In the fourth century B.C., Aristotle no-ticed that seawater was blue and trans-parent, while river water was yellowish

and murky. Scientists have spent the last2,000 years trying to understand why. Al-though there was speculation for centuries,an empirical answer could not be deriveduntil the penetration and color of light inthe ocean could be measured. With the ad-vent of useful instruments and techniquesin the late 1800s and early 1900s, peoplebegan to make measurements that couldexplain Aristotle’s observations. Through-out the twentieth century, improved meth-ods have led to an increasingly detailed un-derstanding of the physical, chemical, and

Two Thousand Years of Ocean Optics

SOPHIA JOHANNESSENSOPHIA JOHANNESSEN

Two Thousand Years of Ocean Optics

biological processes controlling the distri-bution of light in the ocean. Our under-standing of ocean color and transparencycan be traced historically through the dif-ferent approaches and motivations ofsome of the investigators.

The transparency of the oceanAlthough Aristotle suggested in the fourthcentury B.C. that suspended particlesmight explain why river water was lessclear than ocean water, his insights werenot pursued for centuries. However, in themid-1800s people began to measure oceanlight penetration out of curiosity. A roughestimate of ocean transparency required

only an object to submerge and a string toretrieve it.

In 1815, Captain Kotzebue made thefirst recorded ocean measurement. Kotze-bue lowered a piece of red material into theocean and measured the depth at which itdisappeared. The first optical expeditionwas then mounted a few years later onboard the Coquille, during which CaptainDuperrée lowered a white plate into thewater at a number of stations.

Michael Faraday used a similar tech-nique in 1855 in the Thames River in Eng-land. He reported to the local newspaperthat he had torn up pieces of white cardand dropped them into the river in several

1047-6938/01/04/0030/7-$0015.00 © Optical Society of America

ried out a number of experiments and in-troduced some innovations still used inoptical measurements in the 1990s. Theseinnovations included using an amplifiedtelephone to detect unbalanced current,and attaching a diffuser over the photo-electric cell to simplify path-length calcula-tions. In addition, Poole and Atkins cor-rected depth measurements for changes insurface light intensity by using a surfacereference radiometer. With such tech-niques available, it became possible toquantify the effect of light on photosynthe-sis—a link that previously had been madeonly qualitatively.

In 1933, Penelope Jenkin made meas-urements that linked photosynthesis withocean light penetration for the first time.At the time, marine plant photosynthesiswas of considerable interest to people whowanted to understand the productivity ofthe sea. The International Council for theExploration of the Sea (ICES) was formedin 1902 to carry out research benefiting thefisheries of the member states. Britain haddirected much of its share of the ICESmoney to the Plymouth Laboratory forstudies of productivity in the EnglishChannel and in the North Sea. In 1921, W.R. G. Atkins was appointed as a physiolo-gist at the Plymouth Laboratory. It was

Atkins who suggested that Jenkin under-take the simultaneous measurement oflight and photosynthesis in the EnglishChannel.

In 1933, Jenkin also began to measurelight and photosynthesis together. Teamedwith Atkins and Poole aboard the MarineBiological Association’s steam trawlerSalpo, Jenkin hoped to show that light con-trolled marine plant photosynthetic rates.While Jenkin performed photosynthesisexperiments, Atkins and Poole measuredlight in the water using a photocell withcolored filters. The team hoped that lightmeasurements could be used as a proxy forthe much more time-consuming measure-ments of photosynthesis.

During the cruise, Jenkin exposed di-atoms to light at different depths in the wa-ter. She then measured the light and oxy-gen production at those depths. Jenkinused a monospecific culture of Coscinodis-cus excentricus—a centric diatom that shehad collected from the English Channel inthe early spring. She placed six bottles con-taining the cultured diatom at each depth.Three clear glass bottles were open to lightincident from all directions, two bottleswere entirely blackened except on one side,and one bottle was totally dark. Jenkinplaced the bottles into wire baskets at-tached to a fixed cable for a number ofhours.

