JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 95, NO. C6, PAGES 9393-9409, JUNE 15, 1990
Observationsof Phytoplanktonand Nutrients From
a Lagrangian Drifter off Northern California
MARKR. ABBOTT,
1 KENNETH
H. BRINK,
2 C. R. BOOTH,
3 DOLORS
BLASCO
4
L. A. CODISPOTI,
5 PEARN
P. NIILER,
6 ANDSTEVEN
R. RAMP
7
A Lagrangiandrifter was deployedin a cold filament off northern California as part of the Coastal
TransitionZone program.The drifter was equippedwith an opticalpackage(consistingof a spectroradiometer,
a fluorometer,and a beam transmissometer)
suspendedat 8.5-m depth and a water samplersuspendedat
16.3-m depth. The drifter was recoveredafter 8 days. Optical, chemical,and biologicalpropertieschanged
considerablyas the drifter moved offshorein the cold filament. Concentrations
of phytoplanktonchlorophyll
increasedrapidly in the first 2 days, in parallel with the disappearance
of nitrate and nitrite. After this
initial period,chlorophylldecreasedgraduallyover the next 6 dayswith prominentdiurnalfluctuationspresent
in the last 3 days. Water transparencyalso showedsimilar long-termas well as diurnal fluctuations.The
phytoplankton
communitybecameincreasinglydominatedby largecentricdiatomsthroughoutthe deployment.
Althoughtotal cell volumewas highertowardsthe middle of the deployment,this increaseoccurredwithouta
parallel increasein chlorophyll.In addition,total particulateconcentrations
were highestnearshore.Although
the drifter slippagewas approximately1 cm/s, the biological,chemical,and physicalcharacteristics
of the
water were affectedby both in situ changesand verticalmotionsof the water. Theseresultsare generally
consistentwith resultsfrom other upwelling studies.
pressuregradients[Davis, 1985a,], and offshoreeddies[Mooers
1. INTRODUCTION
Unattendedobservations
of biologicalandopticalpropertiesof
theupperoceanarerelativelynewin oceanographic
research[e.g.,
Dickey, 1988]. Recentstudieshavebeenreportedusingfluorometers[Whitledgeand Wirick, 1983; Fukuchiet al., 1988; Walsh
et al., 1988] and spectroradiometers
[Dickeyet al., 1986]. Such
mooringswill providehigh-resolution
time seriesdataon biological processes
thatshouldgreatlyimproveourunderstanding
of the
and Robinson, 1984].
Becausethese filaments are relatively persistentwhen comparedwith phytoplanktontime scales,it has been proposedthat
they may be especiallyimportantto the primary productionof
the California Current system. Estimatesof the transport(of the
orderof 2 Sv [Kosroand Huyer, 1986]) when coupledwith the
higherthan averagenutrientconcentration
nearshoresuggestthat
they may alsobe importantin the overall nutrientbalance.It has
dynamics
oftheupper
ocean.
Inthis
paper
wepresent
results
frombeen
suggested
that
thefilament
may
bearelatively
isolated
fea-
aLagrangian
drifter
that
was
equil•ped
with
aspectroradiometer,
ture
with
relatively
little
exchange
between
itself
and
surrounding
afluorometer,
atransmissometer,
and
anautomated
water
sampler
waters.
Such
isolation
would
result
indistinct
changes
inthebiaspart
oftheCoastal
Transition
Zone
(CTZ)
program
offnorthern
ological
properties
ofthefilament
asthewater
moves
offshore
California
in1987
[Coastal
Transition
Zone
Group,
1988].
and
"ages."
Bymaking
Lagrangian
observations
ofbiological
and
Thelarge
filaments
observed
offthewest
coast
oftheUnitedchemical
properties,
wehoped
toestimate
thetime
scales
ofthese
States
have
been
noted
in satellite
imagery
forseveral
yearschanges
aswellasassess
therelative
roles
ofinsituprocesses
[Breakerand Gilliland, 1981; Traganzaet al., 1983; Coastal versusexchangeprocessesin creatingthesechanges.
TransitionZone Group, 1988]. They can extendseawardseveral
Our resultssupportthe generalview of upwellingecosystems
hundred
kilometers
andareoften
characterized
byhighwaterthat
has
been
developed
over
thelast2 decades.
That
is,freshly
velocities
[Kosro
and
Huyer,
1986;
Kosro,
1987;
Flament
etal., upwelled
water
ischaracterized
byhigh
nutrients,
lowphytoplank1985]. Severalmechanisms
havebeenproposedfor the formation ton biomass,and cold temperatures.As this water advectsaway
andmaintenance
of the filamentsincludingbaroclinicinstabilities from the upwellingcenter,it warms,phytoplankton
take up nutri[Ikeda and Emery, 1984; Willmott, 1984], interactionbetween ents,and biomassincreases.As phytoplankton
continueto grow,
wind
forcing
andcoastal
topography
[Kelly,
1985],
alongshore
nutrients
become
depleted.
Phytoplankton
growth
isgreatly
reduced,eventuallyresultingin a decrease
in phytoplankton
biomass
as lossesdue to sinkingor grazingdominate.Thesefour stages
1College
of Oceanography,
Oregon
StateUniversity,
Corvallis.
2Woods
Hole
Oceanographic
Institution,
Woods
Hole,
Massachusetts.
correspond
tothezones
proposed
byJones
etal. [1983]
andde3Biospherical
Instruments,
Incorporated,
SanDiego,
California.
scribed
byMacIsaac
etal. [1985]forthePeruupwelling
system.
4Bigelow
Laboratory
for OceanSciences,
WestBoothbay
Harbor,We describe
a numberof processes
thatoccuron smalltimeand
Maine.
space
scales
withinthislargerframework.
Wewill compare
the
nia.
observedbiologicalvariabilitywith the physicalenvironmentand
5Monterey
BayAquarium
Research
Institute,
PacificGrove,Califor-
6Scripps
Institution
ofOceanography,
University
ofCalifornia,
Sandiscuss
howthebiological
response
tothefilament
changes
over
Diego,La Jolla.
time.
7Department
of Oceanography,
U.S.NavalPostgraduate
School,
Monterey, California.
Copyright 1990 by the AmericanGeophysicalUnion.
Paper number 90JC00251.
0148-0227/90/90JC-00251 $05.00
2. INSTRUMENTATION
AND METHODS
Lagrangiandriftershavebeenusedsuccessfully
in recentyears
in studiesof surfacelayer currents[Davis, 1985a, b; Niiler et al.,
1988; Poulain et al., 1987]. The possibilityof attachinginstrumentationto suchdriftersis appealing,as it will help reducethe
9393
9394
ABBOTT ET AL.: LAGRANGIAN
co-minglingof temporaland spatialvariability. There havebeen
numerousstudieswhere biological samplinghas been guidedby
drifter tracks[Rytheret al., 1971; ScrippsInstitutionof Oceanography, 1974; Maclsaac et al., 1985; Mackas et al., 1989]. Althoughnot all of thesestudiesusedLagrangiandrifters,the basic
goal of the studieswas to separateadvectiveeffects (Eulerian)
from in situ (Lagrangian)effects. Sucha separationwould allow
the study of temporalvariability without being concernedwith
spatialvariabilitythat may be confusedwith temporalvariability
throughthe processof advection. However, distinguishing
between spatialand temporalvariabilityis not a simpletask with
Lagrangiandriftersthatare fixedat onedepth,sinceverticalmovement of water (and biologicalmaterials)may be significant.
