JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 106, NO. C5, PAGES 9499-9507, MAY 15, 2001
Spatial and temporal variability of absorption by dissolved
material
at a continental
shelf
Emmanuel Boss,W. Scott Pegau,J. Ron V. Zaneveld, and Andrew H. Barnard
Collegeof Ocean and AtmosphericSciences,Oregon State University,Corvallis
Abstract. Opticalpropertiesof dissolved(coloreddissolvedorganicmaterial (CDOM))
and particulatematter and hydrographicmeasurementswere obtainedat the Mid-Atlantic
Bight during the fall of 1996 and the springof 1997 as part of the CoastalMixing and
Opticsexperiment.To assess
the temporal and spatialvariability,time serieswere
obtained at one location and cross-shelftransectswere carried out. On short timescales,
variabilityin the vertical distributionof the dissolvedfractionwas mostlydue to highfrequencyinternalwaves.This variabilitywas conservative,
resultingin no changeson
isopycnals.
Over longerperiodsand episodically,CDOM variabilitywas dominatedby
storms.The stormswere associated
with sedimentresuspension
eventsand were
accompaniedby an increasein the absorptionby the dissolvedmaterials.Data from
spatialtransectsshowthat near the bottom, over the shelf, and in both springand fall,
increasedparticulateabsorptionand increasedCDOM absorptionco-occur.These data
supportthe hypothesisthat bottom sedimentscan act as a sourceof dissolvedorganic
carbonduring sedimentresuspension
events.
cleshave also been suggestedas sinksfor CDOM [Thurman,
1985; M. Moran, personalcommunication,1999].
Colored dissolvedorganic material (CDOM) absorption
The observeddistributionof CDOM is complicatedby physstronglyimpactsoceancolor [Bricaudet al., 1981;Carderet al., ical advectionand local mixing, which act to transport and
1989],especiallyin coastalregionswhere it is a major absorb- erase the signaturesassociatedwith the above sourcesand
ing substancein the blue portion of the visible spectrum sinks,respectively.The objectiveof thispaper is to evaluatethe
[Bukataet al., 1995]. In order to invert reliably the remotely contributionof these processesto the observedvariability of
sensedocean color to obtain the concentrationof optically CDOM on a continental shelf.
significantmaterial, it is of major importance to determine
The data analyzedhere were collectedon the continental
CDOM distributionand its effect on oceancolor.The spectral shelfat the Mid-Atlantic Bight.Hydrographicpropertiesat the
1.
Introduction
absorption
by CDOM (here denotedby aa) at the visible Mid-Atlantic Bight are stronglyaffectedby estuarinesources
wavelengthscan be described,to a high degreeof accuracy,by
a singleexponentialor a sumof two exponentialsthat decrease
with increasingwavelength[Carderet al., 1989;Jerlov,1968;
Roesleret al., 1989]. As in previousstudies,we use the term
CDOM to denote coloredmaterial that passesthrough a 0.2
/•m filter, regardlessof its origin and chemicalcomposition.
Terrestrial
and oceanic sources and sinks contribute
to the
at the land-seaboundaryand by the Gulf Stream, its eastern
boundary.The yearly mean circulationis predominantlyalong
the shelf (westward),oppositeto the direction of the Gulf
Stream[Beardsley
and Boicourt,1981].A densityfront is often
found at the shelf break separatingthe cold and fresh shelf
water from the salineand warm slopewater offshore.Associatedwith thisfront is a geostrophicwestwardjet, with observed
observeddistributionof CDOM (alsoreferredto as Gelbstoff, velocities
of the orderof 0.5 m s-• [Barthet al., 1998].A
Gilvin,andyellowsubstance),
in theworld'soceans[Bricaudet weaker across-shoresecondarycirculationwith vertical velocal., 1981;Carderet al., 1989;Jerlov,1968].Terrestrialrunoff is ities of the order of 10 m d -• associated with this front has
consideredto be the main source of CDOM in coastal waters,
beenobserved[Barthet al., 1998].This circulationis associated
resultingin a tight salinityCDOM relationshipthere. An ad- with the upwelling of sediment-richbottom waters on the
ditional sourceof CDOM comesfrom benthic flux; poorly shoreside of the front and downwellingchlorophyll-richnearoxygenatedsedimentsin highlyproductivecoastalareas have surface waters on the offshore side of the front.
beenobservedto flux dissolved
organiccarbon(DOC) into the
Severalstudieshave addressedthe seasonaland spatialvariwatercolumn[Burdige
andHomestead,1994;Chenet al., 1997]. abilityof surfaceCDOM in the Mid-Atlantic [DeGranpreet al.,
Decompositionof particulateorganiccarbon is the primary
1996; Vodaceket al., 1997] and South Atlantic Bight [Nelson
source of the DOC released. Other potential sourcesfor
and Guarda, 1995]. These studieshave concludedthat the
CDOM in the water columnare its releaseas a byproductof
major sourcefor CDOM absorptionis riverine runoff, resultprimary[Bricaudet al., 1981;Twardowski,
1998]and secondary
ing in a strongseasonalvariability and a stronglinear relationproductivity[Nelsonet al., 1998]. The main known sink for
ship between increasingsalinity and decreasingCDOM for
CDOM is photooxidation[Miller, 1994; Vodaceket al., 1997].
eachseason(thoughthe relationshipvariedbetweenseasons).
