JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 101, NO. C9, PAGES 20,579-20,593, SEPTEMBER 15, 1996
Mixing of chlorophyll from the Middle Atlantic Bight cold
pool into the Gulf Stream at Cape Hatteras in July 1993
A. Michelle Wood and Nelson D. Sherry•
Department of Biology,Universityof Oregon, Eugene
Adriana Huyer
Collegeof Ocean and AtmosphericSciences,Oregon State University,Corvallis
Abstract. In July 1993we collectedhydrographicdata and informationon chlorophyll
distributionon the continentalshelfnorth of Cape Hatteras and acrossthe shelfbreak at
Cape Hatteras. The data showthat a warm, transparentmixed layer lies over much colder,
euphotic,chlorophyll-richbottomwater on the shelf.This layer has temperatureand
salinitypropertiescharacteristicof the Middle Atlantic Bight (MAB) cold pool, a
distinctive
mass of cold bottom
water formed
when cold water from the Gulf of Maine
and ScotianShelf is isolatedfrom surfacewater by vernal warming and seasonal
stratification[Houghtonet al., 1982].The constantdensityof this chlorophyll-rich
water
(o-0 = 25.0-25.6) combinedwith a strongchlorophyllgradientalongthe 25 o-0 isopycnalat
the shelfbreak indicatesthat chlorophylladvectedoff the shelfat Cape Hatteras in July
1993. TS diagramsfurther indicatethat cold pool water, and the chlorophyllit contained,
mixed into upper levelsof the Gulf Stream.Thus the MAB may contributeto the nutrient
budgetof Atlantic surfacewatersthrougha long loop of circulationthat transportsdeep
water from the Labrador Sea to Cape Hatteras.
al., 1986; Chapmanand Beardsley,1989; Walshet al., 1987].
Vernal warming isolatescold shelf water to form the "cold
The fate of primaryproductionin the Middle Atlantic Bight pool" of the MAB, a distinctivemassof bottom water which
(MAB) has been the subjectof intensedebateand scientific spreadsalong the shelf and shelf break from Hudson Canyon
inquiry for severaldecades,primarily in responseto evidence to Cape Hatteras duringthe summer[Houghtonet al., 1982].
that much of the springbloom of the MAB is exportedoff the Net current flow in the MAB is to the southwest. This flow
shelf [Walshet al., 1981;Malone et al., 1983].Two major ex- converges
with northwardflow from the SAB at Cape Hatteras
perimentsdesignedto documentoff-shelftransportof organic where the closeapproachof the Gulf Streamleadsto mixingof
carbon showedthat most primary productionon the MAB
shelf,slope,and Gulf Streamwater [Churchilland Cornilion,
shelf is Oxidized in the water column and/or shelf sediments
1991a,b; Gawarkiewiczet al., 1992;Churchillet al., 1993].
[Roweet al., 1988;Biscayeet al., 1988, 1994;Falkowskiet al.,
Becauseof the convergenceof flow at Cape Hatteras, or1988, 1994;Kemp,1994;Wirick,1994].Sinceremineralization
ganic matter in the water which reachesCape Hatteras must
of nutrients accompaniesoxidationof organicmaterial, these either exit the shelf in water which is mixed into the Gulf
resultsdo not resolvethe shelf/slopenutrient balancewhich
Stream and slopewater, be depositedlocallyin sediments,or
actually determineswhether or not continental marginsare
be oxidized locally. Oxidation would result in net release of
importantsourcesof organicmatter to the slopeand deep sea
CO2 at Cape Hatteras and off-shelftransportof remineralized
[Mamaet al., 1990; Walsh,1991, 1994]. Rapid recyclingcould
nutrients.
Since water leaving the shelf at Cape Hatteras is
supportboth highratesof on-shelfproductivityand respiration
likely
to
be
enriched in either organic material or inorganic
and high off-shelffluxesof organicmatter. In this contextit is
nutrients
or
both, when comparedto the upper levelsof the
important to determinethe fate of new nutrientsenteringthe
Gulf Stream and slopewater, this region representsa potenMAB.
Intermittent intrusionsof nutrient-rich slopewater and ex- tially important sourceof nutrient enrichmentfor the Gulf
changeof MAB shelf water with slopewater are two of the Streamand, throughGulf Streamtransport,more oligotrophic
Introduction
water
offshore.
main sourcesof nutrientsin the MAB [Flagget al., 1994;Walsh
The Cape Hatteras region is a primary study site for the
et al., 1987]. A third major sourceof new nutrientsfor the
MAB is cold, nutrient-richslopewater that apparentlyorigi- ongoingOceanMarginsProgram(OMP), an interdisciplinary,
multiinstitutioninvestigationof the fate of organic matter at
nates in the Labrador
Sea and enters the MAB
from the
ScotianShelf and Gulf of Maine [Bigelow,1933; Chapmanet continentalmargins[Jahnkeand Verity,1994]. Preliminary
cruisesfor this programwere conductedin 1993 and 1994 to
helpidentifytemporalandspatialscalesthatwouldadequately
•Nowat Department
of Oceanography,
University
of BritishColum- describeshelf and shelf-edgeprocesses.
In July 1993we colbia, Vancouver, Canada.
lected hydrographicdata and information on the chlorophyll
Copyright1996 by the American GeophysicalUnion.
distributionon the continentalshelf north of Cape Hatteras
and acrossthe shelf break at Cape Hatteras. Intensive samPaper number 96JC01135.
0 i 48-0227/96/96J C-01135 $09.00
plingwas carriedout in a regionof the midshelfwhere scales
20,579
20,580
WOOD ET AL.: CHLOROPHYLL
EXPORT INTO THE GULF STREAM
tom depthsbetween 149 and 600 m, and eight slopestations
with bottom depthsexceeding1000 m (stations21-27, 33).
36.5
3181118
Thisgridwasalsosampled
in a radiator-like
patternbeginning
4
at the northeastcornerat -0800 hourson July 17 and ending
in the southwestcornerat 0345 on July 18. The positionof the
southern grid was within the zone where maximum benthic
depositionoff Cape Hatteras is thought to occur (L. Benninger,personalcommunication,
1993).
912A7
$
•.
