Biogeochemistry of inter-reef sediments on the northern and central Great Barrier Reef

The deposition and cycling of carbon and nitrogen in carbonate sediments located between coral reefs on the northern and central sections of the Great Barrier Reef were examined. Rates of mass sediment accumulation ranged from 1.9 kg m⁻² year⁻¹ (inshore reefs) to 2.1–4.9 kg m⁻² year⁻¹ (between mid-shelf reefs); sedimentation was minimal off outer-shelf reefs. Rates of total organic carbon decomposition ranged from 1.7 to 11.4 mol C m⁻² year⁻¹ and total nitrogen mineralization ranged from 77 to 438 mmol N m⁻² year⁻¹, declining significantly with distance from land. Sediment organic matter was highly reactive, with mineralization efficiencies ranging from 81 to 99% for organic carbon and 64–100% for nitrogen, with little C and N burial. There was no evidence of carbonate dissolution/precipitation in short-term incubation experiments. Rates of sulfate reduction (range 0–3.4 mmol S m⁻² day⁻¹) and methane release (range 0–12.8 μmol CH₄ m⁻² day⁻¹) were minor or modest pathways of carbon decomposition. Aerobic respiration, estimated by difference between total O₂ consumption and the sum of the other pathways, accounted for 55–98% of total carbon mineralization. Rates of ammonification ranged from 150 to 1,725 μmol NH₄ m⁻² day⁻¹, sufficient to support high rates of denitrification (range 30–2,235 μmol N₂ m⁻² day⁻¹). N₂O release was not detected and rates of NH₄ ⁺ and NO₂ ⁻ + NO₃ ⁻ efflux were low, indicating that most mineralized N was denitrified. The percentage of total N input removed via denitrification averaged ≈75% (range 28–100%) with little regenerated N available for primary producers. Inter-reef environments are therefore significant sites of energy and nutrient flow, especially in spatially complex reef matrices such as the Great Barrier Reef.     

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.     

Significance of <Emphasis Type="Italic">Halimeda</Emphasis> bioherms to the global carbonate budget based on a geological sediment budget for the Northern Great Barrier Reef,Australia

Since the correlation between carbon dioxide (CO₂) levels and global temperatures was established in the ice core records, quantifying the components of the global carbon cycle has become a priority with a view to constraining models of the climate system. The marine carbonate budget is still not adequately constrained and the quantitative significance of the calcareous green alga Halimeda still remains particularly poorly understood. Previously, it has been suggested that Halimeda bioherms on the shelf of the Great Barrier Reef may contain a volume of carbonate equal to or greater than that contained within the shelf edge coral reefs. This study uses published datasets to test this hypothesis in the Northern Great Barrier Reef (NGBR) province. It is estimated that Halimeda bioherms on the outer shelf of the NGBR contain at least as much (and up to four times more) CaCO₃ sediment as the adjacent ribbon reef facies. Globally, if these findings are even only partially applicable, the contribution of shallow water carbonate sediments to the global carbon budget based on coral reefs alone is currently substantially underestimated.     

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.     

Soft coral abundance on the central Great Barrier Reef: effects of Acanthaster planci, space availability, and aspects of the physical environment

 The distribution and abundance of soft coral genera on reefs of the central Great Barrier Reef was investigated in relation to reef position, recent history of disturbance, wave exposure, substratum slope and depth. Eighty-five 25 m long transects were surveyed at 10 m depth on windward sides of 14 mid- and outer-shelf reefs. A further 75 transects in different zones on one mid-shelf reef (Davies Reef) between 5 and 30 m depth were investigated. The crown-of-thorns starfish Acanthaster planci had caused large-scale mortality of scleractinians on eight of these reefs five to ten years prior to the study, and as a result, scleractinian cover was only 35–55% of that on the six unimpacted reefs. On the impacted reefs, stony corals with massive and encrusting growths form had smaller average colony diameters but similar or slightly lower numerical abundance. In contrast, mean colony size, cover and abundance of branching stony corals showed no difference between impacted and unimpacted reefs. Twenty-four genera of soft corals (in eight families) were recorded, and none showed different abundance or cover in areas of former A. planci impact, compared to unaffected sites. Similarly, no difference was detected among locations in the numbers or area cover of sponges, tunicates, zoanthids, Halimeda or other macro-algae. Mean soft coral cover was 2 to 5% at 10 m on sheltered mid-shelf reefs, and 12 to 17% on more current-exposed reefs. Highest cover and abundances generally occurred on platforms of outer-shelf reefs exposed to relatively strong currents but low wave energy. On Davies Reef, cover and colony numbers of the families Nephtheidae and Xeniidae were low within the zone of wave impact, in flow-protected bays and lagoons, on shaded steep slopes, and at depths above 10 and below 25 m. In contrast, distributions of genera of the family Alcyoniidae were not related to these physical parameters. The physical conditions of a large proportion of habitats appear “sub-optimal” for the fastest growing taxa, possibly preventing an invasion of the cleared space. Thus, in the absence of additional stress these shallow-water fore-reef zones appear sufficiently resilient to return to their pre-outbreak state of scleractinian dominance. Accepted: 20 August 1996     

