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Predicting the impact of present and future human land-use on the Great Barrier Reef

An ecohydrologic model, verified against field data, suggests that land-use has contributed to degradation of the health of the Great Barrier Reef and to an increased frequency and intensity of crown-of-thorns starfish infestations. The model also predicts that the health of the Great Barrier Reef will significantly worsen by the year 2050 as a result of global warming. However, the model also suggests that much-improved land-use practices will enable some regions of the Great Barrier Reef to recover, even with global warming. Finally, the model suggests that, if global warming proceeds unchecked, biological adaptation is necessary to avoid a collapse of the Great Barrier Reef health by the year 2100.\u

Does Trophic Status Enhance or Reduce the Thermal Tolerance of Scleractinian Corals? A Review, Experiment and Conceptual Framework

Global warming, and nutrient and sediment runoff from coastal development, both exert increasing pressures on coastal coral reefs. The objective of this study was to resolve the question of whether coastal eutrophication may protect corals from thermal stress by improving their nutritional status, or rather diminish their thermal tolerance through the synergy of dual stressors. A review of previous studies on the topic of combined trophic status and heat exposure on the thermal tolerance of corals reveals a broad range of outcomes, including synergistic, additive and antagonistic effects. We conducted a 90-day long experiment exposing corals to realistic levels of elevated nutrients and sediments, and heat stress. Colonies of two common scleractinian corals (Acropora millepora and Montipora tuberculosa) were kept in coastal seawater, or coastal seawater that was further organically and nutrient enriched (OE), and/or enriched with nitrate. Batches of OE were created daily, facilitating nutrient uptake, plankton succession and organic enrichment as observed in coastal waters. After 10 days of acclimation, 67% of the colonies had their temperature gradually increased from 27° to 31.2°C. After 3–7 weeks of heat stress, colonies of both species had significantly greater reductions in fluorescence yields and lower survival in OE than without addition of OE. Furthermore, photophysiological recovery was incomplete 31–38 days after ending the heat stress only in the OE treatments. Nitrate alone had no measurable effect on survival, bleaching and recovery in either species. Skeletal growth rates were reduced by 45% in heat-stressed A. millepora and by 24% in OE-exposed M. tuberculosa. We propose a conceptual trophic framework that resolves some of the apparently contradictory outcomes revealed by the review. Our study shows that management actions to reduce coastal eutrophication can improve the resistance and resilience of vulnerable coastal coral reefs to warming temperatures

Spatial distribution of deepwater seagrass in the inter-reef lagoon of the Great Barrier Reef World Heritage Area

Seagrasses in waters deeper than 15 m in the Great Barrier Reef World Heritage Area (adjacent to the Queensland coast) were surveyed using a camera and dredge (towed for a period of 4 to 6 min); 1426 sites were surveyed, spanning from 10 to 25°S, and from inshore to the edge of the reef (out to 120 nautical miles from the coast). At each site seagrass presence, species, and biomass were recorded; together with depth, sediment, secchi, algae presence, epibenthos, and proximity to reefs. Seagrasses in the study area extend down to water depths of 61 m, and are difficult to map other than by generating distributions from point source data. Statistical modeling of the seagrass distribution suggests 40000 km2 of the sea bottom has a probability of some seagrass being present. There is strong spatial variation driven in part by the constraint of the Great Barrier Reef’s long, thin shape, and by physical processes associated with the land and ocean. All seagrass species found were from the genus Halophila. Probability distributions were mapped for the 4 most common species: Halophila ovalis, H. spinulosa, H. decipiens, and H. tricostata. Distributions of H. ovalis and H. spinulosa show strong depth and sediment effects, whereas H. decipiens and H. tricostata are only weakly correlated with environmental variables, but show strong spatial patterns. Distributions of all species are correlated most closely with water depth, the proportion of medium-sized sediment, and visibility measured by Secchi depth. These 3 simple characteristics of the environment correctly predict the presence of seagrass 74% of the time. The results are discussed in terms of environmental dynamics, management of the Great Barrier Reef province, and the potential for using surrogates to predict the presence of seagrass habitats

Effects of organic enrichment and nitrate on the survival of heat-stressed and control corals.

<p>Survival of <i>Acropora millepora</i> (a, b) and <i>Montipora tuberculosa</i> (c, d) at the end of the recovery period for the treatments of temperature (25°C (a, c) vs. 31°C (b, d), organic enrichment (+OE), and/or nitrate addition (+NO3; +OE +NO3). The horizontal bars indicate mean percent survival for each treatment; circles mark the percent of surviving colonies for each tank (four colonies per species and tank, four tanks per treatment; points are jittered for clarity).</p

Mean fluorescence yields in <i>Montipora tuberculosa</i> at the beginning of the experiment (Day 3), at the end of the heat stress (Day 59), and after recovery (Day 90<i>;</i>Fig. 3).

<p>Values are mean yields and SE across four tanks (four colonies per tank) for each of the four heat stressed treatments (31°C). The last three rows show the differences in mean fluorescence yields between days, with significant differences (>2 SE) marked in bold.</p

Relationship between nutrient availability and thermal tolerance in corals.

<p>The conceptual diagram illustrates the commonly non-linear and divergent relationships between thermal tolerance of heat-stressed coral species and the trophic status of their environment and/or food provision in experiments. See text for definitions and explanations.</p

Effects of organic enrichment and nitrate on the fluorescent yields of heat-stressed and control <i>A. millepora</i>.

<p>Time series of the fluorescence yields in <i>Acropora millepora.</i> Two treatments (Controls and +OE +NO3) remained at ambient temperature (mean: 25°C) throughout the experimental period (a, b). Tanks in the other treatments were exposed to heat stress (31.2°C) between Days 10 and 33, followed by a recovery period at ambient temperature (c-f). The nutrient treatments applied to these tanks were (c) Controls, (d) +NO3, (e) +OE and (f) +OE +NO3. Points represent means across colonies for each tank; solid lines are estimated temporal trends and dashed lines are 95% confidence intervals.</p

Effects of organic enrichment and nitrate on the fluorescent yields of heat-stressed and control <i>M. tuberculosa</i>.

<p>Time series of the fluorescence yields in <i>Montipora tuberculosa.</i> For details see the legend for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0054399#pone-0054399-g002" target="_blank">Fig. 2</a>. In this species, heat stress (31.2°C) was applied between Days 10 and 59.</p