Bacterialmetabolismand CDOM adsorptionto sinkingpatti- Photo-oxidation of CDOM resulted in lower levels of CDOM
Copyright2001 by the American GeophysicalUnion.
near the surfacein the summer[Vodaceket al., 1997]. These
studieswere mostly confined to the surface and used instrumentation that could not detect the low CDOM absorption
Paper number 2000JC900008.
0148-0227/01/2000JC900008509.00
9499
9500
BOSS ET AL.: COLORED
DISSOLVED
ORGANIC
ABSORPTION
VARIABILITY
than 5 m were not usedbecauseof contaminationby bubbles
generatedby the shipmotion. The data analyzedhere include
162profiles(41 bursts)from CMOI beforehurricaneEdouard,
41 ø 30'N
40 profiles(9 bursts)from CMOI after Edouard,and 136 (40
bursts)from CMOII.
During the analysisof the data the near-surfacevalues of
•"• •
40 ø 30'N
MATTER
•
• ,•transects
line ag(676) werefoundtobeoftheorderof -0.01-0.02m-• and
decayingto ---0 at 10 m depth and below. Sincewe expect
CMO
time-series
site
•
negligible
absorption
by CDOM at the infraredag(676) was
subtractedfrom the dissolvedabsorptionat all wavelengths.
This baseline offset removal procedure assumesthat this
depth-dependent
error wasnot spectrallydependent.We have
not been able to identify fully the sourceof this effect, but it
appearsto be related to changesin flow rate causedby the
39 ø 30'N
72 ø 30'W
71 ø 30'W
70 ø 30'W
69 ø 30'W
addition
of the filter.
A noticeable
difference
in CDOM
concentration
was found
Figure 1. Studysite of the CoastalMixing and Opticsexperbetween days3 and 4 of CMOII, when the instrumentmeaiment (CMO). The locationof the time seriesstationis indicatedby an open circle,and the transectcourseis denotedby suringCDOM was changed.A vicariouscorrectionwas comthe bold line. Bottom depthis basedon the ETOPO5 database. puted by matchingthe CDOM absorptionalong the 6ø isotherms(---40m depth)beforeand after the instrumentchange.
The 6 ø isotherm
signalin the visiblerangeover the midshelfregion(detection
of UV absorptionwas possiblesinceit is higher than in the
visible).In thisstudywe analyzeCDOM absorptiondataat 440
nm collectedat the Mid-Atlantic Bight that spansthe whole
water columnand a period of a few weeksin the late summer
and spring.
was chosen because at this location
neither
surface nor bottom water mass intrusions were observed. The
offsetwasfoundto be +0.0095m-• for the first3 days.Similarly, a differencein salinityand densitywasobservedbetween
days6 and 7 of CMOII when the CTDs were changed.The
offsetswere computedsimilarlyto CDOM (salinitywas +0.05
practical
salinity
units(psu),anddensity
was+0.03kgm-3 for
the first 6 days).These offsetswere added to the data. The
adjustments
were madeto the shortertime recordin eachcase.
2.
Methods
The offsetsto the salinityand densitymake variationsin the
Data were collectedduring two cruisesto the Mid-Atlantic ancillary data more clear. Since these data are not used in
Bight as part of the CoastalMixing and Optics experiment calculations,
it doesnot matterwhichperiodwasadjusted.The
(CMO) duringAugust 17 to September7, 1996 (denotedby offsetto aa(440) wasaddedto the datacollected
duringthe
CMOI), andApril 26 to May 13, 1997(CMOII). A time series first3 daysbecausethe spectralslopeof the CDOM absorption
station was occupiednear 40.5øN, 70.5øW,and three across- during that time was inconsistentwith the rest of the data
that the calibrationerror most likely
shelftransectswere performed,two in CMOI, before (August collected.This suggests
17-18, 1996) and after (September5-7, 1996) the time series occurredduringthe first 3 days.This temporaladjustmentto
and one after the time seriesfor CMOII (May 12-13, 1997, the CDOM absorptionwasfound to be spectrallydependent,
Figure 1). Sosiket al. [thisissue]and Gardneret al. [thisissue] but only the value at 440 nm is necessaryfor the discussion
in
summarizethe variabilityin opticaland hydrographic
proper- this text.
ties duringthe time series.
