JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. C7, PAGES 13,345-13,367,JULY 15, 1995
Scalesof variability of bio-optical properties as observed
from near-surface
drifters
Mark R. Abbott,1 KennethH. Brink,2 C. R. Booth,3 DolorsBiascoil
MarkS.Swenson,
5 Curtiss
O. Davis,6 andL. A. Codispoti7
Abstract. A drifter equippedwith bio-opticalsensorsandan automatedwater samplerwas
deployedin the CaliforniaCurrentaspart of the coastaltransitionzoneprogramto studythe
biological,chemical,andphysicaldynamicsof the meanderingfilaments.During deployments
in 1987 and 1988, measurements
were madeof fluorescence,
downwellingirradiance,upwelling
radiance,andbeamattenuationusingseveralbio-opticalsensors.Sampleswere collectedby an
automatedsamplerfor later analysisof nutrientsandphytoplanktonspeciescomposition.Largescalespatialandtemporalchangesin the bio-opticalandbiologicalpropertiesof the regionwere
drivenby changesin phytoplankton
speciescompositionwhich,in turn,were associated
with the
meanderingcirculation.Variancespectraof the bio-opticalparameters
revealedfluctuationson
bothdiel and semidiurnalscales,perhapsassociated
with solarvariationsandinternaltides,
respectively.Offshore,inertial-scalefluctuationswere apparentin the variancespectraof temperature,fluorescence,andbeamattenuation.Althoughcalibrationsamplescanhelpremove
someof thesevariations,theseresultssuggestthat the useof bio-opticaldatafrom unattended
platformssuchasmooringsanddriftersmustbe analyzedcarefully.Characterizationof the
scalesof phytoplankton
variabilitymustaccountfor the scalesof variabilityin the algori.
thms
usedto convertbio-opticalmeasurements
into biologicalquantifies.
1. Introduction
optimalestimatesfor temporaland spatialfields rely on estimates
of the subgrid-scalevariability to map the larger-scalefields of
Satelliteobservations
of oceancolorprovidea uniqueview of
the large-scalepatternsof phytoplankton
pigmentin the upper
ocean. Coupled with increasingly sophisticatednumerical
models, there is now the potential to study basin-scaleand
global-scalebiogeochemical
processes
in the oceanin greater
detail than before. However, obstaclesremain, especiallyin the
interest.
Suchsmall-scalevariabilityis morethanmerelya nuisancefor
samplingand modeling. As phytoplanktongrowth ratesrespond
on scalesof a day or less (and henceon spatial scalesof tens of
kilometers), this subgrid-scaleis likely the focus of physical/biological coupling [Denman and Powell, 1984; Harris,
1986; Abbott, 1993]. As these scalesare difficult to observe, it is
parameterization
of the unresolvedscales.Similar problems
essentialthat we begin to characterizethis variability sufficiently
confrontmodelsof oceanphysics.For example,the parameterization of small-scale turbulence and diffusion significantly to derive statisticallyrobustfields and modelsat larger scales.
B io-optical measurements have been used for decades as
affectsthe large-scalepatternsof heatflux [Bryan, 1987]. The
surrogates
for direct measurementsof biological properties
characterizationof small-scalevariability is also necessaryfor
[ZaneveM, 1989; Smith et al., 1991; Marra, 1995]. With
optimalinterpolationof sparsedatasets.Cheltonand Schlax
[ 1991] usedestimatesof the variancespectrumto deriveoptimal improvements in data storage, batteries, and sensor design,
routine bio-optical measurementscan be made from unattended
estimates
of meanfieldsfor irregularlysampledtime series.They
useda sequence
of coastalzonecolorscanner(CZCS)imageryto platforms such as mooringsand drifters [Dickey, 1991]. Such
that were
showthat compositeaveragesprovidelittle informationwhere observationstrategiesnow allow the studyof processes
previously difficult to resolve using conventional shipboard
thereare longgapsin the series.Usingmapsof field measurements, Denman and Freeland [1985] estimated the structure techniques[ Dickey et al., 1991; Smithet al. , 1991;Marra , 1995].
With their ability to resolve small-scalevariability, these stratefunctionsof chlorophylland a setof physicalvariablesto progies will allow us to derive statisticsnecessaryfor techniques
duceobjectivemaps.Thusbothparameterization
for modelsand
suchas optimal interpolation.
During the spring/summerupwelling season,large meanders
1College
of Oceanic
andAtmospheric
Sciences,
Oregon
State are apparentin the California Current[Strubet al., 1991]. Inshore
University,Corvallis.
of thesemeanders,cold water is presentnear the surfaceand is
2Woods
HoleOceanographic
Institution,
Woods
Hole,
Massachusetts.
3Biospherical
Instruments,
Incorporated,
San
Diego,
California. characterizedby high nutrient and chlorophyll concentrations.
The coastal transition zone (CTZ) program was designed to
4Rimouski,
Quebec,
Canada.
5Atlantic
Oceanographic
andMeteorological
Laboratory,
NOAA, characterizethe physical,biological, and chemicaldynamicsof
Miami, Florida.
thesemeandersor filaments[Brink and Cowles,1991]. Using an
6Naval
Research
Laboratory,
Washington,
D.C.
array of bio-optical sensorsand automated water samplers, a
7Office
ofNaval
Research,
Arlington,
Virginia.
drifter (droguedepth 13 m) was deployedoncein 1987 and once
in 1988. The water-followingcharacteristics
of thisdrifter should
Copyright1995by theAmericanGeophysical
Union.
allow separationof temporal variability from spatial variability
[Niiler et al., 1987; Paduan and Niiler, 1990; Brink et al., 1991;
Paper number 94JC02457.
0148-0227/95/94JC-02457505.00
Swensonet al., 1992], althoughvertical water movementswill
13,345
13,346
ABBOTT
ET AL.: SCALES OF BIO-OPTICAL
appear as temporal variations because the instruments are
droguedat a constantdepth.This meanderingregionis the siteof
relativelylarge verticalexcursions,of the orderof severaltensof
meters per day [Washburn et al., 1991; Kadko et al., 1991;
VARIABILITY
In both yearsthe drifter was deployednear an upwellingcenter
at the nearshore end of a filament.
The drifters were tracked via
variabilitywill includethe scalesof verticalmotionaswell asthe
scalesassociatedwith bio-optical changesin a particularwater
the Argos system.The two driftersfollowedthe main axis of the
filamentoffshore.Figures1a and 1b showthe drifter tracksfrom
1987 and 1988. In both years the drifters startednearshore,off
Point Arena, and proceededin a southwarddirection offshore.
Both drifters stayednear the cold core of the meanderingfila-
mass.
ment. The instrumented
Swenson et al., 1992]. Thus estimates of the timescales of
drifters tracked the water motion as well
We comparedseveralmethodsto derive biologicalproperties as the uninstrumenteddrifters [Swensonet al., 1992]. During
suchas chlorophyllconcentrationand primaryproductivityfrom both deploymentsthe winds were generally southwardin the
the basicbio-opticalsignals.Althoughwe werenot ableto derive rangeof 7 to 15 m/s.
estimates of absolute error, we were able to estimate the relative
Figure 2 shows a spline fit to the drifter positionsin 1988
variability betweenthe different methodson timescalesof days. comparedwith the geopotentialanomaly [Huyer et al., 1991].
There were also considerable
differences
between the two
When comparedwith a similarfigure from the 1987 deployment
deploymentsin the bio-optical propertiesof the phytoplankton [Abbott et al., 1990, Figure 4], it is apparentthat the 1988
which were apparent in these functional relationships. We
deploymentmore nearly sampledthe core of the meander.Both
suspectthattheseare largelydrivenby shiftsin speciescomposi- Kosro et al. [ 1991] and Huyer et al. [ 1991] showedthat acoustic
tion which affect the observedbio-opticalproperties.
Doppler current profiler (ADCP) maps of the velocity field
We use standardtime seriestechniquesto studythe character- reproducedthe patternsin the circulation observationsderived
istic scalesof variability. These resultsare comparedwith mea- from conductivity-temperature-depth
(CTD) measurements.In
surementsof the physicalenvironmentto estimatethe degreeof
1987 the drifter tendedto be inshoreof the high-velocityportion
coupling between physical and bio-optical variability. There is of the meander.The jet appearsto have been strongerin 1988,
stronginteractionbetweenphysicalforcingandbio-opticalprop- basedon estimatesof the geopotentialanomaly [Huyer et al.,
ertiesat a wide rangeof timescales.Futuresamplingprograms, 1991; Kosro et al., 1991]. In the Point Arena regionthe filament
especially those relying on satellite remote sensing,must take was much narrower and more sharply defined than it was in
such small-scale variability into account when scaling up to 1987. The deploymentin 1988 took place as the filament was
orientedprimarily offshore [Huyer et al., 1991; Chavez et al.,
large-scalemodels.
