JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 95, NO. Cll, PAGES 20,237-20,246, NOVEMBER
ON
OF THE VERTICAL
CHLOROP•LL
a IN
THE
NONCORRELATION
STRUCTURE
OF LIGHT
CASE I WATERS
SCATTERING
15, 1990
AND
James C. Kitchen
Sea Tech, Inc., Corvallis,Oregon
J. Ronald V. Zaneveld
Collegeof Oceanography,
OregonStateUniversity, Corvallis
Abstract. The relationshipsbetween beam attenuation,
absorption,
suspended
particleconcentration,
sizedistribution
and pigment contentare examinedfor a region where the
particle concentrationand pigment maxima are widely
separated.Mie scatteringanalysesareperformedon thisdata
to predictthe profiles of backscattering.It was found that
absorptionand scatteringare not and have no theoretical
concentration
is extremelyusefulin the inversionof remotely
sensedoceancolor spectra,we will show in this paper that
theserelationshipscannotbe used to predict the vertical
structureof scatteringand backscattering
as a functionof the
verticalchlorophyllstructure.
reason to be well correlated in a vertical section even if the
concentration <s>. For case I waters Morel [1988] used the
particle concentration is perfectly correlated with
phytoplankton
biomass.Due to therapidchangein irradiance
with depth,pigment concentrationand thereforeparticulate
absorptionare not expectedto be correlatedwith biomassor
productivityas calculatedfrom publishedlaboratoryresults
andtheoreticalmodelsof phytoplankton
pigmentcontent.The
beamattenuationis a goodindicatorof particleconcentration
andits verticaldistributionrelativeto the pigmentdistribution
is consistentwith models of phytoplanktongrowth. An
relationship
b = 0.30 <chl>
0.62where<chl>is thechlorophyll
concentration
to obtain<s>= 0.30 x 103<chl>
0.62.Thisimplies
examination
of data from a meridional
transect of the North
Pacificrand
fromtheNortheast
Pacificcoastal
upwelling
region
suggestedthe universalityof thesefindings. The remotesensing
algorithmsthatpredictbackscattering
fromchlorophyll
concentration
were developedusingnear-surface
data. Use of
thesealgorithmsfor the predictionof vertical structureof
backscattering
is discouraged.
Introduction
The major tool for the global study of phytoplankton
distributionis remote sensingof oceancolor from satellites.
Interpretation
of the remotelysenseddatadependsstronglyon
our ability to measureaccuratelyand routinely the spectral
inherentopticalpropertiesin the ocean. Gordonand Morel
[1983] have reviewed the models for inverting remotely
senseddata to obtain the chlorophyll concentrations.All
modelscontainparameters
of the formFϥbt,/a,whereFϥis
a parameterthat dependson the radiancedistributionandthe
volumescatteringfunction,13(0),of the waterandparticles.bb
is the backscatteringcoefficient and a is the absorption
coefficient. The remotely sensedocean color spectrathus
dependprimarily uponthe absorptionand the backscattering
[Zaneveld, 1982; Gordon and Morel, 1983; Carder et al.,
1986]. The models currently used to extract chlorophyll
concentrations
from the oceancolor spectraassumethat for
largeregionsof the ocean(caseI waters[Morel andPrieur,
1977]) the backscattering,biomassand absorptioncan be
described as being dependent only on the chlorophyll
concentration[Gordon et al., 1988; Platt et al., 1988; Morel,
1988]. Thesemodelsare robustbecausechangesin spectral
ratiosof absorption
arerelativelylargecompared
to changes
in
thespectral
railosof backscattering.
While the statistical dependence of backscattering,
absorptionand diffuse attenuationon the chlorophyll
Gordon and Morel [1983] have shown that the total
scattering coefficient, b, is related to the total seston
that the sestonincreasesmonotonicallywith chlorophyll.
Relationships
derivedfor thedepth-averaged
structure
arenow
being used to study vertical structure [Gordon, 1989;
Sathyendranath
andPlatt, 1989]. We will showin thispaper
that the vertical structure of scattering, sestonand beam
attenuationare significantlydifferent from the chlorophyll
structureand cannotbe derived by meansof the monotonic
relationships
usedin theinversionof remotesensingdata.
Pak [1984] and Pak et al. [1988] have shown that in most
of the tropical and subtropical Pacific there is a seston
maximumthatis distinctfrom the chlorophyllmaximum. The
sestonmaximumtypicallyoccursjust beneaththemixedlayer,
whereasthechlorophyllmaximumoccursat thebottomof the
euphoticzone. The two maximaareoften separated
by 50 m
in depth.The layerfromnearthetopof thethermocline
to the
bottomof the euphoticzone often is a regionof decreasing
sestonand increasingchlorophyllwhich doesnot satisfya
monotonicrelationship.For the purposeof radiativetransfer
studiesthat involve the vertical structureof backscattering,
the
relationship between chlorophyll concentration and
backscattering
thusneedsto be re-examined.
Gordonet al. [ 1988] have shownthat the backscatteringis
the mostdifficult parameterto estimatein the remotesensing
algorithms. Gordon [1989] uses the remote sensing
correlations to study the vertical structure of the diffuse
attenuationcoefficient. Sathyendranath
andPlatt [1989] have
studiedthe influenceof the verticalstructureof chlorophyllon
the surface reflectance, but they also assumedthat the
backscattering
variesmonotonicallywith chlorophyll.Given a
collectionof suspended
particles,it hasbeenshownthat the
shapeof thebeamattenuation
spectrum
is far moredependent
on the slopeof the size distributionthan on the pigment
concentration[Mueller, 1974; Kishino, 1980; Kitchen et al.,
1982]. The beamattenuationcoefficientis thusnot expected
to be well-correlated with chlorophyll or total pigments.
SiegelandDickey[ 1987]haveshownthatin thecentralgyre
of the North Pacific the diffuse attenuationcoefficient Ka(z,x.)
is not significantlycorrelatedwith the beam attenuation
coefficient,but is well-correlatedwith totalpigments.This is
so becauseKa(z,z.) is approximatelyproportionalto (a +
bt,)/cos0s
[Gordon,1989],whereOsis the solarzenithangle,
so that in an oligotrophicregion it is an absorption-like
parameter.
