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Journal of Marine System

Microbial community structure along 18°W (39°N–44.5°N) in theNE Atlantic in late summer 2001 (POMME programme)

Camila Fernández 1, Melilotus Thyssen, Michel Denis⁎

Laboratoire de Microbiologie, Géochimie et Ecologie Marines (LMGEM), CNRS UMR6117, Université de la Méditerranée, Centre d'Océanologiede Marseille, 163 Av. de Luminy, Case 901, 13288 Marseille cedex 09, France

Received 6 February 2006; received in revised form 6 April 2007; accepted 8 June 2007Available online 21 June 2007

Abstract

The spatial and vertical distribution of heterotrophic prokaryotes and ultraphytoplankton (b10 μm) was investigated by flowcytometry in the NE Atlantic Ocean (Programme Océan Multidisciplinaire Méso Echelle, POMME 3 cruise, 39°N–45°N) with theaim to document the relationships between these distributions and the hydrodynamic features prevailing in the NE Atlantic. Aparticular interest was given to the north–south transect 18°W (12 stations) because it crossed an anticyclonic cold-core SWODDY-type eddy (slope water oceanic anticyclonic eddy) and was sampled within less than 3 days. Four ultraphytoplankton (b10 μm)groups (Prochlorococcus, Synechococcus, pico- and nanoeukaryotes) were distinguished by flow cytometry on the basis of theiroptical properties (scatter, natural fluorescence). Three heterotrophic prokaryotes groups were resolved (two High Nucleic Acid:HNA1 and HNA2 and one Low Nucleic Acid: LNA) after nucleic acid staining. Results showed significant latitudinal variations inbiogeochemical parameters as well as in ultraphytoplankton and heterotrophic prokaryote distribution. The inner domain of theeddy also showed variations in heterotrophic prokaryote abundances with respect to the spring situation [Thyssen, M., Lefèvre, D.,Caniaux, G., Ras, J., Dugrais, L., Fernández I.C., Denis, M., 2005. Spatial distribution of heterotrophic bacteria in the North EastAtlantic (POMME study area) during spring 2001. J. Geophys. Res., 110, C07S16, doi:10.1029/2004JC002670], with highervalues of HNA2 during late summer. The same trend was observed on the abundance of cyanobacteria (Prochlorococcus,Synechococcus) and small size ultraphytoplankton. Despite the observed differences in microbial distributions inside and outsidethe eddy, the biological features appeared insufficient to independently characterize this anticyclonic eddy and provide informationon its microbial dynamics.© 2007 Published by Elsevier B.V.

Keywords: Atlantic Ocean 39°N–45°N, 18°W; Heterotrophic prokaryotes; Ultraphytoplankton; Mesoscale eddies; Flow cytometry

⁎ Corresponding author. Laboratoire de Microbiologie, Géochimie etEcologie Marines (LMGEM), Centre d'Océanologie de Marseille, 163Av. de Luminy, Case 901, 13288 Marseille cedex 09, France. Tel.: +33491 829114; fax: +33 491 829641.

E-mail addresses: fernandez@profc.udec.cl (C. Fernández),michel.denis@univmed.fr (M. Denis).1 Present address: Laboratorio de Procesos Oceanográficos y Clima

(PROFC), Universidad de Concepción, Casilla 160-C, Concepción,Chile.

0924-7963/$ - see front matter © 2007 Published by Elsevier B.V.doi:10.1016/j.jmarsys.2007.06.003

1. Introduction

The dynamics of CO2 and other climatically activegases like DMS and halogens are related to the planktoncommunity structure, which in turn is linked to thephysical and chemical parameters of the upper ocean(Malmstrom et al., 2005). Although primary productionsets the upper limit for the biological pump that transfersCO2 into the deep ocean, the main factors controlling the

47C. Fernández et al. / Journal of Marine Systems 71 (2008) 46–62

export and sequestration of carbon are sinking,mixing andremineralization of POC (Particulate Organic Carbone)and DOC (Dissolved Organic Carbone) by heterotrophicmetabolism (Lefèvre et al., 1996; Legendre and Rivkin,2002). It is thus important to know how biotic as well asabiotic forcing influences heterotrophic prokaryote dy-namics as well as autotrophic community structure.