1991] as in 1987. However, shortly after this deployment,the
offshore
flow apparently decreasedand the filament became
2. Sampling Techniques and the Physical
orientedalongshore[Huyer et al., 1991].
Environment
Figures3a, 3b, and 3c showtime seriesof temperature,strobe
The drifter used in this study has been describedby Niiler et fluorescence,
anddownwellingscalarirradianceEo photosynthetal. [ 1987], Paduan and Niiler [ 1990], and Swensonet al. [1992].
ically active radiation (PAR), respectively, from the 1988
The bio-optical instrumentation included a Biospherical deployment. Plots of these variables from 1987 are given by
Instruments MER-2020, a SeaTech 25-cm beam transmissometer,
Abbottet al. [1990]. In both yearsthe patternsof fluorescence
and a SeaTech strobe fluorometer.
Table 1 shows the variables
andtemperaturewere in generalinverselyrelated,asexpectedfor
that were recorded.An automatedwater sampler[Friederich et upwelling regions [e.g., Hood et al., 1990]. Temperatureinal., 1986] was used to collect samplesevery 12 hours for later creasedabout2øC in 1987 and about3øCin 1988. As notedby
nutrientandphytoplanktonanalyses.The bio-opticalsamplerwas Abbott et al. [1990], much of this increase was the result of
placedat 8.5 m depth,andthe water samplerwas at 17.5 m. Bioconvergenceof warmer water along the path of the drifter in
optical sampleswere collectedevery 4 min in the 1988 deploy- 1987, associatedwith a net downwelling of the surfacewater.
ment after averagingfor 45 s (We incorrectlystatedin the work Similar processesoccurred in 1988, as Swensonet al. [1992]
by Abbott et al. [1990] that the samplinginterval was every 4 estimatedthat surface heating was in the range of 0.15ø to
min; it was actually8 min in 1987. This errordoesnot affectany 0.3øC/d,considerablylessthanthe temperaturechangeobserved
of the figuresor analysesin that paper.)In 1988, sampleswere by the drifter.
collectedadjacentto the drifter at noon every day for shipboard
We comparedthe horizontal transectobtainedby the drifter
analysis.
Watersamples
werecollected
fromtheshiponlyatthe with vertical sectionsof chlorophyll, temperature,and salinity,
beginning
andendof the1987deployment.
Thesesamples
were derivedfrom the vertical profiles made at noon eachday of the
usedto estimatechlorophyllconcentrations
usingstandardex1988 deploymentnext to the drifter (Figures 4a, 4b, and 4c).
traction methods.
Theseshowthe strongfront in temperaturethat occurredapproximately 70 km alongthe drifter track on day 189 (Figure4a). The
sectionsalso show the tendencyfor the nearshorewaterswhich
Table 1. Variables Recorded on Biospherical
are characterizedby low temperaturesand high chlorophyllto
Instruments MER-2020 Spectroradiometer
subductasthey move offshore[Washburnet al., 1991].
Figure 5 is the concentrationof nitrate + nitrite (N+N) for the
Variable
Description
1988 deployment.The pattern and absolutevaluesof N+N are
410, 441,488, 520, 560
similar to the resultsfrom the 1987 deployment[Abbottet al.,
Downwelling irradiance,nm
410, 441,488, 520, 683
Upwelling radiance,nm
1990]. AlthoughN+N disappearedat nearly the samerate as it
Other variables
strobe fluorescence,beam
did in the 1987 deployment,muchof thischangemay havebeen
transmission,
temperature,
the resultof changesin watermassesasthe driftercrossedisopydepth
cnals(Figure 2) or verticalwatermotions.Swensonet al. [1992],
Housekeepingvariables
time, tilt, groundvoltage,
using a vorticity budgetmethod,calculatedthat there were was
batteryvoltage
downwellingon day 187 (approximately12 m/d), upwellingon
ABBOTT
ET AL.:
SCALES OF BIO-OPTICAL
VARIABILITY
13,347
.;.-':'.•.
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.:./.::•
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-:.:
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.• •:.;'•.....
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,':3/' .........
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-.
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. $-'::
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•....
:-...".•:..
:.,.,. ;:;?':.':':'"
' w,
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.: ....
.
.:...
•?-•;•?.:....•.....
....
.
ß'.::•t.
."•
.:...... ,:
..., ......•...::::...:•----*-:,•:;•,•
.
'::.
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:";'a.:;•':./.•
.....7'
..
.
. ...•:$
..:';-:" ..}*?":%:5:::,.
?%,::
,:.,.;•:,
Figure la. Drifter path from 1987 superimposedover advancedvery high resolutionradiometer(AVHRR)
imageof seasurfacetemperature
from Abbottet al. [ 1990].
day 188 (approximately7 m/d), and downwelling on day 189
(approximately8 m/d).
3. Methods
and Results
Chlorophyll Estimates
Our first objective was to compare different methodsfor
convertingbio-opticaldatainto estimatesof chlorophyllconcentration.Partof thiscomparison
wasan assessment
of the time and
spacescalesof variationof algorithmperformance.For example,
were a particularset of parametersconstantalong the lengthof
the drifter track or did they vary from water massto water mass?
As more observationsare made remotely(e.g., from satellitesor
unattendedmoorings), the choice of algorithms becomesmore
critical. We must understandthe total performanceof the algorithm which includesboth estimatesof root-mean-square(rms)
erroraswell ashow its performancechangesover time.
The first method convertedthe strobefluorometerdata using
the chlorophyllextractionsthat were collectedat the beginning
and end of the deploymentin 1987 and on a daily basisin 1988.
Clearly, this methodshouldbe less accuratein 1987 than 1988.
Numerouspapershave appearedin recent years discussingthe
problems associatedwith calibration of fluorometers;in this
experiment
we assumed
thatthetemporal/spatial
scaleof changes
in fluorescenceper unit chlorophyllwas larger than the temporal/spatial scalesover which we collectedcalibration samples.
This is not a robustassumptionas fluorescence/chlorophyll
can
changerapidlyfor a varietyof physiologicalreasons[ Kiefer and
Reynolds,1992].
The
Sun-stimulated
fluorescence
method
was based on the
model describedby Kiefer et al. [1989] and Chainberlinet al.
[1990]. Although this method is relatively new, it does show
promise,particularly in the estimationof primary productivity.
Kiefer et al. [ 1989] discussseveral reasonswhy the technique
13,348
ABBOTT ET AL.: SCALES OF BIO-OPTICAL VARIABILITY
'7
..,
1'9'88
Figure lb. Drifter pathfrom 1988superimposed
overanAVHRR imageof seasurfacetemperature.
shouldwork better for estimatesof primary productivityrather
than chlorophyllconcentration.The flux of Sun-stimulated
fluo-
whereEo(PAR) is the downwellingscalarirradiancebetween400
rescence
Ff wascalculated
asfollows:
thespecificabsorption
coefficientbf phytoplankton.
Ff = 4•;[k(PAR)+
a(Chl)]
x Lu(Chl)
(1)
where k(PAR) is the diffuse attenuance for scalar irradiance
between400 and 700 nm [ Kiefer et al., 1989], a(Chl) is the total
absorptioncoefficient,and Lu(Chl) is the spectrallyintegrated
radianceof a chlorophyll-likesubstance.
The 4n termis a geometric constant that transforms the radiance to a volume emission
and700nm,•f is thequantum
yieldof fluorescence,
andOacis
Kiefer et al. [1989] and Kiefer and Reynolds[1992] discuss
the limitations of this approach.Specifically, the fluorescence
signalmay be confoundedby the presenceof otherpigments.The
quantum yield of fluorescencewill change from speciesto
speciesand dependon physiologicalstate.Lastly, the specific
absorptioncoefficientwill not be constant.
In (1) we followed Kiefer et al. [1989] and assumedthat
k(PAR) + a(Chl) -- 0.52. This assumptionworks fairly well in
waterswhereChl• 2.0 mg/m3, a condition
thatwasnotalways
met in the study area. Assuming other parameters remain
(withunitsof steradians,
notstr-• asdescribed
by Chamberlin
et constant, this will result in an underestimateof chlorophyll
al. [1990])./• hasunitsof nanoeinsteins
percubicmeterper concentrations
in regions
withChl• 2.0mg/m
3.
secondandwasconvertedto chlorophyllas
Chl
=
F,f
Eo(PAR)x d)fxøac(PAR)
Eo(PAR) was basedon a linearregressionof the downwelling
irradianceat 520 nm, Ed(520),using
(2)
Eo(PAR) = -0.0011 + 0.00245x E• (520)
(3)
ABBOTT
ET AL.'