Thebeamattenuation
coefficient
ontheotherhand
is notnecessarily
absorption
dominated.
The beam attenuation
Copyright 1990 by the American Geophysical Union.
Paper number 90JC01296.
0148-0227/90/90J C-01296505.00
coefficient
is the sum of the
absorptionand scatteringcoefficients.Kullenberg[1970]
foundthatin theSargasso
Seaparticulatescattering
wastwice
ashighasparticulate
absorption
at 633 nm and4 timesashigh
20,237
20,238
KitchenandZaneveld:LightScattering
andChlorophyll
a
at 440 nm. Petzold[1972] foundabsorption
dominatedwaters
in theTongueof the Ocean. For radiativetransfercalculations
in which the radiance itself is of interest, so that the beam
attenuation coefficient plays a major role, chlorophylldependentalgorithms,whichderivescattering
propertiesfrom
absorption-like
properties,are thusnot sufficient. For those
calculations beam attenuation models might be more
appropriate.A large numberof studies[e.g.,Peterson,1978;
Baker, 1984; Gardneret al., 1985; Bishop,1986; Pak et al.,
1988] have shown the beam attenuation coefficient to be
highlycorrelatedwith the total suspended
particulatevolume
concentrationwhich in caseI watersis of biologicalorigin.
The chlorophyllconcentrationin turn dependson the total
suspended
volume and photo-adaptation.Irradiancelevels
play an importantrole in the determinationof chlorophyll
concentration,
sothattheverticalstructureof chlorophyllneed
not be well-correlatedwith the total suspendedvolume of
biologicallyderivedmaterials.The pigmentconcentration
can
be measured by in situ fluorescence[Pak et al., 1988],
whereasthe particulatevolume is measuredby the beam
attenuation coefficient
as noted above.
While knowledge of the backscatteringcoefficient is
importantin the interpretation
of remotelysenseddatain the
visiblepartof the spectrum,
veryfew measurements
havebeen
made. Jerlov[1961], Morel [1966], Kullenberg[1968], and
stations were designated as "northern", "southern" or
"intermediate"
basedon their temperature-salinity
structure.
Beamattenuation,
densityandin situfluorescence
profilesare
shown in Figure i for typical northern, southern and
intermediate
stations.Itemsof notearea "mixed"layerof one
or two isopycnal layers of widely variable depth, an
attenuationmaximum near the top of the pycnoclineand a
fluorescence
maximumwell belowthe top of the pycnocline
whichdoesnot vary in depthasmuchasthepycnocline.The
attenuationprofile varies in shapewhile the fluorescence
profile is fairly constant except when the mixed layer
approaches
it. The averagethicknessof the mixed layer is
significantlylarger southof the front thanto the north. The
particlemaximumis roughly 5 m below the bottomof the
mixed layer in both cases. The particle concentrationand
attenuation
arehigherandthe pigmentconcentration
lower in
the southfor boththe mixedlayer andthe particlemaximum
layerdownto 80 m depth.
The particlesize distributionswere fitted to a segmented
powerlaw relationship,
dN = NoD-SdD
(1)
where dN is the particle number concentrationbetween
diameters D and D + dD and s is the shape parameter
henceforth
calledthe slopeof the sizedistribution.One slope
Petzold [1972] have carried out measurementsof the volume
wasobtainedfor particlesbetween2 and6 •tm diameterand
scatteringfunction in situ at one or two wavelengthsthat
anotherfor thosefrom 6 to 16 •tm diameter.The slopeswere
included backscatteringmeasurements.Downing [1983]
similarfor all samplesdownto 80 m depthwith the southern
measuredbackscattering
in very turbidwaters. Moore et al.
stationshavinga slightlylargerslope(i.e., the "average"size
[unpublished SRI final report to DARPA, 1982] have
developeda backscattering
devicethatoperates
in theinfrared was smaller). The pigmentto volumeratiosincreasedwith
depthbeneaththe mixed layer and were larger for thicker
at 880 nm. Few of the backscatteringstudiesincluded
mixedlayers. Beneath80 m no significantdifferenceswere
simultaneous
measurements
of chlorophyll. Thus we have
notedbetweentheregionsnorthandsouthof thefront,but the
few datato describethe distributionof backscattering
in the
ocean.
size distributionschangedwith depth. The chlorophylland
phaeophyfin
maximawere at an averagedepthof 99 m in both
Or the other hand we have many profiles of the beam
attenaationcoefficient (665 nm), particle size distribution, cases. The pigment to particle-volumeratios attained a
spectraldiffuse attenuationand chlorophyllconcentrations. maximumnearthepigmentmaximaandstabilizedor declined
slightlybelowthat. To examinethedistribution
of opticaland
Mie scatteringmodelswill be usedwith a data setfrom the
PacificCentralGyre to demonstrate
thatinternallyconsistent particleparameterswe first groupedall samplesinto 10-m
depth bins. Combining bins that were not statistically
estimated
profilesof backscattering
donotresembletheprofile
different, we found that we could characterizethe optical
of chlorophylla concentration,exceptin a shallow surface
variabilityby sevengroups.This characterization
of the data
layer. Other existingdata setswill be presentedto showthe
in Table 1.
universality
of thenon-correlation
of particleandchlorophyll setis presented
The observedrelationshipsbetweenbeam attenuationto
concentrations. This universality will be shown to be
particulatevolume ratios and pigmentto volume ratios were
consistent
with a modelof primaryproductivity.Finally, the
comparedto optical theory using the anomalousdiffraction
algorithms
thatarecurrentlyusedto predictbackscattering
for
approximation [Van de Hulst, 1957] for the attenuation
variousapplications
will be criticallyexamined.
