Ocean water clarity refers to the transparency of the water, indicating how deeply visible light can penetrate and how well objects can be seen underwater. This transparency varies significantly across oceans due to natural influences that determine light penetration and visibility.
Suspended Particles
Non-living particulate matter influences ocean water clarity. These suspended particles, often called suspended solids, reduce transparency by scattering and absorbing light. Water turbidity, or cloudiness, increases with their concentration.
Sources include sediment like sand, silt, and clay from coastal erosion, river runoff, or seafloor disturbances. Decaying organic matter, such as plant and animal fragments or fecal matter, also contributes to suspended material. These particles can form complex flakes that absorb and backscatter light, reducing the solar radiation available for photosynthesis.
Human activities can also increase inorganic pollutants and fine particles, hindering light penetration and making water murky. This can affect marine life relying on light or visibility.
Microscopic Organisms
Microscopic organisms also influence ocean water clarity and color. Phytoplankton, tiny single-celled photosynthetic organisms, are a major component. They contain chlorophyll and require sunlight and inorganic nutrients to grow. When abundant, especially during “blooms,” phytoplankton significantly reduce water clarity and alter water color, often giving it a green, reddish, or brownish tint.
Zooplankton are microscopic animals that float in the water column and feed on phytoplankton. While individually small, their collective presence can contribute to particulate matter, though their impact on clarity is generally less direct than that of phytoplankton.
High concentrations of phytoplankton often signify nutrient-rich waters that can support a thriving ecosystem, even if the water appears less clear. Chlorophyll concentration, a pigment found in phytoplankton, is also used to assess water clarity.
Light Absorption and Scattering
The interaction of light with water and its contents is fundamental to ocean clarity and perceived color. Water molecules absorb different wavelengths of light at varying rates. Longer wavelengths, such as red and orange light, are absorbed rapidly within the upper few meters, typically disappearing by about 10 and 40 meters, respectively. Shorter wavelengths, like blue and green light, penetrate much deeper, which is why clear open ocean water often appears blue.
Light scattering occurs when particles in the water deflect light into new directions. This scattering contributes to water cloudiness by preventing light from penetrating deeply or reflecting cleanly back to an observer. Both suspended non-living particles and microscopic organisms scatter light, reducing the amount of direct light that can pass through the water column.
The absence of a high concentration of these scattering particles allows for greater light penetration and thus clearer water. In very clear ocean water, only about 1% of the surface radiation may remain at a depth of 150 meters, and virtually no sunlight penetrates beyond 1,000 meters. This explains why underwater environments appear predominantly blue, as other colors are absorbed, and blue light is scattered and reflected back.
Environmental Factors
Environmental conditions and geographic features influence ocean clarity. Water depth plays a role, as deeper waters often have less suspended sediment from the bottom due to less disturbance. However, light penetration significantly decreases with depth, with only a small fraction of surface light reaching 100 meters even in clear waters.
Proximity to land and rivers is a major determinant of clarity. Coastal areas and river mouths typically receive more sediment and nutrient runoff, which reduces clarity and can lead to higher concentrations of plankton. For example, estuaries receiving large rivers, like the Río de la Plata or the Mississippi River, can have very low clarity due to high loads of suspended sediment.
Ocean currents and mixing also influence clarity; strong currents can resuspend bottom sediments, increasing turbidity. Conversely, stable, nutrient-poor waters, such as those found in subtropical gyres, often exhibit very clear conditions because they limit plankton growth.
The type of seafloor affects clarity as well; sandy or silty bottoms are easily stirred up by waves and currents, while rocky or coral bottoms contribute less suspended sediment.
Temperature and nutrient availability affect the growth of phytoplankton, which in turn impacts clarity. Warmer, nutrient-rich waters tend to support denser phytoplankton populations, leading to reduced clarity. Conversely, tropical waters, like those in the Caribbean, can be exceptionally clear due to their warmth and relatively low nutrient content, which limits phytoplankton growth.