encyclopedia of inland waters || optical properties of water

Optical Properties of WaterD P Morris, Lehigh University, Bethlehem, PA, USA

ã 2009 Elsevier Inc. All rights reserved.

The Nature of Light

Light is critical in structuring all aquatic ecosystems.Seasonal variations in the heat energy provided bysunlight are responsible for thermal stratificationand mixing regimes of aquatic systems. Sunlightused in photosynthesis (both terrestrial and aquatic)forms the foundation of all but the simplest microbialcommunities of aquatic ecosystems.The term ‘light’ is commonly referred to as that

portion of the electromagnetic spectrum that is visibleto the human eye. In reality, visible light accounts fora very narrow portion of an electromagnetic spec-trum that ranges from cosmic rays to radio waves.Although some electromagnetic radiation originatesfrom outside our solar system (indeed some as rem-nants of the ‘Big Bang’), an overwhelming majority ofthe electromagnetic radiation reaching the Earth ori-ginates from the Sun.All electromagnetic radiation should be viewed as

having two fundamental properties, which includeboth a physical entity (photons/quanta) and waveproperties. Thus, electromagnetic radiation is a trans-verse wave of energy that behaves as a particle with adefined mass. The photon carries energy in the formof a wave; each discrete photon has a distinct wave-length and frequency associated with it. The wave-length (l, m) and frequency (n, cycles s�1) ofelectromagnetic radiation is inversely related by

l ¼ c=� ½1�

where c is the speed of light (3.0 � 1010 cm s�1 in avacuum). When dealing with light, it is more typical(and convenient) to express wavelength in units ofnanometers (10�9m).The energy content of discrete photons (in Joules)

of electromagnetic radiation is inversely related towavelength via the Planck equation

e ¼ h� ¼ hc=l ½2�

where h is Planck’s constant (6.63 � 10�34 J s).The bulk of solar radiation spans wavelengths from

approximately 100 to 3000 nm (Figure 1). A rela-tively small proportion of this spectrum correspondsto ‘visible’ light (400–700 nm). Wavelength bandswithin the visible spectrum are perceived as colorsby the human eye. The perceived primary colorsof blue (450–500nm), green (500–550nm), yellow(550–600nm), and red (650–700nm) are all associated

682

with distinct wavelength bands within the visible spec-trum. Wavelengths of ultraviolet radiation are shorterthan the visible spectrum (200–400nm) while that ofinfrared radiation are longer (700–3000nm).

Planck’s equatio n [2] can be used to calc ulate theenergy content of photons associated with any ofthese wavelengths. Relative to photons with a wave-length of 665 nm (a major absorption peak for chlo-rophyll a), infrared photons at 1000 nm contain only66% of energy. In contrast, photons associated withUV-b radiation at 300 nm contain 221% the energy at665 nm. This explains why UV-b radiation can causeerythemia (sunburn) while infrared radiation pro-duces only a gentle warming sensation.

Evenmore ecologically significant than the phenom-enon of vision, light provides the energy necessary forphotosynthesis. The energy utilized in photosynthesisis restricted to a relatively narrow spectrum referred toas ‘photosynthetically active radiation’ (PAR). Inter-estingly, and probably not coincidentally from andevolutionary perspective, PAR and visible light areessentially identical (400–700nm). The upper limit ofPAR is defined because photons at a wavelengthgreater than 700nm have too little energy to overcomethe biochemical threshold for production of e� by PS-Iand PS-II. In contrast, photons with wavelengths lessthan 400nm are so energetic that they may potentiallydamage photosystems either directly, or via the pro-duction of free radicals. Although vision and photo-synthesis utilize only a narrow band of the solarspectrum, the region from 400 to 700nm containsalmost 45% of the energy output of the sun (at Earth’ssurface). Evolution of vision and photosynthesis to usethis narrow band resulted from a balance betweenavailable solar radiation and the optimum energy con-tent per photon for biological systems.

As a result of the nature of electromagnetic radia-tion, the radiant flux (F) of light may be quantified inone of two ways: by counting quanta or measuringenergy. Each method has utility with respect to thestudy of aquatic systems.

