Water clarity

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A diver enters crystal clear water in Lake Huron. TBNMS - Russ In The Water (31356290535).jpg
A diver enters crystal clear water in Lake Huron.

Water clarity is a descriptive term for how deeply visible light penetrates through 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. [1]

Contents

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. [2] [3] [4] [5] Water clarity also impacts how well visual animals like fish can see their prey. [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.

The Whaleshark Collection at Daedalus Reef, Red Sea, Egypt diver and a shark approaching (6147232689).jpg
Clear water with high visibility at Daedalus Reef, Egypt.
Diver and seal at Pie Rock DSC09722.JPG
More turbid water with lower visibility near Castle Rocks, South Africa.

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. [10] [11] Tourists visiting the Great Barrier Reef were willing to pay to improve the water clarity conditions for recreational satisfaction. [12] Water clarity also influences waterfront property values. In the United States, a 1% improvement in water clarity increased property values by up to 10%. [13] [14] [15] [16] 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. [17] [18] Water clarity is also needed to detect underwater objects such as submarines using visible light. [19] [20] [21]

Water clarity measurements

Metrics used to measure water clarity. Water clarity metrics figure Turner 2022 Limnology and Oceanography Letters.jpg
Metrics used to measure water clarity.

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. [22]

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. [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. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. [25] [26] Secchi depth is most useful as a measure of transparency or underwater visibility.

Light attenuation

Measuring light attenuation, Kd(PAR), from a boat in the Chesapeake Bay. This is a measure of downwelling light attenuation using a flat sensor. (EPA Science Chesapeake Bay 2) 412-DSP-2-2012-08-29 MDNR SusquehannaFlats 015.jpg - DPLA - 8a697e8c61e6c57300120a90fbfacd4f.jpg
Measuring light attenuation, Kd(PAR), from a boat in the Chesapeake Bay. This is a measure of downwelling light attenuation using a flat sensor.

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:

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. [27] [28] Which translates to:

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. [29] [30] [31] 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

Three glass vials used as turbidity standards for 5, 50, and 500 nephelometric turbidity units (NTU). TurbidityStandards.jpg
Three glass vials used as turbidity standards for 5, 50, and 500 nephelometric turbidity units (NTU).

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. [32] [33] [34] [35]

Beam attenuation

Beam attenuation is measured with a device called a transmissometer that has a light source at one end and a detector at the other end, in one plane. The amount of light transmitted to the detector through the water is the beam transmission, and the amount of light lost is the beam attenuation. Beam attenuation is essentially the opposite of light transmission. Clearer water with a low beam attenuation coefficient will have high light transmission, and more turbid water with a high beam attenuation coefficient will have low light transmission. Beam attenuation is used as a proxy for particulate organic carbon in oligotrophic waters like the open ocean. [36]

Concentration-based metrics

Colored dissolved organic matter (CDOM) absorption

Darker colored water can be seen in the right half of this experiment in a lake, with the giant Secchi disk appearing more brown in color due to higher dissolved organic matter concentrations. Kinwamakwad Secchi Disks.jpg
Darker colored water can be seen in the right half of this experiment in a lake, with the giant Secchi disk appearing more brown in color due to higher dissolved organic matter concentrations.

Colored dissolved organic matter (CDOM) absorbs light, making the water appear darker or tea-colored. Absorption by CDOM is one measure of water clarity. Clarity can still be quite high in terms of visibility with high amounts of CDOM in the water, but the color of the water will be altered to yellow or brown, and the water will appear darker than water with low CDOM concentrations. CDOM absorbs blue light more strongly than other colors, shifting the color of the water toward the yellow and red part of the visible light spectrum as the water gets darker. [37] For example, in lakes with high CDOM concentrations, the bottom of the lake may be clearly visible to the human eye, but a white surface in the same lake water may appear yellow or brown.

Total suspended solids (TSS) concentration

Total suspended solids concentration is measured by weighing a filter before and after filtering water through it to calculate the mass of material left on the filter. August 6, 2011 Chlorophyll filtered from our water samples (6016326962).jpg
Total suspended solids concentration is measured by weighing a filter before and after filtering water through it to calculate the mass of material left on the filter.

Total suspended solids (TSS) concentration is the concentration (dry weight mass per unit volume of water) of all the material in water that is caught on a filter, usually a filter with about a 0.7 micrometer pore size. This includes all the particles suspended in water, such as mineral particles (silt, clay), organic detritus, and phytoplankton cells. Clear water bodies have low TSS concentrations. Other names for TSS include total suspended matter (TSM) and suspended particulate matter (SPM). The term suspended sediment concentration (SSC) refers to the mineral component of TSS but is sometimes used interchangeably with TSS. 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. [38]

Chlorophyll-a concentration

Chlorophyll-a concentration is sometimes used to measure water clarity, especially when suspended sediments and colored dissolved organic matter concentrations are low. 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.