Sedimentation on three caribbean atolls: Glovers reef,lighthouse reef and turneffe Islands,belize

Summary The chief mode of carbonate sedimentation on the Belizean atolls Glovers Reef, Lighthouse Reef and Turneffe Islands is the accumulation of organically-derived particles. Variations in the distribution of the composition and grain-sizes of surface sediments, collected along transects across the atolls, are environmentally controlled. Two major sediment types may be distinguished. (1) Reef and fore reef sediments are dominated by fragments of coral, coralline algae andHalimeda. Mean grain-sizes range from 1–2 mm. (2) Back reef sediments contain more mollusk fragments, more fine-grained sediment (<125 μm) and appear to have fewerHalimeda fragments. In addition, sediments from inner platforms and shallow lagoonal parts of Glovers and Lighthouse Reefs comprise non-skeletal grains, namely fecal pellets. Sediments from lagoonal patch reefs may contain up to 20% coral fragments. Mean grain-sizes range from 0.1–1 mm and are finest on the inner platform and lagoon floor of the back reef environment. Within the reef and fore reef environments, it is not possible to distinguish sub-environments on the basis of textural and compositional differences of the sediments. Sediments from patch reefs contrast with those from back reef lagoons and inner platforms and are similar in terms of grain-sizes and compositions to reef and fore reef surface sediments. Non-skeletal grains forming in shallow parts of the back reef in Glovers and Lighthouse Reefs are interpreted to be indurated by interstitial precipitation of calcium carbonate from warm, supersaturated water flushing the sediment. The lack of hardened non-skeletal particles in the back reef sediments of Turneffe Islands is most probably due to the abundance of muddy, organic-rich sediment in the well-protected lagoon. Fine sediment is less permeable and organic films prevent cement overgrowth on particles.     

Dissolution of Dead Corals by Euendolithic Microorganisms Across the Northern Great Barrier Reef (Australia)

Spatial and temporal variabilities in species composition, abundance, distribution, and bioeroding activity of euendolithic microorganisms were investigated in experimental blocks of the massive coral Porites along an inshore–offshore transect across the northern Great Barrier Reef (Australia) over a 3-year period. Inshore reefs showed turbid and eutrophic waters, whereas the offshore reefs were characterized by oligotrophic waters. The euendolithic microorganisms and their ecological characteristics were studied using techniques of microscopy, petrographic sections, and image analysis. Results showed that euendolithic communities found in blocks of coral were mature. These communities were dominated by the chlorophyte Ostreobium quekettii, the cyanobacterium Plectonema terebrans, and fungi. O. quekettii was found to be the principal agent of microbioerosion, responsible for 70–90% of carbonate removal. In the offshore reefs, this oligophotic chlorophyte showed extensive systems of filaments that penetrated deep inside coral skeletons (up to 4.1 mm) eroding as much as 1 kg CaCO₃ eroded m⁻² year⁻¹. The percentage of colonization by euendolithic filaments at the surface of blocks did not vary significantly among sites, while their depths of penetration, especially that of O. quekettii (0.6–4.1 mm), increased significantly and gradually with the distance from the shore. Rates of microbioerosion (0.1–1.4 kg m⁻² after 1 year and 0.2–1.3 kg m⁻² after 3 years of exposure) showed a pattern similar to the one found for the depth of penetration of O. quekettii filaments. Accordingly, oligotrophic reefs had the highest rates of microbioerosion of up to 1.3 kg m⁻² year⁻¹, whereas the development of euendolithic communities in inshore reefs appeared to be limited by turbidity, high sedimentation rates, and low grazing pressure (rates <0.5 kg m⁻² after 3 years). Those results suggest that boring microorganisms, including O. quekettii, have a significant impact on the overall calcium carbonate budget of coral reef ecosystems, which varies according to environmental conditions.     

