Abstract
Corals worldwide are facing population declines due to global climate change and local anthropogenic impacts. Global climate change effects are hard to tackle but recent studies show that some coral species can better handle climate change stress when provided with additional energy resources. The local stressor that most undermines energy acquisition is sedimentation because it impedes coral heterotrophic feeding and their ability to photosynthesize. To investigate if reducing local sedimentation will enable corals to better endure ocean warming, we quantitatively assessed the combined effects of increased temperature and sedimentation (concentration and turbidity) on the survival of coral recruits of the species, Porites astreoides. We used sediment from a reef and a boat basin to mimic natural sediment (coarse) and anthropogenic (fine) sediment (common in dredging), respectively. Natural sediment did not negatively impact coral survival, but anthropogenic sediment did. We found that the capacity of coral recruits to survive under warmer temperatures is less compromised when anthropogenic sedimentation is maintained at the lowest level (30 mg.cm−2). Our study suggests that a reduction of US-EPA allowable turbidity from 29 Nephelometric Turbidity Units (NTU) above background to less than 7 NTU near coral reefs would facilitate coral recruit survival under current and higher temperatures.
Highlights
Coral reefs are one of the most important and biologically diverse ecosystems on Earth
Temperature and anthropogenic sedimentation additively affected the survival of P. astreoides recruits. Increasing both temperature and anthropogenic sedimentation led to higher mortality than increasing just one of the stressors, e.g., recruits reared at higher temperatures and anthropogenic sedimentation (30 °C and 60 mg. cm−2) displayed significantly higher mortality than recruits reared at lower temperature and similar anthropogenic sedimentation (26 °C and 60 mg.cm−2) or recruits reared at similar temperature and lower anthropogenic sedimentation (30 °C and 30 mg.cm−2) (Fig. 2)
The survival of Porites astreoides recruits under warm conditions can be ameliorated if anthropogenic sedimentation and turbidity are kept at low levels
Summary
Coral reefs are one of the most important and biologically diverse ecosystems on Earth. Corals face multiple natural stressors, but the worldwide population declines registered in the past decades are mostly due to anthropogenic impacts[3,4,5,6,7,8]. Severe storms physically destroy reefs, corals evolved in the storm-prone tropics, and in the absence of anthropogenic stressors, corals should be able to recover from these disturbances through natural processes[9]. Increased seawater temperatures lead to coral bleaching[26,27], i.e. corals lose their endosymbiotic algae, Symbiodinium These symbionts typically provide about 95% of the energy for the coral[7,28]. To maintain or increase current coral cover, anthropogenic stressors (including climate change) would have to be reduced. Studies have shown that some species of coral and fish can buffer deleterious effects of ocean acidification and increased temperatures when additional energy resources (feeding) were provided[31,32]
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