Final answer:
The decline of a coral reef due to algae growth smothering the reef could be caused by rising sea temperatures, changing pH levels, and human activities.
Step-by-step explanation:
To summarize, the decline of a coral reef due to algae growth smothering the reef could be caused by a combination of factors. Rising sea temperatures from climate change can cause coral bleaching, where the coral expels its symbiotic algae, leading to a decline in the reef's health. Additionally, rising CO₂ levels can change the pH of the water, interfering with calcification and impacting the reef's ability to grow. Human activities such as runoff of sediment and agricultural chemicals, as well as overfishing, can also contribute to the decline of coral reefs.