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.     

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.     

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.

     

Declining coral calcification in massive Porites in two nearshore regions of the northern Great Barrier Reef

Temporal and spatial variation in the growth parameters skeletal density, linear extension and calcification rate in massive Porites from two nearshore regions of the northern Great Barrier Reef (GBR) were examined over a 16‐year study period. Calcification rates in massive Porites have declined by approximately 21% in two regions on the GBR ～450 km apart. This is a function primarily of a decrease in linear extension (～16%) with a smaller decline in skeletal density (～6%) and contrasts with previous studies on the environmental controls on growth of massive Porites on the GBR. Changes in the growth parameters were linear over time. Averaged across colonies, skeletal density declined over time from 1.32 g cm^?3 (SE = 0.017) in 1988 to 1.25 g cm^?3 (0.013) in 2003, equivalent to 0.36% yr^?1 (0.13). Annual extension declined from 1.52 cm yr^?1 (0.035) to 1.28 cm yr^?1 (0.026), equivalent to 1.02% yr^?1 (0.39). Calcification rates (the product of skeletal density and annual extension) declined from 1.96 g cm^?2 yr^?1 (0.049) to 1.59 g cm^?2 yr^?1 (0.041), equivalent to 1.29% yr^?1 (0.30). Mean annual seawater temperatures had no effect on skeletal density, but a modal effect on annual extension and calcification with maxima at ～26.7 °C. There were minor differences in the growth parameters between regions. A decline in coral calcification of this magnitude with increasing seawater temperatures is unprecedented in recent centuries based on analysis of growth records from long cores of massive Porites. We discuss the decline in calcification within the context of known environmental controls on coral growth. Although our findings are consistent with studies of the synergistic effect of elevated seawater temperatures and pCO₂ on coral calcification, we conclude that further data on seawater chemistry of the GBR are required to better understand the links between environmental change and effects on coral growth.     

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.     

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.     

Large-scale bleaching of corals on the Great Barrier Reef

 The Great Barrier Reef (GBR) experienced its most intensive and extensive coral bleaching event on record in early 1998. Mild bleaching commenced in late January and intensified by late February/early March 1998. Broad-scale aerial surveys conducted of 654 reefs (∼23% of reefs on the GBR) in March and April 1998, showed that 87% of inshore reefs were bleached at least to some extent (>1% of coral cover) compared to 28% of offshore (mid- and outer-shelf) reefs. Of inshore reefs 67% had high levels of bleaching (>10% of coral) and 25% of inshore reefs had extreme levels of bleaching (>60% of coral). Fewer offshore reefs (14%) showed high levels of bleaching while none showed extreme levels of bleaching. Ground-truth surveys of 23 reefs, which experienced bleaching in intensities ranging from none to extreme, showed that the aerial survey data are likely to be underestimates of the true extent and intensity of bleaching on the GBR. The primary cause of this bleaching event is likely to be elevated sea temperature and solar radiation, exacerbated by lowered salinity on inshore and some offshore reefs in the central GBR. Accepted: 30 July 1998     

Light thresholds derived from seagrass loss in the coastal zone of the northern Great Barrier Reef,Australia

There is a world-wide trend for deteriorating water quality and light levels in the coastal zone, and this has been linked to declines in seagrass abundance. Localized management of seagrass meadow health requires that water quality guidelines for meeting seagrass growth requirements are available. Tropical seagrass meadows are diverse and can be highly dynamic and we have used this dynamism to identify light thresholds in multi-specific meadows dominated by Halodule uninervis in the northern Great Barrier Reef, Australia. Seagrass cover was measured at ∼3 month intervals from 2008 to 2011 at three sites: Magnetic Island (MI) Dunk Island (DI) and Green Island (GI). Photosynthetically active radiation was continuously measured within the seagrass canopy, and three light metrics were derived. Complete seagrass loss occurred at MI and DI and at these sites changes in seagrass cover were correlated with the three light metrics. Mean daily irradiance (I_d) above 5 and 8.4 mol m⁻² d⁻¹ was associated with gains in seagrass at MI and DI, however a significant correlation (R = 0.649, p < 0.05) only occurred at MI. The second metric, percent of days below 3 mol m⁻² d⁻¹, correlated the most strongly (MI, R = −0.714, p < 0.01 and DI, R = −0.859, p = <0.001) with change in seagrass cover with 16–18% of days below 3 mol m⁻² d⁻¹ being associated with more than 50% seagrass loss. The third metric, the number of hours of light saturated irradiance (H_sat) was calculated using literature-derived data on saturating irradiance (E_k). H_sat correlated well (R = 0.686, p < 0.01; and DI, R = 0.704, p < 0.05) with change in seagrass abundance, and was very consistent between the two sites as 4 H_sat was associated with increases in seagrass abundance at both sites, and less than 4 H_sat with more than 50% loss. At the third site (GI), small seasonal losses of seagrass quickly recovered during the growth season and the light metrics did not correlate (p > 0.05) with change in percent cover, except for I_d which was always high, but correlated with change in seagrass cover. Although distinct light thresholds were observed, the departure from threshold values was also important. For example, light levels that are well below the thresholds resulted in more severe loss of seagrass than those just below the threshold. Environmental managers aiming to achieve optimal seagrass growth conditions can use these threshold light metrics as guidelines; however, other environmental conditions, including seasonally varying temperature and nutrient availability, will influence seagrass responses above and below these thresholds.     