Light energy was measured as the sumof all illumination from 380 to 720 nm.The ultraviolet and infrared portions ofthe spectrum were excluded, as Jenkin feltthat their influence would be minimal. Thedark bottle at each depth was included as acorrection for respiration, which wasthought to be constant, regardless of thelevel of incident light. In order to obtaintotal production numbers, Jenkin addedthe oxygen loss in the dark bottle to theoxygen increase measured in the light bot-tles.

From these experiments, Jenkin con-cluded that photosynthesis dependedstrongly on available light. In addition, shenoted that oxygen production increasedwith increasing incident daylight. Al-though it was already known that chloro-phyll readily absorbed red light, Jenkin re-ported that red light could not limit photo-synthesis. Her conclusion was based on thefinding that red light was attenuated quick-ly with depth, while the rate of oxygen pro-duction decreased proportionately with to-tal energy.

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April 2001 ■ Optics & Photonics News 31

Throughout the twentiethcentury, improved methodshave led to an increasinglydetailed understanding of

the physical, chemical,and biological processes

controlling the distributionof light in the ocean.

locations. When the cards sank edge-first,he noted that the lower edge of the carddisappeared before the upper edge waseven below the surface of the water. Hegave these findings as graphic evidence ofthe turbid pollution of the river.

Also in the mid-1800s, Father AngeloSecchi became interested in the physics ofthe Sun and sunlight’s effect on Earth. Sec-chi, an Italian Jesuit priest, made major ad-vances in stellar spectroscopy and geo-physics under the patronage of Pope PiusIX. During this period, he published acombined total of 700 articles and books.In 1865, Secchi focused on sunlight pene-tration through seawater. He argued thatnone of the previously collected light pen-etration depth data was useful. Since earli-er scientists had not reported the solar ele-vation or the degree of cloudiness, it wasimpossible to generalize from their obser-vations.

That same year, Secchi made systematicmeasurements of light penetration onboard the Immacolata Concezione. He usedseveral different iron disks (the largest be-ing 3.73 m in diameter) covered withwhite-varnished sail material. At each sta-tion, Secchi made replicate measurementsof light penetration and simultaneouslymeasured the solar zenith angle with a sex-tant. The measurements were then con-verted into equivalent depths for verticallyincident sunlight.

Secchi noted that the depth measuredusing the disk method was not the depthof light extinction. On the contrary, lighthad to travel twice the depth of the sub-merged object to return to the observer’seye. In addition, since the disk disappearedwhen the only colors of light left to reflectoff it were the same color as the surround-ing ocean, he noted that light must traveleven further into the ocean than twice thedisk depth. Based on these conclusions,Secchi speculated that blue and purplelight could penetrate to great depths. Forhis contributions, Secchi’s name was even-tually given to light penetration measure-ment disks used by generations ofoceanographers.

In 1922, V. E. Shelford and F. W. Gailadapted a Kunz gas-filled photoelectric cellto measure light penetration off the westcoast of the United States in Puget Sound.H. H. Poole and W. R. G. Atkins modifiedthe Shelford and Gail design for use in therougher waters of the English Channelnear Plymouth. In 1925 and 1926, they car-

Jenkin found that both total light ener-gy and photosynthesis decreased exponen-tially with depth. Yet near the ocean’s sur-face, photosynthesis did appear to besomewhat inhibited. Since the inhibitionwas less noticeable in the partially black-ened bottle and more noticeable duringthe midday irradiation, Jenkin concludedthat intense light must itself be the limitingfactor. Due to the general increase in oxy-gen production with increasing incidentlight, the depth of maximum oxygen pro-duction was about 5 m below the watersurface.

Jenkin also calculated a compensationdepth at which photosynthesis exactly bal-anced respiration over 24 hours. Based onthese findings, she produced a plot of“photosynthesis vs. irradiance”—a modelso effective that it is still used commonly inthe 1990s. In addition to being the first ex-periment to combine measurements ofphotosynthesis with simultaneous meas-urements of light, Jenkin’s light and darkbottle procedure is still used to determinethe rate of primary production.

In 1935, Hans Pettersson felt that thePoole and Atkins photoelectric method de-scribed earlier, though quantitative, wasrather complicated and unwieldy becauseit required an external amplifier to makethe tiny currents strong enough to meas-ure. Recognizing that submarine daylightwas essential to primary production in theocean, Pettersson invented a more accuratemeasure of ocean penetration.