SCALES OF BIO-OPTICAL
.6
VARIABILITY
13,349
4
38'::'
37
ø , , , ,<16'4•,,,
', I,,,,,
127øW
126'::'
125 ø
,,,,, •øø•1
124'::'
123 ø
Figure 2. Path of 1988 drifter overlain on geopotentialanomalyfrom the July 6-12 cruise(year days 188-194).
Geopotentialanomalyis from Huyer et al. [ 1991] and is calculatedat 50 dbar relativeto 500 dbar.
This is basedon observationswhich show that downwellingradianceat 520 nm is only weakly affectedby pigmentconcentration
[Gordonet al., 1988]. We comparedthis methodwith an estimate
of the integratedradiancebetween400 and 700 nm usingthe full
spectroradiometricdata. The correlationbetween the two esti-
In (4), SPM is suspendedparticulatematter, c is the beam attenuationcoefficient,Cw is the beam attenuationresultingfrom pure
water (assumedto be 0.358), and k is an empirical coefficient
relating beam attenuationto SPM. On the basis of Bishop's
mates was 0.9996.
we assumedthat approximately25% of the SPM was carbon.We
then converted this carbon estimate to chlorophyll, using a
carbon:chlorophyllratio of 60. As the beam transmissometer
failed in the 1988 deployment,we presentresultsonly from 1987.
The beam attenuation method has several assumptionsthat
may not be robustin our studyarea. Bishop[1986] showsthatthe
relationshipbetweenattenuationand SPM can vary considerably
We used beam attenuation
measurements
to estimate chloro-
phyll by convertingthe raw transmissometer
data to beam attenuation using the methodsof Bartz et al. [1978]. We used the
equationsof Bishop [ 1986], where
SPM = (c- cw) / k
(4)
[1986]data,weusedk = 1.15x 10-3. Following
Bishop[1986],
15
14
© 13
E
• 12
10,
186
,
187
188
t89
190
191
192
193
Yeor Do),
Figure 3a. Time seriesof temperature(in degreesCelsius)from the 1988 deployment.
13,350
ABBOTT ET AL.: SCALES OF BIO-OPTICAL
•
VARIABILITY
6
2
0
186
187
188
189
190
191
192
9.3
Yeor Doy
Figure 3b. SameasFigure3a, but for strobefluorescence.
depending on the oceanographicregime, particularly as one
movesfrom coastalto oceanicregions.Bishop [1986] showsthat
errorscan range from 20 to 40%, dependingon the SPM level.
Also, assumptionsabout the carbon content of SPM and a
constantcarbon:chlorophyll
ratio are clearoversimplifications.
The last method used to estimatechlorophyll concentration
was based on irradiance ratios as described by Smith et al.
[1991]. Althoughupwellingradianceis far more sensitiveto
changesin chlorophyll concentration,downwelling irradiance
was usedwith somesuccesson the Biowatt mooringsby Smithet
Ea is the downwellingirradianceat 441 and520 nm, and A andB
are fitted parameters.Smithet al. [ 1991] reportvaluesfor A and
B at 32, 52, and72 m depthfrom the Biowattmooring.The irradiance method should be much less sensitiveto changesin
chlorophyll,especiallyin low-chlorophyllwaters. This is expectedas downwellingirradianceis integratedover a very short
distance.In contrast, the upwelling radiance method used in
oceancolor remote sensingintegratesover one to two optical
depths.
Figure6 showschlorophyllestimatedin 1987usingthestrobe
fluorescence,Sun-stimulatedfluorescence,beam attenuation,and
al. [1991 ]. We estimatedChl as
irradiance ratio methods described earlier. We limited the esti-
Chl=A Ea(441)
(5)
Ea(520)
rY
400
<
300
mates to the period from 2 hours before local solar noon to 2
hoursafter local solar noon (approximately1100 to 1500 LT).
• 200
100
0 ,• I•
186
187
I
188
I,
189
•I
I
190
191
I, ,
192
193
Yeor Doy
Figure3c. SameasFigure3a,butfor photosynthetically
availableradiation
(in microeinsteins
persquare
meter
per second).
ABBOTT
ET AL.'
I
0•-187
SCALES
188
OF BIO-OPTICAL
189
190
VARIABILITY
191
10
2O
13,351
192
1934
•1
•
f+ • - --2
+ + + + +t
30
40
5o
60
70
8O
9O
100
110
•
,
120
0
10
20
30
40
50
60
70
80
90
I
I
I
t
I
100
110
120
130
140
Distancealong Transect(km)
Figure 4a. Horizontal sectionof chlorophyll (milligrams per cubic meter) from vertical profiles collected at
approximatelynooneachday next to the drifter in 1988. Date and locationof eachprofile are markedat the top.
(øac)of 0.04 anda fluorescence
efficiencyqbfof 0.045 as
cence estimates by a factor of 3. We therefore relied on the
Kishino/Collins parametersfor the Sun-stimulatedfluorescence
model for the 1987 deployment.
Although there are differencesin the four chlorophyll estimates, the general patternsof the four time seriesare similar.
This similarity is particularlyapparentin the early days of the
deployment(days 168-171). The beam attenuationmethodalso
predictsmuch higherpigmentvaluesbut showsmuchof the same
describedby Kishino et al. [1984] and Collins et al. [1988]. We
applied the coefficientsdescribedby Chamberlin et al. [1990]
(0.016 and 0.028 for specific absorptionand fluorescenceefficiency) and found that the Sun-stimulatedfluorescencechlorophyll estimateswere consistentlyhigherthan the strobefluores-
obvious discontinuitiesthat appear in the latter stagesof the
deployment(days 172-175) are the resultof limiting the data to
near-noonobservations.In particular,the discontinuitybetween
day 172 and day 173 is especiallyabruptin the beam attenuation
This reducedthe effects of changingSun angle on the observed
levels of irradiance. Note that the plots have vertical lines to
separate the successivemidday periods. The irradiance ratios
used the 72-m coefficientsestimatedby Smith et al. [1991]; the
coefficients for 35 and 50 m resulted in significantly higher
chlorophyllestimates.The Sun-stimulatedfluorescencedata were
convertedto chlorophyll using a specific absorptioncoefficient
-
structure as the fluorescence
I
I
0 -187
10
small-scale
188
189
- + •-_...•11.3+_•__•
•
+
190
\
\
+
\
\
191
192
+
I+/
+ -
20
30
40
50
60
70
80
90
100
110
120
0
10
20
30
40
50
60
70
80
90
100
110
120
-
193-
130
140
Distancealong Transect(km)
Figure 4b. Sameas Figure 4a, but for temperature(degreesCelsius).
methods. Note that the
13,352
ABBOTT
ET AL.'
07187
SCALES
188
l•_
+,
.3•-.3•-•+ .
OF BIO-OPTICAL
VARIABILITY
189
190
191
192
193r•
+
+
+
+
+
30
70
90
110
120
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
DistancealongTransect(km)
Figure 4c. SameasFigure4a, but for salinity(per mil).
estimate.A similardiscontinuityis apparentin the strobefluorescencemethodbetweendays 171 and 172.
The two fluorescencemethodsare surprisinglysimilarover the
tously.The high chlorophyllvaluesnearshorefrom the transmissometerdata are likely an artifact of the high particulate,but
nonfluorescent,
concentrations
in the upwellingcenter[Abbottet
duration
of the deployment
(r2=0.96).Not onlydo thetwo
al., 1990].
fluorescencemethods agree on large-scale patterns,but also
many of the small-scalefeatures are similar between the two
Despite the numerousassumptionsin the transmissometerbasedchlorophyll estimates(coefficientsfor convertingbeam
transmissionto suspendedparticulate matter, to carbon, and
finally to chlorophyll),the fluorescencemethodsand the transmissometermethodare remarkablysimilar,differingby a factor
of 2 to 3. Althoughthisis a largedifferencefor in situsampling,
thediscrepancy
is in line with otherremotesensingmethodssuch
estimates.
Given
that
the
strobe
fluorescence
data
were
"calibrated" (albeit limited) with chlorophyll extractions, we
expectthat theseestimatesare a reasonablemeasureof the 10-m
chlorophyllrecordalongthe drifter track. The patternsare quite
similarto the chlorophyllmap derivedby Hood et al. [ 1990] for
this region. Relying on both fluorescence-basedestimates, as satellites [ Gordon et al., 1983]. Note the monotonicincreases
thisasevidence
chlorophyllconcentrations
are initially moderateat the beginning on days173-175. Abbottet al. [ 1990]interpreted
of the deployment,increaserapidly, and then decreaseprecipi- of daily growth.
lO
•
6
186
187
188
189
190
191
192
Julion Doy
Figure 5. Time seriesof nitrateplusnitrite(N+N) concentrations
(micromoles)from the 1988deployment.