efficiency: The productof the attenuationefficiencies,size
distributionand particle cross sectionswere numerically
integratedfrom 0.5 to 30 gm diameterfor real indices of
Data andAnalyses
refractionvaryingfrom 1.01to 1.25andfor imaginaryindices
(proportional
to pigmentcontent)from0 to 0.1. Theresulting
As part of the OpticalDynamicsExperiment(ODEX), we
beamattenuationto particlevolumeratioswere plottedas a
sampledin excessof 100 stationswithina regionboundedby
32øand34øN Latitudeandby 140ø40' and143øW Longitude. function of the real and imaginary parts of the index of
refraction. The results indicatedthat the imaginarypart
Thisregionis in theNortheastem
PacificGyrein thevicinity
(proportional
to pigmentcontent)did not affecttheattenuation
of theSub-TropicalFront. Beamattenuation
coefficients
(665
nm), fluorescence,
spectralscalarirradianceandhydrographic efficiencies, which is consistentwith our earlier observations
[Kitchenet al., 1982]. For instance,while the chlorophyll
parameterswere measuredin situ. Chlorophyll a and
contentin theparticlemaximumwasalmosttwicethevaluein
phaeophytinconcentrations,
particle size distributionsand
the mixed layer, the attenuationto volume ratios were the
particulateabsorption(on filters) were obtainedaboardship.
same.Thisis not an unexpected
resultsincethecompensating
Chlorophylla andphaeophyfinconcentrations
were obtained
nature of scatteringand absorptionfor a collectionof low
by filtration and extractionby the method of Smith et al.
indexparticles(truefor the sizedistribution
asa whole,notfor
[ 1981]. Particle size distributionsfrom 2 to 16 •m diameter
eachindividualsizeparticle)was describedby Van de Hulst
wereobtainedwith a resistive-pulse
typeparticlecounterusing
[1957] and was also predictedfor diatomsby concentric50 and 100 •m apertures.Particulateabsorptionspectrawere
spherescattering
modelsby Mueller [1974].
obtained using the glass fiber filter technique (GFFT)
Increases in attenuation efficiencies from 80 m to 130 m
describedby Mitchell andKiefer [1984, 1988]. Whatman934
depthwere consistentwith the increasingslopesof the size
AH filterswereusedfor thispurpose.
The SubtropicalFront, which separatesthe East Central
distributionfor the particleslessthan 6 gm. However,the
North Pacific water from the less saline waters derived from
increase
in attenuation
efficienciesfor samples
deeperthan80
Subarctic North Pacific water, was very convoluted and
m relativeto thoselessthan80 m waslessthanexpectedfrom
KitchenandZaneveld:LightScattering
andChlorophyll
a
0 B
24.4
I
24.8
/
I
!I
..
25.2
25.6
I
I
/
2.4.4
26.0
I
24.8
25.2
South
25.6
i
F
1
20,239
26.0
I
i
North
Intermediate
t
I
-
tx
I
120 -
160 -
BEAM
200
0
I
2
:5
4
5
!ON
[n•
m)
i
$
2
0
4
, , ,
0
I
2:
:5
4
FLUORESCENCE (relative units)
Fig. 1. Profilesof fluorescence,
beamattenuation(665 nm) andot for PacificCentralGyre stationstypicalof
watersnorth,southandin the SubtropicalFront,Oct.-Nov.,1982.
thechanges
in theparticlesizedistributions
in spiteof thefivefold increasein chlorophyllcontentof the particles. In fact,
the beam attenuation coefficient (665 nm) was much better
correlatedwith the measured(2.2 < D < 16 gm) totalparticle
volumeconcentration
thanwouldbe predictedby Mie theory
TABLE 1.
usinga constant
realindexof refractionwith animaginarypart
proportionalto pigment content. If the observedsize
distributionis extrapolatedto a 0.5 to 30 gm diameterrange
andthe newvolumeconcentration
is calculated,the agreement
between attenuation and volume concentration is even better
Means and Standard Deviations (in Parentheses)of the Slopes of the
Segmented Particle-Size Distributions, the Particulate Attenuation to Volume Ratios and
the Chloropigment to Volume Ratios for Samples From Various Depth and Water-Mass
Regions in the Central Gyre of the North Pacific
Region
psdSlope
Ratioof Particulate
Ratioof Pigment
Attenuation to Particle
D > 6 gm
Volume,gm-1
Concentration to
n
D < 6 gm
n
Particle
Volume,g/L
North
23
2.97 (0.21)
5.41 (0.41)
1.01 (0.14)
15
2.0 (0.6)
South
32
3.14 (0.31)
5.42 (0.41)
1.13 (0.19)
33
2.9 (1.0)
North
17
2.99 (0.30)
5.16 (0.26)
0.98 (0.16)
12
3.3 (0.8)
South
17
2.94 (0.30)
5.52 (0.64)
1.14 (0.16)
17
5.2 (1.4)
1-70 m, mixed layer,
41-80 m, atten. max.
81-90 m
14
3.44 (0.24)
5.09 (0.66)
1.12 (0.23)
12
8.2 (2.6)
91-120m
63
3.63 (0.33)
4.96 (0.44)
1.17 (0.23)
52
9.8 (2.7)
121-130m
28
3.92 (0.36)
4.96 (0.53)
1.33 (0.27)
19
9.3 (2.8)
North and Southrefer to verticaltemperatureand salinitystructuretypicalof stationsnorthor southof the
SubtropicalFront.
20,240
KitchenandZaneveld:Light ScatteringandChlorophylla
(Figure2). It would seemthat whenphytoplankton
optimize
for light absorption they also decreasetheir scattering
efficiencies.This may be part of the self-shadingadaptation
for populationsin the mixed layer. The endresultis that a
regression
predictingparticlevolumeconcentration
from beam
attenuationwould yield resultscorrectwithin 5 ppb for this
casewhile trying to correctfor sizedistributioneffectswould
producemuchlargererrors(approximately10 ppb).
,
I
,
I
,
I
,
,
I
,
I
,
Mueller [ 1974]. It was found that the index of refraction of
thelargeparticleshada negligibleinfluenceontheattenuation
efficiencies.Sincewe cannotassignan m' valueuntiltheratio
of high to low index particles is known, m' for the fourcomponentmodel was first computedassuminghalf of the
particlesare low index (chlorophyll-bearing).Then the Mie
scatteringcomputations
were repeatedusingan m' consistent
with theratioof low to highindexsmallparticlesnecessary
to
producetheobservedattenuationfor two cases:onewherethe
largeparticlesall havea highindex(modelLH) andonewhere
thelargeparticlesall havea low index(modelLL). The values
obtained for m' at 440 nm varied from 0.003 to 0.028.