Duringthe firstfield yearof the CTZ program[CoastalTransition Zone Group, 1988], we deployedan opticalpackage,a water
sampler,anda thermistorchainon a TriStar-II drifter [Niiler et al.,
1988]. The TriStar-II telemetersits position,along with surface
temperature,via the Argossatellitetrackingsystem.The uninstrumenteddrifter configurationis known to move with the ambient
flow to within about 1 cm/s. Figure 1 is a schematicof the instrumenteddrifter. The optical packagewas mountedapproximately
4.5 m above the droguecenter at a depth of about 8.5 m. The
packageconsistedof a BiosphericalInstrumentsMER-2020 spec-
DRIFTER
OBSERVATIONS
surface package
syntactic
foam
optical
package
8.5rn
•--
9m
drogue
troradiometer [Booth and Smith, 1988], a SeaTech 25-cm beam
transmissometer,
and a SeaTechstrobefluorometer.The spectroradiometermeasuredboth upwelling radianceand downwelling
irradianceat five wavelengths,instrumentattitude,temperature,
depth,and housekeeping
informationwhich were recordedon an
internal data acquisitionsystem. There was an additional upwelling radiancemeasurementmade at 683 nm by a detectoroptimized to respondto sun-stimulated
fluorescence
by chlorophyll.
This systemalso recordedthe transmissometer
and fluorometer
data. The instrumentswere turnedon for 45 s every 4 min, and
the averagefor the last 30 s was stored. This allowed the fluorometer and transmissometer
to warm up and stabilize. Table 1
givesa completelist of the datarecordedon the data acquisition
system. The automatedwater sampler [Friederich et al., 1986]
was mounteddirectlybelow the drogueat a depthof 17.5 m and
was programmedto samplethe water at twenty 6-hourintervals.
At eachsampling
time,two 20-cm
3 syringes,
onefilteredand
water sampler
thermistor
chain
Fig. 1. Schematic
of Tri-StarII drifterwith watersampler,thermistor
chain, and optical package.
one not, drew in water samplesfor nutrient and phytoplankton
counts,respectively.Both water sampleswere preservedin situ.
3. RESULTS
Lugol's solutionwas usedto preservethe phytoplanktonsamples,
and the nutrient sampleswere preservedwith mercuricchloride
Drifter TrajectoryandRelationship
withHydrographic
Structure
and analyzedas describedby Friederich et al. [1986]. Finally,
a 50-m recordingthermistorchain was hung directly below the
Theinstrumented
drifterwaslaunched
onJune17, 1987(Julian
water sampler,but it did not returnenoughgooddatato be useful. day 168) at 1112LT andrecovered
on June25 (Julianday 176)at
TABLE 1. VariablesRecordedon BiosphericalInstrumentsMER-2020 Spectroradiometer
Variable
Description
Downwelling Irradiance,nm
410, 441,488, 520, 560
Upwelling Radiance,nm
410, 441,488, 520, 683
Other variables
strobe and sun-stimulated fluorescence, beam transmission,
temperature,depth
Housekeeping
variables
time, tilt angle (2), groundvoltage,batteryvoltage
ABBOTT
ET AL.'
LAGRANGIAN
1235 LT. Its deploymentlocationwaschosento be at the center
of the nearshore
end of a pronounced
cold filamentdetectedby
hydrographic
surveysoff PointArena,Czlifomia,andby remote
sensing(Figure2). The driftertrackshow in Figure2 is a spline
fit to the actuallocationsprovidedby the .t egosII systemroughly
eight timesper day. Figure3 is the drifte speedestimatedfrom
the drifter displacements,
usingthe smoot'.tt•'d
locationdata. Note
thatthedriftermovedslowlyon days170-172 andrapidlyon days
173-175. The drifter stayedin the core of the cold, densefeature
for as long as the track couldbe meaningfullycomparedwith the
hydrographicsurveywhich was completedon day 170. During
DRIFTER
OBSERVATIONS
9395
4). The extrainstrumentation
(andhencedrag)on this particular
drifterdid not seemto affectits water-following
characteristics;
otherdriftersdeployedat the sametime behavedsimilarly. A
detailedcomparisonof the instrumenteddrifter with four other
drifterssurrounding
it showedthatthe instrumented
drifter moved
about10% moreslowlythanthe othersalongthe filament.This is
not surprising,as 50 m of thermistorcableprovidesa substantial
connection
betweenthe shallowdrogueand the slowermoving,
deepwater. Figure5a is the alongshore
componentof the wind
stressmeasuredby National Data Buoy Center (NDBC) buoy
46013 (38ølYN, 123ø18'W). The wind vectorshavebeenrotated
thesametimeinterval(days168-170),thedriftertendedroughly 47ø sothatthe alongshore
component
is roughlyparallelto the
to followcontours
of constant10/500dbardynamicheight(Figure coast.The tendencyto followdynamicheightcontours,
of course,
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:.:::..;::..:½::•<:..:
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::::•:•:•.:;:i'
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•;:.::.:
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::.:.•,•:::::..
".....:?':':•:
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•, .:.:::.:.
::•:::.:.:.::::.::....:.;..:.
::.:...
---:•-j-...
..:..:,.j
:•::;.::.:
'"'-•:
-•:•:•:...............•--:?-,:
..........................
:::.•<<.
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::::::::::::::::::::::::::::::::::::::::::::::::::::::
"::::.:•;...
======================
--':::;-.':....,:::':i::::"'.:
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Fig. 2. Overlay of smootheddrifter track on AVHRR image of sea surfacetemperatureacquiredon June22,1987. Cooler
temperatures
are lightershades.Open circleson drifter track representwater samplelocations.Solid circlesrepresentlocation
of drifter at 1700 LT on each day of the deployment.
9396
ABBOTT ET AL.: LAGRANGIAN DRIFTER OBSERVATIONS
to a surface
windstress
of about1.0dyn/cm
2 towards
123.5øT
or a surface
Ekmantransport
of 1.1m3/stoward213øT.If we
50
assumea slablike mixed layer of 10- to 20-m thickness,this
would yield a southwestward
ageostrophicdisplacementof 4-7
km. The observeddisplacementof the drifter was 14 km in the
properdirection.Thus Ekman drift is not adequateto accountfor
õ •o
the offsetof the drifter, althoughit is clearly large enoughto be
an importantinfluence.Other factors,suchas incorrectreference
level for the geostrophic
calculationsor otherageostrophic
forces
• 20
besideswind drift, may be importantduringthis period.
Both the driftertemperaturerecords(Figure5b) andthe drifter
track relative to the near-surfacetemperaturefield (Figure 2)
lO
suggestthat the water warmedas the drifter movedoffshore.This
warming may be attributableto the drifter's movementoffshore
through the general cross-shoretemperaturegradient (warmer
I
o
178
168
170
172
174
176
water tendsto be offshore),but at least someof the temperature
Julion Doy
behavior seemsto be related to the local winds. Specifically,
Fig. 3. Drifterspeedasestimated
fromdisplacements
alongsmoothed much of the warming took place during days 170-172 when
drifter track.
the winds were relatively weak (Figure 5a) and the mixed layer
was presumablywaxmingand stabilizingdue to surfaceheating.