ABBOTT
2.5
ET AL.:
SCALES
' '
OF BIO-OPTICAL
VARIABILITY
13,353
[
2.0
ß
Beam
-
Irradiance
Attenuation
-
Strobe
ß
Sun-stimulated Fluorescence
Ratios
Fluorescence
'•",/?,.
.•,..•-.'
...½
'".......'- '?"! J'":
1.5
' :L '""t!c'
[_.,..'.,.....
' '"-' ,•.',.i
.... ....... /\,,•,• •.....,r
1.0
• :,.,"t
..,
.......
,.........
,..._.-.
0,5
/•,
x,
0.0
1 68
169
1 70
1 71
1 72
73
1 74
175
176
Year Day
Figure6. Estimates
of chlorophyll(milligramspercubicmeter)usingfourdatasetsfromthe 1987deployment.
Observations
are constrained
to between1100 and 1500 LT for eachday.Vertical linesseparatethe successive
middayperiodsfor eachday.
The irradiancemethodis much less sensitiveto changesin
concentration,especiallyat low chlorophylllevelsoffshore.The
general decreasein concentrationoffshore is consistentwith the
otherestimates,but many small-scalefeaturesare not apparent.
However, someof the changesin estimatedchlorophyllmay be
the result of physiologicalchanges,especiallyin the fluorescence-based methods.
12
t
fluorescence
and the Sun-stimulated
fluorescence
estimates
which used the values of specific absorptionand fluorescence
efficiency presentedby Kishino et al. [1984] and Collins et al.
[1988]. These values (0.04, and 0.045, respectively)were used
Strobe
Fluorescence
Sun-stim. Fluor. (.016, .028 model)
-
10
--
Irradiance
Ratios
Sun-stim. Fluor. (04,
•6
•
The general patterns of chlorophyll were similar in 1988
(Figure 7), althoughthe levels were much higher than in 1987
(Figure 6). The most noticeabledifference is between the strobe
.045 model)
•,"
•"
',,,'
!i',
4
•
0
186
187
188
189
Yeor Doy
190.
191
192
Figure7. Estimates
of chlorophyll
(milligramspercubicmeter)usingthreedatasetsfromthe 1988deployment.
Observations
are constrained
to between1100 and 1500 LT. Vertical lines separatethe successive
midday
periodsfor eachday. The Kishino/Collinsparameters(0.04, 0.045 model) [Kishinoet al., 1984; Collins et al.,
1988]for the Sun-stimulated
fluorescence
estimates
arecompared
with theChamberlinparameters
(0.016, 0.028
model) [ Chamberlinet al., 1990].
13,354
ABBOTT
ET AL.'
SCALES OF BIO-OPTICAL
VARIABILITY
for the 1987 deployment.The Sun-stimulatedfluorescence
esti- latedfluorescence,
asmodifiedby ChantberlinandMarra [1992]
matesof chlorophyllare significantlylower than the calibrated to includea temperatureeffect. The rate of primaryproduction
(by daily, in situ samplingnext to the drifter) strobefluorescence canbe expressedas
estimatesby nearly a factor of 3. This differenceis especially
apparent nearshore in the high chlorophyll waters. We tried
different values for the specificabsorptioncoefficientoac and
(6)
kc
f
F½
= kcf
+Eo(PAR)
fluorescence
efficiencyq•f of 0.016 and 0.028, basedon
Chainberlin et al. [ 1990]. The match between thesetwo fluorescenceestimatesis muchbetter,especiallyduringthe first 3 days
of the deployment. After day 188 the Sun-stimulatedfluores-
wherekcf is the valueof the irradiance
at 1/2 (•)c/•)f)max,
isthemaximum
valueof'theratioof thequantum
cenceestimatesusing the Chamberlincoefficientsare consis- ((•c/(•f)max
yields of photosynthesisand fluorescence.The temperature
tently higher than the strobe fluorescenceestimates. Similar
dependence
is modeledaslinearfunctionwhere T is temperature,
variability in theseparametersand their effectson estimatesof
k is 0.044, and c is 0.257 as determinedby Chantberlin and
primary productivity from Sun-stimulatedfluorescencewas
Marta [1992]. Using the data describedby Chantberlinet al.
observedby Stegmannet al. [1992].
[1990],weassumed
that(q•c/(•f)max
wasapproximately
2.3atoms
Theseresultssuggestthatthereweresignificantchangesin the
C/photon/einstein
(E)/m3/s
and
that
k
ff
was
approximately
133
fluorescencecharacteristicsof the phytoplanktonbetween the
pie/m2/s(incorrectly
statedasmE/mZ/sby Chantberlin
et al.
1987 and 1988 deployments.The relationshipbetweenthe two
[1990]). After making the necessaryunit conversions,we
fluorescence
estimatesis muchmorecomplicatedin 1988.First,
expressed
the rate of primary productionas nanogram-atoms
of
recall that the strobe fluorescence
estimates were calibrated
Carbonper cubic meter per second,consistentwith Chantberlin
againstdaily in situ observations;
daily changesin this relationet al. [ 1990].
ship can add variability. The strobe fluorescencecalibration
In 1987 (Figure 8a), primary productivitywas surprisingly
coefficient was much smaller on day 189 than at other times,
uniformalongthe drifter track, comparedwith the moreintense
perhapsaccountingfor at leastpart of the discrepancybetween
variabilityin chlorophyll(Figure6). It decreased
by only a factor
the two fluorescencemethods.Second,changesin the Sun-stimuof 2 from the beginningto the end of the deployment,whereas
lated fluorescencemodel parametersare anothersourceof varichlorophylldecreased
by a factorof 4. In comparison,
the pattern
ability. In additionto the apparentlylarge shiftsbetween1987 of productivity was much more variable as the drifter moved
and 1988, there appearto be changesin theseparametersduring offshorein 1988 (Figure 8b), decreasing
from 500 to 100 ngthe 1988 deployment.It is well known,for example,thatfluores- atoms
C/m3/s.Thisvariability
islargelydrivenbythevariations
cenceyield and specificabsorptionvary as a functionof species in chlorophyll,
whichwerealsomuchlargerthanin 1987(Figure
composition[e.g., Bricaud et al., 1988; Falkowskiand Kiefer, 7).
1985].
The productivity:chlorophyll
biomass
ratiop B is shownin
Figure 9. Clearly, this ratio was muchhigherin 1987 than in
Estimatesof Primary Productivity
1988.Evennearshore,
wherechlorophyllvalueswererelatively
We estimatedinstantaneous
primaryproductivitybasedon the highin bothyears,phytoplankton
weregrowingmorerapidlyon
model describedby Chantberlinet al. [ 1990] usingSun-stimu- a perunit biomassbasisin 1987by a factorof 3 to 5 (Figure9a).
250
200
o
Q- 150
'•.
o
100
50
1 68
169
170
171
172
173
174
175
176
Yeor Doy
Figure 8a. Estimatesof instantaneous
primaryproductivity(nanogramatomsof carbonper cubicmeterper
second)basedon Sun-stimulatedfluorescencefrom the 1987 deployment.Observationsare constrainedto
between1100 and 1500 LT. Verticallinesseparatethe successive
middayperiodsfor eachday.
ABBOTT
ET AL.: SCALES OF BIO-OPTICAL
VARIABILITY
13,355
6OO
5O0
,_
-• 4oo
o
ß•- •oo
o
.2,
ø 200
o
lOO
186
187
188
189
190
191
192
Year Day
Figure 8b. SameasFigure8a, exceptfrom the 1988deployment.
In 1987,P8 wasloweronday169,whenchlorophyll
wasatits
highest.
P8 wasalsolow on partof day 171whenchlorophyll
error.Wecompared
ourp Bestimates
in 1987withthoseof Hood levelsagainincreased.
Thissuggests
thatthechanges
in P8were
etal. [1991],whouseddirectC 14measurements.
