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Annual Cycle of Primary Production in the Cariaco Basin: Response to Upwelling and Implications for Vertical Export

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Annual Cycle of Primary Production in the Cariaco Basin: Response to Upwelling and Implications for Vertical Export University of South Florida Scholar Commons Marine Science Faculty Publications College of Marine Science 3-15-2001 Annual Cycle of Primary Production in the Cariaco Basin: Response to Upwelling and Implications for Vertical Export Frank E. Muller-Karger University of South Florida, [email protected] Ramon Varela Estacion de Investigaciones Marinas de Margarita Robert Thunell University of South Carolina Mary Scranton State University of New York Richard Bohrer University of South Florida See next page for additional authors Follow this and additional works at: https://scholarcommons.usf.edu/msc_facpub Part of the Marine Biology Commons Scholar Commons Citation Muller-Karger, Frank E.; Varela, Ramon; Thunell, Robert; Scranton, Mary; Bohrer, Richard; Taylor, Gordon; Capelo, Juan; Astor, Yrene; Tappa, Eric; Ho, Tung-Yuan; and Walsh, John J., "Annual Cycle of Primary Production in the Cariaco Basin: Response to Upwelling and Implications for Vertical Export" (2001). Marine Science Faculty Publications. 56. https://scholarcommons.usf.edu/msc_facpub/56 This Article is brought to you for free and open access by the College of Marine Science at Scholar Commons. It has been accepted for inclusion in Marine Science Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. Authors Frank E. Muller-Karger, Ramon Varela, Robert Thunell, Mary Scranton, Richard Bohrer, Gordon Taylor, Juan Capelo, Yrene Astor, Eric Tappa, Tung-Yuan Ho, and John J. Walsh This article is available at Scholar Commons: https://scholarcommons.usf.edu/msc_facpub/56 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C3, PAGES 4527-4542, MARCH 15, 2001 Annual cycle of primary production in the Cariaco Basin: Responseto upwelling and implications for vertical export Frank Muller-Karger,• Ramon Varela,2 Robert Thunell,3 Mary Scranton,4 Richard Bohrer,• Gordon Taylor,4 Juan Capelo,2 Yrene Astor,2 Eric Tappa,3 Tung-Yuan Ho, 4 and John J. Walsh• Abstract. Monthly hydrographic,primary production,bacterial production,and settling particulatecarbon flux observationswere collectedbetween November 1995 and December 1997 at 10.5øN,64.67øW within the Cariaco Basin, off Venezuela. Upwelling of SubtropicalUnderwater (SUW) startedaround October and lastedthrough approximately Mayof thefollowing year. Wind speeds >7 m s-• wereobserved between January and June,with weaker winds (<5 m s-•) betweenJuly and December. The upwelling cycle was thereforeout of phasewith that of the trade windsby 2-3 months.A seasonalcycle punctuatedby transientextremes associated with subsurfaceventilation events was observedin primary production.High bacterial activityand organiccarbon recycling rates were observednear the oxic-anoxicinterface. Integrated primary productionwas 690 gC m-2 yr-• in 1996and 540 gC m -2 yr-• in 1997.Settling carbon flux measured with sedimenttraps was about 5.6% of integratedprimary productionat 275 m and about 1.7% at 1225m, with no seasonalityin theproportion of verticalflux to primaryproduction. In total,between 10 and11 gC m-2 yr-*were deliveredto thebottom sediment of Cariaco, whichsuggests that between 4 x l0s and1 x 106 t of C yr-• weredelivered to sediments within the upwellingarea of the CariacoBasin. This representspermanent sequestration of carbonpreviously entrained in the North Atlantic gyre in the area of formation of SUW. Resultssuggests that upwelledinorganic nitrogen, rather than nitrogen fixation, is responsiblefor the large productivityand particulatecarbon settling flux in the Cariaco Basin. 1. Introduction Many studieshave focusedon the presentbalance between inorganicand organicpools in the deep ocean [see,e.g., Han- Over geologicaltimescales, geochemical imbalances at the selland Carlson,1998, and referencestherein], but the contri- Earth's surface are closely tied to carbon sequestrationby bution toward this balanceby watersnear continentalmargins marine organismsand to the sinkingflux of particulateorganic remainsunclear. All attemptsat global assessmentsof marine material [Berner,1992]. This variable flux servesas a key to primaryproduction indicate that oceanmargins support on the interpretingpast climatewhen preservedat depth. However, average2-5 timesthe annual productionof open oceanwaters the relationshipbetween fossil flux and oceanographiccondi- [Koblentz-Mishkeet al., 1970; Field et al., 1998]. Indeed, the tions near the ocean'ssurface must be analyzedto understand mean particleflux at 2300 m alongcontinental margins is of the how these records can be used to make inferences about cli- orderof about7.0 gC m-2 yr-•, comparedto ---0.8gC m-2 mate change.There