Full summer sunlight at the surface of the Earthdelivers a flux density (F) of approximately 1.2� 1021

quanta m�2 s�1 of visible light. In the past, F wasexpressed in terms of moles of photons (1mole¼6.02� 1023 photons).According to recent internationalstandards, 1mole of photons is given the unit of Ein-steins (E). Thus, a ‘full sun’ value of F for a typicalsummer day is approximately 2000mE of visible quanta

0.00 500 1000 1500

Sea level

Solar constant(above atmosphere)

Wavelength (nm)

Irra

dian

ce (

Wm

−2nm

−1)

2000 2500 3000

0.5

InfraredVisibleUV

1.0

1.5

2.0

2.5

Figure 1 Spectral distribution of sunlight above the

atmosphere and at sea level.

Light and Heat in Aquatic Ecosystems _ Optical Properties of Water 683

per square meter per second. Similarly, the flux densitymay also be measured in units of energy. A ‘fullsun’ value for the flux density reported in energy unitsis approximately 430 Jm�2 s�1 or 430W m�2 (1Jm�2 s�1¼ 1Wm�2).If the distribution of quanta or energy is known

across the solar spectrum, it is possible to convert be-tween the tw o sets of units using Planck’s equati on[2]. Often inst rumen tation to precisel y meas ure thespectral distribution of quanta or energy in sunlight isnot available. In these cases a conversion factor maybe applied based on a typical spectral distribution ofsunlight at the Earth’s surface. Morel and Smith(1974) found that for solar radiation in the visible(and PAR) spectra, a conversion factor of 2.77� 1018

quanta s�1 W�1 was accurate to a within few percentover a variety of sky conditions for measurementsmade in the air.

Role of the Atmosphere

Solar flux just outside the Earth’s atmosphere isreferred to as the ‘solar constant’ and has a value ofapproximately 1373Wm�2. Reflectance, scattering,and absorption of light in the atmosphere can reducethis amount by 15–80% before reaching the Earth’ssurface. Scattering and absorption do not influenceall wavelengths equally, thus significant changes inthe spectral distribution of sunlight will occur aslight penetrates the atmosphere.Scattering of light by gas molecules in the atmos-

phere is proportional to 1/l4 (as described byRayleigh’s law). Some of this light is back scattered

into space while some is scattered forward andreaches the Earth’s surface as skylight. Since scatter-ing is inversely related to l4, the shortest wavelengthsare preferentially scattered in the atmosphere, caus-ing a cloudless sky to appear blue. Although notvisible, large fluxes of short wavelength UV radiationcan reach the surface of the Earth as scattered skylight(in contrast to direct sunlight). In addition to gasmolecules, particulate material and water vapor (asclouds) in the atmosphere can also contribute to lightscattering.

The absorption of light by the atmosphere is largelya result of naturally occurring oxygen, ozone, carbondioxide, and water vapor. Anthropogenic air pollu-tion may also have significant affects in some regions.Ozone strongly absorbs high-energy UV-b radiationand essentially prevents light of wavelengths shorterthan 300 nm from reaching the surface of the Earth.Water vapor and carbon dioxide strongly absorbsome specific bands of infrared radiation and aresignificant as ‘greenhouse’ gases. Figure 1 comparesthe spectral distribution of light above the atmo-sphere and at sea level.

Cloud cover can be effective in both reflecting andabsorbing light in the atmosphere. The transmissivityof thick cumulus clouds may be a little as 10% whilethat of high stratus clouds may be as high as 70%.The transmissivity of total solar radiation can be ashigh as 86% under dry, cloudless, and ‘clean’ atmos-pheric conditions. According to Gates (1962), theaverage annual transmissivity of the atmosphereover a region in the Northern Hemisphere was47%. Reflectance and back scattering of light intospace accounted for a loss of 34% while 19% waslost via atmospheric absorption.