1316
6
7
I415
36
Data Collection and Processing
73O
35.5
A Sea-BirdElectronics9/11 conductivity-temperature-depth
(CTD) system,equippedwith a SeaTech fluorometerandSea
Tech transmissometer
(660-nmsource),was usedto measure
water propertiesto within 3-5 m of the bottom, or to 500 m,
i
whichever was most shallow. The raw 24-Hz
35
-76
-75
-74
Figure 1. Location of conductivity-temperature-depth
(CTD) stationsmade from R/V Cape Hatteras,July 14-18,
1993, with bathymetriccontoursshowingthe positionof the
shelf break. Stations1 and 2 were sampleden route to the
studyarea;station1 (not shown)waslocatedat 34ø44.59Nand
76ø03.60W. Stations3-18 are in the "midshelfgrid"; stations
21-41 are in the "shelf-breakgrid." Station 20 was locatedat
the same position as station 12. Positionsof the Minerals
ManagementService/Science
ApplicationsInternational Corporation(MMS/SAIC) mooringsare notedwith solidtriangles.
of variabilitywere expectedto be low and at the shelf break
where
the historical
axis of the Gulf
Stream
CTD
data from
the underwaterunit were averagedin the deck unit to produce
a 4-Hz data file, which was processedthrough the Sea-Bird
SeaSoftsoftwareto removespikesand correctfor the thermal
massof the conductivitycell; data were averagedinto 1-dbar
pressurebins,andsalinityanddensity(o'0)were calculatedfor
each1-dbarbin. Salinitydatafrom the CTD were spot-checked
againstwater samplesrun on an Autosalprecisionsalinometer;
themeandifference
(0.008psu)of ninepairswaslessthanthe
standarddeviation(0.012psu),sowe did not correctthe CTD
data for this small negativebias.Water transparencywas estimated at selectedstationsusinga 0.5-m oceanographic
Secchi
disk. Light transmissiondata from the Sea Tech transmissometer on the CTD showa strongnegativecorrelationwith the in
situ fluorometerreading (i.e., Figure 5 below) and are not
discussedseparately.
is closest to the
Carolina shelf.Here we report the distributionof water mass
propertiesandchlorophyllin both regionsand discuss
the data
Calibration
of In Situ Fluorometer
Water samplesfor measurementof chlorophyllconcentrawhich indicate that bottom water with characteristics of the
tion in discretesampleswere collectedfrom Niskin bottles
cold pool was a site of primary productionon the shelf. We trippedat variousdepthsandstationsacrossthe shelfandshelf
alsoprovideevidencethat this water mixedwith Gulf Stream break(Table 1). Calibrationstationsran alongthe diagonalof
water at the shelf break, transportingthe chlorophyllit con- each grid and sampleswere collectedfrom each of thesestatained off the shelf and into slopewaters.
tions at four or more depths(Table 1). Dawn occurredat
approximately0500 local time and sunsetoccurredat approximately
2015 local time. Thus calibration stationswere colMethods
lectedin bothdaylightandnightimehours(Table 1; Figure2).
Study Area
Sampleswere filtered throughWhatman glassmicrofiberfilObservationswere made at 41 stationssampledfrom the ters (grade:GF/F), and chlorophyllwas extractedby freezing
R/V Cape Hatterasduring July 15-18, 1993 (Figure 1). Sea the filter in 1 mL distilledwater. After freezing,sampleswere
conditionswere fairly calm throughoutthe cruise;wind speeds diluted with 9 mL of 100% acetoneand placed at 4øCin the
recordedwhile on stationrangedfrom 3 to 18 knots(1.5 to 9.3 dark for 24 hours.Chlorophyllconcentrationin the samples
m/s), with a medianvalueof 9 knots(4.6 m/s). All but three was then determinedfluorometricallyusinga Turner Designs
stationswere includedin one of two stationgrids,within which AU-10 fluorometer,correctingfor phaeopigments[Parsonset
stations were located at intervals of 5 min of latitude and
al., 1992]. At the midshelfgrid, phaeopigmentswere 30%
longitude.The northerngrid or "midshelf"grid waslocatedin (s.d. -- 4%, n = 20) of fluorometrically
determinedpigments
the midshelfregionand wassituatedover relativelyflat topog- (chlorophyll+ phaeopigment),
and phaeopigments
averaged
raphy -100 km north of Cape Hatteras and -40 km west of 42% (s.d. = 32%, n = 43) in the calibrationdata setoverall.
the shelf break (Figure 1). Station depth rangedfrom 28 to High values (>60% of total pigments)were only found in
42 m, with an averageof 33 m (s.d. = 4 m, n = 16). The grid samplesfrom >200 m (Table 1). Beforethe cruisethe AU-10
wassampledin a radiator-likepatternbeginningshortlybefore was calibratedwith a dilution seriesof standardspinachchlonoon on July 15 at the northwestcorner and ending in the rophyll (Sigma).
northeastcorner around2200 hourson July 15. The southern
Chlorophyll fluorescence, derived from the 1-m bingrid, or "shelf-breakgrid," was situatedover the outer shelf averagedin situ fluorometerreadingsat the depthswhere
and extendedacrossthe shelf break just north of Cape Hat- samplesfor extractedchlorophyll
weretaken,wascalibratedby
teras (Figure 1). This grid includedeight shelfstations(sta- linear regressionagainstthe chlorophyllmeasurementsmade
tions30, 31, 35-40) with bottomdepthsbetween32 and50 m, on the AU-10 fluorometer(Figure 2). Chlorophylldata dismeasurements
confive shelf-breakstations(stations28, 29, 32, 34, 41) with bot- cussedbelowrepresentin situfluorescence
WOOD ET AL.: CHLOROPHYLL
EXPORT INTO THE GULF STREAM
20,581
Table 1. Station Data for SamplesUsed to Calibrate In Situ Fluorometer
Chl a,
Volume
Local
Station
Date
Time
Latitude
1
July 14
1519
34ø44.59
2
3
July 14
July 15
2012
1110
35ø00.05
36ø19.96
Longitude
76ø03.60
75ø39.95
75ø16.45
Depth
9
July 15
1712
14.43
11.64
13
July 15
2030
09.96
06.41
19
21
25
27
July 15
July 16
July 17
July 17
July 17
2211
1620
0748
1248
1512
05.06
36ø14.71
35ø39.21
19.16
24.07
01.39
74ø31.52
74ø42.68
42.83
47.97
July 17
1839
29.08
52.93
July 17
2325
34.04
58.08
s.d.
X-
s.d.