The study of cold and warm core eddy dynamics hasrapidly evolved in the Sargasso Sea, while less attentionhas been given to mesoscale eddies in the eastern part ofthe North Atlantic (Le Groupe Tourbillon, 1983; Krausset al., 1990; Karrasch et al., 1996). Lochte andPfannkuche (1987) described into details the impact ofa cold-core eddy on the northeast Atlantic waters interms of nutrients, distribution and abundance of bothphytoplankton and heterotrophic prokaryotes. The high-est values of chlorophyll a concentration, total phyto-plankton biomass, dinoflagellates and heterotrophicprokaryote abundances were observed in the surfacewaters of the eddy centre. Also, during the NABE (NorthAtlantic Bloom Experiment) experiment, Karrasch et al.(1996) presented evidence of a link between the variabilityof autotrophic and heterotrophic biological processes andrelated hydrographic conditions. Also, it was stated thatthe declining of the seasonal bloom in the north Atlanticcan be under the influence of the unsteady characteristicsof the mesoscale variability (Karrasch et al., 1996; Garçonet al., 2001).

But it wasn't until recently that the study of mesoscalefeatures and their impact on heterotrophic prokaryotedynamics and the fate of organicmatter gained importance(Pingree and LeCann, 1992; McGillicuddy et al., 1999).

The POMME experiment (Programme Océan Multi-disciplinaire Méso-Echelle) was carried out in thenortheast Atlantic Ocean, which is known as a zone ofhigh eddy energy and mesoscale variability (Mémeryet al., 2005). During POMME, some long lastingcoherent eddy structures were identified, thanks toweekly maps of the circulation near the surface (100 m)that were produced from in situ data, satellite altimetryand a quasi-geostrophic model. A full description ofphysical features and hydrological properties of thePOMME area and methods used during the survey areprovided in the JGR special section of the POMMEexperiment (Mémery et al., 2005). The hydrodynamiccharacteristics of the study area and physical–biologicalinteractions observed during the experiment are alsoreported therein (e.g. Reverdin et al., 2005; LeCannet al., 2005; Fernández et al., 2005b; Leblanc et al.,2005). Fernández et al. (2005a) also reported informa-tion on mesoscale hydrographic variability and theimpact of these features on nutrient distribution in the

study area during the entire annual cycle. Thyssen et al.(2005) provided a general overview of heterotrophicprokaryote distribution in the POMME area during thespring season. During that study, the distribution below100 m depth of heterotrophic prokaryote characterisedby a low nucleic acid content (LNA), was affected bythe presence of hydrodynamic features such as cyclonicand anticyclonic eddies and frontal structures.

Altimetry data and SOPRANE (operational system ofoceanic forecasting) weekly analyses (Assenbaum andReverdin, 2005) established the presence of a mesoscalecold-core SWODDY-type anticyclonic eddy (Pingree andLeCann, 1992). This eddy, hereafter referred as A1, wasfollowed by floats for over one year, and often at morethan one depth level (Mémery et al., 2005; LeCann et al.,2005). After drifting south-westward through March2001, it remained almost stationary (43°N, 18°W) untilSeptember 2001 (Mémery et al., 2005; LeCann et al.,2005). The data gathered by floats and in situ surveysindicate that this SWODDY-type eddy (LeCann et al.,2005) had a small coherent core with a radius of 30 kmandmaximum velocity near 300–400m,which decreasedtowards both the surface and 1000 depth (Mémery et al.,2005). The central part of the eddy was often surroundedby a ring of anticyclonic velocity, with a maximum nearthe surface. This larger anticyclonic zone evolved in timeand its diameter approached 150 km (Mémery et al.,2005). The present paper reports on the POMME3 cruise,carried out during the late summer season and focuses onthe distribution of heterotrophic prokaryotes and ultra-phytoplankton (b10 μm) along the 18°W transect thatcrossed the anticyclonic eddy A1. Indeed, particularinterest was given to this transect because it could providerelevant quasi-synoptic information on the impact of boththe latitude gradient and the A1 hydrodynamic feature onthe composition and vertical distribution of the microbialassemblages.

2. Materials and methods

2.1. Study area and sampling strategy

The first leg of the POMME 3 cruise (R/V Thalassa)was carried out August 28th and October 4th 2001. Thestudy area was located in the northeastern AtlanticOcean (39°N–44.5°N; 16.6°W–20.6°W; Fig. 1). Thegrid was composed by 83 stations, sampled along 7north–south transects, 0.5° apart and following a regularpath (55.5 km, Mémery et al., 2005). Although the entiresampling grid was achieved in three weeks, the transectcrossing 18°W was composed of 12 consecutive stationsthat were sampled within 60 h.

Fig. 1. Location of the study area and sampled stations during the POMME 3 cruise (28 August–4 October 2001) in the north-eastern Atlantic. Allstations were sampled for heterotrophic prokaryotes, except stations labelled as Black dots. Grey dots represent transect 18°W through whichultraphytoplankton analyzes were performed.

