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. References bloomsare generallycausedby weswardmeandersof the Gulf Streamand occurfrequentlyin the summer[Yoderet al., 1981; Atkinson,L. P., Hydrographyand nutrientsof the southeasternU.S. continentalshelf, in Oceanography of the Southeastern U.S. ContiAtkinson, 1985], this chlorophyllmay reflect entrainmentof nentalShelf,CoastalEstuarineStud.,vol. 2, editedby L. P. Atkinson, shelf-edgebloomsof the SAB into the Gulf Stream before it D. W. Menzel, and K. A. Bush,pp. 77-92, AGU, WashingtonD.C., 1985. reachesCape Hatteras. Conclusions The cold pool is a well-knownsummertimefeature on the outer shelfof the southernMAB. We providenew data which showthe presenceof the cold pool on the inner shelf of the southernMAB in July 1993and confirmthat the cold pool is entrainedinto the Gulf Streamat Cape Hatteras.There is a gradient of chlorophyllconcentrationalong densitysurfaces which are contiguousbetween shelf bottom water and Gulf Streamwater over the slope.On the slopeand outer shelfthe highestchlorophyllconcentrationsare associatedwith water havingO-S propertieswhichindicatemixingbetweenthe cold pool and Gulf Stream.Both setsof observations supportour conclusionthat organicmatter of shelf origin mixed into the Gulf Stream at Cape Hatteras where it could be rapidly advectedinto the open ocean. The degreeto whichcontinentalshelfenvironmentsexport organicmatter to the deep seais not necessarily a functionof the proportionof total primaryproductionthat is exported.In a large ecosystemlike the MAB, nutrientsmay be recycled many times before they are ultimately lost from the system throughbenthicdepositionor export;the exportrate may be nearly as high as is possiblebut still only a small fraction of total primary production.Under theseconditionsthe significance of export flux is best estimatedfrom the fraction of inorganicnutrientsenteringthe shelf that ultimatelyleave as dissolvedor particulateorganicmaterial.The cold pool of the MAB is an importantreservoirfor new nitrogenenteringthe systemfrom the Scotian Shelf, from the Gulf of Maine, and through exchangewith slopewater. We have provided evidencethat the ultimate fate of someof organicmatter produced in the cold pool of the MAB is entrainmentinto the Gulf Streamat Cape Hatteras.Historicaldata on the position of the Gulf Stream,the regularoccurrenceof the coldpool in the southernMAB, and the frequent presenceof cool shelf water in the Gulf Streamnorth of Cape Hatterasindicatethis is a commonphenomenonand a potentiallyimportantmech- Bigelow,H. B., Studiesof waterson the continentalshelf,CapeCod to Chesapeake Bay,in TheCycleof Temperature, Pap.Phys.Oceanogr. Meteorol.,2(4), 135 pp., 1933. Biscaye,P. E., R. F. Anderson,andB. L. Deck, Fluxesof particlesand constituents to the easternUnited Statescontinentalslopeand rise: SEEP-I, Cont. ShelfRes.,8, 855-904, 1988. Biscaye,P. E., C. N. Flagg,and P. G. Falkowski,Shelfedgeexchange processes in the southernMiddle Atlantic Bight, SEEP-II: An introductionto hypotheses,results,and conclusions, Deep Sea Res., Part H, 41,231-252, 1994. Boicourt, W. C., The circulation of water on the continental shelf from ChesapeakeBay to Cape Hatteras, Ph.D. dissertation,184 pp., JohnsHopkins Univ., Baltimore, Md., 