Ocean water in coastal areas sometimes takes on a distinct brown coloration. This discoloration is a result of light scattering and absorption caused by materials suspended or dissolved in the water column. The specific cause of the brown hue can be categorized into physical, biological, and chemical factors. While the public often connects discolored water with pollution, the browning is usually caused by natural processes involving particles, organisms, and organic compounds transported from the land to the sea.
Sediment and Suspended Solids
The most common cause of brown ocean water near shorelines is the influx of terrestrial materials, a physical phenomenon driven by water movement. This brown color is a direct result of high turbidity, which refers to the cloudiness caused by tiny, suspended particles. These suspended solids primarily consist of silt, clay, and fine sand particles that originate from soil erosion on land.
Heavy rainfall and subsequent runoff carry these fine-grained sediments into rivers, which then discharge large plumes of turbid, brown water into the ocean near estuaries and river mouths. Wave action and strong currents can also contribute by re-suspending bottom sediments, stirring up material from the seafloor. These particles scatter light, reducing its penetration and giving the water a muddy appearance. Because freshwater is less dense than saltwater, these sediment-laden plumes often form a distinct layer that floats on the surface before eventually mixing or dispersing.
Biological Causes: Brown Algal Blooms
Living organisms are another significant contributor to brown ocean water, particularly during periods of rapid population growth known as algal blooms. The brown color comes from photosynthetic pigments within microscopic marine plants called phytoplankton. Diatoms, a type of single-celled algae, are often responsible for these blooms and possess a golden-brown carotenoid pigment called fucoxanthin.
Fucoxanthin is an accessory pigment that helps the diatoms capture light energy for photosynthesis, and when these organisms multiply rapidly, their collective color dominates the water. These blooms occur naturally, but their size and frequency can be exacerbated by excessive nutrient runoff, such as nitrogen and phosphorus from agricultural or urban sources. This leads to dense patches of brown water that can stretch for miles along the coast.
The Influence of Dissolved Organic Material
Colored dissolved organic matter (CDOM) can give the water a “tea-stained” look. This dissolved material is composed of complex organic molecules, such as humic acids and tannins, which are leached from decaying vegetation. When rain passes through heavily forested areas, swamps, and wetlands, it picks up these yellowish-brown compounds before flowing into rivers and subsequently the sea.
This organic matter is chemically dissolved in the water, meaning it will not settle out and cannot be removed by simple filtration. The concentration of these humic substances is high in regions with large freshwater inputs from forests, creating a distinct boundary where the dark-colored river water mixes with the bluer ocean water. These organic compounds strongly absorb blue light, causing the water to appear brown or yellow to the observer.
Implications of Brown Water for Safety and Ecology
The implications of brown water depend entirely on the source of the coloration. Brown water caused by suspended sediment is generally harmless to human health, but it can negatively impact marine ecosystems. The increased turbidity drastically reduces the amount of sunlight penetrating the water, which hinders the growth of submerged plants like seagrass and kelp. Furthermore, as sediment settles, it can smother bottom-dwelling organisms and habitat structures.
Brown water from dissolved organic material (CDOM) is typically safe for humans, though it can affect the way trace metals interact in the water. However, the greatest concern for human health arises from brown water caused by biological blooms. While many diatom blooms are harmless, some species of phytoplankton can produce potent toxins, leading to a harmful algal bloom (HAB) that warrants public caution.
All forms of intense browning can also lead to a decrease in dissolved oxygen (DO) levels. This occurs when a bloom or large influx of organic material dies off and is consumed by bacteria. This process of decomposition consumes oxygen, potentially creating hypoxic conditions that can suffocate fish and other mobile marine life.