Who has never wondered why the sea in the Northeast is turquoise blue, while the ocean in the South appears grayer? Or why are some Amazon rivers so dark they resemble coffee, while others are the color of clay or approaching a translucent green?
The differences are so visible that, seen from above, they create mosaics on the map of Brazil. But what few know is that these color variations tell a story about life and the chemistry of environments.
The tone of a lake, river or sea is a direct result of the interaction between the substances present in the water, whether natural, such as algae and sediments, or the result of human action. Each particle, each molecule and each organism absorbs and reflects light differently and it is this combination of factors that transforms the color of the water in different locations.
This explanation is part of a detailed study conducted by the National Institute for Space Research (INPE). In the article “Remote sensing of water color”, scientists show how observing color can reveal chemical, physical and biological properties of aquatic ecosystems.
The physics behind water colors
According to the study, sunlight, which appears white to human eyes, is made up of different wavelengths, from violet to red. When it penetrates the water, part of this energy is absorbed and another part is scattered.
Pure water, alone, absorbs more light in the red and infrared bands, and scatters more blue. This is why deep, clean dark blues appear: there are almost no particles or organisms interfering with the path of light.
But a small deviation in the composition of the water is enough for the tone to change completely. The presence of phytoplankton (), for example, changes the optical balance and makes the environment appear greenish.
Inorganic sediments, such as sand, clay and minerals, increase the scattering of light, producing brownish or cloudy waters. And when there is a lot of dissolved organic matter, coming from leaves or plant debris, the result can be a yellowish or even black tone, like that of Amazon rivers.
Three components, infinite combinations
According to the INPE study, there are three main groups of substances that determine the natural color of water:
- Phytoplankton: microscopic organisms that carry out photosynthesis and contain pigments such as chlorophyll, responsible for shades of green.
- Suspended inorganic material: mineral particles that reflect and scatter light, leaving the water cloudy or muddy.
- Yellow substances: dissolved organic matter, which gives the water yellowish, reddish or dark tones.
The combination and concentration of these elements vary depending on the environment. In the open oceans, for example, water is dominated by optical purity and low particle content. In coastal regions, rivers and lakes, the meeting between organic matter, sediments and nutrients creates color palettes.
The example of the Amazon
In , the contrast between white, black and clear water is an example of how color reflects complex biogeochemical processes.
The white waters, typical of the Solimões River, are rich in mineral sediments and have muddy tones. Black waters, such as those of the Rio Negro, have low turbidity, but a high concentration of dissolved organic matter, which darkens the tone and acidifies the environment. Clear waters, with a greenish or transparent color, indicate a low sediment content, but variations in pH and nutrients.
These differences influence biodiversity, productivity and even the type of vegetation that grows on riverbanks. And, through orbital images, it is possible to monitor how these flows change throughout the year, especially during periods of high and low water.
By analyzing the behavior of light over rivers, lakes and oceans, science can identify the presence of sediments, algae and organic matter, and even monitor signs of pollution and climate change. Observing the color of the water is a way of monitoring the health of ecosystems. In times of accelerated environmental transformation, understanding what these colors reveal can be decisive in protecting water resources and planning more effective preservation policies.