Pettersson placed a cuprous oxide cellin a submarine actinometer that he hadconstructed with colleagues. He used theactinometer to measure light from aboveand scattered light from below. Althoughthe cuprous oxide cells he used had a num-ber of problems (such as losing sensitivityafter exposure to intense light), Petterssonobtained continuous records of submarinedaylight at various depths for a year, in-cluding some measurements made be-neath a 20 cm sheet of ice.

Since those early measurements of lightpenetration, there has been an explosion ofwork on the subject. Today, more precisesilicon diodes have replaced photocells forlight detection, and instruments are morestreamlined and accurate than those of the1930s. However, the same optical proper-ties—irradiance, scattering, and absorp-tion—are still measured. And the goal re-mains the same: to relate remotely sensedproperties to the physical, chemical and bi-ological states of the ocean.

Absorption studies in transparencyIt is important to note that absorbance af-fects transparency: any light that is ab-sorbed is unavailable to be scattered. In1936, William Powell and George Clarkediscovered that absorbance near the sur-face of the ocean was significant to trans-parency. They quantified light reflectionoff the surface of the ocean to determinethe varying “surface loss” estimates by re-flection. They found that reflection de-pended strongly on the angle of the Sunand the state of the sea. Furthermore, theproportion of light reflected back from thesurface could vary from 3 to 9% while theSun was more than 30 degrees above thehorizon. They attributed the 60% “surfacelosses” reported by previous scientists toabsorption by a particularly opaque layerof water just beneath the surface.

Using both a transparency meter and aTyndall meter to measure scattering, theDanish researcher Nils Jerlov determinedthe relative influence of dissolved and par-ticulate matter on transparency. In 1953, herelated scattering to particulate ab-sorbance. Jerlov then determined dissolvedabsorbance by difference: he subtractedabsorbance by particles alone from ab-sorbance by particles and dissolved mattercombined.

The euphotic zone is the surface layerof the ocean, where there is enough lightfor phytoplankton to live. Phytoplanktonare the microscopic (and sometimes large,but for optical purposes only the littleones are important)plants that live (andusually float) in the ocean. The euphoticzone is sometimes defined as the depth atwhich only 1% of the light incident at thesurface remains. It traps light and heat,and much of the biological activity of theocean is concentrated in that zone.

It is now relatively easy to measure irra-diance throughout and below the euphoticzone, either in discrete bandwidths or overan integrated range of visible wavelengths.However, the topic still has some appeal. In1986, Rudolf Preisendorfer published a re-

view paper that described how to makequantitative measurements using a Secchidisk. In this paper, Preisendorfer coded the“ten laws of the Secchi disk” into mathe-matical form. Although he did not recom-mend using a Secchi disk where more pre-cise, electronic measurements were appro-priate, Preisendorfer noted that there was along record of Secchi disk measurementsavailable for comparison in long-termstudies. In 1988, a team led by MarlonLewis and Charles Yentsch used the Secchidepth measurement database to calculatemean transparency fields in the ocean.From variations in mean transparency,they were able to identify areas of majorupwelling and show temporal variation infronts of new production.

The color of the oceanCertainly, people besides Aristotle musthave noticed before the nineteenth centurythat the ocean was not the same coloreverywhere or all the time. By the early sev-enteenth century, people evidently recog-nized that the color of the ocean was affect-ed by its components. Shakespeare’s Mac-beth lamented that if he tried to wash hisbloodstained hands in the ocean theywould “the multitudinous seas incarna-dine.”

But other than an occasional referenceto the sea’s turning to blood, references tothe cause of ocean color are scarce beforethe work of J. Aitken in the late 1800s.Aitken went to the Mediterranean Sea todetermine why the water was so blue andwhy the water color varied from one placeto another. He wanted to know whetherparticles or impurities in the Mediter-ranean Sea determined its color, orwhether the absorbance of red was respon-sible. Without any complicated equipment,he tested the absorbance hypothesis in sev-eral ways.