This stabilizationis indicatedby a tendencyfor the sea surface
suggests
a substantial
geostrophically
balancedflow component. temperature(SST) (indicatedby the crossesin Figure 5b) to
To the extentthat the drifter doesnot follow the dynamicheight be warmerthan the 8.5-m temperature
by as muchas 0.5øC.
contours,
errorsin referencelevelof thegeostrophic
calculation
or During this presumablystable period, the 8.5-m temperature
40
thepresence
of ageostrophic
velocity
components
canbecredited. seriesshowsconsiderablehigh-frequency(1- to 6-hour period)
Also,thegeostrophic
contours
maynotbe well resolved
in this variability which is likely to be associatedwith internal wave
area. The most likely ageostrophic
effect is Ekman transport activityin a now stratifiedupperocean.However,thetemperature
increaseof 2.5øC in 2.5 dayswould imply a net heatingrate
Duringthe initial 18 hoursof the deployment,
the apparent of the25-m-deep
mixedlayerby 1210W/m2. Sinceonly10%
within the surfaceboundarylayer.
deviationfrom geostrophy
was particularlyevident,sincethe of this upperoceantemperaturechangecan be attributedto the
drifterwentroughlyperpendicular
to the localdynamicheight net heating,this temperatureincreasemust thereforebe due to
contour(Figure4). At the sametime,the windswererelatively the convergence
of warm water at 8 m, a net downwelling,or
strong(8.5 m/s)towardsthesouthwest
(Figure5a). Thisconverts the drifter's moving into warmer water. As the latter effect is
not apparentin the advancedvery high resolutionradiometer
(AVHRR)image(Figure2), we conclude
thatthe11øCwatersank
to deeperdepthsbelowthe drifter and warmerwaterreplacedit
on the surface. On day 171 the winds increased,leadingto a
relativehomogenization
of upperoceantemperatures,
consistent
with the similarityof the two temperaturerecordsduringthe last
4 days of the deployment. Diurnal cyclesof temperatureare
also evidentduringthis last period along with a slightcooling
trend. The steplikechangesin temperatureduringdays 169-170
(typically0.4ø) maybe theresultof the drifter'scrossing
frontal
40øN
Dynamic Height at 10 db
Relative
to 500 db
(6/16/87 - 6/20/87)
39 ø
or interleavingregions.
Optical Data
38 ø
37 ø
i
126øW
i
125 ø
i
i
124 ø
123 ø
Fig. 4. Dynamicheightfromhydrographic
surveyof June18-24, 1987,
with drifter track overlain.
Figures6a, 6b, 6c, and 6d show downwellingirradianceat
520 nm, beamattenuation,
strobefluorescence,
andupwellingradianceat 683 nm. There was considerablevariability in the daily
amountof downwardirradiance(Figure 6a). Days 169, 170, and
171 (June 19, 20, and 21) were relatively cloudy, with higherfrequencyvariationsassociated
with passingclouds.Note thaton
days 172 and 173, winds were high (Figure 5a) and skieswere
clear. This is the usualconditionduringthe upwellingseasonoff
northernCalifornia [e.g., Abbott and Zion, 1987; Simon, 1977],
wherestrongequatorward
windsare associated
with a shallowing
of the marineboundarylayer whichis unfavorablefor the formation of low stratusclouds.As the windsrelaxedon days 174 and
175, cloudinessincreased.Note that we chooseto presentdownwelling irradianceat 520 nm as an indicatorof solar radiation,
as Gordon et al. [1988] have shown that irradiance at 520 nm is
relatively invariantwith changesin phytoplanktonpigmentcon-
ABBOTT ET AL.'
-15
168
I
170
LAGRANGIAN
I
172
DRIFTER
OBSERVATIONS
I
174.
9397
I
176
(3
I
178
Julian Day
15
_
14
_
13
_
12
_
11
_
10
168
I
I
I
I
170
172
174-
176
b
178
Julian Day
Fig. 5 (a) Alongshore
component
of windvelocityfromNDBC buoy46013. Windswererotated47ø to be parallelto coast.
(b) Temperature
alongdriftertrackfrom thermistor
mountedin opticalpackage(8.5 m depth).Crosses
represent
measurements
from the thermistormountedin surfacetransmitterpackage.
centration.Althoughno specialprecautions
weretakento prevent creasein water clarity, consistentwith a decreasein suspended
foulingof the opticalinstruments,
visualinspection
andcleaning particulateconcentration.Second,thereis a diurnalcycle which
afterrecoveryrevealedno signsof fouling.Note thatbeamatten- is particularlyapparentnear the end of the deployment.Figure
uation(Figure6b) decreased
throughthe period,suggesting
that 7 showsthe transmissiondata from the last 3 days of the deployment in greater detail. Note that the attenuationis lowest
foulingwas not significant.
The beam attenuationcoefficient(Figure 6b) was calculated just beforesunrise,increasessteadilythroughthe day, reachesa
usingthe formulasof Bartz et al. [1978]. Thereappearto be maximumat sunset,and then beginsto decreaseagain. This is
with resultsfrom the centralPacificreportedby Siegel
severaltime scalesof variability. First, thereis a long-termin- consistent
9398
ABBOTT ET AL.' LAGRANGIANDRIFTER OBSERVATIONS
120
100
_
o
80
o
60
_
40
_
20
_
o
(3
0
168
170
172
174
176
178
Julion Doy
0.9
_
0,8
0,7
o
c
E
0.6
b
0.5
168
170
172
174-
176
178
Julion Doy
Fig.6. (a)Downwelling
irradiance
(/•Wcm-2 nm-1) at520nm.(b)Beam
attenuation
coefficient
(m-1) at660nmfrom
25-cmbeam
transmissometer.
(c) Strobe-stimulated
fluorescence
(volts).(d)Upwelling
radiance
(t•Wcm-2 nm-• str-1)
at 683 nm (sun-stimulatedfluorescence).
etal.[1989],
who
propose
that
these
changes
arelargely
theresulture7,wesee
that
these
large
decreases
occur
primarily
between
ofphytoplankton
growth
during
thedayand
subsequent
grazing
sunset
and
midnight.
Figures
8aand
8bshow
therelationship
be-
atnight
byzooplankton.
Note
thatinthiscase,
theaccumulation
tween
strobe
fluorescence
andbeam
attenuation
forthenighttime
ofparticles
during
thedayisnearly
balanced
bylosses
(presumdata
only.Note
thatthechanges
inattenuation
arehighly
correablygrazing
andsinking)
atnight
during
thelastdays
ofthe lated
withchanges
influorescence
onthefirsttwonights.
This
deployment.
Thethirdtimescale
isa series
ofsharp
decreases
correlation
breaks
down
onsubsequent
nights.
(Asimilar
lackof
intransparency
thatoccur
primarily
ondays
174,175,and176. correlation
was
present
fortheremainder
ofthedeployment.)
This
If welookindetail
atthelast3 days
ofthedeployment
inFig- isconsistent
withvertical
migration
ofmicrozooplankton
through
ABBOTTET AL.' LAGt{ANGIAN
DRIFTEROBSERVATIONS
9399
1.5
0.5
c
168
170
172
174.
176
178
Julion Doy
3.5
2.5
o
c
o
1.5
o
...
0.5
o
168
170
172
174
176
d
178
Julion Doy
Fig. 6. (continued)
thebeamtransmissometer
at nighRime
during
thelatterpartof donothaveanindependent
estimate
of chlorophyll
foreachday
thedeployment.
In thefirstdaysofthedeployment,
changes
in (thiswould
bepossible
if theupwelling
radiance
signals
hadnot
attenuation
arelargely
theresult
of changes
in concentration
of beencontaminated
by thedrogue).
However,
fluorescence
declinesby roughlya factorof 3 overthetermof thedeployment,
fluorescent
particles,presumablyphytoplankton.
Strobe-stimulated
fluorescence
(Figure6c) showsa strongdi- with the bulk of the decreaseoccurringin the first4 days.These
werehighlycorrelated
withchanges
in beamattenuation
urnalsignalwhicharisesfrombothphysiological
changes
in the changes
phytoplankton
[Kiefer,1973;Harris, 1980]andchanges
in chlorophyll abundance.Fluorescence
responds
to changesin nutrient
status,light history,speciescomposition,
and a varietyof other
factors[Kiefer, 1973; Abbottet al., 1982]. Unfortunately,we
(ta=0.65)overthewholedatarecord.Thusit appears
thatthe
strobe-stimulatedfluorescencemeasurementis a reasonableindi-
catorof phytoplankton
pigmentconcentration
if we account
for
the diurnal changesin fluorescence
response.
9400
ABBOTT ET AL.' LAGRANGIAN DRIFTER OBSERVATIONS
_ 0.8
_.