The spectralslopesof aa were foundto be on average
Two WET Labs ac-9swere usedto measureCDOM absorp- ---0.018
nm-• (excluding
thefirst3 daysof CMOII), withinthe
tion, total absorption,and the absorptiondue to particles(by range of publishedvalues[e.g.,Roederet al., 1989] and obdifference).To determinethe contributionof dissolvedmate- servedby DeGranpreet al. [1996]and Vodaceket al. [1997]in
rialsto the total absorptioncoefficient,a 0.2 p.mfilter (Gelman the Mid-Atlantic Bight, increasingour confidencein our caliSuporcap100) was attached to the inlet of one ac-9. Both bration procedure.In order to estimatethe effect of the subinstrumentswere calibrateddailywith opticallypure water as tractionof aa(676) on the verticalstructure
of aa(440) the
a reference(BarnsteadNANOpure) and correctedfor tem- correctedabsorptiondata were comparedto CDOM fluoresperature and salinityfollowingPegauet al. [1997].The partic- cencemeasurementson the samplingplatform (WET Labs
ulate absorptionwas correctedfor scatteringusingmethod 3 SpectralAbsorptionand Fluorescence
Instrument(SAFIRE),
(CMOI) and method 1 (CMOII) of ZaneveMet al. [1994]. excitation/emission,
265 nm/460 nm) for randomly selected
Physicalpropertieswere measuredon the samepackagewith a measurementprofileswhere both were available.The two inSeaBirdElectronicsSBE-911 conductivity-temperature-depthdependentmeasurements
of CDOM concentrationwere found
(CTD) (a SBE-25 CTD was usedafter April 31, 1997). The to behighlycorrelated
(average
(R2) -- 0.95), increasing
our
data from the CTD and ac-9 were mergedand binned to 1 m confidencein our processing
procedure.
intervals.
For comparisonwith data collectedpreviouslyin the South
The data were collectedduring daylightin samplingbursts Atlantic and Mid-Atlantic Bights we include in our analysis
(on averagethree per day) that lasted an averageof 40 min data from Nelsonand Guarda [1995],DeGranpreet al. [1996],
(+ 10 min) with an averageof four castsper burst.In order to and Vodaceket al. [1997].When absorption
near440 nm was
quantifythe short-termvariabilityin opticalandphysicalprop- not measured(first andlaststudy),we extrapolatedto 440 nm,
erties the mean
standard
deviation
and maximum
standard
assuming
ag(440)= aa(h.)e
-ø'ø18(44ø-x),
onthebasis
ofthe
deviationof burstswere computedfor eachdepth.Daily means valueof the spectralslope(0.018nm-•) observed
in these
were computedby averagingthe burstmeans.Data shallower studies.
BOSS ET AL.'
CMO-I
COLORED
10
DISSOLVED
ORGANIC
ag(440)
[m-1]
MATTER
ABSORPTION
VARIABILITY
9501
o [kg/m3]
%(440)[m-1]
ß
(
20
30
40
50
60
CMO-II
10
20
30
40
50
60
10-3
10-2
10-3
10-2
10-1
10-2
100
Figure2. Mean(bold)andmaximum
of thestandard
deviation
(over40 minsampling
bursts)of aa(440),
ap(440), anddensities
for CMOI (priorto hurricane
Edouard)andCMOII. Theminimums
werelowerthan
the resolution
3.
Results
of the instruments.
ap correlates
lesswith density(r = 0.23 for CMOI andr =
0.31 for CMOII). The short-termvariabilityis higher, on avThe resultsare presentedfor differenttemporaland spatial
erage,in all propertiesin CMOI relativeto CMOII (Figure2).
samplingscales.We presentthe particulatefractionabsorption
3.1.2. Mesoscalevariability. Over scalesof 4-5 daysthe
at 440 nm, ap(440), for comparison,
sincetogetherwith
andaa exhibit
wave-like
variation
(Plate1) in
aa(440) and the (almost)constantabsorption
of water,it CMOIdensity
constitutesthe total absorption.Sosiket al. [thisissue]discuss the thermocline, indicating the potential presenceof lowfrequencyRossbyor Kelvin waves associatedwith the shelf
the mainsources
of variabilityin ap(440). Physical
forcing,
break
front or the bottom topography.These waves cause
sinks,and sourcesnear boundariesare expectedto result in a
vertical
displacements
of 0(5 m) at the pycnoclineandcanalso
variation of optical propertiesthat depend on the physical
be
observed
in
the
more
evenlyspacedCTD data setcollected
distancefrom the boundary(z). In the interior, on the other
hand,opticalpropertiestend to stayconstantalongisopycnals. during the samecruiseGardneret al. [this issue]and in the
In order to separatethe variability of optical properties at currentrecord of Boydet al. [1997].