Phaeo %
of Total
0.28
0.54
0.28
0.23
0.02
0.02
0.02
0.03
0.11
0.38
0.18
0.10
0.02
0.04
0.05
0.01
28.7
41.1
39.8
31.3
-0.03
0.16
0.03
0.03
0.27
0.47
0.94
3.62
0.26
1.68
5.47
5.90
0.24
0.87
0.92
4.25
0.45
0.63
4.98
5.23
0.75
0.06
0.02
0.02
0.02
0.01
0.01
0.15
0.01
0.03
0.10
0.06
0.01
0.04
0.19
0.08
0.02
0.00
0.04
0.18
0.03
0.00
0.00
0.01
0.12
0.21
0.38
1.68
0.12
0.81
2.21
2.19
0.10
0.38
0.42
1.43
0.18
0.23
1.44
1.66
0.35
0.07
0.08
0.08
0.01
0.00
0.02
0.12
0.01
0.08
0.13
0.15
0.01
0.00
0.10
0.08
0.01
0.02
0.04
0.10
0.05
0.00
0.01
0.00
31.3
31.2
29.0
31.7
32.2
32.4
28.8
27.1
29.0
30.6
31.4
25.2
28.2
26.9
22.5
24.1
32.0
53.8
80.3
80.3
-0.06
0.02
0.14
1.12
-0.08
0.18
1.22
1.49
-0.05
0.13
0.14
0.69
0.01
0.16
0.73
0.99
0.21
-0.01
-0.05
-0.25
1.76
0.15
0.30
0.42
0.53
0.25
0.06
0.16
0.49
0.25
0.16
0.02
0.14
0.24
0.34
0.87
0.14
0.18
2.06
1.79
0.11
0.01
0.01
0.00
0.02
0.01
0.01
0.00
0.01
0.01
0.00
0.01
0.01
0.00
0.01
0.04
0.02
0.00
0.06
0.22
0.78
0.05
0.12
0.20
0.44
0.28
0.12
0.06
0.38
0.29
0.24
0.05
0.04
0.14
0.20
0.53
0.07
0.10
1.17
0.67
0.06
0.01
0.01
0.01
0.04
0.02
0.01
0.01
0.01
0.01
0.02
0.01
0.00
0.01
0.02
0.03
0.01
0.00
0.02
0.06
30.8
25.3
28.3
32.1
45.5
52.7
66.7
27.3
43.6
53.3
59.8
70.0
23.1
36.4
37.1
37.9
32.0
34.9
36.3
27.1
0.38
0.02
0.05
0.07
0.13
0.07
0.00
0.01
0.07
0.08
-0.06
0.11
-0.01
0.06
0.09
0.31
0.02
0.03
0.59
0.28
1.86
1.71
21
15
1.72
21
15
1.83
MidshelfGrid Stations
5
100
1.83
5
5
6
100
100
100
1.83
1.85
1.82
100
1.82
100
100
100
1.81
1.85
1.88
100
1.85
100
100
100
1.83
1.82
1.90
100
1.86
10
100
1.88
20
40
100
100
1.93
1.91
100
1.82
100
30
100
1.56
200
250
100
100
1.23
1.22
Shelf-BreakGrid Stations
42
100
1.83
21
100
1.90
51
100
1.86
70
91
120
201
100
100
100
100
1.81
1.65
100
1.88
100
100
100
100
1.68
1.57
1.48
1.38
100
1.93
100
100
100
1.76
1.76
1.74
100
1.82
100
100
100
1.78
1.76
1.87
20
6
18
23
35
37
X-
15
70
90
120
200
31
Ratio
15
10
15
31
15
Acid
mL
16
15
20
27
5
12
30
41
mgm
Filtered,
24
10
16
28
Phaeo,
mgm-'•
vertedto chlorophylla concentrations
usingthisregression.
A
diurnal pattern of fluorescencequenchingduring periods of
high irradiancewould lead to a clusteringof our daytimedata
pointsabovethe regressionline in Figure2. Diurnal changesin
fluorescence
yield do not appearto bias our fluorometercalibration.This is not surprisingsince,as describedbelow, most
of the chlorophyllin the water masseswe were studyingoccurred as subsurfacepatchesor layers.
1.57
1.41
In Situ
Fluorescence
high-resolutionpicture transmission
passat 2041 UT, July 14,
1993.
Results
Remote Sensing
The best satelliteimage of SST near the time of our cruise
is a daytimepasson July14, 1995(Plate 1), whichshowswarm
Remote Sensing
water throughoutthe study region. Very warm water (29øAdvancedvery highresolutionradiometer(AVHRR) satel- 31øC),presumablyassociatedwith the Gulf Stream,fills the
lite imagesfrom the studyregionwere obtainedcourtesyof the southeasterncorner of the study area and creates a sharp
National Oceanicand AtmosphericAdministration(NOAA)
temperature front acrossthe southeastcorner of the shelfCoastWatchand processed
with NOAA's CCoastsoftware(/3 break grid (Plate 1). Lower surfacetemperatures(25ø-27øC)
version).Plate 1 is a 1.4-km pixel, daytime,nonlinear split were seen in an eastward tending tongue between the two
windowseasurfacetemperature(SST) imagefrom a NOAA grids. Satellite-derived SST within the midshelf grid were
20,582
WOOD ET AL.: CHLOROPHYLL
EXPORT INTO THE GULF STREAM
WOOD ET AL.: CHLOROPHYLL
EXPORT
INTO THE GULF STREAM
Shelf-Break
7 -
20,583
Grid
Mixed layer temperaturesfrom CTD data collectedin the
shelf-breakgrid July 17-18 agree well with the satellite SST
observedJuly 14 but showthat the temperaturefront moved
further north into the stationgrid during the interveningperiod (comparePlate 1 and Figure 6). This shift in the position
of the temperaturefront was also apparent in partially cloudx
2covered AVHRR images available for July 16 and 17. Totl.l
getherwith surfacesalinity,whichis >35.5 psuthroughoutthe
•. •
y•4.145lx + 0.2387
1 southernand northeasternpart of the grid (Figure 6), these
•,-p
data suggestthat most of the southeasternhalf of the crossI
I
I
I
-0.5
0.0
0.5
1.0
1.5
shelfgrid wasexperiencingdirectGulf Streaminfluenceat the
Relative Fluorescence
time of our cruise.This interpretationwas confirmedby data
from current meter arraysmaintainedby the Minerals ManFigure 2. Regressionof extracted chlorophyllvalues from
Table 1 againstin situ fluorometer readingsat the depth of agementService(MMS) of the U.S. Departmentof the Intediscretesamples.D, samplescollectedwithin 1 hour of sunset; rior for extendedperiodsincludingmid-Februaryto late AuN, samplescollectedat night (at least 1 hour after sunsetand gust, 1993 [ScienceApplicationsInternational Corporation,
beforesunrise);P, samplescollectedafter localapparentnoon (SAIC), 1994].Three of the MMS mooringswere insideour
but lessthan 1 hour before sunset;A, samplescollectedafter shelf-breakgrid (Figures1 and 6). BetweenJuly 15 and 20,
sunriseand before local apparentnoon.
1993,40-hourlow-pass
filteredcurrents
were80-100cms- • to
PP
R2=0.906
the northeastat 30 m at mooringB3 locatedin about 60 m of
water and 150-170 cm s-• to the northeast at 100 m at moor-
nearlyuniform and similarto thosein the northwesthalf of the
shelf-breakgrid.
Midshelf
Grid
There is little evidencefor persistenthorizontalgradientsin
the upper 15 m of the northern(midshelf)grid, althoughvertical stratificationis strong(Figures3 and 4). A warm, relatively fresh surfacemixed layer occupiesthe upper 8-10 m
(Figures3 and 4). The surfacelayer is very transparentwith
chlorophyll
concentrations
<0.5 mgm-3 throughout
thelayer
(Figures3 and 4). Secchidepthsin the midshelfgrid ranged
from 18 m, determinedat night under ship'slights,to 24 m,
determinedat midday.
The mixed layer is separatedfrom cold bottom water by a
strongthermoclinebetween8 and 15 m (Figures3 and4). The
water below 20 m is nearly uniform in both temperature and
salinitywhichrangefrom 7.7øto 10øCand32.4to 32.9practical
salinityunits (psu), respectively.There is a slighthorizontal
gradient in the bottom water, which tends to be colder and
more salineon the easternsideof the grid (Figure 3). Chlorophyll concentrations
are slightlyhigherwithin the thermocline than in the surfacelayer and increasesharplybelow the
thermocline.The mostdramaticfeature of the on-shelfgrid is
the high chlorophyllcontentof the bottomwater (Figure 4).
