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Assessment of New Production at the Upwelling Center at Point JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. C4, PAGES 8573-8585, APRIL 15, 1997 Assessmentof new production at the upwelling center at Point Conception, California, using nitrate estimated from remotely sensed sea surface temperature Richard C. Dugdale• Department of BiologicalSciences, University of SouthernCalifornia, Los Angeles Jet PropulsionLaboratory, California Institute of Technology,Pasadena Curtiss O. Davis 2 Jet PropulsionLaboratory, California Institute of Technology,Pasadena Frances P. Wilkerson • Department of BiologicalSciences, University of SouthernCalifornia, Los Angeles Abstract. Remotely sensedsea surfacetemperature (SST) and a model originally developedfor Cap Blanc, northwestAfrica [Dugdaleet al., 1989], are used to estimatenew production(i.e., nitrate uptake, in the senseof Dugdaleand Goering[1967]) for the persistentcoastal upwelling feature at Point Conception,California. Parametersrequired to initialize the model and remotelysensed SST (from advancedvery high resolution radiometerimages) were availablefor spring1983, from data collectedas part of the Organizationof PersistentUpwelling Structures(OPUS) study.Some examplesof the spatial extent of new productionare illustratedusing false color images,and temporal variability is shownby the time seriesof depth- and area-integratednitrate uptake obtained from eight images.The model resultsare comparedwith shipboarddata for three different upwellingconditions that occurred during OPUS-83, along with the model resultsand data publishedfor Cap Blanc. These two regions,Point Conception and Cap Blanc, representtwo ends of a spectrumof coastalupwelling performance, with low new productionat Point Conceptionand lesseffective conversion of availablenitrate into particulatenitrogen biomassin contrastto the high levels of new productionat Cap Blanc. The daily new productionat the Point Conceptionupwelling center is about 10% of the Cap Blanc new production,both estimatedfrom the remote-sensingmodel and satellite- derived SSTs. The model is shown to work well for both extremes and should therefore be suitablefor intermediate situations.The long-term objectiveis to produce a model which can be used for coastalupwelling systems globally to provide a estimate of new production from remotely senseddata in theseimportant areas and to assistin understandingthe role of thesecoastal systems in the air-sea exchangeof biogeochemicalelements. 1. Introduction due to downwardparticle flux [Eppleyand Peterson,1979] or into biomassyield [Dugdaleand Goering,1967]. Rates of new The major reservoir of nitrogen in the ocean is nitrate in production are also important to the global CO2 cycle since subthermoclinewater which is incorporatedinto phytoplank- they set the rate at which CO2 upwelledwith nitrate is taken up ton throughprimary productionprocesses when advectedinto and incorporatedinto biologicalparticles. the upper euphoticzone. The fraction of primary production Coastalregions are sitesof relatively intensenew production resultingfrom this or other new nitrogen sourcesrather from compared to open ocean systems.[Dugdale and Wilkerson, i989] and are consideredby someto be of equal importanceto and Goering,1967]. New productionis a key componentin the the open seain global new productionprocesses and the flux of global oceanic nitrogen and carbon budgets since nitrogen biogenic elements to the deep ocean [Jahnke et al., 1990; must be incorporatedfor carbon fixation (photosynthesis)to Walsh,1991]. Although many more measurementsof new pro- occur, and the input of nitrate largely determines the maxi- ductionwith the lSN techniquehave been made in nearshore mum export of nitrogen and carbon from the phytoplankton regionscompared to open ocean systems,the annual new pro- duction of coastal systemsremains poorly known, in part a •Nowat RombergTiburon Centers, San Francisco State University, result of the high spatial and temporal variability characteristic Tiburon, California. of suchareas, particularly easternboundary current upwelling 2Nowat NavalResearch Laboratory, Washington, D.C. regions [Woosterand Reid, 1963]. Copyright 1997 by the American GeophysicalUnion. Eastern boundary coasts typically exhibit a series of up- Paper number 96JC02136. welling plumes,e.g., alongthe coastof SouthAfrica [Taunton- 0148-0227/97/96J C-02136509.00 Clark, 1985] and the coast of South America [Guillen and 8573 8574 DUGDALE ET AL.: NEW PRODUCTION AT POINT CONCEPTION FROM SST 34o45 ' 1000 rn 34o30 ' S77 100rn "•"'"•"• STUDY AREA 34ø15' I 121o0 ' 120o30 ' 120o0 ' Figure 1. Map of the OPUS-83study site showing G-1 (the stationnearest the upwellingcenter) and drifter S77 deployedat G-1 during period 1. Calienes,1981] and along the California coastwhere