Intrusionsof oceanicwarm saltywaterswere observeddurconstantdepth from that on constantisopycnalsurfacesthe
ing
severaloccasionsof both cruises.During CMOI, intrusions
variabilityof the opticalpropertiesaswell asdensityare quantified at constantdepths,and the covarianceof densityand occurredon August22, 25, and 27 andAugust31 to September
opticalpropertiesis computed.Salinityis presentedin order to 1, 1999(Plate 1). W. S. Pegau(Mixing of opticalpropertiesas
compare
it withtheaa(440) distribution;
in thesimplest
case, evidencedin salinityintrusionsobservedover the continental
shelf in the Middle Atlantic Bight, submittedto Journalof
where the only sourceis rivefine input of constantCDOM
Research,2000,hereinafterreferredto asPegauet
concentration,the two shouldinverselycovary.Last, temper- Geophysical
ature is presentedin order to assess
changesin opticalprop- al., submittedmanuscript,2000) discussintrusivefeaturesobwith meandering
ertieswithinwater massesof constanthydrographicproperties. servedbetweenAugust25 and 27 associated
of the shelfbreak front intrudingat the baseof the mixedlayer
3.1. Temporal Variability
and near the bottom. Within these intrusions,Pegau et al.
3.1.1. Short-term variability (40 min bursts sampling). (submittedmanuscript,2000) found CDOM mixed conservaWhen strongstratificationis present(CMOI), CDOM variabil- tively,while particulateabsorptiondid not. In the spring,beity (its standarddeviationas a function of depth) correlates tween May 4 and 6, a salty bottom intrusion is observed,and
with the variabilityin density(Figure 2, correlationcoefficient after May 7 a freshwater massis observedin the upper 20 m.
r of the standarddeviationof all bursts,r = 0.8). This implies The salty bottom intrusion is associatedwith relatively low
that on this timescale,CDOM behavesto a large extent con- CDOM absorption,while the freshsurfaceintrusionis associservativelyand its variabilityis mostlydue to isopycnaladvec- ated with higher valuesof CDOM.
tion. The maximumin variabilityoccursat the pycnocline(s) 3.1.3. Variability due to storms. Intense episodicstorms
>0.4 N m-2) occurred
duringbothcruises;
during
duringtimes of intensehigh-frequencyinternal wave activity (windstress
associated
with solitarywavepackets[Boydet al., 1997;Chang CMOI (September2, 1996) the eye of hurricane Edouard
and Dickey,thisissue].During CMOII, variabilityin CDOM is passed110 km from the time-seriesstation while a seriesof
smallerthantheinstrument
accuracy
(0.005m-•, precision
is springstormstookplaceduringCMOII (April 27, 1997,MaY 4,
-0.002m-•), andthecorrelation
between
density
andaa is 1997,and May 7, 1997). These stormshave contributedto the
weak (r = 0.18). Higher variabilitynear the bottom, where variabilityin opticalpropertiesby verticallymixinggradients,
sediments
densityisnearlyconstant,is suggestive
of nonconservative
pro- reducingthe physicalstratificationandresuspending
cesses
and/oradvection
there.Particulateabsorption
ap in (Plate 1). Edouard reduced the near-surfacegradients in
bothcruises
is morevariablethanaa' In CMOI, variabilityin CDOM while increasingthe near-bottomgradientsin partic-
9502
BOSS ET AL.:
COLORED
DISSOLVED
ORGANIC
ulate absorption(Plate 1). The springstormscausedmixingof
the opticalpropertiesdownto 20 m (Plate 1).
3.1.4. Seasonal variability. The vertically averaged
CDOM value at the CMO site is decreasedby 0.01 _+0.005
m- • fromfall to spring.Thisdifference
is associated
withthe
MATTER
ABSORPTION
VARIABILITY
1.155, computedwithout usingthe absorptionvaluescloseto
the surface,where photo-oxidationmay be present).
The data of Nelsonand Guarda, [1995, Figure 8] from the
SouthAtlanticBighthavevaluesof CDOM absorption
at least
a factor of 2 higher than those observedin the Mid-Atlantic
Bightin the late summerandspringfor the samesalinityvalues
mean salinityduringCMOI being,on average,0.25 _+0.06 psu
fresherthan during CMOII. During the spring,CDOM is less (approximate
linearrelationship:
aa(440) = -0.0727s +
stratifiedthan in the summer(Plate 1). The hydrographicand 2.618). This may be due to (1) higherCDOM concentration
CDOM absorptionpropertiesof the waters denserthan the in riverine inputsfrom southof the Mid-Atlantic Bight (e.g.,
shelf break front (tr > 25.5) and below the pycnoclineare from the ChesapeakeBay). (2) Accumulationof CDOM derived from in-water productivity and/or sediment-released
nearlyconstantthroughoutthe year (Plate 2 and Figure 3).
