The ocean, vast and deep, often conjures images of sparkling blue waters. However, coastal areas and even parts of the open sea can sometimes appear in shades of brown. This phenomenon can be both natural and, at times, a sign of environmental changes. This discoloration is not uniform and results from various factors interacting with sunlight, altering how we perceive the water’s hue.
Sediment and Runoff
One reason for brown ocean water is the presence of suspended sediments and runoff from land. Rivers act as major transporters, carrying eroded soil and rock, including fine particles like silt, clay, and sand, into the ocean. These terrestrial materials are often referred to as lithogenous sediments.
Stormwater runoff also contributes, washing sediment and other particles from urban and agricultural areas into coastal waters. Coastal erosion, driven by wave action and tidal currents, continuously releases sediment from shorelines into the sea. Human activities, such as construction, agriculture, and deforestation, can accelerate erosion and increase the amount of sediment entering marine environments. These suspended particles absorb and scatter light, leading to the water appearing murky or brown.
Microscopic Marine Life
Another cause of brown ocean water is the presence of dense concentrations of microscopic marine organisms, primarily phytoplankton. Phytoplankton are tiny, plant-like organisms that perform photosynthesis, and their abundance can significantly change water color. Blooms, or rapid increases in their populations, often occur when sunlight and nutrients are readily available.
Certain types of phytoplankton contain pigments that give them a brownish, reddish-brown, or even greenish-brown hue. Diatoms, a type of single-celled algae with silica cell walls, are known for their yellowish-brown chloroplasts. Similarly, some dinoflagellates can produce pigments that cause the water to appear red or brown when they bloom in high concentrations, a phenomenon often referred to as “red tide.” While “red tide” is a common term, these blooms can also appear green, purple, or even be colorless. These blooms can reach concentrations of millions of cells per liter, making their collective color visible.
Dissolved Organic Material
Dissolved organic material (DOM), often called “colored dissolved organic matter” (CDOM), “yellow substance,” or “gelbstoff,” also contributes to the brown coloration of ocean water. This material consists of organic molecules primarily leached from decaying plant and animal matter. Terrestrial vegetation, such as peat and leaves carried by rivers from marshes, forests, and wetlands, is a source of CDOM in coastal areas.
CDOM strongly absorbs shorter wavelength light, particularly in the blue and ultraviolet parts of the spectrum. This absorption leaves longer wavelengths, like yellow and brown, to be reflected or transmitted, causing the water to appear yellow or brown. While CDOM concentrations are higher in freshwaters and estuaries, they can extend into coastal waters. These compounds are distinct from suspended particles and living organisms, representing a dissolved stain in the water. High concentrations of CDOM can reduce light penetration, affecting underwater ecosystems.