The
light adaptationas reflected by scatteringefficiencieswas
indicatedby the observationthat from the surfaceto 80 m
depth,80% of the smallparticleshad to havea highindexto
producethe correctattenuation,
while only 50% hadto havea
high index deeperthan 80 m. High scatteringefficiencies
couldalsobe dueto a deviationfrom sphericalshaperesulting
in highercrosssectionto volumeratios. The backscattering
would be especially sensitive to shape. The previous
cpobserved
- -(anomalous
- CpCOnstant
index
model
diffraction
method)
I
observation, however, would indicate that the shallower
samplesdeviatedmorefrom the sphericalwhichwouldresult
in the shapefactor enhancingthe differencebetweenthe
backscatteringand chlorophyll profiles. This is also in
agreementwith the observation[Lewis, 1976] that larger
phytoplankton
deviatemorefrom sphericalasthechlorophyll
maximumhasrelativelymore smallparticles. The rangeof
volumescattering
functionsobtainedfor themixedlayernorth
of the front are comparedto somehistoricaldatafrom central
50
ß Particleconc.(2-16 gm)
. ß Particle
conc.(0.5-30grn)x 0.74
gyresin Figure 3.
101
100
..............
10ø
Model LL
Model LH
ß
Tongueof the Ocean#7
ß
Tongueof the Ocean#8
10-1
150
0.02
'
I
0.03
'
I
'
0.04
I
0.05
'
I
0.06
'
I
0.07
'
I
0.08
10-2
'
0.09
Particlevolumeconc.(ppm)or
particulatebeamattenuation(I/m)
Fig. 2. Particulatevolumeconcentration
andparticulatebeam
attenuation(665 nm) for the North Pacific CentralGyre north
of the SubtropicalFront. The extendedparticleconcentration
(0.5 to 30 gm sphericalequivalentdiameter)is multipliedby
0.74 to ease the comparison with the beam attenuation
coefficient. An indexof refractionequalto 1.09 - m'i with m'
proportional to the chlorophyll content was used in the
anomalous diffraction
model.
and size distributions.
20
40
60
80
'
I
'
I
'
I
'
I
100 120 140 160 180 200
Scattering Angle (degrees)
We have modeled this effect by using a four-component
Mie [ 1908] scatteringmodelconsistingof two sizeclassesand
two indices of refraction. These four componentswere
combinedin proportionsthatyieldedthe observedattenuation
efficiencies
I
I
Fig. 3. Volume scatteringfunctionscomputedfrom our two
modelscomparedto historicaldata [Petzold, 1972] for clear
oceanwaters. Model LH indicatesthattheparticleslargerthan
6 gm in diameterall have high indicesof refractionwhile
ModelLL indicatesthe largeparticlesall havelow indices.
The Fortran subroutine to
computethe Mie scatteringcoefficientswastakenfrom Bohren
and Huffman [ 1983]. We used two size classesof 0.5 to 6.0
gm and of 6.0 to 30 gm diameterwith differentslopesof the
size distribution, and we used indices of 1.02 - im' and 1.15 -
i0. Here m' was chosento be proportionalto the observed
pigment to volume ratios at each depth consistentwith the
values for m' and chlorophyllto cytoplasmratios given by
Brown and Gordon [1973], using size distribution and
scatteringdata obtainedin the SargassoSea, concludedthat
70% of the particlevolumehada low indexwhile 90% of the
scatteringwas due to high index particles. Zaneveldet al.
[1974] analyzedthe samedatausinga minimizationtechnique
and concludedthat more than half the particleshad a high
KitchenandZaneveld: Light ScatteringandChlorophylla
index. Considering the uncertainties, neither result is
qualitativelydifferentfrom the resultsobtainedhere. Most of
the scattering
is dueto highindexparticleswhile bothhighand
low indexparticlesare significantin numberconcentration.
The absorptionprofiles obtained by the above analysis
(Figure 4) agree roughly with the results obtained by the
GFFT methodbutneitherexplainedtheobservedvaluesof the
diffuse attenuation
coefficients
obtained from the scalar
irradiance measurements.
The diffuse attenuation and the
20,241
of the GFF filters. Thus the consistencyof our Mie analysis
basedon a limited size range and pigmentdata collectedon
filters with Mitchell's GFFT absorptiondata is explainedby
the exclusionof small particlesin both cases. The measured
diffuseattenuationcoefficientsdo not excludesmallparticles.
To modelthiseffect,we increasedtheimaginarycomponent
of the index of refractionof the submicronparticlesto match
(at each depth) the absorptionestimated from the diffuse
attenuationdata assumingan averagecosine of • =0.8 for
the radiance field. The profile of m' for the submicron
particlesis not forcedto fit the profile of pigmentcontentand
is higher from the particle maximum to 80 m than at the
fluorescence
maximum. This may merelyreflectan increasein
• with depth. The resultsare labeled Model •K in Figure
4. If the componentof absorptionthat is missedby the LH
measuredand estimatedprofilesof the absorptioncoefficient
closely paralleled the chloropigmentprofiles but not the
particleconcentrationprofile. It is apparentthat the modeled
particulateabsorptionmissesa large portion of the actual
absorption.Both the chlorophyllmeasurement
andthe GFFT
absorptionmethod uses filters which may miss submicron
particles [Phinney and Yentsch, 1985]. Thus the GFFT
and LL models is due to dissolved material rather than to small
methodmaysimplymissparticlesandtheMie modellingcotfid particles, models LL and LH would still be valid for the
be biasedby an underestimateof the imaginarypart of the
scatteringpropertiessince little attenuationfrom dissolved
index of refraction. Absorptionby dissolvedorganicscould
matter
is expected at 665 nm and the particle scattering
alsobe contributing.B.G. Mitchell [personalcommunication,
propertieswould be unaffectedby dissolvedsubstances.