CDOM into the southwardflowingwaters originatingin the
3.2. Spatial Variability
Mid-Atlantic Bight.
3.2.1. Vertical structure of CDOM during the time series.
The correlationbetweensalinityand CDOM absorptionin
as observedin the refThroughout
bothcruises,
valuesof aa monotonically
increase the surfacelayer is season-dependent
with depthexceptfor the last4 daysin CMOII where,near the erencedstudiesas well as here.At eachdepthhorizon,aa
surface,a low-salinity
high-aa watermassis present.
decreasesand s increaseswith distance from shore. However,
3.2.2. Variability in transects. The optical and hydro- the observed
relationship
betweens and aa in the vertical
graphicpropertiesin the first transectare stronglystratified duringthe time seriesstationis, mostfrequently,oppositeto
with increasing
s
(Plate2). Valuesof aa abovethe shelfexhibita subsurfacethe horizontalrelation,with aa increasing
maximum at depths of 40-60 m and a minimum near the
surface.
The bottommaximum
in aa nearthe 60 m isobathis
associated
withthe20-30% elevated
particulate
absorption
ap
values.
(Plate 1).
4.
Discussion
The secondtransect,performedafter the hurricanepassage, 4.1. Role of AdvectionVersus Local Change
showslittle correlationbetween physicaland optical properThe short-term variability is higher in all properties in
ties.While the water columnhasrestratified,intenseresuspenCMOI
relativeto CMOII. Someof thisvariabilityis associated
sionof particleshastakenplacenear the 60 m isobath.Vertical
with isopycnaladvectiondue to higher stratificationand assogradients
of aa are weakwith bottomandsurfacemaximum
ciatedinternalwaveactivity(Plates1 and 2). Other sourcesof
observed
towardshorecoincident
witha maximum
in ap. Horshort-termvariabilitymay be horizontaladvectionof horizon-
izontalgradients
in aa followingthe hurricaneare stronger
tal gradients
in absorption
(uOaa/Ox)aswellasvariability
due
than the gradientsin the first transect.
to
local
processes.
The
mean
horizontal
velocities
during
The spring(CMOII) sectionexhibitslow verticalstratifica-
CMOI wereof the orderof 0.2 m s-• (the upper95% was
0.4
m s-•) [Boydet al., 1997].For a localchangein CDOM
are higher both at the surface and bottom above the shelf
absorption
of theorderof 0.01m-• overa 40 minintervalto
(midwaterminimum).
be totallydueto horizontaladvection
by a 0.2 m s-• current,
the spatialgradienthasto be of the orderof 0.2 m-• over10
3.3. Salinity-ag
Relationship
tionin bothhydrography
andopticalproperties.
Valuesof aa
In the casewhere the only CDOM sourceis riverineinput,
onewouldexpectaa to increase
with decreasing
salinitys as
km. The maximumalong-isopycnal
CDOM gradientsobserved
in the transects were of the order of 0.005 m -• over 20 km.
oceanicwatersdilute the rivefine input. Indeed, at the surface This is consistentwith the vertical advection of isopycnals
in the SouthAtlantic and Mid-Atlantic Bightsa linear inverse beingthe maincausefor CDOM variabilityin CMOI, resulting
in a high correlationbetweenthe variabilityin densityand that
relationship
of salinityandaa hasbeenobserved
in previous
studies[DeGranpreet al., 1996;Nelsonand Guarda, 1995;Vo- in CDOM absorptionduring this time.
daceket al., 1997];thisis alsothe generaltrend observedin our
spatialsections(Figure 3). By dividingthe data into two seasonsand addingthe data of DeGranpreet al. [1996] and Vodaceket al. [1997] (collected3 years earlier and downstream
from the CMO site)we observethe followingtrends:(1) Variability in CDOM concentrationwithin watersof givensalinity
is much higher in the late summer; twofold differencesin
CDOM absorptionare found in waterswith salinityfrom 31.5
to 32.3psuin that time. (2) There is little variabilityin the deep
watersbeyondthe shelfbreak front (s > 32.3) comparedto
the near-surfacewaters. The data of DeGranpreet al. [1996]
and Vodaceket al. [1997] fit the trends observedin our data
well, suggestinga consistencyin the range of CDOM values
overa 3 yeartimescale.(3) The CDOM absorptionvaluesfrom
the fall, measuredafter the hurricanepassage,obey a similar
relationshipwith salinityasthe springdata (approximatelinear
4.2.