_
1.5
0.7
_ 0.6
o
E
_
0.5
0
175
I
I
I
174
175
176
0.5
o
0.4
177
Julion Doy
Fig.7. Beamattenuation
(upper
line)andstrobe-stimulated
fluorescence
(lowerline)fromlast3 daysof drifterdeployment.
The regressionof beam attenuationon strobe-stimulated
flu-
and recoverypoints)decreased
duringthis periodfrom about1
3 to 0.2mg/m
3 (C. Davis,personal
communication,
1987).
orescence
yielded
a slopeof 0.131(m V)-• andanintercept
of mg/m
0.564 m -•. If we assumethat beam attenuationis the sumof the The magnitudeof this decrease
is consistent
with the decrease
fluorescence
observed
attenuationdue to water,fluorescentparticles,and nonfluorescent in the dailymaximumof strobe-stimulated
particles,then the interceptshouldrepresentthe attenuationdue over this period.
to waterandnonfluorescent
particles.Sincethe expectedrangeof
the beam attenuationcoefficientat 660 nm for pure water ranges Water Sampler Data
between
0.31and0.42m-• [Tyleretal., 1974],thissuggests
that
Nitrate + nitrite (N+N) concentrationsas measuredby the
therewasa largecomponent
of nonfluorescent
material
in the drifter's
watersampler
areshown
in Figure10. Initially,
N+N
water.Aszooplankton
maysignificantly
affect
beam
attenuation
concentrations
arehigh(>10/•M), whichtakenwiththelow
andtheyappear
mainly
atnight,werecalculated
theregression
surface
temperatures
(Figure
5b) suggest
thatthedrifterwas
fordaytime
dataonly.However,
theslope
andintercept
were deployed
in freshly
upwelled
water.It would
thenbeexpected
essentially
unchanged.
Wethenexamined
theperiodfromday thattheN+Nconcentration
woulddecrease
monotonically
asit is
168through
day172separately
fromtheperiod
covering
day173 taken
upbyphytoplankton.
Thissmooth
decrease
wasobserved
through
day176.Fortheearly
period,
theslope
was
0.076
m-l only
during
thefirst
2 days
ofdeployment.
After
thestart
ofday
andtheintercept
was0.627(mV)-•. Forthelateperiod,
the 170,theN+Nconcentration
varied
considerably
overthenext2
slope
was0.078-• andtheintercept
was0.564(mV)-•. The days,
asthedrifter
appeared
tomove
through
anareaofdiscrete
decrease
in theintercept
overtimesuggests
thatthereisa decreasetemperature
fronts(Figure5b). Mostlikely,thechanges
in N+N
in the amountof nonfluorescent
particulatematerialin the water
as the drifter moves along its path.
Except for the 683-nm wavelength,we suspectthat the other
upwelling radiancechannelsmay have been contaminatedby
the presenceof the large, multicoloreddrogueunderneaththe
instrument. The 683-nm upwelling radiancebandswere likely
to be uncontaminated
becauseof the strongabsorptionof light
in this portionof the spectrum. Thus radiancedetectedat this
wavelengthwouldoriginatenearthe detector[Kieferet al., 1989].
Figure 6d (683-nm upwellingradiance)is a measureof sunstimulatedfluorescence.
As describedby Boothet al. [1987], this
fluorescence
maybeanindicatorof thephotosynthetic
potentialof
thephytoplankton.
Figures9a and9b showthevariationof strobestimulated
fluorescence
versusdownwelling
irradiance
at 520 nm.
Note the strongdecreasein the slopeof thesecurvesbeginning
on the third day of the deployment.Chlorophyll(as measured
throughextractionsfrom samplescollectedat the drifterrelease
were a resultof changesin water masses,ratherthan beinga
resultof purelybiologicalprocesses.Theseabruptchangesin
watertypessuggest
thatthe drifterdid not follow waterparcels
exactly.Alternatively,
thesampler
couldhavebeenin a nitracline
whichwasmovingasa resultof verticalmotions.As windswere
relativelylight and the upperlayer of the oceanwas stratified
(Figures5a and5b), verticalmotionscouldappearashorizontal
variability.After day 172thedrifterremainedin waterswithlow
N+N
concentrations.
The N+N time seriesgives the impressionthat the drifter
experienced
at leastthreedistinctregimes.Figure11 showsthe
relationship
between
temperature
andN+N duringthedeployment.
Notethatinitiallythe drifterwasin a regionof low temperatures
andhighN+N. Later,thedrifterpassed
througha regionof more
variabletemperature
andN+N with generallyhighertemperature
and lower N+N. Finally, the drifter enteredan area of high
temperature
and barelydetectableN+N.
ABBOTT
ET AL.'
LAGRANGIAN
DRIFTER
OBSERVATIONS
9401
_
1.5
4
4
4
0.5
o
0.4
I
a
I
I
I
0.6
0.7
0.8
0.9
Beam Attenuation Coefficient
1.5
2888
0.5
0.4
0.5
I
I
I
0.6
0.7
0.8
Beam Attenuation
b
Coefficient
Fig. 8. (a) Strobe-stimulated
fluorescence
versus
beam
attenuation
fornighttime
dataonlyforthefirst4 days•
of thedrifter
deployment.
Numbers
correspond
tothedayofdeployment.
(b)Asin Figure
8aexcept
fordays5-8 ofthedeployment.
mixingprocesses,
in addition
to biological
Wecanestimate
primary
productivity
based
onthedisappear-theresultof physical
uptake.
The
considerable
variability
(and
reappearance)
of N+N
anceof N+Nduring
theinitialtwodaysof deployment.
Approxthatwatermasschanges
aswellasbiological
uptake
are
imately
10/tMof nitrate
wastakenupin 2 days.If weassume
a suggest
However,
phytoplankton
cellvolume
(FigC:N ratioof 106:16basedon the Redfieldratio,thenthe uptake likelytobeimportant.
(asindicated
by strobe-stimulated
fiuoof nitratecorresponds
to a carbon
uptake
rateof about465mg ure 10) andchlorophyll
C/m3/day.
This
value
iscomparable
tothat
forthePeru
upwelling
rescence
inFigure
6c)both
doubled
during
thefirst
2days
ofthe
[Brink
etal.,1981;
Ryther
etal.,1971
] andiswithin
therange
of deployment.
values
reported
forCalCOFI
stations
inthis
region
[e.g.,
Scripps Figure
10also
includes
thetotal
cellvolume
based
onphyInstitution
ofOceanography,
1984].
However,
this
isprobably
an toplankton
counts,
using
anaverage
cellvolume
calculated
for
overestimate,
assome
ofthedisappearance
ofnitrate
waslikelyeach
taxonomic
group
thatwascounted.
Note
thatcellvolume
9402
ABBOTT
ET AL.: LAGRANGIAN DRIFTER
OBSERVATIONS
:•2 222
1.5
/•' 1.
1lz' 1 2 2
0.5
3
0
I
0
I
I
I
40
60
80
I
lOO
Q
I
120
Downwelling
Irradiance(52Ohm)
_
1.5
_
8
O•
0
l,
20
5 5 •5555
•,55 5 555 5
I
40
I
60
I
80
I
100
b,
120
Downwelling
Irrodionce(520nm)
Fig. 9. (a) Strobe-stimulated
fluorescence
versus
downwelling
irradiance
at 520rimfor tbefirst4 daysof thedeployment.
Numbers
correspond
to thedayof thedeployment.
(b) As in Figure9a exceptfor days5-8 of thedeployment.
increased
by nearly2 orders
of magnitude
between
days168 dominated
thecommunity.
