The color of the sea varies mainly because water, sunlight and what's in it interact in different ways.
Open water and the color blue
The deep, clear ocean looks blue because water absorbs sunlight selectively. Longer wavelengths (reds, oranges) are absorbed quickly, while shorter wavelengths (blues) are scattered and reflected back toward the observer. This scattering effect, combined with the sheer volume of water, makes the surface appear an intense blue.
Coastal water and the color green
In coastal areas, several factors cause the water to turn green:
- The presence of phytoplankton and other microscopic organisms containing chlorophyll, which absorbs red light and reflects green light.
- Suspended sediments, such as silt or sand, can scatter green light more strongly.
- Dissolved organic matter (humic substances) can give light a yellowish tint that, combined with blue scattering, produces green.
When sunlight travels through a thinner layer of water near the coast, it can reach depths where these elements dominate the color spectrum, making the sea appear green.
Other contributing factors
Water depth, the angle of the sun and atmospheric conditions can also alter the shade. On cloudy days or during sunset, colors can turn orange or brown because the lighting changes, while algal blooms can create bright green or brown patches.
In short, the sea looks blue in open water because pure water scatters blue light, while coastal water takes on a greenish tone due to the presence of phytoplankton, sediments and organic matter.