1989] indicated that changingfrom the Whatman 934AH
Cumulative(with respectto particlediameter)plotsof the
filters used on the cruise described here to a Whatman GFF
attenuation,
absorptionand scatteringat 2 ø and 180ø as
filter on a subsequentcruise resulted in much higher
computed from the•K model areshown in Figure 5. This
absorptionto attenuationratiosfor the GFFF method. This is
attributedto thehigherretentionefficiencyfor smallparticles indicates that particles in the 1 - 6 gm diameter range
contributesignificantlyto attenuationand to scatteringat all
angles,submicronparticles (cyanobacteria?)can contribute
significantlyto the total attenuationand absorptionwhile the
ß GFFT
largeparticles(D > 6 gm) can contributeto the near-forward
o px K- 0.016
and near-backward scattering in spite of their very low
--- ap LH
concentrations
in the studyregion.
...........
ap LL
--.--ap•tK
50
m•
0.08'
0.06
150
0.00
0.01
0.02
0.03
0.04
0.05
...... CpMXD
Non-waterabsorptionat 440 nm (I/m)
440nm ....................
ap MXD
0.02
.......... apFM
CpFM
ParticleVolumeConcentration
(ppm)
0.00
0
0.02
ß
i
0.04
ß
i
0.06
ß
i
0.08
ß
0.10
I
.
0.12
i
0.00
0
ß
ß
0
•
'
•
o
•
.. ß ['3•
a•
100
ß
OO
øß%ßo
0.0o
_---e
[I'
ß
o
o
_--"l
ß
150
, '
ß ..---I
•'•
oß ß O
'
o
o
ß
'
coefficients.
15
0.002
,,,.,,,.,....,..,....'''"
...............
,...
o
Also shown are the
chlorophylland particleconcentrationprofiles. Models LL
andLH areasin Figure3. For Model •K enoughabsorption
is added to the submicronparticles to match absorption
computedfrom theobserveddiffuseattenuation
coefficients.
•
/
/ ./
o.o
Fig. 4. Particulateabsorption
profilesat 440 nm and665 nm
wavelengthscomputedfrom the Mie scatteringmodels,
measuredwith the glassfiber filter techniqueand estimated
attenuation
10
......8(2
ø)
FM
ß
•
•...•--•
.............
........................
......................................
l
•
o,.I
ji• iiiIIIIIIIIIII
ParticulateAbsorption665 nm (I/m)
from diffuse
5
ap
......
ap LH
....................
ap LL
ß
ap GFFI'
do2
15
...............
2
ß
•
10
Diameter (!.tm)
o (chl+pha)/5
,•
5
' 'W•
..................
0
5
o.•
10
15
Diameter (•)
Fig. 5. Cumulative (as a function of particle diameter)
particulateabsorption,
attenuation,
andvolumescattering(all
at 440 nm) for the mixed layer (MXD) and the fluorescence
maximum(FM) in the Pacific CentralGyre basedon our Mie
scattering
modelassumingtheunexplainedabsorption
is found
in thedissolved
matteror submicron
particles
(modelPK).
20,242
KitchenandZaneveld:Light ScatteringandChlorophylla
The range of backscattering
profilesobtainedfrom models
LH, LL and •K by numerical integration of the volume
scatteringfunctionfrom 90 to 180ø is shownin Figure6. As
is obviousfrom the figure,backscattering
is distributedvery
muchlike the particleconcentration
whichis proportionalto
the beam attenuationand is not distributedlike the pigment
concentration. Also shown is the backscatteringprofile
derivedfrom the pigmentconcentration
via Morel's [1988]
equation:
b
samewatermasses.The universalityof the non-correlation
of
chlorophyllandparticleconcentration
canbe demonstrated
by
a quick look at data from a meridionaltransectof the North
Pacific and from a coastalcruise. The relationshipbetween
pigment content and particle concentration (or similarly
fluorescence
andbeamattenuationsincethey are more easily
obtained) is shown for the meridional transectof the Pacific
Ocean in Figure 7. We see a A shape with the left leg
representingsamplesbeneaththe chlorophyllmaximumand
the right leg thoseaboveit. The left leg is fairly linear and
_.
06
0.0•--•
ß 2BONN
35 ø
152 ø W
ß
[] 40 ø N, 150ø W
(2)
There is only slight disagreement between Morel's
predictionand the modelLH down to the particlemaximum,
but they are quite divergent after that. Significant
••
•
disagreement
is notseenin thefirstopticaldepthwhichmay
explainwhythemodelworksfor oceancolorremotesensing
•
•
•'
4
3
applications.
Notethattheincreased
smallparticle
absorption •
inmodel
•Kresultsin
decreasedbackscatteringat
the
pig- • 2
mentmaximum,a contradiction
to Morel'smodelequation(2).
Particleshapecouldaffectthemagnitudeof backscattering,
but
since
shape
isnotcorrelated
systematically
withchlorophyll,
1
no improvementin verticalstructureis expected.
0
ß
ß
8
'
'
I
ß
'
ß
0.40
0.35
,
I
'
I
,
I
ß 44ON,
148ow•
6
ß
'
'
0.45
i
ß
I
....
0.50
I
i
0.55
I
-
[]42øN'la9øWw
ß 50oN, 145oW
ß 57øN, 148ø
surface waters
150
0.000
-
0.d01
0.•02
-
0.(•03
-
Backscattering
440 nm (l/m)
I
I
:
I
o
0.3
I
i
%
% 'I
='
bb •K
.......... bb Morel
..•.....,5.
0, '--.
100
0.000
0.•01
0.•32
0.4
0.5
0.6
0.7
0.8
Beam Attenuation (l/m)
Fig. 7. Fluorescenceversusbeam attenuation(665 nm) for
variouswatertypesin theNorthPacificduringthe summerof
1985. Note thechangein scalesbetweentheupperandlower
graphs.
....................
bb LL
50
150
surface waters
0.004
0.003
Backscattering665 nm (l/m)
Fig. 6. Backscatteringcomputedfrom our variousmodels
comparedto the resultsof applyingMorel's [ 1988] algorithm
to theverticalstructureof chlorophyllconcentrations.