The Role of Latitude

Latitude plays a critical role in determining solar fluxat the Earth’s surface (E0), by its influence on sunangle and day length (Figure 2). Insolation generallydeclines with increasing latitude through its influenceon sun angle. However during summer, this phenom-enon is countered by increasing day length. The netresult is that during mid-summer, high latituderegions may actually receive greater daily insolationthan the equatorial regions receive. Thus, the maxi-mum daily insolation received at the surface of theEarth does not vary significantly with latitude (dis-regarding any variations in atmospheric conditions).The most profound influence of latitude on insolationis in determining the minimum daily insolation. Dur-ing winter, increasing sun angle is accompanied bydecreasing day length at high latitudes. In fact, daily

Minimum

00

10

20

30

40

50

10 20 30 40Latitude (deg)

MJ

m2

day−

1

50 60 70 80

Mean

Maximum

Figure 2 Maximum, minimum, and mean daily irradiance at

different latitudes at the surface of the Earth. Data do not account

for regional differences in cloud cover.

684 Light and Heat in Aquatic Ecosystems _ Optical Properties of Water

insolation falls to zero in polar regions during thewinter.Reflectance, scattering, and absorption of light by

the atmosphere also increase with sun angle. Loss oflight by scattering and absorption are both a functionof the amount of atmosphere traversed. The atmos-pheric path length is proportional to the cosine of thesolar angle. Thus, the atmospheric path length isroughly doubled when the sun is 30� above the hori-zon than when being directly overhead. For this rea-son, the transmissivity of the atmosphere will varygreatly according to both latitude and time of day.

Reflectance at the Air–Water Interface

Light at the surface of a water body may be reflected.Unless rereflected or scattered back to the aquaticsystem from the atmosphere or the surrounding land-scape, this light is lost from the system. The amountof light reflected from the water’s surface is related tothe incident angle of the sun (angle from the vertical,often referred to as the zenith angle), as described inthe Fresnel equation

r ¼ 1sin2ðYa �YwÞ2sin2ðYa þYwÞ

þ 1tan2ðYa �YwÞ2tan2ðYa þYwÞ ½3�

where r is the reflection of unpolarized light as afraction of the incident light, Ya is the zenith angleof the sun, and Yw is the angle of refraction (i.e., thechange in the angle of a light beam as it penetrates thesurface–water interface). Reflectance for a flat bodyof water with the sun directly overhead (i.e., Ya¼ 0�)is approximately 2% and remains relatively low forzenith angles of up to about 60� (r < 6%). As thezenith angle approaches 90�, reflectance increasesrapidly (e.g., r is approximately 60% at 80�). Atsmall zenith angles, surface disturbances (i.e., small

waves) have very little influence on reflectance but atmore extreme zenith angles (>60�) reflectance can beincreased by as much as 20%. Large waves in combi-nation with high zenith angles may reduce reflectanceby decreasing the effective zenith angle between thewater’s surface and the sun. Reflectance of light fromfrozen water bodies has been estimated to be as muchas 75–95%; however, this value depends on the qual-ity of the ice or snow.

This discussion of reflectance deals solely withdirect solar radiation. Reflection of diffuse solarradiation striking the surface of the water as a re-sult of scattered skylight is difficult to determineprecisely because the angular distribution of lightis unknown.

Inherent Optical Properties

Once photons penetrate the air–water interface ofaquatic systems they may only be scattered orabsorbed. The extent to which either of these phe-nomena may occur is governed by three parameters:absorption coefficient, scattering coefficient, and thevolume scattering function. These three parametersare often referred to as ‘inherent optical properties’because they only depend on constituents ofthe aquatic medium and not on the geometry of thelight field.

To define these parameters, it is necessary to con-sider an infinitesimally thin layer of medium beingilluminated at right angles by a parallel beam of light(Figure 3). The fraction of flux that is absorbed acrossthe medium relative to the incident flux is defined asthe absorptance (A) and the fraction of flux thatdiverges from the parallel beam of incident flux isreferred to as the scatterance (B)

