIN. The trends found in this study are the bases for more detailed research on microbial indicator species for changes in water quality.     

Linking land to ocean: feedbacks in the management of socio-ecological systems in the Great Barrier Reef catchments

The Great Barrier Reef (GBR) off Australia’s northeast coast is one of the natural wonders of the world. As a consequence it has high value, not only for biodiversity, but also for tourists who come to see the GBR and the biodiversity associated with it, bringing in over A$3.5B per annum to the Australian economy. However, there are a number of natural and anthropogenic factors that are threatening the health of the reef ecosystems. One of the major anthropogenic factors is the impact of sediments and nutrients that run off the land, via the rivers, into the lagoon of the reef. Extensive beef production is one of the major land uses of the GBR catchment, and brings in over $1B to the national economy annually and employs nearly 9,000 people, many of them in rural communities. Over 70% of terrestrial sediments and nutrients deposited in the GBR lagoon affecting the health of vulnerable reef ecosystems originate from the extensive grazing lands of Queensland’s interior. Recent research indicates that the quantity of sediments and nutrients lost from these grazing lands is strongly dependent upon grazing management practices; grazing leads to degradation of soil and vegetation resources, reduced infiltration and vegetation production. This has led to a growing concern amongst the Australian public about the environmental performance of the beef industry and increasing pressures on graziers to change their management practices to decrease the off-farm impacts. Given the constraints within the system improvements in water quality draining into the GBR lagoon can best be achieved by demonstrating the productivity and economic benefits of science-based improved grazing management practices for graziers, leading to “AllWin” outcomes for all concerned. In the longer term, only when the range of stakeholders involved approach catchments as linked biophysical, social and economic systems, will truly integrated adaptive catchment management be applied to the GBR.     

The large-scale influence of the Great Barrier Reef matrix on wave attenuation

Offshore reef systems consist of individual reefs, with spaces in between, which together constitute the reef matrix. This is the first comprehensive, large-scale study, of the influence of an offshore reef system on wave climate and wave transmission. The focus was on the Great Barrier Reef (GBR), Australia, utilizing a 16-yr record of wave height from seven satellite altimeters. Within the GBR matrix, the wave climate is not strongly dependent on reef matrix submergence. This suggests that after initial wave breaking at the seaward edge of the reef matrix, wave energy that penetrates the matrix has little depth modulation. There is no clear evidence to suggest that as reef matrix porosity (ratio of spaces between individual reefs to reef area) decreases, wave attenuation increases. This is because individual reefs cast a wave shadow much larger than the reef itself; thus, a matrix of isolated reefs is remarkably effective at attenuating wave energy. This weak dependence of transmitted wave energy on depth of reef submergence, and reef matrix porosity, is also evident in the lee of the GBR matrix. Here, wave conditions appear to be dependent largely on local wind speed, rather than wave conditions either seaward, or within the reef matrix. This is because the GBR matrix is a very effective wave absorber, irrespective of water depth and reef matrix porosity.     

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.     

Host-Specific Interactions with Environmental Factors Shape the Distribution of Symbiodinium across the Great Barrier Reef

Background

The endosymbiotic dinoflagellates (genus Symbiodinium) within coral reef invertebrates are critical to the survival of the holobiont. The genetic variability of Symbiodinium may contribute to the tolerance of the symbiotic association to elevated sea surface temperatures (SST). To assess the importance of factors such as the local environment, host identity and biogeography in driving Symbiodinium distributions on reef-wide scales, data from studies on reef invertebrate-Symbiodinium associations from the Great Barrier Reef (GBR) were compiled.

Methodology/Principal Findings

The resulting database consisted of 3717 entries from 26 studies. It was used to explore ecological patterns such as host-specificity and environmental drivers structuring community complexity using a multi-scalar approach. The data was analyzed in several ways: (i) frequently sampled host species were analyzed independently to investigate the influence of the environment on symbiont distributions, thereby excluding the influence of host specificity, (ii) host species distributions across sites were added as an environmental variable to determine the contribution of host identity on symbiont distribution, and (iii) data were pooled based on clade (broad genetic groups dividing the genus Symbiodinium) to investigate factors driving Symbiodinium distributions using lower taxonomic resolution. The results indicated that host species identity plays a dominant role in determining the distribution of Symbiodinium and environmental variables shape distributions on a host species-specific level. SST derived variables (especially SSTstdev) most often contributed to the selection of the best model. Clade level comparisons decreased the power of the predictive model indicating that it fails to incorporate the main drivers behind Symbiodinium distributions.

Conclusions/Significance

Including the influence of different host species on Symbiodinium distributional patterns improves our understanding of the drivers behind the complexity of Symbiodinium-invertebrate symbioses. This will increase our ability to generate realistic models estimating the risk reefs are exposed to and their resilience in response to a changing climate.     

Dynamics of phytoplankton in the Great Barrier Reef Lagoon

The characteristics of the phytoplankton crop in the centralregion of the Great Barrier Reef were analyzed through two annualcycles together with basic oceanographic parameters. Chlorophylla standing crop and primary production were size fractionatedinto nanoplankton and microplankton components. Community compositionwas determined using the Utermöhl settling technique andcommunity diversities estimated by the Shannon-Weaver equation.The data and analysis are the most comprehensive in existencefor the region, and the first detailed study since the 1928–29Great Barrier Reef expedition. A marked seasonal cycle was identified,contrary to most assumptions, closely associated with precipitationpatterns and nutrients introduced by land drainage. The regionwould rank as mesotrophic with some eutrophic areas in the innerreaches of the Lagoon. Oscillatoria spp. accounted for a largefraction of the majority of phytoplankton maxima and were inverselyrelated to diatom crop densities even under conditions favoringdiatom growth. Microplankton crop species diversities usuallydecreased during extended Oscillatoria blooms. The reduced diversitypersisted after the bloom suggesting that Oscillatoria spp.were the source of extracellular metabolites and/or decompositionproducts adversely influencing diatom microplankton.     

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.     

First-generation fitness consequences of interpopulational hybridisation in a Great Barrier Reef coral and its implications for assisted migration management

The translocation of populations within their natural distribution ranges to instigate crossings between genetic stocks may enhance adaptive potential and resilience. Colonies of the reef-building coral, Acropora millepora, collected in the warmer central Great Barrier Reef (GBR) were experimentally crossed with conspecific colonies from the cooler southern GBR. Fertilisation success was high in all purebred and regional hybrid crosses (>83 %). After 4 months in the field at the southern location, survival rates differed as follows: native purebreds > regional hybrids > central GBR purebreds. The southern GBR purebreds were smaller at settlement compared with the other groups, but this difference disappeared towards the end of the grow-out period. While no benefit of genetic mixing in the F1 generation of this species was evident from our work, it is possible that hybrid vigour exists for other traits, such as thermal tolerance, and over different spatial scales, for different species, or in later generations.