Aitken initially looked at submergedwhite objects through a glass-ended tube.He also submerged several colored objects,including oranges and lemons, and ob-served their underwater color. All the re-sults, including the apparent unripening ofthe fruit, showed that the water preferen-tially transmitted blue and absorbed red.Aitken also determined that color bright-ness depended on the size and color of sus-pended particles.

To be sure that the red absorption wasnot due to any impurity, Aitken condensedwater into several containers made of dif-ferent materials. He wanted to be sure that

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32 Optics & Photonics News ■ April 2001

In the late 1800s, J. Aitkenwent to the MediterraneanSea to determine why the

water was so blue and whythe water color varied from

one place to another.

the resulting water always looked blue. Itdid, and Aitken was quite satisfied with theresult. He commented: “The addition ofimpurities to water seems generally tochange its color from blue to green or yel-low… No attempt was made to find outwhat the discoloring substances in waterare. The task would evidently be an endlessone, and of little value.” Other scientistsdisagreed with Aitken’s assessment of thevalue of further inquiry into this matter. Asa result, research into the cause of oceancolor did not end in 1882 (as will be dis-cussed in later sections of this article).

During the first decade of the 1900s,both Germany and Norway conducted ex-periments related to the spring diatombloom. In 1904, this research led FranzSchütt, a botanist in Kiel, to comment thatthe color of the water was determined byits physical structure and the physiology ofthe organisms living in it. Schütt’s col-league Victor Henson, along with otherKiel scientists, noticed that phytoplankton,in high concentrations, turned the watergreen (although a detailed absorbancespectrum had to wait until Yentsch’s workin 1960).

In the 1920s and 1930s, the realizationthat phytoplankton did not absorb all col-

ors equally drove efforts to determine theattenuation rates of different colors of lightthrough seawater. Sir Isaac Newton pub-lished results in 1686 showing that whitelight was made up of all the colors of therainbow, and that individual colors couldbe separated from the rest by refraction.This information, coupled with the workby James Clerk Maxwell and HeinrichHertz on the wave theory of light in the late1800s, allowed scientists to design experi-ments that used selective cutoff filters tomeasure penetration of light of differentcolors through the ocean.

The photocells used in the 1920s and1930s to measure sunlight penetration ofthe ocean were also useful for measuringthe penetration of different colors of light.In 1926, Victor Shelford and J. Kunz usedphotoelectric cells covered with plates ofcolored glass to measure the penetration ofred, yellow, blue, and green light throughthe ocean. In order to collect light incidentat a wider range of angles, they used ahemisphere-shaped filter to cover the topof the photocell. After the attenuation dueto pure water had been subtracted,Shelford and Kunz discovered that some-thing absorbed the short wavelengths oflight, leaving green and yellow at depth.

C. Utterback and J. Boyle (1933) and R.Oster and George Clarke (1935) also made

measurements of red, blue and green lightpenetration in situ with similar results.Their efforts were apparently the earliestrecorded measurements of absorbance bycolored dissolved organic matter in theocean (CDOM). CDOM, called “yellowsubstance” by W.V. Burt in 1958 (see be-low), is one of the most biogeochemicallysignificant constituents of seawater.

CDOM is now known to absorb mostof the UV radiation that enters the ocean.It protects phytoplankton and other organ-isms from intense UV radiation, and reactsphotochemically to form a variety of eco-logically and chemically important prod-ucts.

Narrow bandwidth filters that meas-ured light in precisely defined wavebandseventually replaced the older, broadbandfilters used by researchers in the 1930s.However, J. Tyler pointed out a problem as-sociated with these new filters in 1959. Sea-water acts as a strong monochromator.Eventually seawater absorbs short and longwavelengths, leaving a narrow band of lightcentered on 510 nm. Tyler believed thatmany of the narrow band filters let in toomuch stray light from other wavelengths.As a result, once the water had filtered outall the light at the wavelength to be meas-ured, the detector would still measure asignificant amount of 510-nm light. The

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April 2001 ■ Optics & Photonics News 33

The Mediterranean Sea off the coast of Cyprus.

leakage might only be a small percentageof the total measured light at the surface,but it could be 100% of the signal at depth.Such waveband leakage is still a problemfor ocean optical measurements. The nar-row band filters, which have proved toovaluable to discard, are still in use today.