Nearly all the chlorophyllin the water columnat the mid-shelf
grid is containedwithin this thick (---15m) bottomlayerwhich
containsthe densestwater observedon the shelfin either grid
(rro= 25.31 _+0.16 at 20 m (n = 16), 25.30 _+0.20 at 25 m
(n = 16), and 25.39 _+0.16 at 30 m (n = 11); valuesare
mean _ s.d.;Figure 4).
Within the bottom water, chlorophyll concentrationsincreasedwith increasingdepth to a point slightly above the
bottom. Typical vertical profilesfor an individualstation are
shownin Figure 5; acrossthe grid, maximumchlorophyllcon-
centrations
rangedfrom 3 to 8.6 mg m-3, andthe depthat
whichmaximumvalueswere observedrangedfrom 21 to 35 m,
with an average depth of 26 m. As with temperature and
salinity,there was a slighthorizontalgradient,with generally
lower chlorophyllvaluesobservedat the westernmoststations.
ing B4 locatedin approximately2000 m of water.
Horizontal propertydistributionsshowa thermohalinefront
running diagonallyacrossthe grid at the interface between
very warm, salty Gulf Stream water present throughoutthe
water column in the southeastcorner of the cross-shelfgrid
and relativelycool,freshwater presentin the north and northeastquadrantsof the grid (Figure6). The front is mostapparent in bottomwater,whichis coolerand fresherthan overlying
surfacewater (Figure6). Outer-shelfbottomwater is similarto
the midshelfbottomwater, thoughnot as cold (10.5øC;compare 8øC) and fresh (33.7; compare32.7 psu). Outer-shelf
surfacewater was slightlywarmer (by <IøC) and muchmore
salinethan surfacewater in the midshelfgrid: at 5 m, salinity
was <31 psu at all stationsin the midshelfgrid (Figures3 and
4) and >34 psu at all of the stationsin the shelf-breakgrid
(Figure 6).
East-westtransectsthrough the shelf-breakgrid (Figures
7-10) showa definitenorth-southgradientin water properties
over the upper slope and outer shelf. Along the southern
boundary(35ø 19' N), all of the water is relativelywarm and
very saline comparedto the waters within the midshelf grid
(compareFigures7 and 8 to Figures3 and 4; see alsoFigure
12). More northerlysectionsshowprogressively
more evidence
of cool (<12øC) low-salinity(<35 psu)water over the upper
slope,just seawardof the shelf break. The salinityof bottom
water (or waterat 100m for slopestations)was<33 psuat all
of the midshelfgrid stations(Figures3 and 4) and >34 psuat
all but five of the stationsin the shelf-breakgrid (Figure 6).
Stationswherebottom or 100 m salinitieswere <35 psuwere
concentratedalong the most northern transect of the grid
(Figures6 and 8) and, at thesestations,bottomtemperatures
were alsolow, rangingfrom 10.5øto 12.7øC(Figures6 and 7).
Thisis in contrastto bottom(or 100m) temperatures> 17øCin
the rest of the grid (Figure 6).
Chlorophyll concentrationswere lower at the shelf-break
gridthanat the midshelfgrid anddid not exceed4 mg m-3
anywherein the grid (Figure 9). Surfacewatershad very low
chlorophyllcontentthroughoutthe grid. The highestchlorophyll concentrationsoccurredin bottom water at shelfstations
20,584
WOOD ET AL.: CHLOROPHYLL
EXPORT INTO THE GULF STREAM
Station Number
6
7
-75.3
14
-75.2
15
-75.1
6
-75.3
7
14
-75.2
15
-75.1
6
7
-75.3
-75.2
14
15
-75.1
6
7
-75.3
14
-75.2
15
-75.1
-75.0
Longitude
Figure 3. West-to-eastsectionsof (a) temperature,degrees(b) salinity,practicalsalinityunits;(c) density
anomaly,kilogramsper cubicmeter;and (d) chlorophyll,milligramsper cubicmeter,at the southernend of
the midshelfgrid (stations6, 7, 14, 15, along36ø05'N).Other sectionsthroughthe midshelfgrid are similar.
and between
40 and 80 m at shelf-break
stations. Patches of
elevated chlorophyllconcentrationin bottom water at the
outer shelfstationsof this grid appearto be contiguouswith
patchesof high chlorophyllin slopewater.
Density contoursalso suggestcontinuity acrossthe shelf
break in the northernhalf of the shelf-breakgrid (Figure 10).
In the transectsalong35ø34' N and along35ø29' N, water with
they did at the two northern transects(e.g., 60-100 m as
comparedto 45-80 m; seeFigure 10).
While the depth of the chlorophyllmaximumat stationsin
the shelf-breakgrid rangedfrom 21 to 98 m, o-0 at the depthof
the chlorophyllmaximumwasrelativelyinvariant(X = 24.9,
s.d.= 0.4,n = 19 stations).Water thisdensewasnot observed
at stations35, 36, and 39, which were located on the shelf in the
a density(o-0)between25.0and25.5kg m-3 liesnearthe southerntwo transectsof the shelf-breakgrid, althoughextrapbottom over the shelf (Figure 10) and continuesbeyondthe
shelf break, forming a horizontallayer between45 and 80 m
over the continentalslope(Figure 10). In transectsalong35ø
24' N and 35ø 19' N, isopycnals
with o'0between23.5 and 24.5
olation betweenstations31 and 38 suggests
that water with a
densityanomalyasgreatas25 kg m-3 probablyoccurredat
station36 belowour deepestsamplingdepthof 34 m (station
depth-37 m; Figure 10). At these,asat other shelfstationsin
kg m-3 followthe bottomcontoursof the shelf,continue the grid, the chlorophyllmaximumoccurredin the bottom
beyondthe shelfbreak, and form horizontallayersbetween30 water and o-0 ranged from 23.5 to 24.8 at the chlorophyll
and 60 m over the slope.In the two southerntransects,iso- maximum.When chlorophyllconcentrationis plotted on the
pycnals
witho-0between
25.0and25.5kgm-3 donotoccuron 24.5, 25.0, and 25.5 isopycnalsurfaces,a gradientof concenthe shelf,and they occurslightlydeeperat slopestationsthan tration can be seenextendingdiagonallyacrossthe grid, with
0
'
-10 -
-20 -
-30 -
0
5
10
15
20
25
Temperature
(øC)
29
30
31
32
Salinity
(psu)
33
19
21
23
25
27
Sigma
Theta
(kgm'3)
0
1
2
3
4
5
Chlorophyll
(mgm'3)
Figure 4. Vertical profilesof averagewater propertiesat stations3-18 in the midshelfgrid; error bars are
_+1 standarddeviationcalculatedfrom all observations
at eachdepth;only a few stationswere deeperthan
33 m (N = 16 between4 and 26 m andN -> 8 at all depths-<33 m).
WOOD ET AL.' CHLOROPHYLL
Sigma-theta
(kgm-3)
18
20
22
I I I I
24
26
EXPORT
INTO THE GULF STREAM
20,585
RelativeFluorescence
(Ioglo)
highvaluesin the north andwest(Figure11). Thesesurfaces
0.0
are inclined,shoalingin the north and west.
0.5
1.0
1.5
2.0
I,,,,I,,,,I,,,,I,,,,,
, , , I , , , I , , •
0-S Analysis
•.