The aboveanalyseswerecardedout in the CentralGyre on
eithersideof the SubtropticalFrontbecauseof the statistical
advantages
of the largenumberof profilesavailablefrom the
constantin slope,while the right leg varies a greatdeal in
shapeandslope. A commonoccurrence
is for therightleg to
be perfectlyvertical. The A shapewas alsofoundin coastal
waterswherethe euphoticzoneis lessthan60 m deepandthe
chlorophyllandparticlemaximacoincide(Figure8). The light
adaptation
canalsobe seenfrom stationto stationfor thesame
cruise. Samplesfrom the mixedlayer(opencirclesin Figure
9) showan exponential
increasein pigmentconcentration
with
particleconcentration.This is explainedby the lower light
levelsin the more turbidmixedlayers. We haveneverfailed
to observethe A shapes,includingcruisesoff the coastof
Peru, in the equatorialregion betweenHawaii and Tahiti,
transecting
the CaliforniaCurrent,duringspringbloomin the
North Atlantic and in the SargassoSea. Mueller and Lange
[1989] have also reported the universality of exponential
increasesin fluorescencewith depth.
KitchenandZaneveld:LightScattering
andChlorophyll
a
2.0
1.8
1.6
20,243
2.0
ß
>60m
ß
30- 60 m
0
<30m
o < 30m
ß .50-60 m
1.8 _
ß >60m
x INL
+ BNL
1.6
o
o
ß
1.4
o
o
o
1.2
z
o
ß
oo
ß
1.0
ß
8
ß
o
•J
0.8
ß
ß
ß
ß
ß
ß
ß ß
• 0.8
0.6
o
ß
ß
ß
o
o
ß
+
ß
ß
ß
ß
+
ß
ß
o
0.4
0.6
•,
0.2
.t.•.
o
0.4 ..•2•.•
. ß' ß
--
••:
-
+
0
'.• o. o
'
0
o
oo
o +
0
ß
0
+
ß , •- • x O•xoj %o+
0.0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Particle Volume Concentration(ppm)
Fig. 8. Pigment concentrationversus particulate volume
concentration
demonstrating
the^ shapefor a eutrophicstation
in the upwellingzoneoff Washington(July 1979), wherethe
chlorophyllandparticlemaximacoincide.
0.2 -
O. I
'
•<88
0.:5
øøoEø
.•
0.5
O.7
o
o
0.9
I. I
I.:5
PARTICLE VOLUME CONCENTRATION(ppm)
Fig. 9. Chlorophyll + phaeophytinconcentrationsversus
suspended
volumeconcentrations
off thecoastof Washington
duringJuly 1979. BNL refersto bottomnepheloidlayers,and
INL refersto intermediatenepheloidlayers(deeperthan60 m
but separated
from thebottom)
Discussion
The theoreticalbasisfor the non-correlationof chlorophyll
andparticleconcentrationis providedby the modelof Kiefer
andKremer [ 1981]. They describethe particulatechlorophyll
concentration
by
CHL
!'Ln+ r
CHL
PN-Ea*tlt(E) 0.2_<PN_<2.0 (3)
wherePN is the particulatenitrogenconcentration,
gn is the
nitrogen specific growth rate, r is the respiration,E is the
scalar quantumirradiance,a* is the chlorophyllspecific
absorption
coefficientand•(E) is thequantumefficiency.The
decreasein pigment contentbelow the maximum has been
observed in the laboratory [Falkowski, 1980] for viable
phytoplanktongrown at extremelylow light levels. From
equation(3) we would expectthe verticalprofile of pigment
contentto vary as l/E, which increasesexponentiallywith
depth,providedthe othervariablesare slowlychanging.For
nutrient-limitedcasesit is reasonableto expect [tn+ r to be
slowlyvarying and•(E) to be constant[Kiefer andMitchell,
1983].
This is observedin the Pacific Central Gyre. Since the
nutrientconcentrations
thereare extremelylow downto 10 or
20 m beneaththe chlorophyllmaximum,thereis no reasonto
expectlargechangesin thegrowthrateor quantumefficiencies
with depth. Plotsof theratioof chlorophyll+ phaeophytin
to
particle volume concentration (Figure 10) show that the
pigmentcontentof the cellsincreasesexponentiallyfrom the
surface to the chlorophyll maximum and then decreases
slowly. There are no real featuresin thiscurveto indicatethat
the particle maximum at 60 m deviates from the general
relationship. Thus, the non-correlationof chlorophyll and
particleconcentrationis shownto be a resultof the processof
primaryproductivityratherthandifferencesbetweenlayersin
particleorigin. Emphasizingthe universalityof thesefindings,
the roughlyexponentialincreasein pigmentto volumeratios
with depth are also observed (Figure 11) in the coastal
upwellingregionwherethe chlorophyllandparticlemaximum
coincideand thereis no reasonto expectgrowthratesto vary
slowly with depth. On a basinwide scale, Kiefer and
Kremer'srelationshipwas shownto predictthe light level at
which the chlorophyll maximum occurs on an entire
meridionaltransectof the Pacific from Tahiti to Kodiak [Pak et
al., 1988].
Due to the effect of light adaptation,an oceanin which the
vertical structureof particulatevolume or massis constant,
would exhibit a chlorophyllconcentrationthat increaseswith
depth to the depth of the chlorophyll maximum. This is
predictedby modelssuchas Kiefer and Kremer [1981] and
hasbeenobservedby Pak et al. [ 1988]. If the particulatemass
is constant,we would expectthe beam attenuationcoefficient
to be constant as a function of depth as well as the
backscattering.As a result the monotonicrelationsbetween
scattering and chlorophyll used to derive chlorophyll
concentrations
from remotesensingdatacannotbe usedwhen
studyingverticalstructurebeneaththe mixed layer. Figure7
(bottom) showsthat the total particulatevolume (as indicated
20,244
KitchenandZaneveld:LightScattering
andChlorophylla
Chlorophyll+ Phaeophytin
Particle
Volume Concentration
2
4
6
8
I
I
I
I
(g/•)
Chl + Pha
10
(Chl+Pha)/P.V.C
Part Vol. Conc.
Station 1
30
2o
40
4o
50
60
6o
I
70
E (488)
8o
80
_/
0
Chlorophyll+ phaeophytin(gg/1)
ParticleVolume Concentration(ppm)
Chlorophyll
+Phaeophytin
(g/l)
Concentration
120
Particle Volume
1
0
2
0
140
10
Chl + Pha
160
(Chl+Pha)/P.V.C
Part Vol. Conc.
