Brown beach water can be concerning, often leading to questions about its cause. While a brown hue might suggest pollution, it is frequently the result of natural processes. Several elements contribute to water discoloration, including suspended particles, microscopic life, and dissolved compounds.
Sediment and Coastal Runoff
Suspended particles, primarily sediment, are a common reason for brown beach water. Rivers carry soil, silt, and clay from inland areas to the coast. After heavy rainfall, increased water flow enhances a river’s ability to transport more sediment into the ocean. Rivers deliver hundreds of billions of tons of sediment to oceans globally each year.
Coastal erosion also contributes to this phenomenon. Waves and currents wear away shorelines, stirring up sand and fine sediments from the seabed. This process is active in shallow areas where the bottom is easily disturbed. Storm systems can drive coastal erosion, further increasing suspended material.
Stormwater runoff from land is another factor. Rain washes soil, organic debris, and pollutants from urban and agricultural areas into coastal waters. Strong winds, waves, and currents create turbulence, which can resuspend sediments settled on the seafloor.
Microscopic Organisms
Microscopic organisms are another natural cause of water discoloration. Phytoplankton, tiny plant-like organisms, multiply rapidly under favorable conditions, forming dense concentrations known as blooms. These blooms can tint the water various shades, including brown, reddish-brown, or greenish-brown, depending on the dominant algae species. Diatoms, for instance, are a type of golden-brown algae that contribute to this color.
Algae contain distinct pigments that give them their characteristic colors. While some blooms appear green, others, like certain dinoflagellates, produce reddish or deep yellow water, which might also appear brownish. Brown tides, caused by specific microalgae such as Aureococcus anophagefferens, turn waterways a distinct brown color.
Nutrient runoff from land, often from agricultural activities, wastewater discharge, and urban areas, fuels phytoplankton blooms. Excess nitrogen and phosphorus act as fertilizers, accelerating algal growth. Some dense algal blooms are termed “harmful algal blooms” (HABs) or “red tides.” These can appear in various colors, including brown, orange, or yellow. While natural, their frequency and intensity can increase with nutrient pollution.
Dissolved Organic Matter
Dissolved organic compounds can impart a brown hue to beach water, creating a “tea-colored” appearance. These compounds include tannins and humic substances, natural byproducts of decaying vegetation. When leaves, wood, peat, and other plant materials decompose, they release organic acids into the surrounding water.
Rivers and estuaries flowing through forested areas, wetlands, or marshlands are common sources of these compounds. As water moves through these environments, it leaches tannins and humic substances, carrying them into coastal waters. This process is similar to how tea leaves steep, releasing their color into the liquid.
The water may appear clear but still have a distinct brown tint. Unlike suspended sediments that settle out, dissolved organic matter remains dispersed throughout the water column. This discoloration is often stable and does not indicate turbidity or cloudiness. It is a common phenomenon in aquatic systems connected to terrestrial organic matter sources.