Potential temperature-salinitycharacteristicsof all data
pointsfrom all stationsfor both gridsare plotted in Figures
12a-12c.Four end-memberscan be identified:warm (25øC),
relativelyfresh(29.5-30.5psu)waterfoundonlyin the surface
layer in the midshelfgrid; cold (7ø-10øC)water with salinity
between32.0 and 32.8 psufoundprimarilyat deeperdepthsin
the midshelfgrid but alsoat somestationsin the shelf-break
grid;highsalinity(35.0-35.1psu),low-temperature
(4.8ø-7øC)
water found only at depths>300 m at stationson the continental slope (stations21-28 and station 33); and warmer
(>15øC)high-salinity
(35.7-36.3psu)waterfoundonlyat stations in the cross-shelf
grid. Chlorophyllvaluesexceed1 mg
10
_
o
20
_
F
3O
'
'
'
'
0
I
'
'
10
'
'
I
'
'
20
'
'
30
PotentialTemperature(øC)
I,•,,l,,,,I
30
....
31
32
76
80
84
88
Relative LightTransmittance
I,,,,I,,,,I
33
34
35
Salinity(PSU)
with all of thesewatermasses
Figure 5. ObservedCTD profilesof temperature(T, left m-3 at somepointsassociated
panel),salinity(S), densityanomaly(D), fluorometer
voltage exceptthewarmrelativelyfreshwater(<30.5 psu)whichcom(F), andlighttransmission
(T, rightpanel)at station6, located prisedthe surfacewaterof the midshelfgrid(Figure12d).
at 36ø04.9'N, 75ø i6.6'W, overthe 32-m isobath,madeat 1754
Water of salinitybetween33 and 35 psuwasonly observed
UT, July15, 1993.Theseprofilesare typicalof stationsin the in the shelf-breakgrid and only at stations21, 28-32, 37, and
midshelfgrid.
38 in the northernhalf of thisgrid (Figure12). At all of these
35.70
A
B
.
C
ß
37
45
600 /
•200
ß
/'
i
/
/
i
i
i
i
35.70
E
J
35.50
75.00
74.80
75.00
74.80
F . .
•
30m
75.10
• 30m
ß
74.90
,oo
74.70
Figure6. Distributions
of near-surface
andsubsurface
temperature
(degrees
Celsius)
andsalinity
(practical
salinity
units)in theshelf-break
grid,withseparate
panels
showing
bottomdepths
andaverage
currents:
(a)
temperature
and(b) salinity
at 5 m; (c) bottomdepths
in metersof CTD stations;
(d) temperature
and(e)
salinity
at 100m,or at maximum
depthif lessthan100m;(f) average
currents
at MMS/SAICmoorings
within
thegridforthe1-week
period
beginning
0000UT, July12,1993,scaled
sothat5 cms-• = 1 km;thelongest
vectorrepresents
a speedof 133cms-•.
20,586
WOOD ET AL.' CHLOROPHYLL
EXPORT INTO THE GULF STREAM
Station Number
40
34
33
26
25
39
35
32
27
24
38
36
31
28
23
37
30
29
22
I-50
-100-
-lOO
-150.
-15o
-250
_c_ -250-
-300-
-350-
-400-
-400
-450-
-7•.9
-74.7
-74.9
-74.7
-74.9
-74.7 -74.9
-74.7
Longitude
Figure 7. West-to-eastsectionsof temperature(degreesCelsius)throughthe shelf-breakgrid;sectionsare
shownin orderfrom southto north:(a) along35ø19'N(stations40, 34, 33, 26, 25), (b) along35ø24'N(stations
39, 35, 32, 27, 24), (c) along35ø29'N(stations
38, 36, 31, 28, 23), and (d) along35ø34'N(stations
37, 30, 29, 22).
stations,there is a portion of the O-S curvein which temperature increaseslinearly with salinity,from water with low values of 0 < 12øC,S < 35 psu to waters of variable high
temperature(20ø-25øC)but nearly constantsalinity (-36.1
psu). These straightline segmentsindicatetwo-pointconservative mixing between the cooler, fresher waters and warm,
saltywaters.The valuesat the cool end of thesestraightline
segmentsoccurnear a straightline joining the O-S characteristicsof the bottom water seenover the midshelf(0 -8øC, S
diagramfor station21; thisis cross-isopycnal
mixingapparently
facilitatedbywind-driventurbulencein the surfacelayer.Mixing of the intrudedwater with deeperGulf Streamwater below
the intrusionis alsoapparentin Figure13a;mostof thismixing
occursalong the 26 rroisopycnal.In both instancesof mixing
between the intruded water and Gulf Stream water, chloro-
phyllvaluesare > 1 mgm-3 in waterwhichcontains
a strong
componentof intrudedwater (Figure 13a).
---32.7psu,o-0 -25.3 kg m-3) withthe O-Scharacteristics
of Discussion
water at a depth of 200 m over the upper slope(0 -12øC, S
---35.5psu,o-o-26.8 kgm-3).Someof thesecool,freshvalues Shelf Bottom Waters Near Cape Hatteras
were observedin the bottomwater over the outer shelf (e.g.,
The cold, chlorophyll-richwater observedas a layer of botstations30, 37). Others occurredas local extremain stations tom water on the midshelfhaspropertiessimilarto that of the
over the upper slope(stations21, 28, 29). Samplesin which Mid-Atlantic Bight cold pool. This is a benthic water mass
chlorophyll
concentrations
were> 1 mgm-3 eitheroccurred
in
which is formed
deep slopewater, in midshelfbottomwater, or in water where
there was a strongindicationof two- or three-point mixing of
water from cold subsurface
midshelf
bottom
water
with
one or more
sources of Gulf
Streamwater (Figure 12d).
A layer of very cool, relativelyfresh water, lying between
layersof Gulf Streamwater is apparentbetween40 and 80 m
at station 21, which was located on the continental slope
slightlynorth of the other stationsin the shelf-breakgrid (Figure 13e). Chlorophyllconcentrations
in this layer are among
the highestobservedin the cross-shelf
grid and greatlyexceed
the chlorophyllconcentrations
aboveand below the intrusion
(chlorophyll
<0.15mgm-3 at 10m, 2.9mgm-3 at 46 m, and
0.19 at 100 m). Two-pointmixingbetweenthe intrudedwater
and overlyingGulf Streamsurfacewater is apparentin the O-S
as summer
stratification
water
isolates the surface
that has entered
the MAB
from the Gulf of Maine [Bigelow,1933;Ketchurnand Cotwin,
1964].Extensivedata setsfrom a cross-shelf
mooredarrayand
surveysspanningthe entire MAB in 1979 [Houghtonet al.,
1982]and the southernMAB in 1971and 1972[Boicourt,1973]
showthat the coldestcold pool water (5ø-6øCin July) is locatedon the outer shelf and slopeof the northeasternMAB.