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Hydrographic stationswere studied using aCTDprobe(Seabird SBE 9) mounted on a rosette carrying 21 Niskinbottles of 12 l. The hydrological data set of the POMMEproject (encompassing 38°N to 45°N; 15°W to 22°W)combined Expendable BathyThermograph drops (XBT),XCTD andCTD casts (Mémery et al., 2005). This data setwas used to produce high-resolution temperature andsalinity analyses. The resulting data set was firstinterpolated onto 65 vertical levels (5 m near the surfaceand 300 m at depth), and then objectively analyzed onto a5 km horizontal grid for all vertical levels. From theseanalyses, currents were computed at each level throughthe geostrophic approximation by using a level of nomotion located near 1400 dbar according to the estimatesof Stramma (1984). The determination of the level of nomotion for the geostrophic currents was based on severaltests and the final choice of a compromise, allowingcurrents to be fairly close to ADCP (Acoustic DopplerCurrent Profiler) currents and the level of no motion to beclose to estimates found in the literature (Stramma, 1984).Details of this procedure are provided in a previous paper(Fernández et al., 2005a).

2.2. Nutrients

Samples for nutrient measurements (NO3−, NO2

−,NH4

+, PO43−, Si(OH)4) were obtained at each station of

the main CTD grid (Fig. 1) and at each depth level.Sampling was carried out using 20 cm3 (NO3

−, NO2−,

PO43−, Si(OH)4) and 250 cm3 (NH4

+) polyethyleneflasks. For NO3

−, NO2−, PO4

3−, and NH4+, analyses were

performed on board immediately after sampling using aTechnicon Autonanalyser®, according to Tréguer andLeCorre (1975). Samples of Si(OH)4 were poisonedwithHgCl2 (20 μg cm− 3; 50 mm3 for 20 cm3 of seawatersample) to be later on analysed in the laboratory. Si(OH)4concentrations were measured following the protocolproposed by Mullin and Riley (1965), modified byStrickland and Parsons (1972). Detection limits are0.05 μM, 20 nM, 0.02 μM, 10 nM and 0.05 μM for NO3

−,NO2

−, PO4−, NH4

+ and Si(OH)4, respectively. To ensurereproducibility in nutrient measurements between cruises,a unique type of in-house standards was used fromPOMME 1 through POMME 3, which was regularlycompared to commercial products (OSIL). Accuracy was

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also tested, through the participation in the Europeaninter-calibration exercise QUASIMEME.

2.3. Flow cytometry (FCM)

For heterotrophic prokaryote analysis by flow cyto-metry (FCM), seawater was sampled at all stations of thegrid, at 13 depths (5, 10, 20, 40, 60, 80,100, 150, 200, 300,400, 500 and 600 m). The seawater (about 100 cm3)collected from Niskin bottles was prefiltered through a100 μm mesh size net to prevent clogging of the flowcytometer. This volumewas homogenized before taking asubsample to fill one cryovial. The subsequent flowcytometric analysis is therefore representative of abun-dances averaged over 100 cm3. Under these conditions,replicates would not improve the information nor theaccuracy defined by the reproducibility of the instrumentand the Poisson law (Shapiro, 2003) regarding countlevels. Separate subsamples for ultraphytoplankton(5 cm3) and heterotrophic prokaryote (2 cm3) analyseswere preserved with 2% paraformaldehyde (Troussellieret al., 1999), frozen onboard and stored in liquid nitrogenuntil analysis in the laboratory. For ultraphytoplanktonanalysis, subsamples were limited to the upper 9 depths(between surface and 200 m depth). Due to technicalreasons, the ultraphytoplankton distribution could only bedetrmined for the 18°W transect.

For FCM analysis, seawater samples were thawed atambient temperature and analyzed with a flow cytometer(Cytoron Absolute, Ortho Diagnostic Systems ®),equipped with an air-cooled 488 nm argon laser. Thesample and sheath fluid rates were 1 and 100 mm3 s−1,respectively.

Heterotrophic prokaryotes were analyzed after stain-ing their nucleic acids. To this purpose, 1 cm3 ofseawater sample was supplemented with 10 mm3 Sybr-Green II (Molecular Probes®, solution diluted 1 /5000in final concentration) and incubated for 15 min in thedark before initiating analysis. The overlapping ofstained Prochlorococcus in the green fluorescence ver-sus side scatter cytograms was accounted for by usingthe red fluorescence that distinguishes Prochlorococcusfrom heterotrophic prokaryotes (Sieracki et al., 1995) asdescribed in Fig. 2b of Thyssen et al. (2005). Synecho-coccus were out of range with respect to the side scattersignal.