A ¼ Fa=F0 ½4�

B ¼ Fb=F0 ½5�where F0 is the incident flux of the parallel beam oflight, Fa is the flux absorbed and Fb is the flux scat-tered by the thin medium. If we represent the changein absorptance (DA) and scatterance (DB) with infini-tesimal changes in the thickness of the medium (Dz),we can estimate the absorption coefficient (a) and thescattering coefficient (b)

a ¼ �A=�z ½6�

b ¼ �B=�z ½7�The beam attenuation coefficient (c), an additionalinherent optical property, is simply an expression ofthe amount of flux lost from the beam of light as a

Parallel incidentmonochromatic fluxf0

fb light scatteredby media

fa light absorbed by media

fz= fo− fa−fb flux resulting afterscattering and absorption by a mediawith infinitesimal thickness, z

Figure 3 Schematic diagram illustrating absorption andscattering of light by an infinitesimally thin layer of water.


result of both absorption and scattering, per unitthickness of the medium

c ¼ a þ b ½8�The absorption coefficient, scattering coefficient, andbeam attenuation coefficient each have units of1/length and are typically expressed as per meter.Another inherent optical property of the aquatic

media is the volume scattering function (b). As lightis scattered within a media, the photons may be dis-persed in variety of different directions. The volumescattering function defines the angular distribution ofthe resulting scattered flux. This function differsaccording to the medium but generally describes aradially symmetrical cone of distribution around thedirection of the incident beam.

Scattering

Scattering of light in aquatic ecosystems can bethought of as the deflection (reflection) of photonsat a large number of angles from water and its dis-solved and particulate constituents. Although scat-tered photons are largely distributed in a radiallysymmetrical cone around the direction of the incidentbeam, there is a finite statistical probability thatphotons may be deflected in any direction. Scatteredphotons that are deflected back may leave the aquaticecosystem along with their energy content. Scatteringcan vary with wavelength according to differentialabsorption and scattering characteristics of the aque-ous media. In very clear aquatic ecosystems, thegreatest amount of scattering occurs in blue wave-lengths (these systems appear blue because backscat-tered light is detected by the eye). Systems with largeconcentrations of dissolved organic carbon appear

yellow/brown because blue photons are stronglyabsorbed and thus not available to be backscatteredto the eye of the observer.

Scattering may be greatly influenced by the natureof particulate material in an aquatic ecosystem. Forinstance fine glacial ‘flour’ may be highly reflectiveand can enhance scattering at the mouth of riversdraining glaciers. White calcium carbonate (marl) atthe bottom of lakes may also enhance scattering(reflectance) in shallow systems or in the water col-umn of deeper systems if sediments are resuspendedby wind driven mixing. In contrast, organic particu-late material (colloids of humic substances, detritus,etc.) may strongly absorb rather than scatter photons.

The net result of scattering is that it can greatlyincrease the path length (Dz) taken by a photon as itpasses through an aqueous media. For instance, it isnot uncommon for the ‘average’ photon to travel apath of 1.2m (as a result of multiple scattering events)before reaching a depth of 1m in an aquatic ecosys-tem. Scattering also has important implications foraquatic photosynthetic organisms because scatteredphotons are available from all directions (includingthe bottom) and not just that oriented toward thedownward welling light.

Absorption

Photons of light may interact with the electron shellof molecules and be absorbed. During this process,the photon is destroyed but its energy is transferred tothe molecule that absorbed it. This energy is used totransition electrons from the ground state to higherenergy levels. Molecules can have very distinctabsorption spectra because the energy content ofsome photo ns (eqn [2]) mat ch exact ly the energyrequired for the electron transition. The energy ofabsorbed photons may be used to break chemicalbonds or generate electrons (in the case of photosyn-thesis). However, because energy has to be conserved,the energy of all absorbed photons is eventuallyreleased in aquatic ecosystems as heat.