High levels of acrylate in the Great Barrier Reef coral Acropora millepora

High concentrations of acrylate, 542–683 μmol g⁻¹ of the non-skeletal dry mass (DM), were measured in the Great Barrier Reef coral, Acropora millepora, using quantitative nuclear magnetic resonance spectroscopy (qNMR). As the amount of NaCl salt in the samples was substantial but variable, the total carbon (TC) in the coral extracts was determined, and the carbon due to acrylate found to represent 13–15% of the TC present in the total organic extracts (TOE). Acrylate, a C₃ compound, is thus a substantial carbon source in the coral holobiont and is known to be derived from dimethylsulfoniopropionate (DMSP), which has previously been found in corals and other organisms that harbor Symbiodinium spp. The reason for such high levels of acrylate in the corals is unknown; possible functions include antimicrobial and/or antioxidant roles, as well as playing a role in the structuring of the healthy resident coral bacteria.     

Importance of foraminifera for the formation and maintenance of a coral sand cay: Green Island, Australia

 CaCO₃ production by reef-building organisms on Green Island Reef in the Great Barrier Reef of Australia is estimated and compared with the contribution of benthic foraminifera to the sediment mass of the vegetated sand cay. Major constituents of the cay are benthic foraminifera (mainly Amphistegina lessonii, Baculogypsina sphaerulata, and Calcarina hispida), calcareous algae (Halimeda and coralline algae), hermatypic corals, and molluscs. Among these reef-building organisms, benthic foraminifera are the single most important contributor to the sediment mass of the island (ca. 30% of total sediments), although their production of CaCO₃ is smaller than other reef-building organisms. Water current measurements and sediment traps indicate that the velocity of the current around Green Island is suitable for transportation and deposition of foraminiferal tests. Abundant foraminifera presently live in association with algal turf on the shallow exposed reef flat, whose tests were accumulated by waves resulting in the formation and maintenance of the coral sand cay. Accepted: 30 June 1999     

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     

Settlement behaviour of larvae of the Stripey Snapper, <Emphasis Type="Italic">Lutjanus carponotatus</Emphasis> (Teleostei: Lutjanidae)

Larval behaviour is important to dispersal and settlement, but is seldom quantified. Behavioural capabilities of larval Lutjanus carponotatus in both offshore pelagic and reef environments at Lizard Island, Great Barrier Reef were observed in situ to determine if they were sufficient to influence dispersal. Offshore, larvae swam with higher directional precision and faster on the windward side of the island (28 cm.s⁻¹) than on the leeward side (16 cm s⁻¹). Most larvae swam directionally. Mean swimming directions were southerly in the windward area and northerly in the leeward area. Larvae avoided the surface and remained mostly between 3–15 m. Larvae released near reefs were 2–3 times faster swimming away from reefs (19 cm s⁻¹) than swimming toward or over them (6–8 cm s⁻¹). Speed swimming away was similar to that offshore. Of 41 larvae released near reefs, 73% reached the reef, 59% settled, and 13% of those reaching the reef were eaten. Larvae settled onto hard and soft coral (58%), topographic reef features (29%) and sand and rubble (13%). Settlement depth averaged 5.5 m (2–8 m). Before settling larvae spent up to 800 s over the reef (mean 231 s) and swam up to 53 m (mean 14 m). About half of the larvae interacted with reef residents including predatory attacks and aggressive approaches by residents and aggressive approaches by settling larvae. Settlement behaviour of L. carponotatus was more similar to a serranid than to pomacentrids. Settlement-stage larvae of L. carponotatus are behaviourally capable, and have a complex settlement behaviour.     

Coral reef ecosystems as a source of atmospheric CO2: evidence from PCO2 measurements of surface waters

 Precise measurements of the partial pressure of CO₂ (PCO₂) in oceanic and lagoonal surface waters from Palau Barrier Reef and Majuro Atoll were taken to ascertain the net effect of these coral reef systems on air-sea CO₂ exchange. PCO₂ in the surface water in the lagoon of Palau Barrier Reef and Majuro Atoll was 48 and 14 μatm higher than in the surrounding oceanic water, respectively. A total carbon dioxide vs. total alkalinity diagram indicates that calcification is mainly responsible for CO₂ evasion in these coral reef systems. Accepted: 31 March 1997     

Bioerosion caused by foraging of the tropical chiton Acanthopleura gemmata at One Tree Reef, southern Great Barrier Reef

The bioerosive potential of the intertidal chiton Acanthopleura gemmata on One Tree Reef was determined by quantification of CaCO₃ in daily faecal pellet production of individuals transplanted into mesocosms after nocturnal-feeding forays. Mean bioerosive potential was estimated at 0.16 kg CaCO₃ chiton⁻¹ yr⁻¹. Bioerosion rates were estimated for populations on two distinct chiton habitats, reef margin (0.013 kg CaCO₃ m⁻² yr⁻¹) and beachrock platform (0.25 kg CaCO₃ m⁻² yr⁻¹). Chiton density on the platform was orders of magnitude greater than on the reef margin. The surface-lowering rate (0.16 mm m⁻² yr) due to bioerosion by the beachrock population is a substantial contribution to the total surface-lowering rate of 2 mm m⁻² yr⁻¹ previously reported for One Tree Reef across all erosive agents. At high densities, the contribution of A. gemmata to coral reef bioerosion budgets may be comparable to other important bioeroders such as echinoids and fish.     