Ultraphytoplankton was analysed without stainaddition . Flow-Set™ fluorospheres (3.6 μm; BeckmanCoulter) were added to samples for ultraphytoplanktonanalysis as internal standard but not for heterotrophicprokaryote analysis. For each cell, 5 optical parameterswere recorded: two diffraction parameters, namely for-

ward scatter (related to the cell size) and side scatter(related to the cell structure), and three fluorescenceparameters measuring emissions in the red (N620 nm),orange (565–592 nm) and green (515–530 nm) wave-length ranges. Data were collected in list mode andstored as FCS files by using the IMMUNOCOUNTsoftware (ORTHO Diagnostic Systems ®). The FCSfiles were further handled with the WINLIST software(VERITY Software House ®) to resolve cell clusters.

Flow cytometry does not give access to speciesidentification (except in the case of in situ hybridization)and the clusters resolved by FCM may be composed ofseveral species (Zubkov et al., 2001). No reference tospecies will be made in this study and any observedvariability in the analysed cluster properties may eitheroccur within the same species or be the result of changesin species composition, or both.

Heterotrophic prokaryote abundances were expressedin terms of carbon biomass by using the conversion factorof 15 fg C cell−1 (Caron et al., 1995). Ultraphytoplanktonabundanceswere expressed in terms of carbon biomass byusing the same conversion factors as in Karagianni et al.(2005), i.e. 49, 250, 671 and 3498 fg C cell−1 for Pro-chlorococcus, Synechococcus, pico- and nanoeukaryotesrespectively.

The Cytoron absolute was equipped with volumetricinjection for both sample and sheath fluid. The linearity ofthe flow rates was insured by the stepping motors thatmoved the pistons of the syringes. The sample volumeanalysed per time unit was thus accurately defined andweekly controlled by analysing bead suspensions of knownconcentrations. This value was integrated in the instrumentsoftware to directly calculate absolute concentrations.

The larger phytoplankton cells are, the less concentratedthey are. As a result, they cannot be accounted for by FCManalysis of small volumes in conventional flow cytometers(b0.6 cm3 in the present study). Consequently, we referhere to total FCMcarbon as a combination of heterotrophicprokaryote (heterotrophic FCM carbon) and ultraphyto-plankton carbon (autotrophic FCM carbon) only.

2.4. Pigment analysis

At each station and depth level, 2.8 dm3 of sea waterwere filtered onto 25 mm Whatman ® GF/F glass-fiberfilters. After filtration, filterswere stored in liquid nitrogenuntil laboratory analyses. Samples were extracted in3 cm3 HPLC-grade methanol and injected into a reversedphase C8 Hypersil MOS column and analyzed with anAgilent Technologies 1100 series HPLC system coupledto a Thermoquest AS3000 autosampler as described inClaustre et al. (2005). The HPLC procedure derived from

Fig. 2. Mixed layer depth, euphotic layer, sea surface temperature andsalinity along 18°W. a) Latitudinal evolution of the Mixed Layer Depth(Zm, main axis) and the euphotic layer (Ze, secondary axis) alongtransect 18°W. b) Latitudinal evolution of Sea Surface Temperature(SST, main axis) and Sea Surface Salinity (SSS, secondary axis) alongtransect 18°W. Location of Eddy A1 is indicated by a black rectangle.

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the protocol of Vidussi et al. (1996) is detailed by Claustreet al. (2004). Total chlorophyll a (Tchl a) refers here tothe sum of chlorophyll a and divinyl–chlorophyll a.

2.5. Mixed and euphotic layer depths

The mixed layer depth (Zm) along 18°W wascalculated from a change in potential density (σ) higherthan 0.001 kg m−3 in two consecutive determinationswithin a CTD cast (Fernández et al., 2005a).

The euphotic layer depth (Ze) was calculated fromprogressive integration over depth of chl a verticaldistribution using the model developed by Morel andMaritorena (2001).

2.6. Statistics

In order to compare abundances between stations, theKruskal–Wallis non parametric multiple comparison testwas run between stations using R freeware (www.cran.com). It is similar to one-way analyses of variance whichtest the equality of population medians within groups.

3. Results

3.1. Hydrological background

During POMME 3 (Fig. 2a), the minimal Zm (∼8 m)was observed at 43°N, while the general trend showedvalues close to 35 m in southern stations. The depth of theeuphotic layer (Ze, Morel and Maritorena, 2001) showedtypical summer characteristics (deeper than Zm) andfollowed the distribution trend of the Zm. Deeper Ze values(up to 103m)were observed in the south while in northernstations Ze reached 73–74 m. Values of Zm were lower inthe northern area, reaching in average 20mdepthwhile theaverage for southern stations reached 30 m depth.