1973. Chapman,D.C., and R. C. Beardsley,On the originof shelfwater in the Middle Atlantic Bight,J. Phys.Oceanogr.,19, 384-391, 1989. Chapman,D.C., J. A. Barth,R. C. Beardsley,andR. G. Fairbanks,On the continuity of mean flow between the ScotJanshelf and the Middle Atlantic Bight,J. Phys.Oceanogr.,16, 758-772, 1986. Cho, B.C., and F. Azam, Biogeochemicalsignificanceof bacterial biomassin the ocean'seuphoticzone, Mar. Ecol. Prog. Set., 63, 253-259, 1990. Churchill, J. H., and P. C. Cornilion, Gulf Stream water on the shelf and upperslopenorth of Cape Hatteras,Cont.ShelfRes.,11, 409431, 1991a. Churchill,J. H., and P. C. Cornilion,Water discharged from the Gulf Streamnorth of Cape Hatteras,J. Geophys.Res.,96, 22,227-22,243, 1991b. Churchill,J. H., P. C. Cornilion, and P. Hamilton, Velocity and hydrographicstructureof subsurfaceshelfwater at the Gulf Stream's edge,J. Geophys.Res.,94, 10,791-10,800,1989. Churchill, J. H., E. R. Levine, D. N. Connors, and P. C. Cornilion, Mixing of shelf,slope,and Gulf Streamwater over the continental slopeof the Middle Atlantic Bight, DeepSeaRes.,Part I, 40, 10631085, 1993. Falkowski,P. G., J. Vidal, T. S. Hopkins,G. T. Rowe,T. E. Whitledge, and W. G. Harrison, Summer nutrient dynamicsin the Middle AtlanticBight:Primaryproductionandutilizationof phytoplankton carbon, J. Plankton Res., 5, 515-537, 1983. Falkowski,P. G., C. N. Flagg, G. T. Rowe, S. L. Smith, T. E. Whitledge,and C. D. Wirick, The fate of a springbloom:Export or oxidation?,Cont. ShelfRes.,8, 457-484, 1988. Falkowski,P. G., P. E. Biscaye,and C. Sancetta,The lateral flux of biogenicparticles from the eastern North American continental margin to the North Atlantic Ocean,Deep Sea Res.,Part II, 41, 583-602, 1994. Fisher,A., Entrainmentof shelfwaterby the Gulf Streamnortheastof Cape Hatteras,J. Geophys.Res., 77, 3248-3256, 1972. WOOD ET AL.: CHLOROPHYLL EXPORT INTO THE GULF STREAM Flagg, C. N., R. W. Houghton, and L. J. Pietrafesa,Summertime thermoclinesalinitymaximumintrusionsin the Mid-Atlantic Bight, Deep SeaRes.,Part H, 41,325-340, 1994. Ford, W. L., J. R. Longard, and R. E. Banks,On the nature, occurrence, and origin of cold low salinitywater along the edge of the Gulf Stream, J. Mar. Res., 11,281-293, 1952. Gawarkiewicz, G., T. M. Church, G. W. Luther III, T. G. Ferdelman, and M. Caruso,Large-scalepenetrationof Gulf Streamwater onto the continentalshelfnorthof CapeHatteras,Geophys. Res.Lett., 19, 373-376, 1992. Gibbs,C. F., Chlorophyllb interferencein the fluorometricdetermination of chlorophylla and 'phaeo-pigments', Aust. J. Mar. Freshwater Res., 30, 597-606, 1979. Harris, G. P., Phytoplankton Ecology,p. 63, Chapmanand Hall, New York, 1986. Harrison, W. G., D. Douglas, P. G. Falkowski, G. T. Rowe, and J. Vidal, Summernutrient dynamicsof the Middle Atlantic Bight: Nitrogen uptake and regeneration,J. Plankton Res., 5, 539-556, 20,593 Marra, J., R. W. Houghton,and C. Garside,Phytoplanktongrowthat the shelf-breakfront in the Middle Atlantic Bight,J. Mar. Res.,48, 851- 868, 1990. Parsons,T. R., Y. Maia, and C. M. Lalli, A Manual of Chemicaland BiologicalMethodsfor SeawaterAnalysis,173 pp., Pergamon,Tarrytown, N.Y., 1992. Rowe, G. T., R. Theroux,W. Phoel, H. Quinby, R. Wilke, D. Koschoreck,T. E. Whitledge, P. G. Falkowski,and C. Fray, Benthic carbon