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Climate Change and Sedimentation Can Severely Affect Corals in the Future by Nina Cupaníková

Coral reefs are very important ecosystems with more than 25% of all marine species being reliant on them at some point in their lives. Furthermore, over 500 million people worldwide depend on coral reefs for food, jobs and coastal defense.

However, coral reefs are threatened by climate change, ocean acidification, ocean warming, mass bleaching events and local water-quality degradation. Local water quality degradation is overlooked when studying coral reef resilience in the future climate, though. Yet almost 40% of the world’s population lives within 100 km of the coast and coastal development is a significant contributing factor in declining water quality in near-shore reef systems and therefore endangers coral reef resilience. That is why when predicting how resilient coral reefs will be in the future, we need to jointly investigate the effects of declining water quality and climate change in order to protect them effectively.

Especially after what the study done by Brunner et al. shows us. When the temperature will continue to rise and will rise to 2˚C compared to the preindustrial times the coral recruits according to this study will be twice as sensitive to sediment deposition than are corals currently. And even corals under current climate conditions have problems with sediment depositions typically found near river runoff or dredging operations. For instance, only half of the 5-week-old corals was able to survive this deposition amount normally found near river runoffs. This emphasizes the need of efficient water-quality guidelines for managing sediment concentrations in order to protect future coral recruits and by that coral reef replenishment and conservation of these precious ecosystems.

But let dive into the study conducted by Brunner et al. step by step. So, first question is why the sediment deposition and coral recruits?

Declining water quality is mostly caused by sediment from human activities. And when investigating the coral reef resilience and how the decline in water quality affects them, we should mainly take a look at coral recruits, since post-settlement survival in coral life cycle is critical for the replenishment of coral reefs. This is due to the fact that coral recruits are particularly sensitive to being smothered by sediments, because of their small size. So, looking ahead and seeing what effects will climate change have together with the decline in water quality, especially on coral recruits, is a key step in coral reef replenishment, protection and conservation. And that is exactly what Brunner et al. did in their study.

To test how the recruits reacted to the deposited sediment, the researchers picked 6 sedimentation intensities that represented the values observed at calm inshore reefs, wet season river runoff and near dredging operations (ranging from 0 to 80 mg.cm-2). To test how corals react to sediment under different climate conditions, they implemented 3 climate scenarios – current, medium and high. In the current climate scenario, the temperatures ranged from 26.2 to 28.7˚C and the partial pressure of CO2 (pCO2) was 410 ± 50 ppm. The medium climate scenario was represented by a temperature of current climate scenario + 0.6˚C and the pCO2 was 680 ppm ± 50. The high climate scenario had a temperature of current climate scenario + 1.2 ˚C and the pCO2 was 940 ± 50 ppm. And given the fact that the temperature of the water around the Great Barrier Reef has already risen by 0.9˚C since the preindustrial times, the temperatures of medium and high climate scenarios roughly corresponded with 1.5˚C and 2˚C targets of the Paris Agreement. Furthermore, diurnal cycle was used – 12 hours day, 12 hours night. To simulate the deposition of sediment, coarse silt sediment was used, due to the fact that it is of the most common particle size. (Table 1)

Two experiments were carried out. In Experiment 1 the recruits of Acropora millepora were exposed to the sediment twice – during week 5 and 10, the response was photographically captured 4 weeks later.- during week 9 and 14. In Experiment 2 the corals were exposed to sediment only during week 10, response was captured again during week 14. The corals’ ability to remove sediment was recorded one hour after the sediment was deposited. (Image 1a)

Regarding the sediment deposition, the discs where the coral recruits grew were enclosed with a transparent PVC tube and then the sediment was deposited into the PVC tubes. After 12 hours, the upper part of the PVC tube was removed in order to allow water to circulate, but a PVC rim was left around the discs just in case so that the sediment would not be totally carried away by the water flow. Three days after the deposition this was stopped by removing the PVC rims and the remaining sediment was then collected. (Image 1b)

And what does the study show us then? And why is it even important?

So, let’s say that the temperature has just met the target of the Paris Agreement and has risen 2˚C compared to preindustrial times. The corals we are observing are near a dredging site, so the sediment concentration is really high. What is the survival rate? Definitely not high, according to the study, under the high climate scenario and greatest amount of sediment deposited the lowest number of recruits survived. According to the data, the survival rate dropped by 85%. In both experiments, sediment deposition loads that were cleared by half of the recruits were mostly recruits grown in current climate conditions. Furthermore, the only recruits that survived the greatest amount of sediment (80 mg.cm-2) were the recruits raised under the current climate scenario and that were exposed to the sediment only once as10-week-olds. Even when we would meet the most adequate target of the Paris Agreement – 1,5˚C the recruits have only a really small chance of surviving the sediment deposition from dredging operations according to the study. Moreover, recruits were more resilient to sediment deposition when exposed to the sediment for the first time at 10 weeks, not 5 weeks, so the older the better.

What this study shows us is that in order to protect corals effectively in the future, we will need to have water-quality guidelines that sufficiently manage sediment deposition. Especially taking into account the life stage in which corals at a particular site are, since as we now know younger coral recruits are more sensitive and the conservation of coral reefs depends on the coral recruit replenishment and survival.

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