Although
thecentric
diatoms
in our
and173,although
thereisconsiderable
variability
between
days. samples
wereidentified
asa mixture
between
Actinocyclus
and
Some
of these
changes
mayresult
fromtravelthrough
differentThalassiosira
spp.,it isdifficult
todistinguish
between
these
two
watermasses
bythedrifter,
rather
thanjustin situchanges.
Note groups
without
usingelectron
microscopy.
Hoodet al. [1990]
thesignificant
decrease
in totalvolume
in thelastsample
onday collected
a sample
fromthisregion
a fewdaysbefore
thedrifter
173.
deployment.
Their(in collaboration
withE. Venrick)
scanning
Thecomposition
of thephytoplankton
community
is shownelectron
micrograph
results
showed
thatthecentric
diatoms
were
in Figure12asa percentage
of totalcellvolume.
Chaetocerus
largely
Actinocyclus.
Notethatchanges
inthedominance
ofthese
andlarge,centric
diatoms
dominate
thecommunity
fromthe largecentric
diatoms
(hereinafter
referred
toasLCD)arelargely
beginning
untilday170.Afterthispoint,
thelarge
centric
diatomscorrelated
withchanges
in totalcellvolume.Thereappear
to
ABBOTT ET AL.' LAGRANGIAN DRIFTER OBSERVATIONS
9403
10
12
11
8
10 ,..,
•
6
,,
,,
-I-
•
4
2
6
168
170
172
174
176
178
Julion Doy
Fig. 10. Nitrate+ nitrite(N+N)concentrations
(solidline)andlogarithm
of totalphytoplankton
cellvolume
in/tm3/10ml
(dashedline) during the deployment.
fairly well
havebeenthreedistinctphytoplankton
assemblages.
The first largely LCD. Note that theseperiodscorresponded
wasa nearshare
community,
consisting
primarilyof ½haetocerusto the delineationsbasedon temperatureand N+N concentration
andLCD. The totalcell volumewasfairly low in thisregion.The described earlier.
diatomcommunity
in the second
regime(beginning
late on day
4. DISCUSSION
169andextending
throughday 171)wasprimarilyRhizosolenia
As the slippagecharacteristics
of the drifter are of the orderof
andNitzschiawith a decreasing
percentage
of Chaetocerus
and
an increasing
percentage
of LCD. The third assemblage
was 1 cm/s,we expectthat the drifter may be asmuchas 10 km away
12
_
+
10
+
+
+
+
+ +
o
lO
11
+++
12
+++
13
Temperature
Fig. 11. N+N concentrations
versustemperature
at 8.5 m.
+
14
9404
ABBOTT ET AL.: LAGRANGIAN DRIFTER OBSERVATIONS
lOO
•
•••'..
Coccolithophores
andsmall
flagellates
Small
dinoflagellates
::::•
••_•x•
Large
dinøflag
ellates
••i!i i!i!:
:•.
•.'•Rhizøsølenia
'•....
80
Nitzschia
60
40
•..........
'•
Chaetocerus
.....................................
Large centric diatoms
20
................................................
o
168
.............
:1..................
170
:1:.............
172
I
I
I
174
176
178
Julian Day
Fig. 12. Percent
of totalcellvolume
forthemajorphytoplankton
groups.
fromtheoriginal
watermassbytheendof thedeployment.
This Microscopic
observations
reported
byHoodetal. [1990]showed
couldbea considerable
deviation
in regions
suchasthefilament,thatthisarea1 weekearlierwasdominated
by thesamespecies
wherehorizontal
gradients
arelarge.Further,
thepossibility
of assemblage
alongwithan abundance
of phytoplankton
detritus.
changes
in waterproperties
through
mixing,exchange
withsur- Figures
9a and9b showthattherewerelargechanges
in fiuorounding
water,or through
subduction
of thewatermass,may rescence
yieldin thisregime.Falkowski
andKiefer[1985]have
complicate
matters.
Although
theTristar-II
hasexcellent
horizon-suggested
thatlargechanges
in fluorescence
yieldwithchanges
tal water-following
properties,
it is constrained
to a fixeddepth in incident
sunlight
aretypicalof rapidlygrowing
populations.
so,strictlyspeaking,
it is onlya quasi-Lagrangian
drifter.Thus Similarpatterns
wereseenin naturalpopulations
by Abbottet
thetemporal
changes
thatwe observed
weresomecomplicated
al. [1982],whoalsosuggested
thatrapidlygrowing
populations
function
of slippage,
in situchanges,
andchanges
inwater
massshould
havea strong
fluorescence/light
response.
characteristics
resulting
frommixing
andvertical
motions
of the Thepresence
of anupwelling
center
hasbeennoted
in this
water.However,
thedriftertrackitselfappears
to followthe location
southwest
ofPointArena
(Figure
2) inother
years
from
strong
temperature
signature
of thefilament
present
atthewater bothshipandsatellite
observations
[Kelly,1985;Simpson,
1985;
surface
(Figure
2),suggesting
thatthelarge-scale
changes
maybe Abbott
andZion,1985].
Given
thatthewinds
were
onlymoderate
duetoin situprocesses.
atthetimeofdeployment,
it islikelythattheupwelling
center
Usingall of these
datasets,wecanidentify
threedistinctwasweakening.
Water
velocities
at thispointasmeasured
by
regimes
ortypes
onthebasis
of theirphysical/chemical
charac-thedrifter
motion
were<20crn/s,
compared
withmeasurements
teristics
aswellasontheirbiological
properties.
Thefirsttypewas of >50cm/sin thesame
location
atothertimes[Davis,
1985a].
located
nearshore
andischaracterized
bylowtemperatures,
high Thepresence
of thechain-forming
diatom,
Chaetocerus,
is also
N+N,lowlighttransmission,
highstrobe-stimulated
fluorescence
consistent
withthenear-coastal
originof thiswater.Hoodetal.
andsun-stimulated
fluorescence,
lowtotal
phytoplankton
cellvol- [1990]
reported
onanextensive
survey
made
during
the10days
umes
andhighrelative
proportions
ofLCDandChaetocerus.
On priortothedrifter
deployment,
andtheynoted
thatChaetocerus
thebasis
ofthehydrographic
survey,
thistypeislocated
near
the wasconstrained
tothenearshore
portion
oftheirsurvey
grid.
origin
ofthecold,seaward-moving
filament.
Although
thisappar- Asthedrifter
moved
offshore
during
days170-172,
thewaentlyfreshly
upwelled
water
hadallofthecharacteristics
typicalterbecame
significantly
different.
Temperatures
increased
(al-
ofsuch
conditions
noted
inupwelling
regions
[e.g.,
Beers
etal., though
notsmoothly
asonemight
expect
if onlywarming
due
1971;Jones
et al., 1983;Brinket al., 1981;Abbott
andZion, to solarradiation
wasoccurring),
andN+Nconcentrations
de1985],notethatbeamattenuation
(Figure
6b)ishighdespite
the creased
to nearlyundetectable
levels.Bothsun-stimulated
and
relatively
lowtotal
cellvolume
(Figure
10)butconsistent
withthe strobe-stimulated
fluorescence
decreased
during
thisperiod,
alhighstrobe-stimulated
fluorescence
values
(Figure
6c).Wesus-though
there
wasconsiderable
variability.
Water
clarity
(asinpectthatthiswater
alsocontained
significant
amounts
ofdetritusdicated
bybeam
attenuation)
increased
generally,
although
there
andother
suspended
material
ashasbeen
seen
infreshly
upwelled
wasconsiderable
variability
inthesignal.
Much
ofthisvariability
water
offPoint
Conception
[Jones
etal.,1983].
However,
changes
inclarity
wasassociated
withnonfluorescent
material,
particularly
inwater
transparency
were
generally
wellcorrelated
withchanges
atnight.