In 1958, W. V. Burt measured light pen-etration through seawater in 13 wavebandsover a range of 400-800 nm and at depthsof 1170 m. Although he did not specify thedepth used to measure any of the wave-lengths, he noted that transmission in-creased with increasing wavelength. He at-tributed the rapid attenuation of shortwavelengths to a high concentration of“yellow substance,” which presumably ab-

sorbed the blue and violet light. This find-ing was another example of the early meas-urement of the biogeochemically impor-tant colored dissolved organic matter inseawater.

Absorption studies in colorLaboratory studies of absorption comple-mented the field measurements ofShelford, Kunz, and Burt. In 1926, E. O.Hurlburt measured visible radiation ab-sorption with three open ocean water sam-ples using a monochromator in the labora-tory. Although the spectral resolution ofHurlburt’s measurement is unclear in his

report, he is credited with making the firstdirect measurements of light absorption byseawater.

Hurlburt observed that if sunlightpassed through an atmosphere that con-tained the equivalent of 40 meters of water(salt or fresh), the resulting, selectively at-tenuated light spectrum coincided exactlywith the sensitivity curve of the human eye.From this observation, Hurlburt speculatedthat our eyes developed their sensitivity at atime when the atmosphere contained muchmore water vapor than it does today.

In 1927, Hurlburt went on to determineabsorption coefficients for UV radiation inseawater. He reported that UV was attenu-ated more quickly than visible radiation

through both seawater and fresh water.Magnesium chloride and calcium sulphatecontributed significantly to the absorptionof UV, while the other salts absorbed little.Speculating further about the ancientEarth, Hurlburt commented that it seemedunlikely that the atmosphere and oceanwould be transparent to the most intenseparts of the solar spectrum (the visibleportion) by coincidence. He suggested thatvisible radiation might have effectivelyburned its way through whatever chro-mophores had originally absorbed it. Al-though Hurlburt’s ideas about ancientEarth have never been verified, they remaininteresting speculations.

Following Hurlburt, George Clarke andHenry James at the Plymouth Laboratorymade light absorption measurements inseawater in 1938. The two hoped to discov-er the percentage of light attenuation inseawater attributed to suspended particlesand dissolved components, termed “color.”Using a monochromator with a one-metersample path length, Clarke and Jamesmeasured the absorption of different sea-water components from a number of loca-tions over a range of 365 to 800 nm. Theyfiltered their water through both coarseand fine filters and compared the colors ofthe filtrates. Since the filtered seawatersamples had almost the same absorption asdistilled water in that region of the spec-trum, they determined that particles wereresponsible for almost all of the absorptionat longer wavelengths (473 to 800 nm). Incontrast, they found that blue and ultravi-olet radiation was absorbed strongly bysomething in solution.

Clarke and James speculated that thisunknown, UV-absorbing material mighthave come from the disintegration of phy-toplankton. In order to compare the trans-parency of seawater in situ to that meas-ured in the laboratory, they deployed asubmarine photometer while collectingtheir samples. Clarke and James hoped tofind that laboratory absorbance measure-ments would provide a convenient proxyfor in situ light attenuation. Unfortunately,their preservation method changed thesample color too much for such compar-isons to be practical.

By the late 1950s, it was widely acceptedthat the blue color of the open ocean wasdue to the scattering of the shortest lightwavelengths and the absorption of thelongest light wavelengths. It was also ac-cepted that inshore waters turned greendue to the influence of dissolved organicmatter. However, although it was knownthat phytoplankton were affected by thecolor of incident light, the effect of phyto-plankton on ocean color was not clear.

In 1960, for the first time, it becamepossible to quantify precisely the contribu-tion of phytoplankton to spectral attenua-tion. Charles Yentsch published a spectrumof particulate phytoplankton absorbance.He determined that phytoplankton ab-sorbance shifted the reflectance of waterfrom blue to green because of its strong ab-sorption at 443 nm.Yentsch also noted thatmost of the colored material seemed to becontained in cells that did not pass througha 5-mm filter. This filter size was much

OCEAN OPTICS

34 Optics & Photonics News ■ April 2001

By the late 1950s, it was widely accepted that the blue color of the open ocean was due to the scattering ofthe shortest light wavelengths and the absorption of the longest light wavelengths.

larger than pore sizes generally used today,but it helped to standardize the measure-ment of phytoplankton absorbance.