Cross-shelf variation in browsing intensity on the Great Barrier Reef

Herbivory is widely accepted as a key process determining the structure and resilience of coral reefs, with regional reductions in herbivores often being related to shifts from dominance by coral to leathery macroalgae. The removal of leathery macroalgae may therefore be viewed as a critical process on coral reefs. However, few studies have examined this process beyond a within-reef scale. Here, browsing activity was examined across the entire Great Barrier Reef shelf using bioassays of the leathery macroalga Sargassum to directly quantify algal removal. The assays revealed marked cross-shelf variation in browsing intensity, with the highest rates recorded on mid-shelf reefs (55.2–79.9% day⁻¹) and decreasing significantly on inner- (10.8–17.0% day⁻¹) and outer-shelf (10.1–10.4% day⁻¹) reefs. Surprisingly, the variation in browsing intensity was not directly related to estimates of macroalgal browser biomass; rather, it appears to be shaped primarily by the local environment and behaviour of the component species. Removal rates across the inner- and mid-shelf reefs appear to be related to the attractiveness of the assays relative to the resident algal communities. Controlling for the influence of the resident algal communities revealed a positive relationship between removal rates and the biomass of a single macroalgal browsing species, Naso unicornis. In contrast, the low removal rates on the outer-shelf reefs displayed no relationship to algal or herbivore communities and appeared to reflect a negative behavioural response by the resident fishes to a novel, or unfamiliar, alga. These findings not only highlight the complexities of the relationship between fish presence and ecological function, but also the value of examining ecological processes across broader spatial scales.     

Oxygen detoxification in algal-invertebrate symbioses from the Great Barrier Reef

Summary Examination of 34 species of symbiotic invertebrates in four phyla has confirmed the generality of a direct relationship between chlorophyll concentration and the activities of superoxide dismutase and catalase, two enzymes involved in the detoxification of active oxygen. On a finer scale, activities of these enzymes also depend on the localization of the algal symbions (intracellular or extracellular) and hence on the extent to which photosynthetic O₂ actually contacts animal cytoplasm, and on the solar irradiance experienced by the symbionts. Differences in SOD activity among organs of Tridacna crocea are not fully explained by local O₂ levels but are further related to organ-specific retes of O₂ consumption. This result is discussed in terms of known mechanisms of superoxide radical production in mitochondria and differences in O₂ utilization concentrations among various organs in bivalve molluscs.     

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     

Recruitment Variability of Coral Reef Sessile Communities of the Far North Great Barrier Reef

One of the key components in assessing marine sessile organism demography is determining recruitment patterns to benthic habitats. An analysis of serially deployed recruitment tiles across depth (6 and 12 m), seasons (summer and winter) and space (meters to kilometres) was used to quantify recruitment assemblage structure (abundance and percent cover) of corals, sponges, ascidians, algae and other sessile organisms from the northern sector of the Great Barrier Reef (GBR). Polychaetes were most abundant on recruitment titles, reaching almost 50% of total recruitment, yet covered <5% of each tile. In contrast, mean abundances of sponges, ascidians, algae, and bryozoans combined was generally less than 20% of total recruitment, with percentage cover ranging between 15–30% per tile. Coral recruitment was very low, with <1 recruit per tile identified. A hierarchal analysis of variation over a range of spatial and temporal scales showed significant spatio-temporal variation in recruitment patterns, but the highest variability occurred at the lowest spatial scale examined (1 m—among tiles). Temporal variability in recruitment of both numbers of taxa and percentage cover was also evident across both summer and winter. Recruitment across depth varied for some taxonomic groups like algae, sponges and ascidians, with greatest differences in summer. This study presents some of the first data on benthic recruitment within the northern GBR and provides a greater understanding of population ecology for coral reefs.     

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.     

Measurement and modelling of tropical cyclone waves in the Great Barrier Reef

Recorded wave data from four wave measuring instruments located at various points within a section of the Great Barrier Reef during the passage of a tropical cyclone are presented. A spectral wave prediction model is used as an aid to the interpretation of the data. The tropical cyclone generated significant wave heights of approximately 10 m seaward of the reef complex. The many scattered reefs, however, act to reduce this value to approximately 6 m landward of the reef complex. Individual reefs appear to act as complete barriers to waves at typical oceanic periods, even at high tide. In a scattered reef complex such as the Great Barrier Reff, wave energy can penetrate the inter-reef gaps. Although such inter-reef gaps may appear large in comparison to the sizes of individual reefs, wave attenuation is still significant.     

Cumulative bleaching undermines systemic resilience of the Great Barrier Reef

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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.