As this water is advectedslowlyto the south, the cold pool
watersundergogeneralwarming.In 1979 the cold pool was
still clearlydefinedsouthof HudsonCanyonwhere temperatures on the outer shelf were between8ø and 10øCin July
[Houghtonet al., 1982],and in 1971and 1972the coldpool was
easilyrecognizedon the outer shelf (depths>50 m) as far
southas 36ø20N[Boicourt,1973]. In June 1988 the cold pool
WOOD ET AL.: CHLOROPHYLL
EXPORT
INTO THE GULF STREAM
20,587
Station Number
40
34
33
26
25
39
35
32
27
24
38
36
31
28
23
37
30
29
22
-50
-50-
-100
--150
-200
-250
-300
-350
-400
-450
-500
I
-74.9
-74.7
-74.9
-74.7
-74.9
-74.7
-74.9
-74.7
Longitude
Figure 8. West-to-eastsectionsof salinity(practicalsalinityunits)throughthe shelf-breakgrid;sectionsare
shownin the same order as in Figure 7.
could be identified
on the midshelf and outer shelf in a cross-
shelftransectoff the northernDelmarva peninsula[Biscayeet
al., 1994].Salinityassociated
with the coldestcold pool waters
in July 1979 was between32.5 and 33 psu [Houghtonet al.,
1982], identical to that of the bottom water of our midshelf
grid. Water of similar salinity,but not quite as cold as the
bottom water we observedat our mid-shelf grid, formed a
bottomlayeron the inner shelfnorth of Cape Hatterasin July
1971 and July 1972 [Boicourt,1973].
The only other sourceof coldbottomwater for the shelfoff
Cape Hatteras would be deep slopewater. Gulf Stream meandersthat eject water onto the shelfnorth of Cape Hatteras
are common[Churchilland Cornilion,1991a],but 10øCwater
that couldbe intrudedfrom the slopeor Gulf Streamwouldbe
of higher salinity(S > 35.0 psu) than the bottom water we
observedin the midshelfgrid (S < 33.0 psu).The combination
of temperatureand salinitypropertiesand the slightgradient
towardcolder, saltierwater from west to eastwithin the layer
(Figure 3) are consistentwith spreadingof the cold pool
slightlyfurther southin the MAB in 1993than in 1979 or 1980
[c.f. Houghtonet al., 1982;Falkowskiet al., 1983] and further
firmed by examiningplankton samplescollectedin the midshelf grid. Phasecontrastand epifluorescence
microscopyrevealedabundantplanktonicdiatomswith intactchloroplastsin
the bottomwater (Leptocylindrus
danicus,Corethronsp., and
Skeletonema
costatum
).
Since the transmissometer
data cannot be used to infer ir-
radiancelevelsat a given depth becausethe instrumentmeasureschangesin beam attenuationrather than diffuseattenuation (R. Zaneveld, Oregon State University, personal
communication,
1996),it is unfortunatethatwe were unableto
measureirradiancedirectly.However, we can infer that the
chlorophyll-richbottom water was within the euphotic zone
from the 24-m daytimeSecchiDisk readingwe obtained.As is
apparentfrom Figure 4, the Secchidepthwas at least 4 m
deeper than the transitionfrom the relativelytransparentupper layer of the water columnto the lesstransparentchlorophyll-richbottom water. Using the relation K d = 1.44/Zs•,
where K• is the diffuseattenuationcoefficientand Zs• is the
Secchidepth [Kirk, 1994], we calculatea very conservative
estimateof 0.06 for K• in the transparentsurfacelayer and of
0.24 for K• in the chlorophyll-richlayer. The latter value is
basedon a presumed"Secchidepth"of at least6 m withinthe
westthan in 1971or 1972[Boicourt,1973].
During our surveythe cold pool water in the midshelfgrid chlorophyll-richlayer since the observedSecchiDisk depth
containedhigh concentrationsof chlorophyll.The low esti- involvedround-trip travel of light throughat least 8 m in the
matedcontributionof phaeopigments
to total fluorometrically chlorophyll-richlayer and 40 m in the surfacewater. Comderivedpigmentconcentrationat stationsin the midshelfgrid bined, these values lead to the prediction that irradiance at
(Table 1) indicatesthat the chlorophyllwe measuredwas 30 m wasroughly2.7% of surfacevaluesand that irradianceat
presentin healthy cells.Although high concentrationsof ac- 33 m is close to 1% surface irradiance. The combined Secchi
supcessorypigmentscouldcauseartifactsin the determinationof Disk data, chlorophylldata, and microscopicobservations
phaeopigmentconcentration[Gibbs,1979; Treeset al., 1985], port our interpretationthat chlorophyllin the bottom water
the presenceof photosynthetically
competentcellswas con- resultedfrom in situ production.It also appearsthat the sed-
20,588
WOOD ET AL.: CHLOROPHYLL
EXPORT INTO THE GULF STREAM
Station Number
40
34
33
26
ß
25
39
35
32
'
27
24
38
36
31
28
23
37
30
29
22
0.3
ß
:
•.
--200
•
-250
:0.3
..
-oo
ß
o.3
":':
•'•
i
i
:'•
--400
:•
-450
.
.
-501
•
-74.90
'
'
-74.70
B
...........
•'
-74.90
'
' "':C
:..
-74.70
-74.90
•,::'
-74.70
.p
'
-74.90
-500
-74.70
Longitude
Figure 9, West-to-eastsectionsof chlorophyll(milligramsper cubicmeter) throughthe shelf-breakgrid;
sectionsare shownin the same order as in Figure 7.
imentsurfacewasprobablywithin the euphoticzone,at leastin Shelf-Break Transport and Mixing
the westernhalf of the midshelfgrid where total depthwasless
As noted above, bottom water found over the outer shelf in
than 33 m.
the northerntransectsof the shelf-breakgrid, thoughnot quite
There is more varietyin the benthicwatersof the outer shelf
as cold (10.5øC;compare8øC) or fresh (33.7; compare32.7
than the midshelf. At stations 35, 39 and 40, in the southeastpsu), is similar to the cold pool water in the midshelfgrid
ern cornerof the shelf-breakgrid (Figure 1 and Plate 1), warm,
(Figures4 and 6). The cool, relativelyfreshwater seenin the
saltyGulf Stream surfacewater fills the entire water column
shelf-breakgrid may be contiguouswith the water masswe
over the shelf: even the deepestsampleshave temperatures
sampledas bottom water in the midshelfgrid, or it may rep>24øC and salinities>36 psu.These stationstend to have low
resent fragmentsof that water mass.As would be expectedif
concentrationsof chlorophyllat all depths.At stations30, 31,
midshelfcoldpoolwater were transportedto the shelfbreak by
36, 37, and 38, in the northwesterncorner of the grid, warm
southeastwardflow, chlorophyllconcentrationsare relatively
saltyGulf Streamsurfacewatersat middepthor in the surface
high in the cooler, relativelyfresh water at the shelf break,
layer lie over a cooler, fresher bottom layer; bottom water at
compared
to concentrationsin other water massesfound on
thesestationsis warmer and fresherthan cold pool water, but
the
outer
shelf
and slope(Figures7 and 9). Along the 25 tro
cold pool influenceis apparent from comparisonof the O-S
isopycnal,
which
correspondsto the densityof the midshelf
diagramsfrom thesestations(e.g., Figures13b-13d)with the
O-S diagramfor the midshelfgrid (Figures12a and 12b). At cold pool, chlorophyllconcentrationsat the shelf-breakgrid
somestations,e.g., 37 and 38, the deepwater is a mixture only alsoshowa clear gradientfrom high valueson the shelf in the
of undilutedcold pool water and Gulf Stream surfacewaters; northwestto lowvaluesoverthe slopein the southeast(Figure
that is, the deep O-S characteristicslie on the mixing line 11). This is alsoconsistentwith transportof cold pool water
joining the cold pool and Gulf Streamsurfacewater types.At from the midshelfregionsnorth of Cape Hatteras to the shelf
other stations,e.g., station36, the deep water is a mixture of break by the prevailingsouthwardshelfcurrents.We therefore
three water types:Gulf Streamsurfacewater, cold pool water, interpret the cooler, lesssalinewater in the shelf-breakgrid,
and cold,saline,upperslopewater (0 -12øC, S -35.5 psu,tro and the chlorophyllit contains,as being of shelf origin.
