The mesmerizing hues of the sea have captivated human imagination for centuries. From the deep blues of the open ocean to the turquoise of tropical waters, the colour of seawater can vary dramatically.

This phenomenon can be attributed to several factors, including the absorption and scattering of light, the presence of dissolved substances, the depth of the water, and the organisms living within it.

Understanding these factors can give us a deeper appreciation of the ever-changing palette of the world's oceans.

Absorption and Scattering of Light

The primary reason for the colour variations in seawater is the way light interacts with water molecules. When sunlight penetrates the ocean, it is made up of different colors, or wavelengths. Water absorbs red light from the end of the spectrum more efficiently than blue light. As a result, red, orange, and yellow wavelengths are absorbed quickly as they penetrate the water, while blue and green wavelengths penetrate deeper. This selective absorption makes the water appear blue or green to our eyes.

Scattering also plays a crucial role in the colour of seawater. When light hits water molecules and small particles in the water, it is scattered in different directions. This scattering process, known as Rayleigh scattering, is more effective at shorter wavelengths, such as blue. Therefore, the scattered blue light enhances the blue appearance of the water. In areas with less scattering, such as deeper waters, the absorption effect dominates, giving the water a darker blue hue.

The Role of Dissolved Substances

The presence of various dissolved substances in seawater can also influence its colour. For instance, in coastal regions where rivers flow into the ocean, the water often appears brown or green due to the presence of suspended sediments and organic matter. These substances can absorb and scatter light differently than pure water, altering the overall colour. Additionally, the presence of dissolved organic compounds, such as tannins from decaying vegetation, can give the water a yellow or brown tint.

In areas where there is a high concentration of phytoplankton, and microscopic plants that live in the ocean, the water can appear green. This is because phytoplankton contain chlorophyll, a pigment that absorbs red and blue light and reflects green light. During algal blooms, when phytoplankton populations explode, the water can take on a vivid green colour. This phenomenon is particularly common in nutrient-rich waters where upwelling brings nutrients from the deep ocean to the surface, fueling phytoplankton growth.

Depth and Color Perception

The depth of the water also affects its colour. In shallow waters, the seabed reflects sunlight, which can enhance certain colours depending on the composition of the seafloor. For example, sandy bottoms can reflect more yellow and red light, giving the water a turquoise or aquamarine appearance. In contrast, areas with dark, rocky, or muddy bottoms may absorb more light, resulting in darker or murkier water.

In deeper waters, the colour tends to be darker. The greater the depth, the more light is absorbed, and less light is available to be scattered back to the surface. This is why the open ocean often appears a deep blue or even nearly black in deep regions. The water clarity also plays a role--clearer water allows light to penetrate deeper, enhancing the blue colour.

Marine Life and Water Color

Marine life, particularly large populations of certain species, can significantly impact the colour of seawater. For instance, coral reefs often thrive in clear, shallow waters and can contribute to the vibrant turquoise colour often associated with tropical seas. The coral itself, along with the symbiotic algae living within it, can influence light absorption and reflection, creating a dazzling array of colours.

Jellyfish and other gelatinous zooplankton can also affect water colour. Some species of jellyfish are bioluminescent, meaning they can produce light. While this effect is more noticeable at night, large swarms of bioluminescent organisms can give the water a glowing appearance.

Human Activities Impact

Human activities can also change the colour of seawater. Pollution, such as oil spills, can create a sheen on the water's surface, reflecting light in ways that change its colour. Runoff from agriculture can carry nutrients into the ocean, leading to algal blooms that can turn the water green or red. Additionally, construction and dredging can stir up sediments, making the water appear brown or murky.

The colour of seawater is a complex interplay of various natural and human-induced factors. Light absorption and scattering, the presence of dissolved substances, water depth, marine life, and human activities all contribute to the diverse and dynamic colours of the ocean. By understanding these factors, we can better appreciate the beauty and complexity of our planet's oceans and the importance of preserving their delicate balance for future generations.