Sea surface temperatures (SST, Fig. 2b) showed asouth–north gradient with maximal values at 39°N(22.5 °C) and minimal values near 44.5°N (20.2 °C).SST values inside A1 (43°N to 44.5°N) were 1 °C lowerthan in surrounding waters. Sea Surface Salinity (SSS,Fig. 2b) also showed a south–north gradient withmaximal values close to 36 in southern stations and asignificant decrease northward. Inside eddy A1, salinityvalues were generally close to 35.8, with a minimal valueof 35.6 at 43°N.

3.2. Biogeochemical parameters

Fig. 3a illustrates the vertical distribution oforthosilicic acid along 18°W. Values were lower than

1 μM in the first 50 m, except at stations 31 and 32, andinside the domain of A1 where they were clearly higherthan in the rest of this transect (exceeding 1.5 μM insurface waters and reaching 2.5 μM at 50 m). WithinA1, silica concentrations between 100 and 300 m rangedbetween 3 and 3.5 μM.

The vertical distribution of NO3− (Fig. 3b) was

homogeneous in the upper 200 m between 39°N and42.5°N. Surface waters showed nitrate concentrationsbelow 1 μM in the first 50 m of the water column, whileinside A1 values at 50 m could reach 5 μM. Valueswithin the lens of seawater inside A1 (at 100–300 mdepth), could reach between 7 and 9 μM (Fernándezet al., 2005a).

Although nitrite values (Fig. 3c) did not exceed0.03 μM at depths below 200 m, concentrationsexceeding 0.05 μM were observed at 40.5°N and alsobetween 42.5 and 43°N. Both locations corresponded tothe anticyclonic zones, A31 and A1, respectively(LeCann et al., 2005; Mémery et al., 2005).

Vertical distribution of Tchl a (Fig. 4) showed valuesdecreasing southward (0.1–0.2 mg m−3 in the top 75 m

Fig. 3. Vertical distribution along 18°W of a) Si(OH)4 (μM), b) NO3−(μM) and c) NO2

−(μM). The location of eddy A1 is labelled.

Fig. 4. Vertical distribution along 18°W of Tchl a (mg m−3). The location of eddy A1 is labelled.

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Fig. 5. Cytograms of the different clusters resolved by flow cytometry.a) Green fluorescence versus side scatter (RT-SC) cytogram showingthree heterotrophic prokaryote clusters (HNA1, HNA2, LNA). Pro-chlorococcus cells were removed from this plot on the basis of their redfluorescence. (a.u.): arbitrary unit. b) Red fluorescence versus Orangefluorescence cytogram resolving Prochlorococcus and Synechococcusclusters. (a.u.): arbitrary unit. c) Red fluorescence versus Greenfluorescence cytogram resolving Nanoeukaryote I, Nanoeukaryote IIand Picoeukaryote autotrophic cell groups. (a.u.): arbitrary unit. Notethat the 3 cytograms of the figure correspond to different instrumentalsettings.

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south of 42°N). In the northern area, within the domainof A1 (43–44.5°N), maximal values were close to0.25 mg Tchl a m−3.

3.3. Microbial community composition

The FCM analyses allowed distinguishing threeheterotrophic prokaryote clusters based on their nucleicacid staining and side scatter signals (Fig. 5a).Following the approach described by Thyssen et al.(2005) for the spring season, these clusters were labelledas Low Nucleic Acid heterotrophic prokaryotes (here-after referred as LNA), High Nucleic Acid heterotrophicprokaryotes 1 (hereafter referred as HNA1) and HighNucleic Acid heterotrophic prokaryotes 2 (hereafterreferred as HNA2).

Five ultraphytoplankton groups were identified byFCM analysis with regard to size and fluorescencecharacteristics: cyanobacteria (Prochlorococcus andSynechococcus), nanoeukaryotes I (hereafter referredas NANO I), nanoeukaryotes II (hereafter referred asNANO II) and picoeukaryotes (hereafter referred asPPK). Fig. 5b shows a typical Red fluorescence versusOrange fluorescence cytogram for Prochlorococcus andSynechococcus populations. Fig. 5c shows a Redfluorescence versus Green fluorescence cytogram forNano I, Nano II and PPK clusters.