Some of these cool, fresh values are found in the bottom
--•26.8).The outer-shelfstationsin which the bottom water
showssignificantcoldpool influencetend to havehigh(_>2mg water on the outer shelf; others are seen as local extrema in
m-3) chlorophyll
valuesin the bottomwater,intermediate stationsoverthe upperslope(e.g.,stations21, 28, and29) and
chlorophyll
values(1-2 mg m-3) at the interfacebetween indicate interleavingof waters transportedoff the shelf with
bottomwater and overlyingGulf Streamwater, and low chlo- ambient Gulf Streamwaters.Figure 13 showsthat the salinity
rophyllvaluesin surfaceGulf Streamwater (e.g.,Figures13b, minimum at station 21, the most northern station of the shelfbreak grid, occursat 40 m, aboutthe bottom depth of the outer
13c,and 13d).
Station Number
40
34
33
26
25
39
35
32
27
24
38
36
31
28
23
37
30
29
22
-50.
-100
O0
-150.
50
-200-
-250.
-300.
-350.
-400-
-450-
-500
-74.90
-74.70
-74.90
-74.70
-74.90
-74.70
-74.90
-74.70
Longitude
Figure 10. West-to-eastsections
of densityanomaly(O'o,kilogramsper cubicmeter)throughthe shelf-break
grid; sectionsare shownin the sameorder as in Figure 7.
24.5
25.0
25.5
35.70
• • • ,•I • •
Chl(mgm-3)
35.50
35.30
35.70
[
,
[
•
1_..
P(dbar)
35.50
35.30
75.00
74.80
75.00
74.80
75.00
74.80
Figure11. (top)Chlorophyll
onthe24.5,25.0,and25.5kgm-3 isopycnal
surfaces
and(bottom)thedepth
of theseisopycnals.
The shadedregionin eachpanelrepresents
areaswherebottomwaterwaslessdensethan
the isopycnal
of interest.The densestwaterwasfoundat the bottomat stations35, 39, and 40 and had o-o
valuesof 24.3,24.3,and23.8kgm-3 andchlorophyll
concentrations
of 1.38,0.95,and0.34mgm-3 at each
station,respectively.
20,590
WOOD ET AL.: CHLOROPHYLL
30
30
32
,
,
,
34
I
,
,
,
I
EXPORT INTO THE GULF STREAM
36
,
,
,
I
,
o
20
that a large extrusionof shelfwater was subsequently
overridden by Gulf Stream surfacewater.
Such an expulsionof water from the shelf into the open
ocean is the result of strongconvergenceof the alongshore
flow near Cape Hatteras. Figure 6 showsthat currentsduring
our surveywere stronglynortheastwardat the shelf break
withinourgrid(-30 cms-• at 55m, 75 cms-• at 30m at B3;
10
•
0
/
26
A
seelegendfor Figure 6), while the coreof the Gulf Streamlay
over the outer continentalslope.Meanwhile,shelf-breakcur-
rentsweremuchweaker(-2 cms-• to thesoutheast
at 55m,
-10 cms-• to the southeast
at 30 m) at anothershelf-break
mooring(D2, at 35.72N, 74.93W) just north of the arrayin
our shelf-breakgrid [SAIC, 1994].Suchstrongalongshoreconvergencemust be balancedby an increase in the offshore
velocityat the shelf break; that is, shelf waters are advected
offshoreacrossthe shelfbreak.The 6-monthaverages(February-August1993) from MMS/SAIC mooringsnear our midshelf grid (A2 and A3 along 36ø14.7N at 75ø12.4W and
74ø54.4W,respectively)showbottomwater flow to the south-
30
20
10
eastat -12-18 cms-•, indicating
approximately
4-6 daytran-
26
/
o
sit time from our midshelfgrid to the shelf break. Six-month
averagecurrentvelocityand directionat the shelf-breakmoorings
(B3, B4, andD4) are similarto thoseobservedat the time
E
of our surveybut not as strong[SAIC, 1994].Thus conditions
I- 20
favoredmixingof coldpoolwater into the Gulf Streamat Cape
Hatteras during most of the summerof 1993.
Opportunitiesfor mixingof shelfwater transportedto Cape
Hatteraswith the upperlevelsof the Gulf Streamare frequent,
and mixing of cold pool water with the Gulf Stream may
26
partially explain the lossof cold pool signatureobservedin
/
temperaturedata collectedbetween36ø and 37øNat the shelf
30
break in the summer of 1980 [Falkowskiet al., 1983]. The
historicalaxisof the westwall of the Gulf Stream comesvery
close to the shelf break at Cape Hatteras. During July and
2O
August,Gulf Streamdisplacement
from the historicalaxisasit
movesnorth and eastfrom Cape Hatteras is mostfrequentlyto
the northwest [Traceyand Watts, 1986], providing extended
10
contact with outer shelf and slope water before the Gulf
Streammovesoffshore.Low-temperature(8ø-10øC)water of
low salinity(33-34 ppt) in the upper200 m of the Gulf Stream
0
north of Cape Hatteras is a well-known phenomenon;this
30
32
34
36
water was first identifiedasbeingof shelf origin by Ford et al.
[1952]. Additional studieshave shownthat large volumesof
Salinity(PSU)
shelfwater are often found in the Gulf Stream north of Cape
Figure 12. Potential temperature-salinitycharacteristicsof Hatteras [Fisher, 1972; Kupferman and Garfield, 1977;
water at all stationsin three groups:(a) shelfwater (stations
Churchill et al., 1989; Churchill and Cornilion, 1991b], and
2-18, midshelfgrid); (b) stations21, 28-32, 37, and 38, preBoicourt
wasthe first to proposethat the specificoriginof the
dominantlyover the outer shelf, (c) Gulf Streamwater (stawater
might
be the cold pool [Boicourt,1973].Lillibridgeet al.
tions22-27, 33-36, 39-41, predominantlyin the southernhalf
of the shelf-breakgrid. (d) The O-S characteristics
of water [1990]providethe only databesidesourswhichincludeinforwithchlorophyll
concentrations
greaterthan1.0mgm-3 at all mationaboutthe biologicalcontentof water entrainedinto the
Gulf Stream at Cape Hatteras. In their study, conductedin
stationsin either grid are also shown.
