Water clarity

In addition to light penetration, the term water clarity is also often used to describe underwater visibility.

The clarity of the underwater environment determines the depth ranges where aquatic plants can live.

[6] [7] [8] [9] 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.

[10] [11] Tourists visiting the Great Barrier Reef were willing to pay to improve the water clarity conditions for recreational satisfaction.

[17] [18] Water clarity is also needed to detect underwater objects such as submarines using visible light.

[23] [24] 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.

Turbidity is a measure of the cloudiness of water based on light scattering by particles at a 90-degree angle to 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 is used as a proxy for particulate organic carbon in oligotrophic waters like the open ocean.

[36] Colored dissolved organic matter (CDOM) absorbs light, making the water appear darker or tea-colored.

If desired, the concentrations of volatile (organic) and fixed (inorganic) suspended solids can be separated out using the loss-on-ignition method by burning the filter in a muffle furnace to burn off organic matter, leaving behind ash including mineral particles and inorganic components of phytoplankton cells, with TSS = volatile suspended solids + fixed suspended solids.

Chlorophyll-a concentration is a proxy for phytoplankton biomass, which is one way to quantify how turbid the water is due to biological primary production.

[39] Chlorophyll-a concentration is most useful for research on primary production, the contribution of phytoplankton to light attenuation, and harmful algal blooms.

Chlorophyll-a concentration is also useful for long-term monitoring because these sensors are often low cost and sturdy enough for long deployments underwater.

The clearest waters occur in oligotrophic ocean regions such as the South Pacific Gyre, tropical coastal waters, glacially-formed lakes with low sediment inputs, and lakes with some kind of natural filtration occurring at the inflow point.

[44] The clearest recorded water on Earth is either Blue Lake, New Zealand or the Weddell Sea near Antarctica, both of which claim Secchi depths of 80 meters (230 to 260 feet).

[43] [40] Very low water clarity can be found where high loads of suspended sediments are transported from land.

The Río de la Plata shows long-term mean TSS concentrations between 20 and 100 grams per cubic meter, higher than most estuaries.

In recent years, as acidity is reduced and watersheds become more forested, many lakes are less clear but also ecologically healthier with higher concentrations of dissolved organic carbon and more natural water chemistry.