Although
Hood dominatedby changesin chlorophyll contentin 1987. However,
et al. [1991] sampledat somewhatdifferent locationsand times the generalpatternis for p B to increase
as thedriftermoves
in 1987,the rangeof PB is aboutthe same,accounting
for offshore.In 1988 the patternis much lessvariable (in contrastto
However,thereare numerousassumptions
in the productivityand
chlorophyll calculationsthat could causetheseestimatesto be in
changesin units. Chavezet al. [ 1991] publishedmapsof chlorophyll and estimatesof integratedprimary productivity(from a
regressionmodel of PAR and chlorophyll)for 1988. While these
arenotdirectlycomparable
to ourestimates
of near-surface
pB,
the ratio of integrated productivity to integrated chlorophyll
approximately doubled from inshore to offshore, a relative
increasesimilarto that shownin Figure9b. Thus the patternsof
relativechanges
in P8arereasonable.
the productivity and chlorophyll valuesin Figure 8b and Figure
7). As noted above, the drifter in 1988 more nearly followed the
core of the much strongermeander,whereasin 1987 the drifter
stayed inshore of the weaker meander. Even offshore, where
chlorophyll
valueswerenearlythesamein bothyears,PB was
much higher in 1987. This is consistentwith the general pattern
of higher absoluteproductivityin the freshly upwelled water in
the core of the meander and lower productivity offshore or in
2.5
2.0
-••
1.5
-• 1.0
0..5
0.0
1 68
169
170
171
1 72
t 73
1 74
1 75
i 76
Year Dc•y
Figure9a. Productivity
perunitchlorophyll
pBfromthe1987deployment
(inmilligrams
carbon
permilligrams
chlorophyllper hour).Vertical linesseparatethe successive
middayperiodsfor eachday.
13,356
ABBOTT ET AL.' SCALES OF BIO-OPTICAL VARIABILITY
2.5
2.0
o15
•._
.
.-•_
.>
•
1.0
o
0.5
0.0
186
187
188
189
190
191
192
Yeor Ooy
Figure 9b. SameasFigure9a, exceptfromthe 1988deployment.
places
oryears
where
themeander
isweaker.
Thepatterns
ofpB, The one notableexceptionis on day 169 in 1987 (Figure10a);
however,arethereverse.A similarpatternwasnotedby Hoodet
al. [1991];thehighestprimaryproductivity
rateswereinshoreof
the meanderingjet.
We took all daytime observations(as opposedto a 4-hour
window centered around local solar noon) of the flux of Sun-
chlorophylllevelswere quite variable(Figure6), and species
composition
changedconsiderably
[Abbottet al., 1990,Figure
12]. We expectthe linearrelationship
to breakdownat higher
light levelsas strobefluorescence
(whichwasusedto estimate
chlorophyll)will show someinhibitionat high light levels
stimulated
fluorescence
/• andplotted
themversus
PAR.This [Harris, 1980]. However,the nearlylinearrelationshipstill holds
resultedin a family of curvesthatresembledtypicalphotosynthe- evenat high PAR levels.
by phytoplankton,
it canbe usedeitherin
sisversus
irradiance
(P-I) curves.
We normalized
/• tochloro- As lightis absorbed
transformed
intoheator reemittedas
phyll as estimatedfrom strobefluorescence.
Figures10a-10d thecarbonfixationprocess,
[ Falkowskiand Kiefer, 1985].At low to moderate
showtherelationship
between
normalized
/• andPARforboth fluorescence
1987and1988.ForPARlevelsbelow200!.tE/m
2/s,therelation- light levels, there shouldbe nearly linear partitioningbetween
whichis consistent
withtheseobservations.
shipsare fairly linear,althoughthe slopesdiffer betweendays. thesethreeprocesses,
1500
a_ 1250
o
c
+
•
0
A
1000
Doy
Doy
Doy
Doy
168
169
170
171
o
u_
750
:
E
•
o+oo
500
I
-
•
•
,
250
0
I
0
100
200
300
400
•
,
5OO
PAR
Ei[•re lea. •lux of Su•-stimulated
fluorescence
peru•it chlorophyll
(•a•oei•stei•spermilligramchlorophyll
persecond)
•ersusphotosy•thetically
a•ailableradiatio•(PAR).Dataareshow•fori•shorere,ionsi• ]98?.
ABBOTT
ET AL.'
SCALES
OF BIO-OPTICAL
VARIABILITY
13,357
1500
•o
1250 _
c
+
ß
0
Day 172
Day 173
Day 174
-
A
Day 175
'_..
El
Day176
_
-
o
1000
•_•
-
o
L,_ 750
E
'•
ß
. •:k'••
'-+:•,- +
._
500
I
•
250
0
0
100
200
500
5O0
400
PAR
r•gure 10b."•lIll•
OllbllOl•
asFigme 10a,butIOl
...........
re onsin
just anotherway of presenting
F c (whichis a function
However, the partitioning should change, depending on the in essence,
physiological state of the phytoplankton.As phytoplankton of PARandFf), thisresult
doesshowthestrong
change
in the
becomemore nutrient limited, the amountof light reemittedas partitioningof absorbedlight energyandthe associated
temporal
fluorescence
scales. We note that the transition between inshore and offshore
should increase relative to the amount used to fix
carbon
[KieferandReynolds,
1992].We usedlinearregression
to
propertiesis more abruptin 1988 (occurringbetweenday 188
calculatethe slopeof the lines for each day exceptday 169 in andday 189) thanin 1987 whenthe transitionwasmoregradual.
1987 as there appearedto be a much higher level of temporal This is consistentwith the changesin the strengthand width of
of Kosroet al.
variabilitythanthe otherdays.Theseslopesare shownin Table themeanderasseenin thephysicalmeasurements
2. In 1987 the slope increasedby 50% as the drifter moved [ 1991] andHuyer et al. [ 1991].
offshore, whereas it more than doubled in 1988. Note that the
Stramskaand Dickey [ 1992] notethat the bio-opticalpropercan changerapidlyon diel scales,thereby
slopesweremuchsmallerin 1988 thanin 1987. Althoughthisis, ties of phytoplankton
1200
•o
1000
c
800
Day
Day
Doy
Doy
Doy
186
187
188
189
190
L,_ 600
E
'•
._
400
I
200
0
•
0
100
200
300
,
400
,
I
500
•
6OO
PAR
Figure 10c. Sameas Figure 10a, but for inshoreregionsin 1988. Note that day 190 hasbeenplottedwith both
the inshoreand offshoreregions.
13,358
ABBOTT ET AL.: SCALES OF BIO-OPTICAL
VARIABILITY
1200
• •ooo
+
ß
0
o
Doy 190
Doy 191
Doy 192
g 800
o
•
600
._
•
o
400
I
.+ t<t>..,e.<•
+•
o
^
0
o
20O
o
.
0
1 O0
200
,300
400
500
600
PAR
Figure 10d. SameasFigure10a,butfor offshoreregionsin 1988.Notethatday 190hasbeenplottedwith both
theinshoreandoffshoreregions.
confounding simple interpretation of beam attenuation and
fluorescencein termsof either chlorophyllbiomassor primary
productivity.We thus plotted the ratio of beam attenuationto
complicatethis interpretation[Marra, 1995].We noteno apparent quenching of fluorescenceby PAR, similar to results of
Stramskaand Dickey [1992]. The offset between the inshore
strobe fluorescence from 1987 as a function of PAR as Stramska
waters and the offshore waters implies that there was more
and Dickey [ 1992, Figure 17]. Figure 1l a showsthe nearshore scattering and absorptionoffshore per unit of fluorescence
(days 168-171) and Figure 1lb, the offshore(days 172-176) (chlorophyll)asnotedby Abbottet al. [1990].
data.Theseare quitesimilarto the 10-m datafrom Stramskaand
Dickey [1992], which suggestthat muchof the diel variationis Small-ScaleVariability
causedby changesin the bio-opticalpropertiesof the phytoTo examine small-scale patterns, we calculated variance
plankton. In particular, changesin cellular chlorophyll may
changebothfluorescence
andbeamattenuation
in parallelduring spectrafrom severalof the physicaland bio-opticaltime series.
the daylighthours,althoughprocesses
suchasinternalwaveswill We analyzedonly thosetime serieswheretherewas a continuous
record(temperature,beam attenuation,and strobefluorescence).
We used maximum entropy methods(MEM) to calculatethe
Table 2. Slopesof Linear Regression
spectra;as we limited our discussionto timescalesgreaterthan 5
Between the Rate of
hours, this method should not produce spurious results.
Sun-Stimulated
Fluorescence
Comparison with structure functions showed that the MEM
(Normalized to Chlorophyll Concentration)
spectracapturedthe same scalesof variability at theselonger
and PhotosyntheticallyAvailable Radiation
Day
Slope
1987
168
1.4
n/a
1.7
1.5
1.7
2.1
2.1
2.1
2.1
169
170
171
172
173
174
175
176
1988
186
187
188
189
190
191
192
0.4
0.4
0.4
1.0
1.0
0.8
1.0
timescales.We varied the number of polesused to calculatethe
MEM spectra as well; we used these results to estimate the
minimum period that could be reasonablystudiedbasedon the
robustness
of the variousspectralpeaks.In our case,periodsless
than 5 hours showedconsiderablevariability in the number of
peaksasa functionof the numberof polesusedin thecalculation.
