What causes the Red Sea to appear red?

The Red Sea appears red primarily due to the presence of red phytoplankton, specifically a type called Trichodesmium. During certain times of the year, these phytoplankton can bloom in large numbers, imparting a reddish hue to the water. The phenomenon is often enhanced by sunlight reflecting off the blooms, creating a visual effect that gives the sea its name.

In contrast, the other options do not provide valid explanations for the color of the Red Sea. While krill and shrimp might have some impact on the coloration in marine environments, they do not lead to a noticeable red appearance in significant concentrations. Copper deposits and other minerals at the sea's bottom do not affect surface coloration in the same way phytoplankton do. Lastly, stating that the Red Sea does not appear red is inaccurate; it indeed has instances of reddish coloration due to these biological blooms. Therefore, the presence of red phytoplankton is the key reason behind the Red Sea’s name and its occasional red appearance.