Wespeculate
thatthenocturnal
variations
were
largely
instrobe-stimulated
fluorescence,
indicating
thatthismaterial
may theresult
ofzooplankton
vertically
migrating
through
thetranshave
been
ofphytoplankton
origin
aslongaswecanassume
that missometer
path,assuggested
earlier.Thewater
samples
also
strobe-stimulated
fluorescence
is a reasonable
indicator
of phy- showed
increased
abundance
ofnauplii
inthisarea
beginning
on
toplankton
biomass
(Figure
6c).Ata minimum,
wehypothesize
day171.Although
neither
of these
measurements
willgivean
thatthissuspended
material
covaried
withphytoplankton
biomass.
accurate
quantitative
estimate
ofzooplankton
abundance,
theydo
ABBOTT
ET AL.:
LAGRANGIAN
suggestthat the relative abundanceof zooplanktonis higher in
DRIFTER
OBSERVATIONS
9405
beam attenuationand strobe-stimulated
fluorescence,
ratherthan
this regionthan closerto shore. This resultis also consistentcell volume. This assumption
is basedon the observation
that
with the long-termaveragederivedfromthe CalCOFIdatathat changes
in cell volumeare largelydrivenby changes
in LCD
showsa zooplanktonbiomassmaximumapproximately100 km abundance,
whichare characterized
by a largevacuole.
offshore[Chelton,1982]. The phytoplankton
communityhas a
We cannotcalculatechorophylldirectlyfrom thesedata, but
largecomponentof LCD alongwith RhizosoleniaandNitzschia. if we assume
that strobe-stimulated
fluorescence
is a goodinThe coastalspecies,Chaetocerus,
becameincreasingly
rare. LCD dicatorof relativechlorophyllcontent,then this third, offshore
also increasedduringthis period,as did the overallcell volume. regionhad aboutone-thirdthe chlorophyllof the first,nearshore
The fluorescence
responseof the phytoplankton
showedlessvari- region. (Note that the relativechangebetweeninshoreand offationas a functionof solarradiationthanin the firstregion,sug- shorechlorophyllextractionsdiscussedearlier are similar to this
gestingthat the phytoplankton
were growingmore slowly than estimate.)If oneusesthe formulaand parameters
proposed
by
nearshore(Figures 9a and 9b).
Bishop [1986] for the conversionof beam transmissionto susDuring this secondperiod, the winds had relaxed and there pendedparticulatematerial (SPM) concentrations,SPM concenwas evidenceof near-surfacetemperaturestratification. There trationsin the offshoreregionwereapproximately
one-halfthose
was alsoconsiderable
high-frequency
variabilityin temperature, nearshore.Althoughthe reliability of theseestimatesof biomass
presumably
a resultof increased
internalwave activity. Basedon is not high (eventhe extractedchlorophyllestimates
may not be
theuptakeof N+N, phytoplankton
weregrowingrapidly,andthere representative
of the drifter data as the sampleswere not taken
were strongshiftsin the speciesstructureof the phytoplankton, adjacentto the drifter but ratherwere collectedseveralkilometers
particularlyof the diatoms.This regionis similarto the "typical" away), we note that the general trend is similar for both SPM
upwellingcommunity,which is characterized
by diatoms[e.g., and estimatedchlorophyllwith similarmagnitude.The lone exBrink et al., 1981; Estrada, 1984; Jones et al., 1983]. The ception was cell volume, which increasedas the drifter moved
sinkingratepastthe drifterduringthe secondperiodis estimated offshore. If we estimatethe relativechangein the volumeto
to be w = -(dT/dt)/(OT/Oz). With dT/dt =l.0øC/dayand chlorophyll
ratio(basedonthecell countsandtherelativechange
OT/Oz •0.06øC/m, thenw •-16 m/day. Thissinkingratein in strobe-stimulated
fluorescence)
betweenthe nearshore
region
the filamentis comparable
to the upwellingrate alongthe coast. andthe offshoreregion,it is about300 timeslargeroffshorethan
The third regime covereddays 172 through 174 and was nearshore. Results of Hood et al. [1990] indicate the cell volcharacterizedby strongwinds on days 172-173. The drifter was ume to chlorophyllratio was about3 timeslargeroffshorethan
movingmostrapidlyat this point (about40 cm/s). Temperature nearshore. However, the resultsfrom Hood et al. [1990] are
increasedinitially and then leveledoff around13øC, although based on Coulter counter volumes and thus include detritus as
a strongdiurnal cycle was present. N+N concentrations
were well as living phytoplankton
cellsin the estimateof total volume
essentiallyzero. Water clarity was fairly high with a strong whereas
theresultsherearebasedon microscopic
countsof phydiurnaltrend.Transparencies
decreased
from earlymorninguntil toplankton.If we make the assumption
that the estimatedSPM
near sunset,after which they began to increaseuntil the next concentrations
are reasonable
indicatorsof total particulatevolmorning.This increasewasinterrupted
intermittently,
perhapsby ume (analogousto the Coulter countermeasurements),then the
verticallymigratingzooplankton.During this period,the water ratioof particulate
volumeto chlorophyll
for the offshoreregion
samplercaughtthehighestnumberof nauplii.The phytoplankton was abouttwice that of the nearshoreregion,similarto the Hood
community
wasdominated
by LCD. Thefluorescence
response
of et al. [1990] estimates.This suggests
thatthe opticalproperties
the phytoplankton
was lessaffectedby changinglight levelsthan of thenearshore
regionat theupwellingcenterweresignificantly
during the secondregime.
affectedby thepresence
of suspended
particulates
otherthanphyDespitethe strongwinds,the nitraclineappearedto havebeen toplanktonand that Coulter counterdata from suchareascannot
exclusivelywith phytoplankton.It alsoshowsthat
deeperthan the mixed layer as therewas no apparentincrease be associated
in near-surface
N+N concentrations
as might be expecteddur- changesin chlorophyllcontentper cell may furthercomplicate
ing strongerwinds. However,distributions
of otherproperties, interpretationof fluorescencedata.
We do not have much confidence in the absolute estimates of
notablytemperature,strobe-stimulated
fluorescence,
and beamattenuationshowedmuchlesssmall-scalevariabilitycomparedto SPM fromthebeamtransmissometer
sincemanyresearchers
have
earlierin the deployment.This may havebeenpartiallythe result shownthattherecanbe considerable
variabilityin theparameters
of intensemixing,causedby the strongwindsduringthisperiod usedto estimateSPM from changesin beamattenuation[Spin(Figure 5a).
rad, 1986; Bartz et al., 1978;Bishop,1986]. However,the SPM
Althoughtotalcell volumeswerenearly2 ordersof magnitude estimates,
alongwith estimatesin chlorophylland total cell volhigherin this third periodthan in the first period,beamattenua- umefromphytoplankton
counts,do fit intoa consistent
patternof
of suspended
particulates
asonemoves
tionwasmuchlower(roughly
0.73m-• versus
0.59-•). At first changein the composition
thisseemsparadoxical,but as wasdiscussed
earlier,watersin the alongthe core of the filamentfrom its originto offshore.
firstperiodwereprobablydominated
by smallphytoplankton
and
Thisthird(offshore)
regimeappeared
to bein balancebetween
covaryingdetritalparticlesasnotedby Hoodet al. [1990]several phytoplankton
growthand zooplankton
grazing,particularlyon
daysearlier.The Actinocyclus
species
thatformeda largecom- days173-176(Figure7). Beamattenuation
showsa strongdiurponentof theLCD grouppresentin thethirdregionis a largecell nalcycle,increasing
duringthedayanddecreasing
(although
with
withlittlechlorophyll
(consistent
withtheoveralldecrease
in fluo- considerable
high-frequency
variation)at night.Strobe-stimulated
rescence
andrelativeincrease
in downwelling
irradiance
at 488 nm fluorescence
is more difficultto interpret,as changesin fluoresnotedin thisregion)anda largevacuole.Hood et al. [ 1990]noted cencethat may result from changesin chlorophyllcontentare
that samplesfrom this regionwere almostexclusivelyActinocy- entangled
with changes
in fluorescence
thatresultfrom photoineffects,we
cluswith hardlyany debris.Thuscell volumeis apparentlynot hibition.However,after allowingfor photoinhibition
fluorescence
decreases
at nightat
an accurateindicatorof phytoplanktonchlorophyllin this area. notethat the strobe-stimulated
Althoughwe do nothaveindependent
estimates
of phytoplankton about
thesametimeasthebeamattenuation
databecomes
"spiky"
carbon,we suspectthat theymorecloselyfollow the patternsin (presumably
asresultof verticalmigration
of zooplankton).