budgetsfor the continentalshelf south of New England, Cont. ShelfRes.,8, 511-527, 1988. ScienceApplicationsInternational Corporation,A physicaloceanographicfield programoffshoreNorth Carolina:Current meter, surface wind, and water level data products,February-August,1993, data report, Raleigh,N. C., 1994. Tracey, K. L., and D. R. Watts, On Gulf Stream meander characteristicsnear Cape Hatteras,J. Geophys. Res.,91, 7587-7602,1986. Trees, C. C., M. C. Kennicut, and J. M. Brooks, Errors associatedwith the standard fluorimetric determination of chlorophylIs and pheopigments, Mar. Chem.,17, 1-12, 1985. Houghton,R. W., R. Schultz,R. Schlitz,R. C. Beardsley,B. Butman, Walsh,J. J., Importanceof continentalmarginsin the marinebiogeoand J. L. Chamberlin,The Middle Atlantic Bight cold pool: Evoluchemicalcyclingof carbonand nitrogen,Nature,350, 53-55, 1991. tion of the temperaturestructureduringsummer,J. Phys.Oceanogr., Walsh,J. J., Particleexportat Cape Hatteras,DeepSeaRes.,Part H, 41, 1983. 12, 1019-1029, 1982. 603- 628, 1994. Houghton,R. W., F. Aikman III, and H. W. Ou, Shelf-slopefrontal Walsh,J. J., G. T. Rowe, R. L. Iverson,and C. P. McRoy, Biological exportof shelfcarbonis a sinkof the globalCO2 cycle,Nature,291, structureand cross-shelf exchangeat the New Englandshelf-break, 196-201, 1981. Cont. ShelfRes.,8, 687-710, 1988. Jahnke,R., and P. Verity, OceanMarginsProgram:ScientificFrame- Walsh, J. J., T. E. Whitledge,J. E. O'Reilly, W. C. Phoel, and A. F. Draxler, Nitrogencyclingon GeorgesBank and the New York shelf: work,48 pp., U.S. Dep. of Energy,WashingtonD.C., 1994. A comparisonbetweenwell-mixedand seasonallystratifiedwaters, Kemp, P., Microbial carbonutilizationon the continentalshelf and in GeorgesBank, edited by R. H. Backus,pp. 232-246, MIT Press, slopeduring the SEEP-II experiment,Deep Sea Res.,Part II, 41, 563-582, 1994. Cambridge,Mass., 1987. Ketchum,B. N., and N. Corwin,The persistenceof "winter"water on Wirick, C. D., Exchangeof phytoplanktonacrossthe continentalshelfthe continental shelf south of Long Island, New York, Limnol. slopeboundaryof the Middle Atlantic Bight during spring 1988, Deep Sea Res.,Part II, 41, 391-410, 1994. Oceanogr.,9, 467-475, 1964. Kirk, J. T. O., Light and Photosynthesis in Aquatic Ecosystems, pp. Yoder, J. A., L. P. Atkinson, T. N. Lee, H. H. Kim, and C. R. MacLain, 119-120, CambridgeUniv. Press,New York, 1994. Role of Gulf Stream frontal eddies in forming phytoplankton patcheson the southeastern shelf,Limnol. Oceanogr., 26, 1103-1110, Kupferman,S. L., and N. Garfield,Transportof low salinitywater at 1981. the slopewater-GulfStreamboundary,J. Geophys. Res.,82, 34813486, 1977. Li, W. K. W., p.M. Dickie, B. D. Irwin, and A.M. Wood, Biomassof bacteria, cyanobacteria,prochlorophytes and photosyntheticeukaryotesin the SargassoSea, Deep Sea Res.,Part A, 39, 501-519, 1992. Lillibridge,J. L., G. Hitchcock,T. Rossby,E. Lessard,M. Mork, and L. Golmen, Entrainment and mixing of shelf/slopewaters in the near-surfaceGulf Stream,J. Geophys.Res.,95, 13,065-13,087,1990. Malone, T. C., T. C. Hopkis, P. G. Falkowski,and T. E. Whitledge, Productionand transportof phytoplanktonbiomassover the continental shelf of the New York Bight, Cont. ShelfRes., 1, 305-337, 1983. 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.)