Case studies

High water clarity

Sand Harbor, Lake Tahoe - Nevada State Park, Incline Village, Nevada (112791492).jpg
Clear water in Lake Tahoe, California, United States.
Shoreline, Crater Lake NP, OR 8-13 (15064119167).jpg
Clear water in Crater Lake, Oregon, United States.

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. Blue Lake in New Zealand holds the record for the highest water clarity of any lake, with a Secchi depth of 230 to 260 feet. Blue Lake is fed by an underground passage from a nearby lake, which acts as a natural filter. [40] Some other very clear water bodies are Lake Tahoe between California and Nevada in the United States, [41] Lake Baikal in Russia, [42] and Crater Lake in Oregon in the United States. [43]

Crystal Clear Water, Maldives (4549923261).jpg
Clear water in the Maldives in the Indian Ocean.
Snorkling nanggu.jpg
Clear water for snorkelers near Nanggu Island, Lombok, Indonesia.

In tropical coastal waters, the water is clear thanks to low nutrient inputs, low primary production, and coral reefs acting as a natural buffer that keep sediments from getting resuspended. [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]

Low water clarity

Rio de la Plata NASA image.jpg
Turbid water in the Río de la Plata estuary between Uruguay and Argentina.
Murky water, checking turbidity. (4606483974).jpg
Turbid water in the Gulf of Mexico near the coast of Louisiana, United States.

Very low water clarity can be found where high loads of suspended sediments are transported from land. Some examples are estuaries where rivers with high loads of sediments empty into the ocean. One example is the Río de la Plata, an estuary in South America between Uruguay and Argentina where the Uruguay River and the Parana River empty into the Atlantic ocean. The Río de la Plata shows long-term mean TSS concentrations between 20 and 100 grams per cubic meter, higher than most estuaries. [45] Another example is the gulf coast of North America where the Mississippi River meets the Gulf of Mexico. Turbid water from snowmelt and rain washes high loads of sediment downstream each spring, creating a sediment plume and making the water clarity very low. [46] Water bodies can also experience low water clarity after extreme events like volcanic eruptions. After the eruption of Mount St. Helens, the water of Spirit Lake, Washington was darkened by decaying trees in the lake and had a Secchi depth of only 1 to 2 centimeters. [43]

Water clarity vs. water quality

Water clarity is more specific than water quality. The term “water clarity” more strictly describes the amount of light that passes through water or an object’s visibility in water. The term “water quality” more broadly refers to many characteristics of water, including temperature, dissolved oxygen, the amount of nutrients, or the presence of algal blooms. How clear the water appears is only one component of water quality. [1] [5] [47]

An underwater ecosystem can have high water clarity yet low water quality, and vice versa. Scientists have observed that many lakes are becoming less clear while also recovering from acid rain. This phenomenon has been seen in the northeastern United States and northern Europe. In the past, some lakes were ecologically bare, yet clear, while acidity was high. 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. [48] [49] [50]

See also

Related Research Articles

The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological processes that supply nutrients into the upper water column. The photic zone is home to the majority of aquatic life due to the activity of the phytoplankton.

<span class="mw-page-title-main">Limnology</span> Science of inland aquatic ecosystems

Limnology is the study of inland aquatic ecosystems. The study of limnology includes aspects of the biological, chemical, physical, and geological characteristics of fresh and saline, natural and man-made bodies of water. This includes the study of lakes, reservoirs, ponds, rivers, springs, streams, wetlands, and groundwater. Water systems are often categorized as either running (lotic) or standing (lentic).

<span class="mw-page-title-main">Turbidity</span> Cloudiness of a fluid

Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and water quality.

<span class="mw-page-title-main">Secchi disk</span> Circular disk used to measure water transparency or turbidity

The Secchi disk, as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) in diameter used to measure water transparency or turbidity in bodies of water. The disc 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. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the disk has also been used in a modified, smaller 20 cm (8 in) diameter, black-and-white design to measure freshwater transparency.

<span class="mw-page-title-main">Turbidity current</span> An underwater current of usually rapidly moving, sediment-laden water moving down a slope

A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. Turbidity currents can also occur in other fluids besides water.

High-nutrient, low-chlorophyll (HNLC) regions are regions of the ocean where the abundance of phytoplankton is low and fairly constant despite the availability of macronutrients. Phytoplankton rely on a suite of nutrients for cellular function. Macronutrients are generally available in higher quantities in surface ocean waters, and are the typical components of common garden fertilizers. Micronutrients are generally available in lower quantities and include trace metals. Macronutrients are typically available in millimolar concentrations, while micronutrients are generally available in micro- to nanomolar concentrations. In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport.