This
9406
ABBOTT
ET AL.:
LAGRANGIAN
decreasein fluorescencecontinuesafter sunrise,reachinga minimum around noon, and then recovers in the afternoon until the
following midnight. Note that at midnight,beam attenuationand
strobe-stimulated
fluorescencebegin at the samelevels for both
days 174 and 175. On day 176, the beam attenuationis higher
and the strobe-stimulated
fluorescenceis higher than that on the
previousnights. Thus there appearsto havebeenroughequilibrium betweenthe productionof organicmatterduringthe day and
its subsequentremoval at night.
Althoughwe do not have water samplerdatato reinforceour
position,we suggestthatthe latterportionof the deployment(days
175 and 176) represented
a distinctfourthregime.Total cell volume droppedprecipitouslyin the last sampleon day 173, while
both fluorescencesignalsdecreasedand water transparencyincreased.Althoughthe winds were calm duringthis period,there
was considerablylesssmall-scalevariabilityin fluorescence
and
beam attenuation.This regime was probablymore typical of an
oceanicregimewith very low nutrientconcentrations,
warm temperatures,anda roughbalancebetweenphytoplankton
growthand
removalby zooplankton.Note that on the lastday, the drifterappearedto beginto leavethisregimeandenteran areaof increased
DRIFTER
OBSERVATIONS
after the initial upwelling). Thus the use of cell volume as an
indicatorof phytoplanktonabundancemay be somewhatmisleading. A more reasonableindicatormay be strobe-stimulated
fluorescence(after accountingfor the diurnal signalin fluorescence
yield). If we look at the midnightvalues,they showan initial
increaseon the first 2 days, reachinga maximum on day 170.
Afterward,they declinesteadily,reachinga minimumon day 174
and increasingslightlyuntil the end of the deployment.This pattern is consistentwith the generalpatternof beam transmission.
While thereare severaltime scalespresentin all of the various
data sets,there doesappearto be a dominantscaleof 2-3 days
for changesin N+N concentration,
phytoplankton
speciescomposition,total chlorophyll,andfluorescence
yield (as an indicatorof
photosynthetic
potential).Similar time scaleshavebeenobserved
in other upwelling systems[e.g., Ryther et al., 1971; Brink et al.,
1981; Jones et al., 1983]. However, we caution that our estimates
of the time scalesmay be inaccurate
becauseof slippage,mixing,
and exchangewith other water masses.
As the Tri-Star drifter doesnot follow water massesexactly,
particularlyin areasof largeverticalmotions,someof our conclusionsmay not be accurate.We tend to favor the explanationthat
the filamentmay be subductingas it movesoffshore,leadingto
changein the opticalandbiologicalpropertiesasmeasuredby the
fixed-depthdrifter. There is strongevidencefor subduction
from
other CTZ data sets(D. Kadko and T. Cowles, personalcommunication,1989). Satelliteimageryfrom 1981 of phytoplankton
pigmentandSST suggestthatthe offshoreend of filamentsis characterizedby relatively high SST and high pigmentoff northern
California [Abbottand Zion, 1985]. As the effective measurement
depthof the coastalzonecolorscanneris largerthanthe SST measurement(severalmetersto a few tensof metersversusthe upper
few microns), this observationis consistentwith the scenariothat
the cold, high pigmentwater subductsunderneathwarmer, low
pigment water. The net effect is that the satellitesensors"see"
the high pigmentwater at depth and the warm water at the surface. Given the rapid heatingof the surfacewaterbetweendays
170 and 173, it appearsthat the filamentwas subducting
during
this period. DecreasedN+N concentrationsare also consistent
with this picture of subductionand subsequent
convergence
of
phytoplankton
biomass(as indicatedby strobe-stimulated
fluorescence)anddecreased
watertransparency.The drifterhadchanged
direction1 day earlier and was headingtowardshore(Figure2),
perhapsresultingin this increaseon the last day as the drifter
enteredmore productivenearshorewater.
The main goal for the drifter deploymentwas to estimatethe
time scalesof variabilityindependently
of the spacescales.However, the processes
of subduction,mixing, and entrainmentcan
still confusespatialvariationswith temporalvariations.In particular, estimatesof the sinkingof cold waterpresentedearliersuggest
that vertical motionsare importantalong the filament. However,
for the moment,let us assumethat the dominantchangesoccurin
situ;that is, the drifter is movingwith a waterparcel. If we focus
on changesin phytoplanktoncommunitycomposition,we note
that the communitypresentin the presumablyfreshlyupwelled
water is dominatedby Chaetocerusspp. and LCD with a high
concentrationof covaryingdetrital particles. Within 36 hours,
Chaetocerusis largely replacedby Nitzschia and Rhizosolenia,
and LCD becomesdominant. One day later, Nitzschiais almost warmer, more N+N-deficient water.
absent,and the communitynow becomesdominatedalmostexclusivelyby LCD. Althoughthere is a decreasein the relative
5. CONCLUSIONS
abundanceof LCD aroundday 172 (fourth day of deployment),
we ascribethis to changesin water masses,ratherthanbiological
Thereis a considerable
bodyof work on the patternsof nutri-
changes.
In summary,
we notea shiftfroma rapidlygrowingents,phytoplankton
biomass
andprimary
production
in upwelling
Chaetocerus/Nitzschia/Rhizosolenia/LCD
community
to a more systems.
Thefundamental
pictureis thattheupwelling
centeris
slowlygrowing
LCD community
by theendof thewatersam- initiallycharacterized
bylowphytoplankton
biomass,
lowtemperplersequence.
Giventhepresence
of LCD at thebeginning
of ature,andhighnutrient
values.Rapidgrowthby phytoplankton
the deploymentat the upwellingcenterand that the overall in- reducesthe nutrientconcentrations
asthe waterwarmsandmoves
creasein totalcell volumeis largelydrivenby changes
in the offshore
fromtheupwelling
center.Zooplankton
abundance
also
abundance
of LCD, we suspect
thatit behaves
muchlike a "seed" increases
in response
toincreased
foodavailability.Phytoplankton
populationwithin the filament. After the initial high N+N val- growthratesthendecline,andphytoplankton
biomassdecreases.
uesare reduced,it appearsthat the growthof LCD continues
for Typically,oneexpectslargediatomspresentinshorewith smaller
about2-3 daysduringwhichtime its growthrate slowssubstan- formsoffshorein the low nutrientenvironment.
tially. However,duringthisperiodthe warmingof the surface
Ourresults
donotcontradict
thisbasicpicture,although
they
water occursat a rate suchthat sinkingof cold water mustalso do differ in someof the details.The particularfilamentthat was
be occurring.