Light absorption can be measured in a spectropho-tometer, which is an instrument specifically designedto measure the absorbance of light by a medium ofknown thickness. Values obtained by spectropho-tometers are known equivalently as absorbance (A)or optical density (OD). Using a spectrophotometer,absorbance (A) can be calculated as

A ¼ log10I0IZ

½9�

where I0 is the intensity of incident light and I is theintensity of light transmitted through the medium. Ifwe arrange the system so that any light scattered in


the medium is also included in our measurement of I,we can use eqn [9] to de termine the absorpt ion coef-ficient (a) of the medium

a ¼ 2:303A

Z½10�

where 2.303 is the constant to convert base 10 tonatural logarithms and Z is the path length of themedium. The absorption coefficient is given in unitsof 1/path length (typically m�1).In aquatic ecosystems, photons may be absorbed by

the water itself, dissolved substances, and particulatematerial. Pure water is not a colorless liquid. As isobvious by observing very deep, clear systems, wateris a blue liquid. This color implies that there is signifi-cant spectral variation in absorption over the visibleregion. As see in Figure 4, the absorption of light bypure water is lowest in the blue range of the visiblespectrum and increases substantially from 430 to760 nm. Following a small decline centered around

0

10

20

30

40

50

60

200 300 400 500 600 700 800 900 1000Wavelength (nm)

a (m

–1)

Figure 4 Spectral distribution of the absorption coefficient

for pure water. Measurements were made with a

spectrophotometer, with air as reference.

0

20

40

60

80

Wavele

a (m

–1)

Lake Giles (1 mgC/l) Lake Laca

300 400 500 600

Figure 5 Spectral distribution of absorption for filtered water from l

apparent color of four lake water samples ranging from approximate

were made with a spectrophotometer referenced to air.

810 nm, absorption increases markedly through theinfrared region, peaking at about 975 nm. A gradualincrease in absorbance is also evident through theUV-a and UV-b regions, although this may actuallybe an artifact of small amounts of dissolved organiccarbon contamination present in even the most care-fully purified water. The strong absorbance peak inthe IR region attests the importance of infrared radi-ation in influencing the heat budgets of aquaticecosystems.

Dissolved organic matter (DOM) plays the mostcritical role in influencing the absorption spectrumof natural waters. The colored fraction (typically yel-low/brown and referred to as gelvin or gelbstoff) ofDOC is referred to as chromophoric dissolvedorganic matter (CDOM). This material consistsmainly of humic and fulvic compounds originatingfrom the decomposition of terrestrial plant materialunder anaerobic conditions. The global mean concen-tration for DOM in freshwater systems is approxi-mately 5mg C l�1 but can vary widely (�1 mg C l�1

in very clear systems to >100mg C l�1 in bogs). Asseen in Figure 5, the presence of CDOM causes anexponential increase in absorption from the shortvisible wavelengths through the UV-b region of thespectrum. The slope of this exponential curve (S, ln(a)vs. l280–400nm) has been measured for a variety offreshwater systems and averages �0.0181m�1 nm�1.S typically ranges between �0.020 and �0.010 andseems to be related to qualitative differences inCDOM (likely due to CDOM source or photobleach-ing). CDOM has the greatest influence on short wave-length UV radiation but also strongly affects theabsorption of visible light (especially blue). The optical

ngth (nm)

wac (5 mgC/l) Neuse River (9 mgC/l)

700 800 900 1000

akes with different concentrations of CDOM. The inset shows the

ly 1 to 15mg C l�1 of dissolved organic carbon. Measurements


property often referred to a ‘color’ is typically mea-sured as the absorption coefficient at 440nm and is areflection of CDOM concentration for a particularaquatic ecosystem. In systems with high CDOM con-centrations, absorption of PAR can significantly reducelight available for photosynthesis. Because of the shapeof the absorption spectrum, much smaller concentra-tions of CDOM can effectively absorb damaging solarUV-b radiation, producing UV refugia in aquatic sys-tems. Above approximately 600nm, CDOM has littleinfluence on the absorption spectrum. Other dissolvedconstituents of aquatic systems (e.g., nitrate) may alsoabsorb light but their specific absorption coefficientsare very low compared to that of CDOM. Thus, thesecompounds will only be optically significant in systemswith very low concentrations of CDOM.Suspended particulate matter may also absorb light

in aquatic systems (Figure 6). Phytoplankton evolutionhas produced photosynthetic pigments that are highlyefficient at absorbing light in the PAR and UV spec-trum. These cells often contain a variety of accessorypigments (e.g., carotinoids, chlorophyll b, and chloro-phyll c, etc.) aswell as chlorophyll a. As seen inFigure 6,the absorbance peaks of chlorophyll a (425 and665nm) are often apparent in the spectral analysis ofseston over and above that of accessory pigments.Systems with high concentrations of CDOM mayalso have significant amounts of suspended organiccolloids composed of humic matter. The chemicalcomposition and absorption spectrum of this particu-late matter is often similar to that of CDOM (compareFigures 5 and 6). Figure 6 also shows an absorptionscan for mineral clay particles. In contrast to either