<span class="mw-page-title-main">Underwater vision</span> The ability to see objects underwater

Underwater vision is the ability to see objects underwater, and this is significantly affected by several factors. Underwater, objects are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water. They are also blurred by scattering of light between the object and the viewer, also resulting in lower contrast. These effects vary with wavelength of the light, and color and turbidity of the water. The vertebrate eye is usually either optimised for underwater vision or air vision, as is the case in the human eye. The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact. Provision of an airspace between the cornea and the water can compensate, but has the side effect of scale and distance distortion. The diver learns to compensate for these distortions. Artificial illumination is effective to improve illumination at short range.

<span class="mw-page-title-main">Colored dissolved organic matter</span> Optically measurable component of the dissolved organic matter in water

Colored dissolved organic matter (CDOM) is the optically measurable component of dissolved organic matter in water. Also known as chromophoric dissolved organic matter, yellow substance, and gelbstoff, CDOM occurs naturally in aquatic environments and is a complex mixture of many hundreds to thousands of individual, unique organic matter molecules, which are primarily leached from decaying detritus and organic matter. CDOM most strongly absorbs short wavelength light ranging from blue to ultraviolet, whereas pure water absorbs longer wavelength red light. Therefore, water with little or no CDOM, such as the open ocean, appears blue. Waters containing high amounts of CDOM can range from brown, as in many rivers, to yellow and yellow-brown in coastal waters. In general, CDOM concentrations are much higher in fresh waters and estuaries than in the open ocean, though concentrations are highly variable, as is the estimated contribution of CDOM to the total dissolved organic matter pool.

<span class="mw-page-title-main">Ocean color</span> Explanation of the color of oceans and ocean color remote sensing

Ocean color is the branch of ocean optics that specifically studies the color of the water and information that can be gained from looking at variations in color. The color of the ocean, while mainly blue, actually varies from blue to green or even yellow, brown or red in some cases. This field of study developed alongside water remote sensing, so it is focused mainly on how color is measured by instruments.

<span class="mw-page-title-main">Forel-Ule scale</span> Method to approximately determine the color of bodies of water using a standard colour scale

The Forel-Ule scale is a method to estimate the color of bodies of water. The scale provides a visual estimate of the color of a body of water, and it is used in limnology and oceanography with the aim of measuring the water's transparency and classifying its biological activity, dissolved substances, and suspended sediments.

<span class="mw-page-title-main">Trophic state index</span> Measure of the ability of water to sustain biological productivity

The Trophic State Index (TSI) is a classification system designed to rate water bodies based on the amount of biological productivity they sustain. Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.

The deep chlorophyll maximum (DCM), also called the subsurface chlorophyll maximum, is the region below the surface of water with the maximum concentration of chlorophyll. The DCM generally exists at the same depth as the nutricline, the region of the ocean where the greatest change in the nutrient concentration occurs with depth.

<span class="mw-page-title-main">Lake</span> Large body of relatively still water

A lake is a naturally occurring, relatively large body of water localized in a basin surrounded by dry land. A lake generally has a slower-moving flow than the inflow or outflow stream(s) that serve to feed or drain it. Lakes lie completely on land and are separate from the ocean, although, like the much larger oceans, they form part of the Earth's water cycle by serving as large standing pools of storage water. Most lakes are freshwater and account for almost all the world's surface freshwater, but some are salt lakes with salinities even higher than that of seawater.

<span class="mw-page-title-main">Ocean turbidity</span> Measure of cloudiness of sea water

Ocean turbidity is a measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification. Highly turbid ocean waters are those with many scattering particulates in them. In both highly absorbing and highly scattering waters, visibility into the water is reduced. Highly scattering (turbid) water still reflects much light, while highly absorbing water, such as a blackwater river or lake, is very dark. The scattering particles that cause the water to be turbid can be composed of many things, including sediments and phytoplankton.

<span class="mw-page-title-main">Mille Lacs Lake</span> Lake in the state of Minnesota, United States

Mille Lacs Lake is a large but shallow lake in the U.S. state of Minnesota. It is located in the counties of Mille Lacs, Aitkin, and Crow Wing, roughly 75 miles north of the Minneapolis-St. Paul metropolitan area.

<span class="mw-page-title-main">Water remote sensing</span> System to measure the color of water by observing the spectrum of radiation leaving the water.

Water Remote Sensing is the observation of water bodies such as lakes, oceans, and rivers from a distance in order to describe their color, state of ecosystem health, and productivity. Water remote sensing studies the color of water through the observation of the spectrum of water leaving radiance. From the spectrum of color coming from the water, the concentration of optically active components of the upper layer of the water body can be estimated via specific algorithms. Water quality monitoring by remote sensing and close-range instruments has obtained considerable attention since the founding of EU Water Framework Directive.