Calcium carbonate budgets for two coral reefs affected by different terrestrial runoff regimes, Rio Bueno, Jamaica

A process-based carbonate budget was used to compare carbonate framework production at two reef sites subject to varying degrees of fluvial influence in Rio Bueno, Jamaica. The turbid, central embayment was subjected to high rates of fluvial sediment input, framework accretion was restricted to ≤30 m, and net carbonate production was 1,887 g CaCO₃ m⁻² year⁻¹. Gross carbonate production (GCP) was dominated by scleractinians (97%), particularly by sediment-resistant species, e.g. Diploria strigosa on the reef flat (<2 m). Calcareous encrusters contributed very little carbonate. Total bioerosion removed 265 g CaCO₃ m⁻² year⁻¹ and was dominated by microborers. At the clear-water site, net carbonate production was 1,236 g CaCO₃ m⁻² year⁻¹; the most productive zone was on the fore-reef (10 m). Corals accounted for 82% of GCP, and encrusting organisms 16%. Bioerosion removed 126 g CaCO₃ m⁻² year⁻¹ and was dominated by macroborers. Total fish and urchin grazing was limited throughout (≤20 g CaCO₃ m⁻² year⁻¹). The study demonstrates that: (1) carbonate production and net reef accretion can occur where environmental conditions approach or exceed perceived threshold levels for coral survival; and (2) although live coral cover (and carbonate production rates) were reduced on reef-front sites along the North Jamaican coast, low population densities of grazing fish and echinoids to some extent offset this, thus maintaining positive carbonate budgets.     

Coral-reef hydrology: field studies of water movement within a barrier reef

Water movement through the framework of Davies Reef, a coral reef in the central Australian Great Barrier Reef, was studied using field and laboratory determinations of permeability, tide gauge measurements of water levels, dye tracers, and pore water chemistry. Flow is driven by current, wind-induced, or tide-induced water level differences which were shown to occur between reef front and lagoon. The reef is hydraulically very heterogeneous with bulk flow occurring through high permeability zones (voids and rubble) at a velocity on the order of 10 m/d. Pore water exchange in less permeable zones occurs at a much slower rate. Vertical components of flow are significant. Chemical data indicate that carbonate precipitation and solution occur so that porosities, permeabilities, and flow paths may change with time. Implications for nutrient transfer through the benthic sediments and for fresh water resources on reef islands are discussed.     

Carbonate production of an emergent reef platform, Warraber Island, Torres Strait, Australia

Complex relationships exist between tropical reef ecology, carbonate (CaCO₃) production and carbonate sinks. This paper investigated census-based techniques for determining the distribution and carbonate production of reef organisms on an emergent platform in central Torres Strait, Australia, and compared the contemporary budget with geological findings to infer shifts in reef productivity over the late Holocene. Results indicate that contemporary carbonate production varies by several orders of magnitude between and within the different reef-flat sub-environments depending on cover type and extent. Average estimated reef-flat production was 1.66 ± 1.78 kg m⁻² year⁻¹ (mean ± SD) although only 23% of the area was covered by carbonate producers. Collectively, these organisms produce 17,399 ± 18,618 t CaCO₃ year⁻¹, with production dominated by coral (73%) and subordinate contributions by encrusting coralline algae (18%) articulated coralline algae, molluscs, foraminifera and Halimeda (<4%). Comparisons between the production of these organisms across the different reef-flat zones, surface sediment composition and accumulation rates calculated from cores indicate that it is necessary to understand the spatial distribution, density and production of each major organism when considering the types and amounts of carbonate available for storage in the various reef carbonate sinks. These findings raise questions as to the reliability of using modal production rates in global models independent of ecosystem investigation, in particular, indicating that current models may overestimate reef productivity in emergent settings. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Phytoplankton biomass and primary production in semi-enclosed reef lagoons of the central Great Barrier Reef,Australia

