Final answer:
Coral bleaching occurs when corals expel their symbiotic algae due to stressors like rising ocean temperatures and increased CO₂, leading to decreased biodiversity and problems for species dependent on reefs.
Step-by-step explanation:
Coral reefs become "bleached" due to various environmental stressors, particularly rising ocean temperatures and increasing levels of carbon dioxide (CO₂). The main stressor leading to bleaching is an increase in water temperature, which causes corals to expel their symbiotic algae called zooxanthellae. This loss of algae leads to a loss of color in the corals and if prolonged, the death of coral animals.
Additionally, ocean water temperature change and increased CO₂ contribute to ocean acidification, hindering the corals' ability to calcify and build their characteristic calcium carbonate skeletons.
The effects of global climate change and increasing CO₂ levels significantly impact the local ecosystem. As the coral loses its algae and begins to die, biodiversity and productivity decrease dramatically, leading to a reduction in species diversity and a decline in the populations of animals that rely on coral reefs for food and shelter.