3.3.1. Distribution of heterotrophic prokaryotesFig. 6a displays the vertical distribution of HNA2

along 18°W. Abundances did not exceed 0.5×105 cellscm−3, except in the southern area (40°N–40.5°N) wherean increase in surface HNA2 abundance was observed.Values inside A1 reached 0.1×105 cells cm−3 in surfacewaters and increased to over 0.2×105 cells cm−3 below150 m depth. Note the inversion of both distributionpatterns, abundances of HNA2 being higher in surfacewaters at 40°N and below the 1% of light penetrationzone in the north. Maximal values reached 0.5×105

cells cm−3 in the first 30 m depth of the southern area(40°N–40.5°N; st. 34 and 33). This area is coinciden-tal with a secondary anticyclonic eddy A31 whose struc-ture is less well known but seemed to be active during ourstudy (LeCann et al., 2005). The observed HNA2abundance in deeper layers was 0.2×105 cells cm−3.

The abundance of HNA1 ranged from 2.5 to4.0×105 cells cm−3 in the first 100 m of the watercolumn (Fig. 6b). However, abundances in deeper layerswere lower at the northern area and particularly insideeddy A1. At station 26, values reached only 1.5×105

cells cm−3 below 150 m, which represents a decrease ofabout 50% compared to the station located at 40.5°N.

LNA distribution in the upper 100 m (Fig. 6c)showed a progression from lower values south of 43°N(Fig. 6c; 4×105 cells cm−3) to higher values inside eddyA1 (6×105 cells cm−3). In deeper layers, values werehigher in the domain of A1 (i.e. between 42°N and

Fig. 6. Vertical distributions of heterotrophic prokaryote clusters along transect 18°W: a) HNA2, b) HNA1 and c) LNA. The location of eddy A1 islabelled.

53C. Fernández et al. / Journal of Marine Systems 71 (2008) 46–62

44.5°N). As seen in Tchl a distribution (Fig. 4), isolinesbecame 50 m shallower than in southern stations andvalues at 150 m depth reached 2.5×103 cells cm−3

inside A1 and 3.5×103 cells cm−3 in the south.Comparison between stations inside (st 26–27) and

outside (st 51–52–53) the eddy domain showeddifferences in the distribution of heterotrophic prokar-yotes (Fig. 7a). The Kruskal–Wallis non parametric testwas used to make evidence of differences betweenstations. HNA2 abundances inside A1 (st 26–27) weremuch higher in the first 600 m than at stations located at19.5°W (st 51–52–53; pb0.001). Differences in thedistribution of HNA1 showed less clear trends. Still,station 26 at 18°W (centre of A1) showed the highest

abundances in the upper 50 m within northern stationsof the 18°W transect (Fig. 7b), with significantdifferences only with st 29 (pb0.05) and showedthe lowest HNA1 values under 50 m compared to st25, 27 and st 28 (pb0.05). The distribution of LNAshowed no clear difference compared to 19.5°W stations(Fig. 7c).

3.3.2. Integrated heterotrophic prokaryote abundancesand mesoscale variability

Heterotrophic prokaryote abundances were integrat-ed between the surface and 300 m, using the trapezoidalmethod and superimposed to geostrophic currentsdetermined as described in Section 2.1.

Fig. 7. Vertical profiles of heterotrophic prokaryote abundances at stations located in the domain of eddy A1 (43°N–44.5°N; 18°W) and stationslocated outside of the influence of A1 (43°N–44.5°N; 19.5°W). a) and b) HNA2 cluster, c and d) HNA1 cluster, e and f) LNA cluster. Stations atcommon latitudes were identified with identical symbols.

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Integrated LNA abundances (Fig. 8a) were distrib-uted in a patchy way and appeared affected bymesoscale features. High integrated abundances wereobserved specifically in the north-eastern border ofanticyclonic eddy A1 (N10×1013 cells m−2), as well asin its core (8×1013 cells m−2). Maximal integratedvalues were close to 14×1013 cells m−2 at the north-western border of the eddy A1. LNAwere also abundantin highly hydrodynamic zones of the grid, like the

zone located between 42°N and 44°N, along 20°W.HNA1 integrated abundances (Fig. 8b) were generallylower than LNA integrated abundances, and werehomogeneously distributed around 7×1013 cells m−2

in the entire study area. Exceptions to this areillustrated by minimal values (b3×1013 cells m−2)along transect 17.5°W and particularly inside eddy A1and a cyclonic eddy (C32) located southward. Maximalvalues, on the other hand, reached 11 to 12×1013 cells

Fig. 8. Distribution of heterotrophic prokaryotes coupled with geostrophic fields for late summer in the POMME area.

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m−2 in the north-western and south-western border ofboth eddies, respectively. It is interesting to note thatthis distribution reminds the distribution of LNA.Indeed, as already seen for LNA, the area between42°N and 44°N, following transect 20°W, showed highheterotrophic prokaryote abundances (∼10×1013 cellsm−2).