Thussomeof theincrease
in LCD mayresultfrom sampled
by thedrifterin 1987wassimilarto thisgeneral
picture,
convergence
of waterscontaining
LCD.
although
it differedin somerespects.For example,the rapidly
It appears
thatmuchof thisapparent
"growth"in phytoplank- increasing
portionof the phytoplankton
populationappeared
to
ton cell volumeis accomplished
withoutany increasein to- be largecentricdiatomsthatapparently
havea very low chlorotal chlorophyll.That is, as cell volumeincreases
dramatically, phyll:volume
ratio. Typically,oneexpectssmallerformssuchas
chlorophyllas estimatedby strobe-stimulated
fluorescence
actu- chain-forming
diatoms(e.g.,Chaetocerus,
Rhizosolenia)
to domally decreases
overtime(exceptonthefirstdaywhenit increasesinate suchupwellingsystems.However,LCD appearto have
ABBOTT
ET AL.: LAGRANGIAN
an advantage,particularly where nutrient concentrationsare decreasing. The role of the large vacuole in LCD in maintaining
the positionof the populationin a high-light environmentmay
be importantin suchconditions.That is, it may be able to survive low-nutrientperiodsand then "shift up" [Joneset al., 1983;
Maclsaac et al., 1985] rapidly when it entersanotherareaof high
nutrients.This particularclassof speciesmakesinterpretationof
cell volumedata somewhatdifficult,as cell volumemay increase
althoughchlorophyllfluorescence
may be decreasing.
Nonfluorescentparticulatesare abundantnearshore,particularly within the upwellingcenter. These particulatesin general
covarywith fluorescentmaterial. Apparentlytheseparticlessink
out rapidly as the upwelledwater movesoffshore.The decreasein
their abundance,
alongwith the decreasein phytoplankton
abundance(as measuredby chlorophyllconcentration),is largely responsiblefor the overall increasein water transparency. Total
cell volume increases,though,largely becauseof the increasein
LCD abundance.Short-termchangesin transparency,
especially
after sunset,appearto be causedby zooplankton.Diurnalchanges
apparentlyresultfrom phytoplanktongrowth duringthe day and
zooplanktongrazing(or sinkingof phytoplankton)at night. As
expected,photosynthetic
(as indicatedby fluorescence
response)
rates are highestnearshoreand decreaseas the upwelledwater
movesoffshore.This decreasemovesin parallelwith the decrease
in N+N. Populationsoffshoreare still viable althoughapparently
any biomassaccumulationduring the day is balancedby losses
at night.
DRIFTER
OBSERVATIONS
9407
could examine the distributionof LCD along the length of the
California Current in relation to the upwelling centers. If such
a model were correct, one would expect to find a fairly stable
"pool"of LCD with maximumvaluesbeinglocatedseawardof the
upwellingcentersalong the meanders.This picturediffers from
that presentedby Joneset al. [1988] for the Point Conception
upwellingsystem.Their model was of an asymmetricupwelling
center with low phytoplankton,high nutrient concentrationson
the polewardside of an upwelling center and high chlorophyll,
high nutrientconcentrations
on the equatorwardside. Our results
suggestthat much of this "asymmetry"may result from rapid
temporalchangesalong the filamentrather than spatialchanges.
However, it is likely that the dynamicsof upwelling off Point
Conception,where the coastlineis highly irregular,are different
than those off northern
California.
This presentdata set is not sufficientto make conclusivestatements concerningexchangerates between the cold filament and
the surrounding
waters. Slippageof the drifter cannotbe easily
be mistakenfor mixing, but sincewe lack detailedinformationon
the propertiesof the surrounding
water, slippagecan be a problem. Bucklin et al. [1989] show that particle advectionin the
geostrophic
flow in a filamentis not affectedby morethana few
kilometerswith the expectedsmall-scalehorizontaldiffusivities.
However, the phytoplanktonspeciesinformationcan be usedto
indicatethe relativeimportance
of exchangeandin situprocesses
for theperiodwhenwatersampleswereavailable(days168-173).
The nearshoreupwellingcenteris initially dominatedby chain-
The"aging"of theupwelled
water,asmanifested
in changes forming diatoms and LCD. As the water advectsoffshore, the
rapidlybecomesdominatedby LCD. We suggest
in phytoplankton
biomass,
species
composition,
primaryproduc- phytoplankton
composition
betweendays168
tivity,nutrient
levels,temperature,
andwatertransparency,
occurs thatthis changein phytoplankton
rapidly.Largechanges
in these
variables
cantakeplacein 2-3 and 173 is largelyan in situprocess,as LCD is generallyfound
waters[Hood
days.Giventhatthedominant
scales
of windforcingandsurface only within filamentsratherthanin the surrounding
currentsin this areaare about4-5 days[Kosro,1987; Winantet
et al., 1990]. That is, the increasein LCD wasa resultof growth
al., 1987],we notethatthereis a closecorrespondence
between of the initialpopulation,ratherthanmixingandexchangealthough
thescales
of biological
variability
andphysical
forcing.Thecon- convergenceprobablyplays somerole in this increase.
Futureexperiments
usingclustersof similarlyequippeddrifters
sequent
couplingis probablyimportant
in maintaining
the high
productivityof upwellingecosystems.
will help unraveltemporalvariationsfrom spatialvariations.We
Onemodelfor the filamentssuggests
thattheyaretheresult also shouldbe able to estimateLagrangiandecorrelationtime
of theinteraction
of a meandering
southward
current(in thiscase, scales with such clusters of instrumenteddrifters. However,
the CaliforniaCurrent)with cold, nutrient-rich
upwelledwater we also note that the availability of water samplesconcurrent
nearthe coast.Becauseof variationsin alongshore
wind stress with more automatedopticalsensorsis essentialat this early
and coastaltopography,upwellingoccursin distinct"centers," stagein researchwith Lagrangiandrifters. We need to gain
andthis alongshore
variationmay leadto localintensification
of considerablymore experiencewith such comparisonsbetween
theseaward
movingportionof themeander
[e.g.,Davis,1985b]. opticalinstrumentsand traditionalsamplingtechniquesin order
This intensification
may be the cold filamentreadilyobserved to interpretthe opticaldata properly.
in satelliteimages[CoastalTransition
ZoneGroup,1988]. If
sucha modelis correct,thenonemightexpectphytoplankton Acknowledgments.
We wish to thankG. Friederichfor technicalasspecies
thatarepresent
at the northern
endof the meanderingsistance
withthewatersampler
andT. BaynesandA. C. Amesonfor
withtheopticalpackage.
R. Limeburner
andC. Davisassisted
currentto be advectedsouthward,
alternatelybeingnearshore
in assistance
deployment
andrecovery.
Thisworkwassupported
bythe
nutrient-replete
conditions,
and thenbeingoffshorein nutrient- withdrifter
(grants
toM.R.A.,K.H.B.,P.P.N.,andS.R.R.)
deficient
conditions.
Certainly,
therewill bedepartures
fromsuch Officeof NavalResearch
andby theNational
Aeronautics
andSpaceAdministration
(contract
to
a simplemodelas we expectthereto be significant
exchange C.R.B.).
betweenthe meanderwatersand the surrounding
oceanicand
coastalwaters. Given typical near-surfacewater velocities,we
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ABBOTT ET AL.: LAGRANGIANDRIFTER OBSERVATIONS
9409
P. P. Niiler, ScrippsInstitutionof Oceanography,
Universityof California, San Diego, La Jolla, CA 92093
K. H. Brink, WoodsHole Oceanographic
Institution,WoodsHole,
S. R. Ramp,Department
of Oceanography,
U.S. Naval Postgraduate
MA 02543.
School,Monterey,CA 93943
C. R. Booth,BiosphericalInstruments,Inc., 4901 Morena Boulevard,
San Diego, CA 92110.
D. Blasco,Bigelow Laboratoryfor OceanSciences,McKown Point,
(Received October 13, 1989;
West BoothbayHarbor,ME 04575.
revisedJanuary8, 1990;
L. A. Codispoti,MontereyBay AquariumResearchInstitute,160
acceptedJanuary15, 1990.)
Central Avenue, Pacific Grove, CA 93950
M. R. Abbott,Collegeof Oceanography,
OregonStateUniversity,
Corvallis, OR 97331.