Storms and SeasonalVariability
The effect of a singlestorm can causeas muchvariabilityin
CDOM absorptionas is observedbetweenseasons.Over the
shelfthedifference
in valuesof aa at thebeginning
andendof
CMOI (At • 3 weeks)is largerthan the differencein aa
betweenthe end of CMOI and CMOII (At • 8 months,Plate
2 and Figure 3). Seasonalvariability,in addition to winter
verticalmixing,
includes
potential
changes
in aa - s in riverine
sourcesand effectsof seasonallymodulatedprimary and secondary productivity.Our data are by no means adequateto
resolvetheseprocesses.
4.3. Variability of CDOM in the Water
off the Shelf Break
Front
CDOM propertiesbeyondthe shelfslopefront are foundto
that prorelationship
aa(440) = -0.0157s + 0.58). Thisrelationship varylittle throughoutthe year (Figure3), suggesting
is different from that computedwith the data taken in the fall
prior to the hurricanepassage(aa(440) = -0.032s +
cessesaffectingCDOM in thesewatershave timescaleslonger
than 0(6 months).It is interestingthat a nearly linear rela-
BOSSET AL.: COLORED DISSOLVED ORGANIC MATTER ABSORPTION VARIABILITY
9503
Data fromAugustand September
(a) 0.2 ......
0.15
(b)
**•.
20
.
E
• 15
t3.1
0.05
. .
,
I
......
Data fromApril andMay
(C)0.2
(d)
20
0.15
t3.1
10
0.05
0 30
i 31
i 32
i 33
I 34
i 35
i
1
53
32
Salinity(psu)
33
34
35
36
Salinity(psu)
Figure3. (a) and(b) Theaa(440)-salinity
andtemperature-salinity
diagrams
compiled
withthedata
collected
during
thesummer-fall
transect
(August
17-18,1996,soliddots)andthepost-hurricane
Edouard
transect
(September
5-7,1996,
shaded
dots)anddatacollected
3years
earlier
intheMid-Atlantic
Bightfrom
Vodacek
etal. [1997,
Table1] (extrapolating
to 440nmassuming
s - 0.018onthebasis
of Vodacek
etal.
[1997,
Figure
7],denoted
bystars)
andbyDeGranpre
etal.[1996,
Table1](and442nmabsorption
given
by
Vodacek
etal.[1997,
Figure
7],denoted
bysolidcircles].
(c)and(d)Thedatafromthespring
transect
(May
12-13,1997).Squares
denotethe 5 m dataof eachsampling
station.
tionshipstillexistsbetweensalinityandCDOM withinthese
4.5.
watersdespitetheir distancefromriverineinput.
A back of the envelopecalculationbasedon observed
coastalbenthicfluxesof DOC in Chesapeakebay (2 mmol
4.4. Primary ProductivityAs Sourcefor CDOM
Primaryproductionover GeorgesBank and the MidAtlanticBight,wherethe CMO siteis located,is nearlyfive
timeshigherthanthemeanoceanic
productivity
[O'Reilly
etal.,
1987].Primaryproductivity
is an important
sourceof DOC at
Georges
BankandtheMid-Atlantic
Bight;onaverage,
14%of
the primaryproduction
is released
as DOC [O'Reillyet al.,
1987].In our datathereis no correlation
betweenparticulate
andCDOM absorption,
thoughon the basisof DOC budgets,
phytoplankton
isanimportant
source.
Evenin theopenocean,
Benthic
Source
DOG m-2 d-•) [e.g.,Burdige
andHomestead,
1994]suggests
a
440nmabsorption
fluxof theorderof 0.01m-• m-2 d-•
(assuming
thathalfofDOC isCDOCanda specific
absorption
at440of0.7m-• (mgL-•) onthebasisofBlough
etal. [1993]).
O'Reillyet al. [1987]foundthe annualprimaryproduction
in
thevicinityof theCMO siteto be of theorderof 210-450g C
m-2 [O'Reillyet al., 1987,Table21.3].Walshet al. [1987]
estimated that between 5 and 10% of the annual primary
production
is releasedas organicDOC from the sediment
et al., 1987,Table22.2].Thesenumbers
suggest
an
where CDOM is believedto be mostlythe result of primary [O'Reilly
fluxof the sameorderof magnitude
asthoseobproduction,
no correlation
hasbeenfoundbetweenbiomass absorption
[1994].In orderfor the
and CDOM [Bricaudet al., 1981],suggesting
that the other servedby Burdigeand Homestead
benthic
flux
to
have
a
significant
effect
relativeto the backprocesses
controlling
CDOM distribution
havea highertemground
values,
near-bottom
CDOM
has
to
accumulate
overa
poralfrequency
of variability,
masking
the contribution
from
this source.
periodof the orderof a few weeks.