30
180
Station 15
40
200
50
Fig. 10. An examplefrom the PacificCentralGyre showing
the ratio of pigment to suspended particle volume
concentrationto be roughly proportionalto the inverseof the
irradiancedown to the chlorophyllmaximum.
70
80
by beam attenuation) and chlorophyll fluorescenceare
correlated in the surface waters.
This correlation
does not
Fig. 11. Theverticalprofileof pigment
to particle
volume
concentration
compared
toparticle
andpigment
profries
forthe
upwelling
zoneoff Washington
duringJuly'1979.
persistvery deepinto thewatercolumn.
Many biological oceanographers use chlorophyll
concentration as a measure of biomass. We have seen that the
biologicallyderived total suspended
mass(as indicatedby
beamattenuation)
is not simplycorrelatedwith thechlorophyll
concentration. If "mass"is usedin the physical sense,i.e.,
quantityof matter, it is seenthat chlorophyllconcentration
cannotuniversallybe usedasan indicatorof biomass.
In thispaperwe haveshownthattheverticalstructure
of the
backscattering
coefficientcannotbe deriveddirectlyfrom the
chlorophyllstructure.It is thereforeof interestto derivethe
dependence
of theremotelysensedreflectanceon thevertical
structure of the particulate matter and the pigment
concentration.Zaneveld [1982] hasderivedan expressionof
the remotely sensedreflectance. If we usea zenith sun,we
may seta + bt,= K; the remotelysensedreflectance(RSR)is
RSR =
bbm
m
Zm
]+e_2K
m
Zm
-I;m
2K(z')dz'
2K
m,[
1- e_2K
l;mb
b(z)e
dz
(5)
Theintegralin equation(5) is thereflectance
beneath
themixed
layer RSR•t. Thus
-2Kmzm
]+e-2KmzmRSR
2
bbm
RSR
= 2K
m[1-e
thengivenby
The contributionto RSRby thereflectancebeneaththe mixed
layeris approximately
-I•2K(z')
dz'
dz
RSR
=Iobb(Z)e
(4)
Assuming a constant K in the mixed layer which has a
thicknesszm, and denotingall mixed layer propertiesby the
subscriptrn, we get
e
-2Kmzm
RSR
If the mixed layer is 0.5 opticaldepthsthick andRSRais
twice RSRM then 42% of the total RSR is contributedby
KitchenandZaneveld:LightScattering
andChlorophyll
a
RSRB. In the case of the Pacific Central Gyre, where the
mixed layer is 1.4 optical depthsthick, we have computeda
differenceof only 1% betweenMorel's RSR model and that
based on our backscatteringcoefficient estimates. We
multiplied the backscatteringcomputedby equation (1) by
0.75 to make it agree with the values we estimatedfor the
mixed layer. Performing the integration of equation (5)
produceda RSR of 0.0223 for Morel's estimate(Equation2)
and 0.0220 for our estimate from Mie theory. In more
productiveregionswherethechlorophyllmaximumis foundat
a muchsmalleropticaldepths[Paket al., 1988] thedifferences
couldbe larger.
We seefrom equation(5) thatthecontributionto theremote
sensingreflectanceat the surfaceby a layerjust belowdepthz
is attenuated
approximately
as
20,245
diffuse attenuation coefficient of ocean water, Limnol.
Oceanogr.,34, 1389-1409, 1989.
Gordon, H. R. and A. Morel, Remote Assessmentof Ocean
Color for Interpretation of Satellite Visible Imagery, 114
pp., Springer-Verlag,New York, 1983.
Gordon, H. R., O. B. Brown, R. H. Evans, J. W. Brown, R.
C. Smith, K. S. Baker, and D. K. Clark, A semianalytic
radiance model of ocean color, J. Geophys. Res., 93,
10,909-10,924, 1988.
Jerlov, N. G., Optical measurementsin the easternNorth
Ariantic, Medd. Oceanogr.Inst. G6teborg, Ser. B, 8, 40
pp., 1961.
Kiefer, D. A., and J. N. Kremer, Origins of vertical patterns
of phytoplankton and nutrients in the temperate, open
ocean: a stratigraphichypothesis,Deep Sea Res., 28,
1087-1105, 1981.
e
Kiefer, D. A., and B. G. Mitchell. A simple, steady state
descriptionof phytoplanktongrowth basedon absoprtion
crosssectionand quantumefficiency,Limnol. Oceanogr.,
-2.[•K(z)dz
Layersbelow the mixed layer thuscontributevery little to
the remote sensingreflectance. The correlationsused for
calculations
of chlorophyllfrom reflectancethusonly needto
take into accountthe upper few 1/K depths,which rarely
extendbelow the mixed layer. For the oligotrophicregime
examinedhere we saw that the backscattering
coefficientas
obtained from chlorophyll concentration [Morel, 1988]
matchedwell in the mixed layer with thosecalculatedfrom
optical propertiesand particle size distributions. Regions
belowthe mixedlayerare not includedin the remotesensing
correlations. When dealing with predictionsof visibility,
predictionof LIDAR performanceor bioluminescent
radiance
belowthe mixed layer, the remotesensingcorrelationsshould
not be used.
Acknowledgments.This work was supportedby the Office
of Naval
Research, contracts N00014-86-C-0784
and
N00014-90-J-1132. We are gratefulto GreggMitchell for use
of the particulateabsorptiondata. HasongPak first pointed
out to us the universalityof the A shapefor the relationship
between fluorescence and attenuation.
References
28, 770-776, 1983.
Kishino, M., Studiesof the optical propertiesof sea water,
Application of Mie theory to suspendedparticlesin sea
water, Sci. Pap., Inst. Phys. Chem. Res., 74, 31-45,
1980.
Kitchen, J. C., J. R. V. Zaneveld, and H. Pak, Effect of
particlesize distributionand chlorophyllcontenton beam
attenuationspectra,Appl. Opt., 21, 3913-3918, 1982.
Kullenberg,G., Scatteringof light by SargassoSea water,
Deep Sea Res., 15, 423-432, 1968.
Kullenberg, G., A comparison between observed and
computedlight scatteringfunctions,Rep. 13, 22 pp., Univ.
Copenhagen,Inst. of Phys.Oceanogr.,1970.