are few placeswhere flux observations yr-• at 2300m in thedeep sea [Deuser et al., 1990;Walsh, 1991; have been linked to climatic and oceanographicforcing con- Pilskalnet al., 1996;Thunell, 1998b]. Globally, over 65% of the ditions[Deuser et al., 1990;Karl et al., 1996; Thunell,1998a]. particulatecarbon that falls below 1000 m is derivedfrom the Indeed, only in a few locationsdo the underlyingsediments slopeand rise of continentalmargins [Jahnke, 1996], and this preservevariability spanning a wide range of timescales.This estimatedoes not include areas shallowerthan 1000 m [see hasmade it difficult to establishthe relationshipbetween pro- alsoHonjo et al., 1982;Spencer, 1984; Walsh et al., 1992].These ductivityin surfacewaters and biogeochemicalfeedbacks act- simple statisticssuggest that the biologicalpump [Volk and ing over decadaland centurytimescales [see Falkowski et al., Liu, 1988] is much more efficientin the vicinityof continental 19981. marginsthan in the ocean'sinterior. To try to understandthe connectionbetween surfacepro- 1Collegeof MarineScience, University of SouthFlorida, St. Peters- duction and vertical carbon flux in a productive setting,we burg, Florida. starteda seriesof monthlyhydrographic and productivityob- 2FundacionLa Sallede CienciasNaturales, Estacion de Investiga- cionesMarinas de Margarita, Isla de Margarita, Venezuela. servationsand deployeda set of sedimenttraps in the Cariaco 3Departmentof GeologicalSciences, University of SouthCarolina, Basin, off the coast of Venezuela. These observationsform the Columbia, South Carolina. basisof the Carbon Retention in a Colored Ocean (CARI- 4MarineSciences Research Center, State University of New York, ACO) study.Here we discussthe first 2 yearsof observations Stony Brook, New York. at the CARIACO site. Copyright2001 by the American GeophysicalUnion. The CariacoBasin (Figure 1) offers a unique geographical Paper number 1999JC000291. settingthat permits studiesthat are difficult or impossibleto 0148-0227/01/1999JC000291509.00 conductat other sites.This is a large (---160 km long, 70 km 4527 4528 MULLER-KARGER ET AL.: CARIACO BASIN PRIMARY PRODUCTION CYCLE 20' 10' 66 ø 50' 40' 30' 20' 10' 65 ø 50' 40' 30' 20' 10' 64 ø 40' 40' 30' 30' 20' 20' 10' DE MARGARITA 10' DELA t 'RTUGA.... ISLA DE MARGARITA 11 ø 11 ø 50' 50' SILLA DE CUBAGUA-. 40' ILLA CENTRAL 40' 30' 30' 20' LATAFORMA DE UNA 20' 10' 10' 10 ø 10 ø 20' 10' 66 ø 50' 40' 30' 20' 10' 65 ø 50' 40' 30' 20' 10' 64 ø Figure 1. The CariacoBasin: bathymetry and locationof the CARIACO time seriesstation [modified from Febres-Ortegaand Herrera, 1975]. wide) and deep (--•1400 m) basinwithin a continentalshelf. to decadal-scalechange [Petersonet al., 1991; Hughen et al., It is bound to the north by a sill connectingMargarita Island 1996, 1998;Haug et al., 1998;Black et al., 1999]. The evidence to Cabo Codera at a mean depth of about 100 m. There are suggeststhat evenweak disturbanceswith a muted expression two channelsbreaching this sill (Figure 1): one in the north- in the deep ocean are recordedtemporarily within the water east of 135 m depth (La Tortuga) and a narrower one in the columnin Cariaco and permanentlyin the sedimentsat the northwestof 146 m depth (Centinela) [Richards,1975; Lidz bottom of the basin. et al., 1969]. In additionto the recentimportance of the CariacoBasin as Current conceptualmodels attribute a marked cyclein up- the site of an important paleo-oceanographictime series,the wellingand associatedsea surface temperature (SST) changes CariacoBasin has servedas a natural laboratoryfor biogeo- in this region primarily to the seasonalintensification of the chemistsfor over 40 years. This basin has been key in con- tradesalong an east-westcoastline [Richards, 1960, 1975;Her- structingstoichiometric models of organicmatter remineral- reraand Febres-Ortega, 1975; Muller-Karger and Aparicio, 1994]. ization [Redfieldet al., 1963; Richards, 1975], developing Similar and concurrentvariations in upwellingoccur along the residencetime and box models,studying metallic sulfides [Ba- entire south/central Caribbean Sea, but we show here that con et al., 1980], and numerousother studies.However, very winds lag behind the onset of upwellingby up to 3 months. little is known about the variabilityin primary productionor Within the CariacoBasin the seasonalupwelling process pro- aboutthe actualprocesses that controlthe carbonand nutrient videsa sourceof nutrientsthat leadsto vigorousphytoplank- fluxes within the basin. ton growth near the surface. Previous studies suggestthat muchof this remainsungrazed and sinks,allowing pigments to 2. Methods reach the sediment [Richards,1975; Richardsand Vaccaro, 1956]. Twenty-sixCARIACO cruiseswere conductedbetween No- In contrastto the situationat other continentalmargins, the vember1995 and December1997 to examinethe hydrography, CariacoBasin has a muted advectiveregime
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