9504
BOSSET AL.: COLOREDDISSOLVEDORGANICMATTER ABSORPTIONVARIABILITY
Hurricane Edouard
•
ag(440)[m-•]
Spring•.4N/m2)
0.08
20
0.07
0.06
0.05
a•(440) [m-•]
0.2
0.15
0.1
0.05
60
cr[kg/m3]
26
[
20
ß
25
24
40
23
' ß i •!
i
•
22
33
20
32.5
40
32
31.5
60
31
Temperature[øC]
18
16
14
12
10
6.5
8
60
6
19
21
23
25 27
29 31
August
Date in 1996
2
4
6
September
26
28
30
2
4
April
6
8
10
May
Date in 1997
Plate1. Contour
plotof dailymeanofag(440),ap(440),salinity,
anddensity
at thetimeseries
station.
Contour
density
isdifferent
forthetwocruises
because
ofa weaker
stratification
inCMOII.In bothcruises,
ag(440)
increases
nearthebottom
associated
withbottom
resuspension
(higher
ap(440))
following
storm
passage.
BOSS ET AL.:
COLORED
DISSOLVED
8/17-18/96(CMO-I)
ORGANIC
MATTER
ABSORPTION
9/5-7/96 (CMO-I)
VARIABILITY
9505
5/12-13/97(CMO-1I)
20
0.08
40
0.06
60
0.04
I
80
I
100
ag(4-40)
(44o) l,
20
0.02
i ,• It••[o.
! I-"
I
40
0.1
---
>>0.12
60
0.05
80
%(440)
%(440)
lOO
{
• yP %(440)
27
I
20
''
40
I
I
I
I
26
I
I
!
25
60
24
80
I [•
lOO
ø't
23
35
20
I
I
I
I
I
I
34
i
I
!
I
33
32
lOO
•
31
20
t8.5
6.5
60
15
ß
10
80 •
100
40
•
40.2
•
T
40.4
40.6
degreeslatitudeN
40.8
40
T
40.5
degreeslatitudeN
41
[:
T
ß
40
40.4
40.8
degreeslatitudeN
Plate2. Contourplotof aa(440), ap(440), salinity,anddensityof threespatialtransects.
Theboldblack
line indicatesbottom depth at each samplingstation.Dashed lines denote the location of the sampling
stations.Notice the decreasein stratificationfollowingthe hurricanepassage(secondtransect)and spring
transects
compared
to thelatesummer
transect.
Noticeanelevated
valueof aa(440) nearthebottomin all
transects to the north of 40.5øN near the 60 m isobath.
9506
BOSS ET AL.'
COLORED
DISSOLVED
ORGANIC
While diffusive fluxes may be low, suddenstirring of the
sedimentsmay releasewater with very high concentrationof
CDOM. CDOM concentrationsin the upper sediment of
coastalwatershavebeen found to be 5 timeshigherthan those
abovewaters(e.g., D. J. Burdigeet al., Fluorescentdissolved
organicmatter in marine sedimentpore waters,submittedto
Marine Chemistry,1999)while CDOM fluorescence
in the upper meter of anoxicsedimentshasbeen observedto be 10-500
timeshigherthan the overlyingwaters[Chenand Bada, 1994].
5.
MATTER
ABSORPTION
VARIABILITY
Conclusion
Our data set is unique in that it resolvesthe whole water
column and includesboth temporal and spatialinformation.
These data, together with previousstudies,suggestthat the
distribution
of aa is determined
bya coastal
rivefinesource,
a
shelfbottomsource,and a photo-oxidativesinkat the surface.
Advectionand diffusionby the backgroundflow further modulatesthe distributionof CDOM absorption.There may also
be an additional
in-water
source of CDOM
associated with
In all thetransects
weobserve
highervaluesofaa ontopof the phytoplankton[Twardowski,1998],whichwe did not observein
shelf,in correlationwith higher particulatevalues.In the last our data,thoughit maycontribute
to the differentaa - s
two transectsa midwater minimum in CDOM absorptionis
observedon top of the shelf, uncorrelatedwith the hydrographicproperties(Plate 2). A sedimentarysourceis consistent with sucha pattern. Similarly,the stronggradientalong
isopycnalsfollowingthe hurricanesuggeststhat CDOM and
particulateabsorptionhave not mixed conservatively.