Lewis, W. M., Jr., Surface/ volume ratio: implicationsfor
phytoplankton
morphology,Science,192,885-887, 1976.
Mie, G., Beitrggezur OptiktriiberMedien,speziellkolloidalen
Metall-16sungen,
Ann. Phys.,25, 377, 1908.
Mitchell, B. G., and D. A. Kiefer, Determination of the
absorption and fluorescence excitation spectra for
phytoplankton.
in MarinePhytoplankton
andProductivity,
edited by O. Holm-Hansen,L. Bolis, and R. Giles, pp
157-169,Springer-Verlag,
New York, 1984.
Mitchell, B. G. and D. A. Kiefer, Variability in pigment
specificparticulatefluorescence
andabsorption
spectrain
Baker, E. T., Patternsof suspended
particledistributionand
transportin a large fjordlike estuary, J. Geophys.Res.,
89, 6553-6566, 1984.
Bishop, J. K. B., The correctionand suspendedparticulate
matter calibrationof Sea Tech transmissometer
data,Deep
Sea Res., 33, 121-134, 1986.
Bohren,C. F. and D. R. Huffman, Absorptionand Scattering
of Light by Small Particles, John Wiley & Sons, New
York, 530 pp., 1983.
Brown, O. B., and H. R. Gordon, Two component Mie
scattering
modelsof Sargasso
Seaparticles,Appl. Opt., 12,
2461-2471, 1973.
Carder,K. L., R. G. Steward,J. H. Paul, and G. A. Vargo,
Relationships
betweenchlorophyllandoceancolor
constituents
astheyaffectremote-sensing
reflectance
models,Limnol. Oceanogr.,31,403-413, 1986.
Downing,J., An opticalinstrumentfor monitoringsuspended
particulates
in oceanandlaboratory,Proceedings
of Oceans
'83, Inst. ElectricalandElectonicsEng., New York, 1983.
Falkowski,P. G., Light-shadeadaptationin marine
phytoplankton,
in PrimaryProductivityin theSea,edited
by P. G. Falkowski, pp. 99-119, PlenumPress,New
York, 1980.
Gardner, W. D., P.E. Biscaye, J.R.V. Zaneveld, and M.J.
Richardson, Calibration and comparisonof the LDGO
nephelometerand the OSU transmissometer
on the Nova
ScotJanRise, Mar. Geol., 66, 323-344, 1985.
Gordon, H. R., Can the Lambert-Beer law be applied to the
the northeastern
PacificOcean,Deep SeaRes,35, 665-689,
1988.
Morel,A., l•tudeexp6rimentale
dela diffusion
dela lumi•re
par l'eau, les solutionsde chlorurede sodiumet l'eau de
mer optiquementpures,J. Chem.Phys.,10, 1359-1366,
1966.
Morel, A., Optical modelingof the upperoceanin relationto
its biogenousmatter content(caseI Waters), J. Geophys.
Res., 93, 10,749-10,768, 1988.
Morel, A., and L. Prieur, Analysis of variations in ocean
color, Limnol. Oceanogr.,22, 709-722, 1977.
Mueller, J. L., The influenceof phytoplanktonon oceancolor
spectra, Ph.D. thesis, 239 pp., Oregon State Univ.,
Corvallis, 1974.
Mueller, J. L., and R. E. Lange, Bio-opticalprovincesof the
northeastPacific Ocean: a provisionalanalysis,Limnol.
Oceanogr.,34, 1572-1586, 1989.
Pak, H., Characteristicvertical profilesof particles,oxygen,
and chlorophyllpigmentsin the North PacificGyre, Proc.
SPIE, Int. Soc. Opt. Eng., 489, 107-112, 1984.
Pak, H., D. A. Kiefer
and J. C. Kitchen, Meridional
variations in the concentration of chlorophyll and
microparticlesin the North PacificOcean,Deep SeaRes.,
35, 1151-1171, 1988.
Peterson,R. L., A study of suspendedparticulatematter:
Arctic Ocean and Northern Oregon Continental Shelf,
Ph.D. thesis,122 pp. Oreg. StateUniv., Corvallis.,1978.
Petzold,T. J., Volume scatteringfunctionsfor selectedocean
20,246
KitchenandZaneveld:Light Scattering
andChlorophylla
waters,SIO Ref. 72-28, 79 pp.,ScrippsInst. of Oceanogr.,
La Jolla, Calif., 1972.
Phinney,D. A. and C. S. Yentsch,A novel phytoplankton
chlorophyll technique: toward automatedanalysis,J.
Plank. Res., 7, 633-642, 1985.
Platt, T., S. Sathyendranath,C. M. Caverhill, and M. R.
Lewis, Ocean primary productionand available light:
furtheralgorithmsfor remotesensing,Deep SeaRes.,35,
855-879, 1988.
Sathyendranath,
S. and T. Platt, Remotesensingof ocean
chlorophyll:consequence
of nonuniformpigmentprofile,
Appl. Opt., 28, 490-495, 1989.
Siegel,D. A., andT. D. Dickey, Observations
of thevertical
structureof the diffuse attenuationcoefficient spectrum,
Deep Sea Res., 34, 547-563, 1987.
Van de Hulst, H. C., Light Scatteringby Small Particles,470
pp., JohnWiley, New York, 1957.
Zaneveld, $. R. V., Remotely sensedreflectanceand its
dependence
on verticalstructure:a theoreticalderivation,
Appl. Opt., 21, 4146-4150, 1982.
Zaneveld,
$.R.V.,
determination
D.M.
Roach,
of the index
and H.
of refraction
The
of
oceanicparticulates,J. Geophys.Res., 79, 409D4095,
1974.
J.C. Kitchen, Sea Tech,
Corvallis, OR
97339.
Inc.,
P.O.
Box
779,
J.R.V. Zaneveld, College of Oceanography, Oregon
State University, Corvallis, OR 97331.
Smith, R. C., K. S. Baker and P. Dunstan, Fluorometric
techniques
for the measurement
of oceanicchlorophyllin
the supportof remotesensing,SIO Ref. 81-17, Visibility
Lab., ScrippsInst. of Oceanogr.,La Jolla,Calif., 1981.
Pak,
distribution
(Received November 16, 1989;
accepted March 8, 1990.)