Selective deep water coral bleaching occurs through depth isolation.

Area of Science:

• Marine Biology
• Climate Change Ecology
• Coral Reef Science

Background:

• Coral reefs globally face degradation from climate change, with increasing frequency of mass bleaching events.
• Mesophotic coral ecosystems (30-150 m depth) represent a substantial portion of global coral reef area, with potential refuge capacity understudied.
• The role of mesophotic ecosystems as climate change refuges remains unresolved.

Purpose of the Study:

• To investigate coral bleaching events in mesophotic coral ecosystems in the northern Red Sea.
• To assess the impact of rising temperatures on mesophotic coral populations and community structure.
• To evaluate the potential of mesophotic ecosystems to serve as climate change refuges.

Main Methods:

• Monitoring of mesophotic coral ecosystems over an eight-year period in the northern Red Sea.
• Documentation of three distinct mesophotic-specific coral bleaching events.
• Analysis of temperature changes at mesophotic depths and their correlation with coral population dynamics and community structure shifts.

Main Results:

• Three mesophotic-specific coral bleaching events were recorded over eight years in the northern Red Sea.
• Faster temperature increases at mesophotic depths led to approximately 50% decline in coral populations.
• Community structure shifted from hard coral dominance to turf algae dominance following recurrent bleaching events, while shallow reefs remained intact.

Conclusions:

• The northern Red Sea mesophotic ecosystem experienced significant coral decline and community shifts due to recurrent bleaching events.
• While not refuting the Red Sea's potential as a thermal refuge for shallow reefs, these findings question the universal capacity of mesophotic ecosystems to act as climate change refuges.