To developerror estimatesfor the spectralestimates,we calculatedthe MEM spectrumfor 10,000randomtime series.We used
seriesthat had the same number of points as our actual data
recordsand usedthe samenumberof polesin the MEM calcula-
tions.On the basisof thesebootstrapestimates,a factorof 2
aroundeachMEM spectralestimatewill fit within plusor minus
2 standard deviations (nearly equivalent to 90% confidence
intervals).For example,if the MEM estimateis 10, then the "2
standard deviation"
confidence interval extends from 5 to 20.
Figure 12 showsa typical MEM spectrum,in this casebeam
attenuationfrom the 1987 record.A strongpeakis presentwith a
periodof 24 hours,with smallerpeaksat 16, 12, 8, and6 hours.
Temperature and especially strobe fluorescenceshow similar
diurnaland semidiurnalpeaks.The diurnalsignalhasbeennoted
ABBOTT
ET AL.'
SCALES OF BIO-OPTICAL
VARIABILITY
13,359
2.00
1.75
--
Days 168-171
_
-_
_
_
_
_
.
_
_
_
-
_
_
_
-
_
1.50
1.25
_
_
-
_
-
_
.
-
-
_
1.00
-
_
-
_
_
+
_-
0.75
. + ++
_
0.50
.
+
Z,% +
+
_
++_+++
%.+..
• • +
0.25
0.00
I
0
,
,
•
I ....
50
I
1 O0
,_,
,
,
I ....
150
I
....
I
200
250
,
,
• • I
300
• • • • I
• • • • I
550
• • • •
400
450
PAR
Figure 11a. Beam attenuationcoefficientversusstrobefluorescencefor nearshoreregionsfrom the 1987
deployment.
in many other bio-opticalrecords[Dickey et al., 1993; Stramska
andDickey, 1992;Dickeyet al., 1991]. In the caseof strobefluorescence,diel variability is the result of the changesin fluorescence response to changes in solar irradiance [Kiefer, 1973;
Stramskaand Dickey, 1992]. For temperature,diel variabilityis
also the result of changesin solar irradiancewhich causesnearsurface heating during the day and is visible in the offshore
portionsof the deploymentsin both years.Beam attenuationis
thought to change as a result of both growth in the number of
particles as well as changes in their scattering properties
[Acklesonet al., 1990]. A peakat the semidiurnalperiodhasnot
been observed in other bio-optical time series [Dickey et al.,
1991] which have strong diurnal signals, but it is quite pro-
nouncedin the strobefluorescencedata from the 1987 deployment (Figure 13). Smaller peaks were visible in the temperature
and beam attenuationspectraat the semidiurnalperiod.
A semidiurnaltide may be the causeof this peak, similar to the
semidiurnal peaks found in current meter records off northern
California [Noble et al., 1987; Rosenfeldand Beardsley, 1987].
This could result in water motions which would changethe biooptical and temperature properties of the water at the drifter.
Fluorescencecould change either as the result of changesin
chlorophyll content or as a photoadaptiveresponseto changing
light levels as deeper water is brought closer to the surface.
Although the strongdiurnal signal could result in an overtoneto
appearat the semidiurnalfrequency,note that no suchovertone
appearsin the 1987 fluorescencespectrum(Figure 13).
2.00
1.75
Days 172-176
1.50
+
+
+
+
+
1.25
0.75
_
0.25
0.00
0
50
1 O0
150
200
250
300
350
400
PAR
Figure 11b. SameasFigure 1la, exceptfor the offshoreregions.
450
13,360
ABBOTT ET AL.' SCALES OF BIO-OPTICAL VARIABILITY
Beom
1000.000
Attenuotion
.............
100,000
10.000
1.000
O. lOO
O.OLO
0.001
lOO
lO
Period (hours)
Fieure12. Variance
spectrum
derived
usingthemaximum
entropy
method
forbeamattenuation
fromthe1987
deployment.
Strobe
1000.000
'
i
I
i
i
i
i
i
Fluorescence
i
i
i
i
i
i
i
i
-
1 oo.ooo
lO.OOO
1 .ooo
0.100
0.010
o.ool
,
I • • • t
lOO
•
i
,
,
. I I • ,
lO
Period (hours)
Figure13. SameasFigure12,exceptforstrobefluorescence.
ABBOTT ET AL.: SCALES OF BIO-OPTICAL VARIABILITY
13,361
Temperoture
1000.000
First 3 Days (nearshare)
.3 Days (offshore)
100.000
10.000
-_
.
1.000
.
\,
\,
\,
O. lOO
\
,
\,
,
,
,
,
O.OLO
\
_
,
,
\
ß
-
\,
_
\,
[ I I I
o.ool
I
I
I
] I I I
loo
I
I
_
ß
I
I
I
lO
Period (hours)
Figure 14a. Variance spectrumfor temperaturefrom the 1987 deployment.Data were passedthrougha band
rejectfilter, removingthe 24-hourcycle.
Temperature
I ' ' ' '
1000.000
'
'
'
'
I''
lO.OOO
'
'•
'
'
First 3 Days (nearshare)
Second.5 Days (offshore)
.......
lOO.OOO
' '
_
'\'\,•,'••.
1.ooo __..
\,
\,
_
O.lOO
.
-
\,
\,
'
\,
-
\
,
O.OLO
-
,
.
-
\,
\
o.ool
i•
lOO
, ,
,
,
,
,
,
I,
, ,
,
,
,
,
,
lO
Period (hours)
Figure 14b. SameasFigure 14a,exceptfrom the 1988 deployment.
13,362
ABBOTT ET AL.' SCALES OF BIO-OPTICAL
The patterns of variance appear to differ between nearshare
and offshore, so we divided each deploymenttime seriesin half.
Thus we considereddays 168-172 from 1987 and 186-189 from
1988 as the nearsharetime seriesand days 172-176 from 1987
and 189-192 from 1988 as the offshore time series.We passed
eachtime seriesthrougha bandrejectfilter, removingthe strong
diurnal signal from all variables. For strobefluorescencewe
removed both the diurnal and semidiurnalsignals.Figures 14a
and 14b show the MEM spectrumfor temperaturefrom both
deployments.Althoughthe level of varianceis differentbetween
the two deploymentsin the nearshareregion, the slopesare
nearlyidentical(about2). A peak is visibleat 10 hoursin 1987
(Figure 14a), but in general, neither nearsharespectrumshows
much structure. The nearshare spectrum of beam attenuation
(Figure 15) is flatter than the nearshare temperaturespectrum
from 1987 (Figure 14a), with a slopeof about 1.5 and a peak at
15 hours rather than at 10 hours. The shapesof the nearshare
strobe fluorescencespectra(Figures 16a and 16b) are slightly
different, with a slopeof 2 in 1987 and 1.5 in 1988. There is a
noticeablesemidiurnalpeak in 1987 (Figure 16a). Despite these
subtledifferencesthe nearsharespectraof all three variablesare
generally similar in their lack of strongpeaks.This is most likely
a result of the intense, mesoscale variability present in the
nearshareregionthat occursover a continuumof timescales.
The offshorevariancespectrabehavemuch more irregularly.
Figure 14 shows the spectrafor temperaturefrom the offshore
regionsin 1987 and 1988. As with the nearshareseries,both data
setswere passedthrougha bandrejectfilter to removethe diurnal
signal.In 1987 (Figure 14a) the spectrumis dominatedby much
strongerpeaks at 19 and 6 hoursthan nearshare.In contrast,the
1988 temperaturespectrum(Figure 14b) was dominatedby a
Beam
1000,000
I I I I
100.000
I
I
VARIABILITY
peaknear 20 hours.The bio-opticalvariablesalsovariedin a less
continuousmanner offshore. Beam attenuation(Figure 15) is
dominatedby distinctpeaksat 48, 20, and 7 hours.Strobefluorescence(Figure 16) has a peak at 19 hoursin both 1987 and
1988, althoughit is more pronouncedin the secondyear along
with a peak at 8 hoursin 1988 (Figure 16b) and at 6.5 hoursin
1987. Given the lower level of variability in the offshoreregions,
it is not surprisingto see the variance spectradominatedby
distinctperiods.We have no explanationfor the peaksthat occur
in the 6- to 8-hour range. They may be causedby internal waves
which are propagatingpast the drifter. Of more interestis the
consistentpeak in the 19-20 hour range.
