The Red Sea, known for its vibrant marine life and clear waters, bears a name that often piques curiosity. Despite its evocative designation, the sea typically presents shades of intense blue or turquoise. However, on occasion, its waters can indeed take on a distinct reddish hue, leading many to wonder about the origins of its famous name.
The Primary Biological Culprit
The Red Sea’s occasional red coloration comes from a marine cyanobacterium, Trichodesmium erythraeum. These microscopic organisms are often called blue-green algae and are colloquially known as “sea sawdust” or “red tide” during blooms. When Trichodesmium erythraeum multiplies rapidly, forming extensive blooms, they become visible.
The reddish-brown pigment, phycoerythrin, is present within Trichodesmium erythraeum cells. As these cyanobacteria grow and die, they release this pigment into the water. This transforms the sea’s surface into a reddish-brown expanse. The species name erythraeum is derived from Greek and Latin roots meaning “red,” directly linking the organism to the sea’s name.
Other Factors Influencing Perception
Beyond cyanobacteria, other factors can contribute to reddish tones in the Red Sea. Red desert dust from surrounding landmasses can cast a reddish tint onto the water. Atmospheric conditions associated with dust can alter how light interacts with the water.
The angle of the sun, particularly during dawn or dusk, also plays a role in how the sea’s color is perceived. At these times, sunlight travels through more of the Earth’s atmosphere. This increased atmospheric path causes shorter wavelength blue light to scatter more, allowing longer wavelength red and orange light to dominate and reach the observer’s eye, making the sky and reflected water appear redder. The Red Sea is also known for its exceptional clarity and high salinity, which typically give it a clear blue or turquoise appearance. However, changes in optically active components in the water, whether biological or particulate, can subtly influence its perceived coloration.
When the Red Sea Appears Red
The Red Sea does not maintain a perpetual red color; its reddish appearance is an intermittent phenomenon. These noticeable reddish episodes primarily occur during specific times of the year when environmental conditions become particularly favorable for the rapid growth of Trichodesmium erythraeum. Such conditions often include periods of warm water, with surface temperatures frequently exceeding 27°C, alongside sufficient nutrient availability, such as iron and phosphorus.
While some studies indicate blooms can occur in winter, the abrupt appearance and disappearance of these cyanobacterial blooms are characteristic. A bloom can be extensive, sometimes spanning several kilometers, and may last for weeks or even months before receding. Outside of these bloom events, the Red Sea reverts to its more typical and clear blue-green appearance.