In 1981, Annick Bricaud and colleaguesat the Université Pierre et Marie Curie(France) reported that dissolved organicmatter might affect ocean color in muchthe same way as phytoplankton did in adifferent spectrum region. Bricaud’s teampresented absorbance spectra that showedthat dissolved organic mat-ter absorbed UV strongly,and visible radiation less so.This demonstration markedthe first time absorbancespectrum was described asan exponential decay withwavelength—an insight thathas proved useful to subse-quent optical and chemicaloceanographers.

Bricaud’s team foundthat the log-linearized spec-tral slope did not vary great-ly. Therefore, if one knewthe absorbance at one wave-length, one could apply an average slope tocalculate the absorbance of all the otherwavelengths over the UV and visible spec-trum.

Since that time, however, a number ofscientists have disputed the claim that theslope of the log-linearized absorbancespectrum of colored dissolved organicmatter is constant. There have subsequent-ly been many attempts to characterize thechanges in the slope with respect to loca-tion, season, and salinity. So far, there hasbeen no consensus. Often researchers stilluse the single slope calculated by Bricaudin models.

Interpretation of remotely sensed dataSince the mid-1900s, scientists have inter-preted optical variations as indications ofphysical, chemical, and biological processesin the ocean. Based on these interpreta-tions, scientists began applying that knowl-edge to remotely sensed data. The earliestremote sensing data was collected by air-plane. In 1954, C. Cox and Walter Munktook aerial photographs of the sea’s surfacewhile a ship below them measured windspeed. They calculated statistics of the sur-face glitter using the photographs, andcompared them to the actual state of thesea at the time they were taken. From thisrelationship, Cox and Munk found thatglitter could be used as a proxy for sea sur-

face slope and wind speed.Ocean color remote sensing on a global

scale became possible with the advent ofartificial satellites that carried light re-flectance sensors. With the beginning ofthe American National Aeronautics andSpace Administration (NASA) program in1958, people were able to see photographsof the entire Earth for the first time. Pat-terns were visible in the clouds, and in 1964NASA launched Nimbus 1—the firstweather satellite to take cloud cover photo-graphs relating to local weather. A succes-sion of weather satellites followed, eachwith more advanced sensors.

In 1978, NASA launched Nimbus 7, aweather satellite that carried the CoastalZone Color Scanner (CZCS). CZCS meas-ured the reflectance of different light wave-lengths from the ocean’s surface, makingremote sensing of ocean color possible.Now that weather prediction is possible ona local scale, NASA wants to predict long-term trends in climate change and to findpatterns that will help to predict suddenchanges in weather. Ocean color, as earlierresearch has shown, reflects the physical,chemical, and biological conditions of theocean. As a result, it is a natural parameterto study in relation to climate change.

In 1981, R. Austin and T. Petzold atScripps Institute of Oceanography (Cali-fornia) published empirical relationshipsthat related the reflectance ratio at two vis-

ible wavelengths measured by CZCS to thein situ light attenuation at an intermediatewavelength. At the same time, HowardGordon, an influential optical oceanogra-pher based at the University of Miami,published atmospheric correction algo-rithms. These algorithms, when used withAustin and Petzold’s empirical relation-ships, allowed the calculation of chloro-phyll concentration in surface waters. Gor-don also tested a chlorophyll retrieval algo-rithm that did not use the reflectance ratiomethod. This algorithm predicted chloro-phyll concentrations to within 30% of thein situ value.

In 1988, Trevor Platt and ShubhaSathyendranath at the Bedford Institute ofOceanography (Nova Scotia) created an-other algorithm for global scale estimationof chlorophyll concentration. Making thislink between ocean color and remote sens-ing was essential for global calculations ofprimary production by phytoplankton. Al-though the CZCS satellite was no longeroperational at this time, the model was in-tended for use with a satellite to belaunched later. Platt and Sathyendranathalso proposed that there should be large-scale field studies to ground-truth the al-gorithm.