As the inertialperiodat this latitude(39øN)is a little over 19
hours, we suspectthat the drifter was being subjectedto organized inertial motions in the offshore region, similar to those
noted by Paduan and Niiler [ 1990] in the CTZ region. These
organizedmotions may have moved the drifter along a front in
bio-optical properties as well as temperature.We note that the
strengthof the inertial peak is muchlargerin 1988 in both temperatureand strobefluorescencewhen the drifter was nearly in
the core of the meanderingjet. As notedby Paduan and Niiler
[ 1990] and Swensonet al. [ 1992], thejet is nearly geostrophicin
the offshore domain, and there are stronginertial featuresin the
drifter tracks. Paduan and Niiler [ 1990] calculated a horizontal
scale of about 3 km for these features. Swenson et al. [1992],
examiningsimilar scalesof motion, estimatedvertical motionsof
10-20
m/d that were associated with these features. These verti-
cal motions,causedby changesin relative vorticity, are similar to
thosedescribedfor a front in the SargassoSea by Pollard and
Regier [1992]. These strongvertical motionsare likely to have a
strongeffect on the bio-opticalpropertiesobservedby the drifter
package.
Attenuation
[ i i [
I
.......
i
i
i
i
i
First 4.Days (nearshare)
Second4 Days(offshore)
10.000
1.000
O.lOO
O.OLO
o.ool
10o
10
Period(hours)
Figure 15. Variancespectrumfor beamattenuationfrom the 1987 deployment.Data were passedthrougha band
rejectfilter, removingthe 24-hourcycle.
ABBOTT ET AL.: SCALES OF BIO-OPTICAL
Strobe
VARIABILITY
13,363
Fluorescence
1000.000
First4 Days(nearshare)
100,000
_
ß xSecond
4Days
(offshore
-.
10.000
•
,\,\,1,\,
1.000
',. ,.' !
'•-•
0.100
,
0.010
0,001
lOO
lO
Period (hours)
Figure 16a. Variancespectrumfor strobefluorescence
from the 1987 deployment.Data were passedthrougha
bandrejectfilter, removingthe 12- and24-hourcycles.
Strobe
1000.000
[ i i
[
i
i
[
[
Fluorescence
[
[ [ ] [
[
t
i
[
First 3 Days(nearshare)
Second5 Days(offshore)
.......
100.000
]
10.000
1.000
,
\
/
ß
0.100
0.010
0.001
[ I I I
100
I
[
I
t
I
[ I I I
I
I
.
10
Period (hours)
Figure 16b. Same as Figure 16a, exceptfrom the 1988 deployment.
13,364
ABBOTT
ET AL.:
SCALES OF BIO-OPTICAL
We testedthe performanceof our bandrejectfilter with time
seriescomposedof sinusoidfunctionscontaminatedby random
noise.The filter successfully
removedthe desiredfrequencies,
so
we do not think that the 19- to 20-hour peak is an artifactcaused
by the removalof the 24-hoursignal.
VARIABILITY
changesalongthe drifter path.Beforeday 190, temperatures
are
coolandvariable.The temperature
increases
sharplyby 2øCin 12
hoursat day 190, and this front is associatedwith the shift to
dinoflagellates(Figure 17). However,nutrientsdecreaserapidly
beforethe temperaturefront is reached.Swensonet al. [1992]
used two methods to estimate vertical velocities, based on esti-
matesof theheatbudgetandthevorticitybudget.
Although
there
4. Discussion
were significantdifferencesin the magnitudeof the velocity
While we expectthat bio-opticalparameters
shouldchange estimates,the general patternsof upwelling and downwelling
from one oceanic domain to another, these observations show
were quite similar. The larger estimatesfrom the heat budget
thattheyvary on muchsmallerscalesthanmightbe expected. approachrangedfrom 70 rn/ddownwellingto 40 rn/dupwelling,
The interplay between physical forcing and physiological
response
of thephytoplankton
will complicate
theapplication
and
interpretation
of bio-opticalalgorithms.
However,thisvariability
may also be usedto understandthe processes
of responseand
adaptation,if the appropriateancillarydataarecollected.
One suchancillarydatasetwill be informationon phytoplankton speciescomposition.Figure 17 showsthe 1988 species
composition
asa percentage
of totalcell volume.Duringthefirst
3 days of the deploymentthe phytoplanktoncommunitywas
dominatedby Chaetocerosspp. and othercentricdiatoms.On
day 189 thecomposition
changeddramatically,
withRhizosolenia
whereasthe vorticityestimateswere abouta factorof 3 smaller.
However, as noted by Swensonet al. [1992], the heat budget
estimateswere basedon a subclusterof driftersandrepresenteda
region
of about
60km2 compared
with150km2 forthevorticity
budgetestimates.Initially, the bio-opticaldrifter was in a region
of downwelling(about40 m/d, basedon the heat budgetestimates)nearshore.This changedto moderateupwelling(about20
m/d) on day 188. The drifter then experiencedmoderatedownwelling about midway through day 188. Strongdownwelling
occurredduringday 189 at the sametime that Chaetoceroswas
replacedby Rhizosolenia.This downwellingwas associated
with
replacingChaetoceros.
Thefinalshiftwasto a community
domi- an "instability"identified by Swensonet al. [1992] and a westward turn of the drifter. Note the strongfront at 70 km (Figure
nated by dinoflagellates.Although a similar replacementof
4a). As Rhizosoleniaare generallybuoyant,their increaseon day
Chaetocerosby Rhizosoleniaoccurredin 1987 [Abbottet al.,
1990],the magnitudeof the shiftwasnotasdramatic.In 1987the 189 in associationwith this front is not unexpected.This was
shift was from 35% Chaetoceros to 30% Rhizosolenia, whereas
followedby weak upwellingon days 190 and 191 asdinoflagelthe shift in 1988 was from 55%
Chaetoceros
to 45%
latescameto dominatethe phytoplankton.We notedearlier that
Rhizosolenia. In 1987 the community was dominatedby large the normalized rate of Sun-stimulatedfluorescencechanged
centricdiatomsin the offshorewaterscomparedwith dinoflagel- dramaticallyon day 189 (Table 2). This changein physiological
latesin 1988. Given the increasedvariability in speciescomposi- responseis consistentwith the changein physicalforcing and
tion in 1988, it is not surprisingto see large differencesin the speciescomposition.
Given the complex flow patterns in this region, it is not
fluorescence
response.
Changesin the physicalenvironmentare likely the ultimate surprisingto see strong interleaving of various water masses
causeof the specieschanges.Figure 3 showsthe temperature characterizedby differentphytoplanktoncommunities.Following
lOO
0occolithophores
and Flagellates
Dinoflagellates
80-
60_
_
Rhizosolenia
-
40-
Nitzschia
-
Chaetoceros
-
-
20Centric Diatoms
-
-
-
o
186
,
187
188
189
190
191
I
192
,
,
,
I
193
Year Day
Figure 17. Relativephytoplankton
speciesabundance
fromthe 1988deployment
in termsof percenttotal cell
volume.
ABBOTT
ET AL.:
SCALES
OF BIO-OPTICAL
the descriptionlaid out by Strub et al. [1991], we define the
broad,cold, productiveregionsas the filamentswhich are separated from the warmer, less productive offshore waters by a
meanderingjet. As the drifter doesnot exactly follow a single
water parcel, it may traversethese various water types. It is
apparentfrom thesedatathatthe physicaldynamicsof thecoastal
transitionzoneaffectthe timingandnatureof the specieschanges
as observedby the drifter.
A generalpictureemergesfrom the two drifterdeployments.
Changesin the strengthof the meandercorrespondwith changes
VARIABILITY
13,365
dominatedby small forms, primarily flagellates[Hood et al.,
1991].
We applied several methodsto estimate chlorophyll using
downwellingirradianceratios[Smithet al., 1991], beamattenuation [Siegel et al., 1989], Sun-stimulated fluorescence
[Chainberlin et al., 1990], and conventional strobefluorescence.
Although the general patterns were similar (high, variable
chlorophyllvaluesnearshore,decreasingrapidly as the drifter
moved offshore), there was considerable difference in the
detailed structureof the time series. The relationshipbetween
in species
composition,
primaryproductivity,
andP•. Biomass chlorophyllderived from strobefluorescenceand that derived
and primaryproductivitydecreaseoffshoreas nutrientsbecome from Sun-stimulatedfluorescencediffered considerablybetween
depleted,
butpl•increases.
Phytoplankton
become
morestrongly 1987 and 1988. In the latterdeploymenta differentsetof coefficients for Sun-stimulated fluorescencewas required to obtain
scattering per unit chlorophyll in offshore waters. Thus the
meanderrepresents
a sharpdemarcation
betweenboththebiolog- agreementbetween the two fluorescenceestimates,and these
ical and bio-optical propertiesof the phytoplankton.Physical coefficients needed to be altered along the drifter track as it
processes
suchasupwellinganddownwellingalongthe meander moved from nearshore to offshore. It appeared that strong
play an importantrole in the smaller-scalevariability of these changesin the phytoplanktoncommunitywere largelyresponsiproperties.