The HNA2 community was generally less abundantthan HNA1 and LNA (Fig. 8c). This group was notobserved at all stations (stations where HNA2 wereabsent were assigned 0 in the grid in order to perform thespatial interpolation in Fig. 8c) and was mainlydistributed in the eastern part of the study area while itwas virtually absent of the western area. High integratedabundances were observed particularly in the southernand eastern border of eddy A1 (∼1×1013 cells m−2)as well as in its core (∼0.5×1013 cells m−2). Yetmaximal values were observed south of 42°N and eastof 18°W.

3.3.3. Cyanobacteria and ultraphytoplanktondistribution

The vertical distribution of Prochlorococcus along18°W showed high spatial variability (Fig. 9a). Cells

Fig. 9. Vertical distribution of cyanobacteria along transect 18°W. a) Prlabelled.

were mainly distributed in the first 150 m depthand higher average values were generally observed at50–75 m depth (30×103 cells cm−3), coinciding with thebottom of Ze. An increase in cell abundance was observedinside eddyA1. At stations 26 and 25 (centre and northernborder, respectively), Prochlorococcus concentrationsranged between 40 and 60×103 cells cm−3.

Synechococcus abundance along 18°W was lowerthan that of Prochlorococcus (Fig. 9b). Between 39°Nand 42°N, values were homogeneously distributedaround 2.5×103 cells cm−3. Higher values were ob-served from 42.5 to 44.5°N, ranging between 5 and7.5×103 cells cm−3 in the first 100 m depth. Maximalvalues were observed inside eddy A1, and particularly atits centre (st. 26) reaching 12.5×103 cells cm−3 at 50 mdepth.

Abundances of NANO I were higher than 0.25×103

cells cm−3 in the first 150 m (Fig. 10a). Between 40°Nand 43°N, abundances varied between 0.75 and 1×103

cells cm−3 in the upper 100 m while a subsurfacemaximum (1.25×103 cells cm−3) was detected at st. 31(41.5°N). Abundances of NANO I inside eddy A1 werehomogeneously distributed around 0.75×103 cellscm−3 in the first 100 m depth.

ochlorococcus and b) Synechococcus. The location of eddy A1 is

Fig. 10. Vertical distribution of autotrophic cell groups along transect 18°W. a) Nanoeukaryote I, b) Nanoeukaryote II and c) Picoeukaryote. Thelocation of eddy A1 is labelled.

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The distribution of NANO II was characterized bylower abundance values compared to NANO I (Fig. 10b).Abundances in the first 100 m were generally close to0.2×103 cells cm−3, including stations inside eddy A1and reached 0.3×103 cells cm−3 at its southern border.

PPK was essentially present in the upper 200 m andisolines of vertical distribution were 50 m shallower inthe northern part of the 18°W transect compared to thesouthern section (Fig. 10c). Note that the isoline 3×103

cells cm−3 follows the distribution of Ze (shallowerdepth in northern stations). Maximal values (up to4×103 cells cm−3) were observed north of 42.5°N, andparticularly within the domain of eddy A1. In its centre,

a subsurface maximum was detected near 50 m depth,i.e. near the bottom of the euphotic layer and out ofreach of vertical mixing.

Abundances of autotrophic organisms were inte-grated over Ze and plotted against the depth of theeuphotic layer (Fig. 11). No relationship was foundbetween the depth of the euphotic layer and theabundance of Prochlorococcus, PPK and NPK, thesum of NANO I and NANO II. However, Synecho-coccus showed an inverse relationship (R2 =0.705;pb0.05) with Ze, expressing lower integrate values fordeeper euphotic layers. In the case of Prochlorococcus,a large scatter in data does not suggest light

Fig. 12. Latitudinal variation of cluster contributions to FCM carbonbiomass (%). a) contributions of all clusters to total FCM carbonbiomass. b) contributions of autotrophic clusters to total autotrophicFCM carbon biomass.

Fig. 11. Integrated cell abundance of autotrophic organisms plottedagainst the depth of the euphotic layer (Ze, m). The main axis refers toProchlorococcus (Proch), Picoeukaryotes (PPK) and Nanoeukaryotes(NPK). The secondary axis refers to Synechococcus (Synecho) as wellas the trend line and equation.

58 C. Fernández et al. / Journal of Marine Systems 71 (2008) 46–62

dependency, while small size ultraphytoplanktonshowed a constant trend with variable Ze.

3.4. FCM heterotrophic and autotrophic carbonbiomass

In order to understand the importance of differenttrophic groups within the domain of A1 compared tosurrounding waters, we estimated the contribution ofFCM autotrophic and heterotrophic carbon along 18°W.To do so, abundances of the FCM resolved clusters wereconverted in carbon biomass (total FCM carbon) asdescribed in Materials and methods.