Phytoplankton biomass and primary production rates within semi-enclosed reef lagoons of the central Great Barrier Reef were compared with adjacent shelf waters. Chlorophyll concentrations and surface primary production rates were usually higher in lagoons although seasonal differences were only significant during the summer. Nitrate concentrations were higher in lagoons than in shelf waters year-round. Nano- (<20 m size fraction) or pico-phytoplankton (<2 m size fraction) dominated phytoplankton biomass and production within reef lagoons throughout the year. Net phytoplankton (>10–20 m size fraction), however, were relatively more important in both reef lagoons and open shelf waters during the summer. Biomass-specific production within lagoons (range 41–90 mg C mg chl^–1 day^–1) was high, regardless of season. Lagoonal phytoplankton production (range 0.2–1.6 g C m^–2 day^–1) was directly correlated with standing crop and inversely related to lagoon flushing rates. Phytoplankton blooms develop within GBR reef lagoons during intermittent calm periods when water residence times exceed phytoplankton generation times.     

Net uptake of dissolved free amino acids by the giant clam, Tridacna maxima: alternative sources of energy and nitrogen?

 The role of dissolved free amino acids (DFAA) in nitrogen and energy budgets was investigated for the giant clam, Tridacna maxima, growing under field conditions at One Tree Island, at the southern end of the Great Barrier Reef, Australia. Giant clams (121.5–143.7 mm in shell length) took up neutral, acidic and basic amino acids. The rates of net uptake of DFAA did not differ between light and dark, nor for clams growing under normal or slightly enriched ammonium concentrations. Calculations based on the net uptake concentrations typical of the maximum concentrations of DFAA found in coral reef waters (∼0.1 μM ) revealed that DFAA could only contribute 0.1% and 1% of the energy and nitrogen demands of giant clams, respectively. These results suggest that DFAA does not supply significant amounts of energy or nitrogen for giant clams or their symbionts. Accepted: 7 October 1998     

Coastal climate of northwest Australia and comparisons with the Great Barrier Reef: 1960 to 1992

 Significant coral reef ecosystems occur along the northwest (NW) coast of Australia in an oceanographic setting somewhat similar to that of the Great Barrier Reef off the northeast (NE) Australian coast. Seasonal and inter-annual variations of several surface climate variables are described for the NW coastal region of Australia from 10^°–30^°S over the period 1960 to 1992. Average climatic conditions in this region are compared with those for similar latitudes on the Great Barrier Reef. On average, sea surface temperatures (SSTs) along the NW Australian coast are warmer than at similar latitudes along the NE coast north of ∼20^°S and cooler than the NE coast at higher latitudes. The annual range of SSTs along the NW coast is lower than found along the NE coast. There is also lower average cloud amount (and greater incoming solar radiation) along the NW coast compared with the NE coast. Corals reefs off the NW Australian coast are less likely to be influenced by freshwater and associated terrestrial impacts than nearshore reefs of the GBR. Although the latitudinal distribution of tropical cyclone activity is similar along the NW and NE Australian coasts, the total number of tropical cyclones and tropical cyclone days is substantially higher on the NW coast compared with the NE coast. Accepted: 22 June 1998     

Shifting base-lines, declining coral cover, and the erosion of reef resilience: comment on Sweatman et al. (2011)

Formal monitoring of the Great Barrier Reef was initiated in 1986 in response to the clear scientific evidence (and growing public concern) over the loss of corals caused by two protracted outbreaks of crown-of thorns starfish, which began in 1962 and 1979. Using monitoring data from manta tows along and across the Great Barrier Reef, Sweatman et al. (Coral Reefs 30:521–531, 2011) show that coral cover after these outbreaks declined further from 28 to 22% between 1986 and 2004. Pointing to the current levels of protection of the Great Barrier Reef, they state that earlier estimates of losses of coral cover since the early 1960s have been exaggerated. However, the loss of close to one-quarter of the coral cover over the past two decades represents an average loss of 0.34% cover per year across the whole GBR after 1986, which is very similar to previously reported rates of annual loss measured over a longer timeframe. The heaviest recent losses have occurred on inshore and mid-shelf reefs, which Sweatman et al. (Coral Reefs 30:521–531, 2011) attribute to a natural cycle of disturbance and recovery. But there has been very limited recovery. While coral cover has increased for short periods on some individual reefs, it has declined sharply on many more to produce the observed system-wide trend of declining cover. Close to 40% of coral cover on inner reefs has been lost since 1986. Of particular significance is the new evidence that coral cover has remained unchanged or declined further from a low 1986 baseline in 28 out of 29 sub-regions of the Great Barrier Reef, indicating a gradual erosion of resilience that is impeding the capacity of this huge reef system to return towards its earlier condition. This result, and other clear evidence of widespread incremental degradation from overfishing, pollution, and climate change, calls for action rather than complacency or denial.