In 1987, whenthe drifterwassomewhatinshoreof the ble for the changesin the fluorescencerelationships.The beam
main meander,thesepropertiestendedto changemoregradually attenuationdata were availableonly duringthe first deployment,
but they revealed strong onshore/offshoredifferencesin bioover the courseof the deployment.In contrast,the deploymentin
opticalproperties,especiallyin the amountof scatteringper unit
1988 morenearly followed the coreof the meander,and the bioopticalpropertieschangedmoreabruptlyovertime. This resulted chlorophyll.
We examinedsmall-scalevariability throughvariancespectral
in more abrupt changesin the biological/physicalregime as
observed in 1988 and more intense inertial motions.
analysis.A strong diel signal was presentin many of the bioThe presenceof intenseverticalmotionsalongthe drifterpath, optical variables, as expected.In particular, strobefluorescence
showeda consistentpatternin relation to the amountof solar
either through subduction processes[Kadko et al., 1991;
Washburnet al., 1991] or throughvorticity adjustmentscaused radiation.Beam attenuationshoweda similar diel patternin the
offshore waters. We suggestthat this is the result of grazing
by inertial motions,obviouslycomplicatesthe interpretationof
the drifter data.As notedearlier,phytoplanktonspeciescomposi- activity, but changesin the optical propertiesof phytoplankton
tion changesconsiderablyin associationwith changesin the would causea similar pattern.A semidiurnalsignalwas present
directionof vertical motion.However,we are ableto interpretthe in temperatureand strobefluorescenceand is perhapsrelated to
patternsof temporalchangein termsof changesin the physical the semidiurnaltide. An unexpectedpeak was found at the inercirculationfield. While we cannotassumethat the drifter is truly tial period(19 hours)in the bio-opticalvariablesandin temperafollowing a patchof phytoplankton,thesedatashowthe influence ture.This peak,however,waspresentonly in the offshorewaters
of inertial and tidal motionson bio-opticalpropertiesas well as in both 1987 and 1988. Organizedinertial motionsof the drifter
the larger-scale changes that occur between nearshore and in the offshore portion of the meanderhave been observedin
other drifter data sets from northern California.
These inertial
offshoreor betweenyears.
motionsare sometimesassociatedwith strongvertical motions
causedby changesin relativevorticity.The bio-opticalproperties
5. Summary and Conclusions
varied in responseeither to thesevertical motionsor to changes
The two deploymentsof a bio-optical drifter in the coastal in the relativehorizontalpositionof the drifter asit movedalong
the meander.
transitionzone off northernCaliforniawere analyzedin termsof
The proximatecausefor the temporaland spatialfluctuations
the performanceof severalbio-opticalalgorithmsandin termsof
large- and small-scalevariability. With the availability of long- in bio-optical properties was changes in the physiological
term bio-opticalsamplingwe needto understandthe limits and responseof the phytoplanktoncommunitywhich was driven by
advantages of these data sets. Bio-optical relationships can variability in phytoplanktonspeciescomposition.Such species
changedramaticallyon a wide rangeof time and length scales, dependencehas been noted in many studiesand is not unexpected.The ultimatecause,however,appearsto be changesin the
thuscomplicatingany analysis.
The generalfeaturesof the watersoff northernCaliforniawere physicalenvironment.For example,the strengthof the meander
similar in the 2 years.Chlorophyll,particulateconcentration,and as well as distance from the meander appearedto affect the
primary productivity were higher and more variable in the phytoplanktonspeciescompositionasnotedalsoby Chavezet al.
nearshore
waters
thanoffshore.
pl• andscattering
perunitchloro- [1991]. Similarly, vertical velocities associatedwith organized
phyll, however,were lower nearshoreand higher offshore.The inertial motionsin the offshoreregionresultin a pronouncedbioopticalresponse.
separationbetweennearshoreand offshorepropertieswas delinSimilar concerns were raised about fluorometric
methods
eatedby the meanderingjet, which separateddistinctphytoplankton communities,based on speciescounts.Similar distinctions when they were introducednearly 30 years ago. Many of the
were notedby Hood et al. [1991] andChavezet al. [ 1991] for the problemswere addressedsimply by more frequent calibration
samples.However, it is clear that suchcalibrationsamplingmust
coastaltransitionzone region.There were significantchangesin
speciescomposition(and size structureof the community),both be done on a scaleappropriatefor the inherentvariability in the
along the drifter tracks and between the two deployments. processunder study. Unfortunately,we know little about these
Althoughthe phytoplanktonsamplingceasedto operatein the far scalesin the bio-optical realm. Of more concernis the recent
offshorewaters, other observationsshowedthat this region was availability of unattendedsampling devices where calibration
13,366
ABBOTT ET AL.' SCALES OF BIO-OPTICAL VARIABILITY
sampling is not possible.Analysis of such data must be conductedwith the awarenessthat variability in the relationshipsof
interest(suchasbeamattenuationandparticulateabundance)
will
haveits own temporalandspatialscales.Thusstudiesof biologi-
scannerestimatesof chlorophyllconcentration,
J. Geophys.Res., 96,
14,669-14,692, 1991.
Collins,D. J., C. R. Booth,C. O. Davis,D. A. Kiefer,andC. Stallings,A
modelof the photosynthetically
availableandusableirradiancein the
sea,Ocean Optics 9, Proc. SPIE Int. Soc. Opt. Eng., 925, 87-100,
calvariability
mustalsofocusonthevariability
in theprocesses
1988.
usedto convertthesensor
data(beamtransmission,
fluorescence,Denman,K. L., andH. J. Freeland,Correlationscales,objectivemapping
absorption,etc.) into the biological data of interest(particulate
concentration,chlorophyllcontent,etc.).
We arebeginning
to collectinformation
onthetemporal
and
spatial scales of variability that will be useful in future data
assimilation models. While we cannot hope to measureall
processeson all relevantscales,more systematicobservations
of
suchtemporal and spatialvariability mustbe undertaken.Future
anda statisticaltestof geostrophy
overthe continentalshelf,J. Mar.
Res., 43, 517-539, 1985.
Denman, K. L., and T. M. Powell, Effects of physicalprocesseson
planktonicecosystems
in the coastalocean,Oceanogr.Mar. Biol., 22,
125-168, 1984.
Dickey,T. D., The emergenceof concurrent
high-resolution
physicaland
bio-opticalmeasurements
in the upperoceanandtheir applications,
Rev.Geophys.
, 29, 383-413, 1991.
Dickey, T. D., J. Marra, T. Granata, C. Langdon,M. Hamilton, J.
Wiggert, D. Siegel, and A. Bratkovich,Concurrenthigh resolution
work mustextendtheseobservations
overa broad•angeof
bio-opticalandphysicaltime seriesobservations
in the Sargasso
Sea
physicalandbiologicalconditionsasit is app•ent thatthe scales
duringthe springof 1987,J. Geophys.
Res.,96, 8643-8663, 1991.
of variabilitycanchangedramatically,
depending
onthephysical
Dickey, T. D., et al., Seasonalvariability of bio-opticaland physical
environment.
propertiesin the Sargasso
Sea, J. Geophys.Res.,98, 865-898, 1993.
Falkowski,P., and D. A. Kiefer, Chlorophylla fluorescence
in phytoAcknowledgments. We thank P. Niiler for providingthe driftersand
plankton:Relationshipto photosynthesis
and biomass,J. Plankton
Res., 7, 715-731, 1985.
assisting
with designanddeployment.
We thankR. Hoodfor designand
constructionof the drifter packagesfor the seconddeployment.G.
Friederich,G. E., P. J. Kelly, and L. A. Codispoti,An inexpensive
Friederich provided technical assistancefor the water sampler. R.
mooi'edwater samplerfor investigating
chemicalvariability,in Tidal
LimeburnerandT. Cowlesassisted
with deployments.
Usefuldiscussions
Mixing and Plankton Dynamics,edited by M. J. Bowman,C. M.
were held with R. Letelier, and valuablecommentswere providedby J.
Yentsch,and W. T. Peterson,pp. 463-482, Springer-Verlag,New
Marra and an anonymousreviewer.B. Barksdaleassistedwith programYork, 1986.
ming and data analysis.This work was supportedby the Office of Naval
Gordon, H. R., D. K. Clark, J. W. Brown, O. B. Brown, R. H. Evans, and
Research(grantsto M.R.A and K.H.B.) and by the NationalAeronautics
W. W. Broenkow, Phytoplanktonpigment concentrationsin the
andSpaceAdministration(grantsto M.R.A. andC.O.D).
Middle AtlanticBight: Comparisonbetweenshipdeterminations
and
coastal
zonecolorscanner
estimates,
Appl.Optics,22,20-36,1983.
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