By the late 1980s, it had become appar-ent that the errors in the chlorophyll esti-mates were not improving. Something wasinterfering with the estimates made from

OCEAN OPTICS

April 2001 ■ Optics & Photonics News 35

An Illustration of the SeaStar spacecraft,which carries the SeaWiFS sensor(left). A SeaWiFS image of the Caribbean Sea (above).

reflectance data. In 1989, Ken Carder at theUniversity of South Florida showed thatthe CZCS algorithms often made chloro-phyll concentrations too high in areaswhere there was a large concentration ofcolored dissolved organic matter. Carder’steam pointed out that earlier assumptions(such as the premise that dissolved organicmatter absorption at 443 nm was low andthat it co-varied with chlorophyll) were in-correct in many places. They suggested thatthese errors could account for the incorrectchlorophyll estimates.

In 1995, Dave Siegal and AnthonyMichaels at the Bermuda Biological Stationagreed. They calculated that in some areascolored dissolved organic matter could ac-count for up to 60% of the observed ab-sorption. Algorithms for quantifying dis-solved organic matter absorption from re-mote reflectance measurements are still be-ing developed. The Sea-viewing, Wide-Field-of-View Sensor (SeaWiFS) sensor onthe SeaStar satellite, launched in August1997, will provide more data for testingand improving calculated relationships.

ConclusionSince the mid-1800s, the study of light inthe ocean has been driven largely by inter-est in the controls on primary production.More recently, the focus has shifted to theprediction of local weather and global cli-mate change. Today, we seem to have con-vincing explanations for most of the varia-tions in the color and transparency of theocean. As Aitken concluded in 1882, coloris determined by absorption, and the in-tensity of color by scattering. Both scatter-ing and absorption affect transparency.Phytoplankton, dissolved organic matter,detritus, and water itself all contribute tothe color of seawater. We can even plot thespectral effect of each component onocean color. The discovery of the causes ofocean color and transparency required thedevelopment of a number of instrumentsand the curiosity and techniques to usethem. Remote sensing now allows us toapply the ocean process knowledge wehave gained on a global scale, and to ap-proach answers to Aristotle’s two thou-sand-year-old questions.

Further reading1. J.Aitken,“On the colour of the Mediterranean and

other waters,” Proceedings of the Royal Society ofEdinburgh, 1881-1882, 472-83 (1882).

2. O.Krümmel,Handbuch der Ozeanographie (Verlagvon J. Engelhorn, Stuttgart, 1907).

3. H. Pettersson,“Submarine daylight and the trans-parency of sea water,” Journal du Conseil Interna-tionale pour l’Exploration de la Mer, 10, 48-65 (1935).

4. C.S.Yentsch,“Measurement of visible light absorptionby particulate matter in the ocean,” Limnology andOceanography, 7, 207-17 (1962).

5. N.G. Jerlov,Optical Oceanography (Elsevier,Amster-dam,1968).

6. R.W.Austin and T.J. Petzold,“The determination of thediffuse attenuation coefficient of sea water using thecoastal zone color scanner,” Oceanography fromSpace,R.F.Gower, ed. (Plenum Press,New York andLondon , 1981), pp.239-56.

7. A.Bricaud, et al.,“Absorption by dissolved organicmatter of the sea (yellow substance) in the UV andvisible domains,” Limnology and Oceanography, 26,43-53 (1981).

8. R.W.Preisendorfer,“Secchi disk science: Visual opticsof natural waters,” Limnology and Oceanography, 31,909-26 (1986).

9. E.Mills, Biological Oceanography: An Early History,1870-1960, (Cornell University Press,NY,1989).

10. SeaWiFS Project website,http://seawifs.gsfc.nasa.gov/SEAWIFS.html, verifiedMarch 6, 2001,maintained by Gene Carl Feldman(gene@seawifs.gsfc.nasa.gov) (2001).

Sophia Johannessen is an oceanographer. She can bereached by e-mail at johannessen@pac.dfo-mpo.gc.ca.

OCEAN OPTICS

36 Optics & Photonics News ■ April 2001

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