Water clarity is the qualitative property of how deeply visible light penetrates water. In addition to light penetration, the term water clarity is also often used to describe underwater visibility. Water clarity is one way that humans measure water quality, along with oxygen concentration and the presence or absence of pollutants and algal blooms. Water clarity governs the health of underwater ecosystems because it impacts the amount of light reaching the plants and animals living underwater. For plants, light is needed for photosynthesis. The clarity of the underwater environment determines the depth ranges where aquatic plants can live. Water clarity also impacts how well visual animals like fish can see their prey. Clarity affects the aquatic plants and animals living in all kinds of water bodies, including rivers, ponds, lakes, reservoirs, estuaries, coastal lagoons, and the open ocean.
Water clarity also affects how humans interact with water, from recreation and property values to mapping, defense, and security. Water clarity influences human perceptions of water quality, recreational safety, aesthetic appeal, and overall environmental health. Tourists visiting the Great Barrier Reef were willing to pay to improve the water clarity conditions for recreational satisfaction. Water clarity also influences waterfront property values. In the United States, a 1% improvement in water clarity increased property values by up to 10%. Water clarity is needed to visualize targets underwater, either from above or in water. These applications include mapping and military operations. To map shallow-water features such as oyster reefs and seagrass beds, the water must be clear enough for those features to be visible to a drone, airplane, or satellite. Water clarity is also needed to detect underwater objects such as submarines using visible light.
Water clarity measurements
Water clarity is measured using multiple techniques. These measurements include: Secchi depth, light attenuation, turbidity, beam attenuation, absorption by colored dissolved organic matter, the concentration of chlorophyll-a pigment, and the concentration of total suspended solids. Clear water generally has a deep Secchi depth, low light attenuation (deeper light penetration), low turbidity, low beam attenuation, and low concentrations of dissolved substances, chlorophyll-a, and/or total suspended solids. More turbid water generally has a shallow Secchi depth, high light attenuation (less light penetration to depth), high turbidity, high beam attenuation, and high concentrations of dissolved substances, chlorophyll-a, and/or total suspended solids.
Overall general metrics
Secchi depth
Secchi depth is the depth at which a disk is no longer visible to the human eye. This measurement was created in 1865 and represents one of the oldest oceanographic methods. To measure Secchi depth, a white or black-and-white disk is mounted on a pole or line and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. Secchi depth is most useful as a measure of transparency or underwater visibility.
Light attenuation
The light attenuation coefficient – often shortened to "light attenuation" – describes the decrease in solar irradiance with depth. To calculate this coefficient, light energy is measured at a series of depths from the surface to the depth of 1% illumination. Then, the exponential decline in light is calculated using Beer's Law with the equation:
I z I 0 = e − k z {\displaystyle {I_{z} \over I_{0}}=e^{-kz}}
where k is the light attenuation coefficient, Iz is the intensity of light at depth z, and I0 is the intensity of light at the ocean surface. Which translates to:
k = l n I z I 0 − z {\displaystyle k={ln{I_{z} \over I_{0}} \over -z}}
This measurement can be done for specific colors of light or more broadly for all visible light. The light attenuation coefficient of photosynthetically active radiation (PAR) refers to the decrease in all visible light (400–700 nm) with depth. Light attenuation can be measured as the decrease in downwelling light (Kd) or the decrease in scalar light (Ko) with depth. Light attenuation is most useful as a measure of the total underwater light energy available to plants, such as phytoplankton and submerged aquatic vegetation.
Turbidity
Turbidity is a measure of the cloudiness of water based on light scattering by particles at a 90-degree angle to the detector. A turbidity sensor is placed in water with a light source and a detector at a 90-degree angle to one another. The light source is usually red or near-infrared light (600–900 nm). Turbidity sensors are also called turbidimeters or nephelometers. In more turbid water, more particles are present in the water, and more light scattering by particles is picked up by the detector. Turbidity is most useful for long-term monitoring because these sensors are often low cost and sturdy enough for long deployments underwater.
Beam attenuation
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