October 1985,phytoplanktoncommunitiesin cool,freshlayers
in the Gulf Stream were dominatedby neritic diatoms,suggestingto them that theselayershad their originson the inner
shelf, and containswater with chlorophyllcontentand hydro- shelf.
graphiccharacteristics
that are nearlythe sameasthe midshelf
cold pool. A similar salinityminimum layer was observedat Implications for Material Flux From the Continental Shelf
The cold pool is a site of new production throughoutthe
two adjacentupper slopestations(stations29 and 28), indicatingthat this intrusionof shelf water into the Gulf Stream summer[Harrisonet al., 1983], and some of this production
extendedat least 20 km alongshore.There was a definite gra- may leave the shelf in the northern MAB where chlorophyllwith slopewater of similar
dationof O-S characteristics
immediatelyabovethe extremum, rich cold pool water is contiguous
with progressively
lesscold pool and more Gulf Streaminflu- density[Falkowskiet al., 1983;Houghtonet al., 1988].At these
encesouthwestward
alongthe continentalslope.It seemslikely points,energeticbarriersto advectionof organicmatter off the
E 30
i
i
i
i
i
i
i
i
i
WOOD ET AL.: CHLOROPHYLL
EXPORT
INTO THE GULF STREAM
20,591
RelativeFluorescence
(Iog•o)
32
30
33
34
35
36
37
0.0 0.2
0.4
i,,,i,,,i,,,i,,,i,,,
..............
0.6
0.8
1 0
- 50
20
- 100
10
o
/•
s
150
_ 250
-
30 _
500
0
_
-
s
lO
_
_
20-
_
20
_
_
- 30
10-
- 40
0
/26
B
F
50
30 _
0
' 10
T
- 20
- 30
lO
- 40
o /26
c
50
30
0
_
_:-1o
20
•- 20
- 30
10
- 40
0I....I....I....I....I....
32
33
34
35
36
37
Salinity(PSU)
H
50
0
5
10
15
20
25
30
PotentialTemperature(øC)
I,,,,I,,,,I,,,,I,,,,I,,,•1
32
33
34
35
36
37
Salini• (PSU)
Figure 13. Potentialtemperature-salinity
characteristics
andverticalprofilesof water propertiesat stations
wheremixingof chlorophyll-rich
coldpoolwaterandGulf Streamwaterwasapparent.Station21 (toppanels)
is locatedin 1200 m of water at the northeasterncorner of the shelf-breakgrid; the lower panelsare shelf
stationsin the shelf-breakgridwherebottomwater showscoldpool influence(stations36 (Figures13b and
13f),37 (Figures13cand13g),and38 (Figures13dand13h)).Data pointsenclosed
in a triangleindicatewater
inwhichchlorophyll
concentration
was> 1mgm-3. F isin situfluorescence,
Sissalinity,
andT istemperature.
shelf would be low, but unlike the situation we observe the
Gulf Stream is too far offshoreto provide a mechanismof
transportintothe openocean.In contrast,entrainmentof shelf
waterinto the Gulf Streamat CapeHatteraswill resultin rapid
transportof the entrainedwater to the openoceanwherethe
organicmatterand nutrientsit containsmayhavea significant
influenceon Gulf Stream biogeochemistry.
Entrainment of
shelfwater into the Gulf Streamat CapeHatterasmaybe one
of the moresignificantlosstermsin the shelfnutrientbudget.
However, becausethe water is entrained into surfacelayers
and is unlikely to reach more than 200-m depth, advected
organicmattercouldeasilybe oxidizedin the epipelagic.Thus
incorporationof shelf water into the Gulf Stream at Cape
Hatterasis more likelyto be importantasa sourceof nutrients
to the upperlevelsof the Gulf Streamthanasa sinkfor organic
carbon.
It is difficultto quantifythe input of organicnutrientsto the
Gulf Streamat CapeHatterasfrom our data set,but at station
21, morethan 60% of the chlorophyllin the upper200 m of the
Gulf Streamis found in the entrainedcold pool water (com-
pare87.3mgchlm-2 integrated
overtheupper200m to 56.41
mgchlm-2 integrated
overthedepthintervalbetween40 and
80 m where cold pool water intrudedinto the stream;Figure
13). Using a carbonchlorophyllconversion
value of 47 (mg/
20,592
WOOD ET AL.: CHLOROPHYLL
EXPORT INTO THE GULF STREAM
mg) [ChoandAzam, 1990;Liet al., 1992],assumingRedfield anismby whichthe MAB shelfinfluencesthe biogeochemistry
proportionsfor C:N:P (molarratiosof 106:16:1)[Harris,1986], of the open ocean.
and comparingconditionsat station21 to thoseat station25
wherethe Gulf Streamappearsto be undilutedby shelfwater,
Acknowledgments.We thank the captain and crew of the R/V
entrainmentof the coldpool into the Gulf Streamat station21 Cape Hatteras, particularly electronicstechnicianMark Cook, and
represents
an additionof approximately
33 mM nitrogenm-2
and2 mM m-2 phosphorus
asparticulate
organicmatter.If
thismaterialwere remineralizedanddispersedevenlythroughout the upper 100 m, this singleintrusioneventwould increase
localconcentrationof inorganicnitrogenin the euphoticzone
by 0.5
The deep chlorophyllobservedat nearly400 m at station23
is probablynot from the shelfnear Cape Hatteras. Sincethe
shelf is approximately50 m deep at the shelf break in this
region, organic matter entrained into the Gulf Stream at the
shelf break
would
travel
much further
north
in the stream
beforeit wouldhavetime to sinkto suchgreatdepths.Organic
matter associatedwith blooms on the outer shelf of the SAB,
however, could have had time to sink to 400 m. Since these
Chief ScientistLarry Benningerfor the successful
completionof the
hydrographic
samplingon this cruise.We alsothankBerdenaFlesher,
Jane Fleischbein,and Michael Lipsen for technicalsupport;Jim
Churchill, Walter Johnson, and Peter Hamilton for accessto the MMS
current meter data; and K. Brink, J. Walsh, L. R. Pomeroy,and
J. Churchillfor their commentson earlyversionsof the manuscript.
We also thank W. C. Boicourt, R. Zaneveld, J. Cullen, B. Jones, and
T. Cowlesfor helpfuldiscussions.
Thisworkwasfundedbygrantsfrom
theU.S. Departmentof Energy(DE-FG06-92ER61417)andthe Otfice
of Naval Research(96PR00110-00)to the Universityof Oregon and
from the National ScienceFoundation(OCE-9113510)and ONR
(N00014-92J1348)to OregonStateUniversity.The currentmeter data
were fundedby the Minerals ManagementServicethroughcontract
14-35-0001-30599
to ScienceApplicationsInternationalCorporation,
Raleigh,North Carolina.
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A. Huyer, College of Ocean and AtmosphericSciences,Oregon
State University,Corvallis,OR 97331.
N. D. Sherry,Department of Oceanography,Universityof British
Columbia, Vancouver, British Columbia, V6T 124 Canada.
A.M. Wood, Departmentof Biology,Universityof Oregon,Eugene,
OR 97403.(e-mail: miche@darkwing.uoregon.edu)
(ReceivedAugust 19, 1995;revisedApril 4, 1996;
acceptedApril 8, 1996.)