0.0

0.5

1.0

1.5

Wavelength (nm)

a (m

–1 )

Mineral clayHumic POCAlgae

Chlorophyll a

300 400 500 600 700 800 900 1000

Figure 6 Spectral distribution of absorption for three

categories of particulate material commonly found in aquatic

ecosystems. Absolute size of the peaks will depend uponthe concentration of these materials in the water column.

Remnants of the chlorophyll a absorption peaks (665 and

425nm) can be seen in the sample of algae. Data were obtained

using spectrophotometry and the quantitative filter padtechnique.

phytoplankton or colloidal CDOM, kaolin exhibitsvery little absorption above 400nm.

Optical Properties of Aquatic Ecosystems

The transparency of aquatic systems is represented bya value referred to as the extinction coefficient ordiffuse attenuation coefficient (Kd or Z in older liter-ature). This value may be determined empirically foran aquatic system using the following equation:

Edz ¼ Ed0 e�Kd z ½11�where Edz is the downward welling irradiance atdepth z, Ed0 is the downward welling irradiancejust below the surface, and Kd is the extinction coeffi-cient. Kd has units of reciprocal depth (typically m�1).

As demonstrated in the previous section, the extinc-tion coefficient in aquatic systems will be influenced byabsorption and scattering by pure water (Kwater), bymaterials dissolved in the water (Kdissolved), andby particulate material (Kparticulate). Thus, the totalmeasured diffuse attenuation coefficient for a givensystem will be the sum of each of the partial diffuseattenuation coefficients.

As illustrated by data presented in Figures 4–6, eachof these partial diffuse attenuation coefficients have astrong spectral component. Thus, the relative contribu-tion of each of these partial diffuse attenuation coeffi-cients to the total diffuse attenuation coefficient willvary substantially across the solar spectrum andbetween systems. For instance, due to the spectral dis-tribution of CDOMabsorption,Kdissolved in lakes com-prised an average 68%ofKtotal at 305nm compared toonly 33% for PAR. In contrast, Kwater contributes amuch larger fraction (>95%) of Ktotal in the infraredportion of the spectrum than do the other partial dif-fuse attenuation coefficients. For lakes in general, theconcentration of CDOM is the single most importantdeterminant of Kd in the UV region of the spectrumwhile a combination of both CDOM concentrationand phytoplanktom biomass are significant determi-nants of Kd for PAR.

Plots showing the extinction of light in an aquaticecosystem are shown for Lake Giles, PA (Figure 7).The top panel demonstrates that light diminishesexponentially with depth as one would expect fromeqn [11]. The incid ent irradianc e (i.e., at a depthof 0m) follows from the data presented in Figure 1,where the highest value of radiant flux (F) is for PARand a progressive decrease in flux is observed througheach of the wavelengths in the UV spectrum. Light ofeach of the wavelengths diminishes through the watercolumn with a characteristic slope. In fact, the valueof Kd for each of these wavelengths can be derived as

00 5 10 15

0 5 10 15

20

40

60

80

305 nm320 nm340 nm380 nmPAR

0

1

2

3

4

5

Depth (m)

Ed

( µW

or

mW

m–2

)Ln

(Ed)

.16 (29)

.43 (11).74(6)

1.00(5)

1.27 (3.6)

Figure 7 Attenuation of downward welling irradiance (Ed) in

Lake Giles, PA. In the top panel the units for PAR is mW m�2. All

other wavelengths are expressed as mWm�2. In the bottom panel

the line of best fit is plotted through the data points of ln(Ed) vs.depth. The negative slope of this line is the value of Kd for each

wavelength. The values of Kd (z1%) are provided. Data were

obtained using a Biospherical PUV-500 submersible radiometer.

