How can solar irradiance cause coral bleaching?
Solar irradiance can cause coral bleaching by increasing the water temperature of the surrounding ocean. Elevated temperatures cause the coral to expel their zooxanthellae, which are the microscopic algae that live within their tissue and provide them with energy through photosynthesis. The loss of these algae leaves the coral significantly weakened and vulnerable to disease, starvation, and death, leading to coral bleaching. The increased solar irradiance is often the result of climate change induced by human activity and can be exacerbated by natural factors such as El Niño events.
Solar irradiance refers to the amount of solar radiation reaching the Earth's surface, including the ocean. When it comes to coral bleaching, high levels of solar irradiance can contribute to the process in the following way:
1. Heat stress: Solar irradiance increases the water temperature of the ocean. When the water temperature rises above the optimal range for corals (usually around 29-32 degrees Celsius or 84-90 degrees Fahrenheit), they experience heat stress. This increased temperature puts the corals in a state of physiological stress.
2. Photosynthesis disruption: Corals have a symbiotic relationship with tiny photosynthetic algae called zooxanthellae, which live within their tissues. These algae provide corals with essential nutrients through photosynthesis. High levels of solar irradiance can cause the corals to expel the zooxanthellae or reduce their photosynthetic activity. This can lead to decreased nutrient availability, weakening the corals.
3. Bleaching response: When corals are under stress, they undergo a bleaching response. During this process, corals expel the zooxanthellae living within them, leaving their white calcium carbonate skeleton visible through their transparent tissues. The loss of zooxanthellae leads to a decrease in pigmentation, resulting in a bleached appearance.
4. Reduced energy reserves: Without the zooxanthellae, corals lose a significant source of energy from photosynthesis. This puts them in a weakened state and compromises their ability to grow, reproduce, and resist disease. If the stressors persist, the corals can ultimately die.
It is important to note that solar irradiance alone is generally not the sole cause of coral bleaching. Other stressors, such as pollution, ocean acidification, and increased seawater temperatures due to climate change, often interact with solar irradiance, exacerbating the bleaching process.