Follow-
relationshipobservedfarther downstreamat the SouthAtlantic Bight [Nelsonand Guarda, 1995]. This sink/sourcelayout
resultsin a spatialdistributionof CDOM, whichis decreasing
awayfrom shore(with increasein salinity)and increasing
with
depthat the shelf(alsowith increasein salinity).
ingthehurricane,
aa increased
in thefreshest
waters(s < 32
found to be mostly due to conservativeprocessesconsistent
with the advectionof isopycnals.Over periodsof weeks and
psu),whileit decreased
in the saltierwaters(s > 32 psu).This
decreasein CDOM absorptionmay be due to vertical mixing
of saltywaterswith the relativelyCDOM-poor fresherwater.
The increasein CDOM of the freshestwatersmaybe due to a
benthicinjectionof CDOM into the waterson the shallowpart
of the shelf. This is also consistentwith increasingbottom
stressand elevated resuspensionactivity in shallowerwater
causedby the wind and wavesassociatedwith the hurricane.
Advectionof a freshwatermassof a different CDOM-salinity
relationshipthan that previouslyfound on the shelfcannotbe
ruled out.
4.6.
Photooxidation
Short-term
variability
in theverticaldistribution
of aa was
longer,sources
andsinksof aa causea distribution
thatdeviates from that of physicalproperties near the surface and
abovethe shelf.Episodicstormsare found to havean impor-
tantrolein the high-frequency
variability
in ag throughboth
redistributionand resuspension
of bottom sediment.
Over the shelf, salinity and CDOM are found to have a
different relationshipdependingon the seasons,aswas found
previously[DeGranpreet al., 1996;Nelsonand Guarda, 1995;
Vodaceket al., 1997].Beyondthe shelfbreak front, subsurface
aa wasfoundto behavein a conservative
fashionfor a period
longerthan 8 months(Plate 2 and Figure 3). Comparisonof
the (scarce)data obtainedfor this study and those of past
studiesover the Mid-Atlantic Bight shelf suggeststhat the
seasonalcyclerepeatsitself over the years.
Though we do not observea direct reductionof surfacea
duringthe time series,photo-oxidationmay havebeenpresent
Thevariability
inaa(440) isfoundtobesignificantly
smaller
andis probablythe main contributionto the trend of decreased
than
that
in
ap(440)
on
all
timescales
and
space
scales.
HowCDOM in waters of decreasedsalinity observedabove the
ever, the averagemagnitudeof the dissolvedabsorptionis
shelf(Plate1).Themeanaa distribution
asa function
of depth similarto that of the particulatefractionat 440 nm, providing
in the late summercouldbe the result of a balanceof photo- a similarcontributionto the total absorption.This impliesthat
oxidationin the mixed layer (ML) and a diffusiveflux from high-frequencyvariabilityof ocean color in a given surface
lower in the water column(integratingthe conservation
equa- water massis most likely due to variationsin the particulate
tion over the ML):
fraction.
0 = -HMi•'yaa+ K daa/dz (at z = HM•),
where HML is the ML depth, •/is the ML-averagedphotooxidationrate, and K is the eddy diffusivity.Taking HMi• •
Further studyof the spectralshape(the exponentialslope)
of aa isrequiredto distinguish
betweenmarineandterrestrial
sources and to determine
whether
the surface a
has been
photo-oxidized[e.g.,Carderet al., 1989;Vodaceket al., 1997].
10 m, K • 2 x 10-s m-2 s-1 (on the basisof the in situ We do not havethe spectralresolutionto addressthisquestion
microstructuremeasurementof MacKinnonand Gregg(per- fully with our data set.
sonalcommunication,
1998)), and the relativeverticalgradient
of CDOMacross
theML baseaa ldaa/dz• 0.2 m-1 (Plate
1), the averagephoto-oxidation
rate constant•/corresponds
to
a timescale
(•/-1) of theorderof 1 month.GiventhatK varied
Acknowledgments.Discussions
with M. Twardowskyand P. Coble
are gratefullyacknowledged.
We thank JenniferMcKinnon and Mike
Greggfor eddydiffusivities
data,T. Boydand M. Levinefor providing
current data, S. Lentz, A. Plueddernann, S. Anderson, J. Edson, and
the WHOI Upper Ocean ProcessesGroup for providingthe wind
stressdata. Supportwas providedby the EnvironmentalOpticsProgram of the Office of Naval Research.
by a factor of 5 near the ML base,this value shouldbe interpreted as an order of magnitude estimate of the rate. This
value is consistentwith the estimatedphoto-oxidationratesof
Nelson et al. [1998] and Vodaceket al. [1997]. We cannot,
however,rule out the possibilitythat the surfacereductionwas
due to the advectionof lower CDOM waters, though this is
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(ReceivedApril 2, 1999;revisedAugust11, 1999;
acceptedDecember10, 1999.)