<span class="mw-page-title-main">Whiting event</span> Suspension of fine-grained calcium carbonate particles in water bodies

A whiting event is a phenomenon that occurs when a suspended cloud of fine-grained calcium carbonate precipitates in water bodies, typically during summer months, as a result of photosynthetic microbiological activity or sediment disturbance. The phenomenon gets its name from the white, chalky color it imbues to the water. These events have been shown to occur in temperate waters as well as tropical ones, and they can span for hundreds of meters. They can also occur in both marine and freshwater environments. The origin of whiting events is debated among the scientific community, and it is unclear if there is a single, specific cause. Generally, they are thought to result from either bottom sediment re-suspension or by increased activity of certain microscopic life such as phytoplankton. Because whiting events affect aquatic chemistry, physical properties, and carbon cycling, studying the mechanisms behind them holds scientific relevance in various ways.

<span class="mw-page-title-main">Lake metabolism</span> The balance between production and consumption of organic matter in lakes

Lake metabolism represents a lake's balance between carbon fixation and biological carbon oxidation. Whole-lake metabolism includes the carbon fixation and oxidation from all organism within the lake, from bacteria to fishes, and is typically estimated by measuring changes in dissolved oxygen or carbon dioxide throughout the day.

<span class="mw-page-title-main">Ocean optics</span> The study of light interaction with water and submerged materials

Ocean optics is the study of how light interacts with water and the materials in water. Although research often focuses on the sea, the field broadly includes rivers, lakes, inland waters, coastal waters, and large ocean basins. How light acts in water is critical to how ecosystems function underwater. Knowledge of ocean optics is needed in aquatic remote sensing research in order to understand what information can be extracted from the color of the water as it appears from satellite sensors in space. The color of the water as seen by satellites is known as ocean color. While ocean color is a key theme of ocean optics, optics is a broader term that also includes the development of underwater sensors using optical methods to study much more than just color, including ocean chemistry, particle size, imaging of microscopic plants and animals, and more.

<span class="mw-page-title-main">Alpine lake</span> High-altitude lake in a mountainous zone

An alpine lake is a lake at a high altitude in a mountainous zone, usually near or above the tree line, with extended periods of ice cover. These lakes are commonly formed from glacial activity but can also be formed from geological processes such as volcanic activity or landslides. Many alpine lakes that are fed from glacial melt have the characteristic bright turquoise or green color as a result of glacial flour, suspended minerals derived from a glacier scouring the bedrock. When active glaciers are not supplying water to the lake, such as a majority of alpine lakes of the Rocky Mountains in the U.S., the lakes may still be bright blue due to the lack of algae growth resulting from cold temperatures, lack of nutrient run-off from surrounding land, and lack of sediment input. The coloration and mountain locations of alpine lakes attract lots of recreational activity.

References

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  2. ↑ Moore, Kenneth A.; Short, Frederick T. (2006). "Zostera: Biology, Ecology, and Management". Seagrasses: Biology, Ecology, and Conservation. Dordrecht: Springer Netherlands. pp. 361–386. doi:10.1007/978-1-4020-2983-7_16. ISBN   978-1-4020-2942-4.
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  5. 1 2 Tango, Peter J.; Batiuk, Richard A. (2013-09-04). "Deriving Chesapeake Bay Water Quality Standards". JAWRA Journal of the American Water Resources Association. Wiley. 49 (5): 1007–1024. doi:10.1111/jawr.12108. ISSN   1093-474X. S2CID   102492027.
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  45. ↑ Moreira, Diego; Simionato, Claudia G.; Gohin, Francis; Cayocca, Florence; Luz Clara Tejedor, Moira (2013). "Suspended matter mean distribution and seasonal cycle in the Río de La Plata estuary and the adjacent shelf from ocean color satellite (MODIS) and in-situ observations". Continental Shelf Research. Elsevier BV. 68: 51–66. doi:10.1016/j.csr.2013.08.015. hdl: 11336/85098 . ISSN   0278-4343.
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  48. ↑ Warren, Dana R.; Kraft, Clifford. E.; Josephson, Daniel C.; Driscoll, Charles T. (2016-12-15). "Acid rain recovery may help to mitigate the impacts of climate change on thermally sensitive fish in lakes across eastern North America". Global Change Biology. Wiley. 23 (6): 2149–2153. doi: 10.1111/gcb.13568 . ISSN   1354-1013. PMID   27976837. S2CID   787974.
  49. ↑ Kritzberg, Emma S. (2017-06-05). "Centennial‐long trends of lake browning show major effect of afforestation". Limnology and Oceanography Letters. Wiley. 2 (4): 105–112. doi: 10.1002/lol2.10041 . ISSN   2378-2242. S2CID   90943855.
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