offshore dissolvedorganic matter originating from land sourcesare filamentsand jets also occur [Brink and Cowles,1991; Abbott more important then phytoplanktonpigment in determining and Barksdale,1991]. Production cyclesin these coastal sys- the total absorptionof incidentradiation. Also, in this studywe temshave temporal scales that are relatedto the frequencyof usemodel and shipboarddata combinedto obtain a best esti- local winds as well as to remotely forced seasonalchanges in mate of new productionover the area of the upwellingplume. the depth of the thermocline.As a result, coastalsystems are New productionfor Point Conceptionis estimatedfor each of strongly undersampled due to practical limitations of re- eight images available over a period of 34 days. The area- sources,especially ship time. Correctionof the undersampling integratednew productionis then further integratedover the problem in coastalupwelling systemsis only feasible using time encompassedby the satellitedata to providea total area remotesensing [e.g., Platt and Sathyendranath,1988; Platt et al., and approximatelymonthlong total plume new production. 1989],in conjunctionwith data from mooredinstruments and shipboardobservations. Models for evaluationof new produc- tion from remotelysensed ocean color data are underwayin a 2. Study Site: Point Conception, California numberof laboratories[e.g., Pribble et al., 1994;Campbell and The upwellingcenter at Point Conceptionhas the general Aarup, 1992]. Remotely sensedtemperatures from the ad- appearanceand characteristicsof other easternboundary up- vancedvery high resolutionradiometer (AVHRR) sensorcan wellingcenters (15øS, Peru [MacIsaacet al., 1985]; Punta San be usedto constructthe surfacenitrate field [Motin et al., 1993] Hipolito, Baja California [Walshet al., 1974]; the Benguela and to determineproductivity as proposedby Traganzaet al. upwellingin SouthAfrica [e.g.,Shillington et al., 1990;Probyn, [1983] for the upwellingplume off Point Sur, California.Dug- 1992]), with cold, nutrient-rich,low-chlorophyll water being dale et al. [1989] and Sathyendranathet al. [1991] have used upwelledclose to the coastand movingoffshore as a surface AVHRR-SST in conjunctionwith coastalzone color scanner plume with increasingchlorophyll concentration and temper- (CZCS) data to estimatenew productionat Cap Blanc,north- ature and decreasingnutrient concentrations[Jones et al., west Africa, and GeorgesBank, respectively. 1988]. The upwellingplume has its origin betweenPoints Ar- Our present study uses the Dugdaleet al. [1989] remote- guelloand Conception[Jones et al., 1983]as the shorelineturns sensingmodel to estimatenew productionfor the upwelling abruptlyfrom a north-southorientation to east-west,becoming center at Point Conception,California. It differsfrom the pre- the north shore of the Santa Barbara Channel (Figure 1). vious applicationin that the input parameterswere obtained Details of physicalprocesses in the Point Conception up- duringthe sametime period asthe satelliteimages and a series wellingcenter have been describedby Barth and Brink [1987], of AVHRR imageswere used to get a range of values over Brink et al. [1984], Davis and Regier [1984], and Brink and different upwelling and nonupwellingconditions, whereas a Muench [1986]. This upwellingsystem was intensivelyinvesti- singleimage was used in the previouscase study and shipboard gated during the OPUS-83 field studyfrom April 4 to May 11, data from a different year were used to initialize the model. 1983 [Atkinsonet al., 1986],with ships,moorings, instrumented For the Point Conceptionstudy we use the temperature-only aircraft,and satelliteAVHRR imagery(images obtained, Ta- mode (no CZCS) of the model primarilyas a resultof unre- ble 1). Atkinsonet al. [1986] identifiedthree upwellingevents solvedproblems in identifyingthe case2 watersin whichchlo- and two downwellingevents from the wind stressdata obtained rophyll concentrationsare overestimated,especially in the duringOPUS-83, whichwere then organizedinto three periods nearshoreregions. In thesecase 2 waters,inorganic particles or (Table 1) by Dugdaleand Wilkerson[1989], characterizedby DUGDALE ET AL.: NEW PRODUCTION AT POINT CONCEPTION FROM SST 8575 Table 1. UpwellingState and PlumeAppearance at Point ConceptionDuring the AcquisitionPeriod of AVHRR ImagesUsed in This Study Julian Day 1983 Date Period UpwellingState Image Features 104 April 14 1 moderateupwelling classicalcold water plume 107 April 17 2 early relaxation shrinkingplume 114 April 24 2 relaxed plume absent 115 April 25 2 earlyweak upwelling coolwater advectedaway 121 May 1 2 major downwelling plume absent 126 May 6 3 major upwelling developingplume 129 May 9 3 major upwelling plume and eddy to north 138 May 18 3 major upwelling large plumesat Point Conception and
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