Toolik Lake, AK(0.75 m−1)

10

5

Dep

th (

m)

Percent incident PAR

01 10 100

Silver Lake, PA(0.27 m−1)

Lago Correntoso,Argentina (0.10 m−1)

Green Mtn. Res., CO(1.00 m−1)

Little Echo Lake, PA(5.21 m−1)

Figure 8 Penetration of PAR for lakes with widely differing

transparency. Diffuse attenuation coefficients are provided foreach lake (Kd).


the negative slope of the plot of ln(Ed) vs. depth(Figure 7, bottom panel).Since Kd is an exponential rate constant, it is often

difficult for the general public to embrace it as anestimate of water column transparency. A moregenerally understandable parameter is the 1% atten-uation depth for light. This parameter is the depth inthe water column at which 99% of the surface irradi-ance has been attenuated. This parameter can easilybe calculated as

z1% ¼ 4:61

Kd½12�

where z1% is the 1% attenuation depth (m) and Kd isthe diffuse attenuation coefficient (m�1). The value4.61 is simply ln(100%). Lower values of Kd corre-spond to higher water column transparency. In LakeGiles (as well as many other aquatic systems) theprogressive decrease in transparency with decreasingl is due to the presence of CDOM (�1.5mg C l�1 inLake Giles). The relationship between Kd and lfor Lake Giles will generally follow the relationship

between the absorption coefficient (a) and l observedin different concentrations of CDOM (Figure 5).

The transparency of PAR in aquatic ecosystems canvary tremendously. Figure 8 shows the vertical pene-tration of PAR in five lakes that span a range oftransparency typically found in aquatic systems. Inextremely clear Lake Correntoso, z1% is in excess of45m while 99% of PAR is attenuated in the firstmeter of Little Echo Lake, which has moderate con-centrations of chlorophyll a (�6mg l�1) and a highconcentration of CDOM (�23mg C l�1).

One method of estimating the transparency of visi-ble light (PAR) in aquatic systems is the Sceehi disk.First used in the 1860s, the black and white Secchidisk (20 cm diameter) is lowered through the watercolumn on a calibrated line. The depth at which thedisk disappears is termed the Secchi depth (zsd).Although the Secchi depth can vary between users,light conditions, and surface conditions, it is generallythought that z1% for PAR corresponds to approxi-mately 2–3 times zsd. The value of Secchi measure-ments lies not in its accuracy, but in its ease of use andthe extremely long records of transparency based onthis device.

Conclusion

In conclusion, light is critical in structuring aquaticecosystems. Seasonal variations in the heat energyprovided by sunlight are responsible for thermalstratification and mixing regimes so fundamental toaquatic systems. The energy of sunlight used in pho-tosynthesis forms the foundation of all but the sim-plest microbial communities on Earth. Thus, anunderstanding of the nature of light, and how it inter-acts with the atmosphere and the water column, is


fundamental to our understanding of how aquaticecosystems function.

See also: Color of Aquatic Ecosystems; Interactions ofDissolved Organic Matter and Humic Substances;Natural Organic Matter; Ultraviolet Light.

Further Reading

Gates DM (1962) Energy Exchange in the Biosphere. Harper &

Row, New York.

Hargreaves BR (2003) Water column optics and penetration of

UVR. In: Helbling EW and Zagarese HE (eds.) UV Effects inAquatic Organisms and Ecosystems, Comprehensive Seriesin Photochemical and Photobiological Sciences, pp. 59–105.

Cambridge, UK: Royal Society of Chemistry.

Kirk JTO (1994) Light and Photosynthesis in Aquatic Ecosystems,2nd edn. New York: Cambridge University Press.

Morel A and Smith RC (1974) Relation between total quanta and

total energy for aquatic photosynthesis. Limnology and Ocean-ography 19: 591–600.

Morris DP, Zagarese H, Williamson CE, et al. (1995) The attenua-tion of solar UV radiation in lakes and the role of dis-

solved organic carbon. Limnology and Oceanography 40:

1381–1391.

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