The contributions of heterotrophic prokaryotes tototal FCM carbon along transect 18°W (Fig. 12a)increased toward northern stations. Maximal values(77%) were obtained at 42.5°N. Values at borderstations reached 65 to 74% (north and southern bordersrespectively). The contribution of Prochlorococcus tototal FCM carbon (Fig. 12a) was close to 5% at southernstations and decreased to 3% in the north, with theexception of stations 26 and 25 (44 and 44.5°N), bothlocated inside A1, where Prochlorococcus contributionto total FCM carbon reached the highest values (7%).Synechococcus (Fig. 12a) showed lower abundancethan Prochlorococcus in southern stations, where itscontribution to total FCM carbon did not exceed 3%.However in the northern part of the transect, valuesincreased and ranged between 4.7% at 43.5°N to amaximal contribution of 8.9% at the centre of eddy A1that even exceeded the contribution of Prochlorococ-

cus. The contribution of PPK (Fig. 12a) to total FCMcarbon was homogeneously distributed and rangedbetween 4% and 8.7%, reaching 8.3% at 44°N, whichis close to the Synechococcus contribution. Thecontribution of NPK (Fig. 12a) oscillated between12% and 13% south of 42°N and reached a maximalvalue of 24% at 41.5°N. Inside the domain of A1,NANO I and NANO II contributed with 13 to 14% tototal FCM carbon, exceeding the contribution of allother studied autotrophic communities.

Inside eddy A1 (Fig. 12b), the higher contribution ofthe photosynthetic community to FCM autotrophiccarbon came from NANO I and NANO II (30% to50%). The contribution of PPK to FCM autotrophiccarbon inside A1 amounted 20% to 30%. Synechococ-cus represented between 15% and 25% of “FCMautotrophic carbon, exceeding the contribution of Pro-chlorococcus, (10% and 20% of FCM autotrophiccarbon). At 44°N, the contribution to FCM autotrophiccarbon was dominated by NPK (38%), followed by Sy-nechococcus (22%) and PPK (21%) and finally byProchlorococcus (17%). Outside the influence of eddyA1, the contributions of PPK and Prochlorococcusincreased, while that of Synechococcus andNPK showeda decrease of at least 30%.

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4. Discussion

4.1. General considerations

The study of transect 18°W (Fig. 1) aimed at givingsome insights on the biological features of an anticy-clonic SWODDY-type structure. The sampling of eddyA1 was restricted to a few stations and a short period oftime, which nevertheless provides a biological snapshotof the standing stocks of nutrients and planktonicorganisms for a cold-core eddy, at least 10 months old atthe time of sampling. A second anticyclonic structurewas encountered at 40.5°N. In spite of presentingsimilar hydrological properties to A1, it showeddifferent biological characteristics.

Fernández et al. (2005a), described eddy A1 as aSWODDY-like eddy that exhibited an excess of nitrateof the order of 1 μM compared to surrounding waters,in the euphotic zone belonging to the inner lens. At itscentre, nitrate surface concentrations reached 0.1 μMwhile surrounding surface waters were nutrient deplet-ed. Although a general deficit in silica was observed inthe area, eddy A1 also showed an accumulation of Si(OH)4 inside its domain (2.5 to 3.5 μM; i.e. 0.5 μM ofsilica excess compared to the southern area). In spite ofa local excess in Si(OH)4, diatoms are supposed to onlycontribute 3% to the observed seasonal primaryproduction (Leblanc et al., 2005). Nevertheless, thepersistence of diatom-based activity supports the ideaof an isolated active biological system within eddy A1with sustained nutrient assimilation that resulted inintegrated primary production rates comparable withwinter rates (Fernández et al., 2005b). Concerningnitrogen recycling, nitrite (an intermediate step ofnitrification) along transect 18°W showed high con-centrations inside eddy A1, which coincided with thehighest abundances of prokaryotes suggesting intensenitrification (resulting in the observed excess of avail-able NO3

−).

4.2. Distribution of heterotrophic prokaryotes

During the spring season 2001, the highest concen-tration of heterotrophic prokaryotes was observed in thenorthern area (Thyssen et al., 2005), reaching in average5.8±2.9×105 cells m−3 in the first 300 m (Fig. 6).During late summer (POMME 3), values of HNA1 andLNA in the entire study area were almost equivalent tothose observed earlier in the year. The highestconcentration of heterotrophic prokaryotes was alsoobserved in the northern area, although the proportionsof the different clusters did vary. In contrast, HNA2

were more abundant during POMME 3 and particularlyat the centre of eddy A1.

Concerning the relationship between heterotrophicprokaryote abundances and mesoscale processes, Thys-sen et al. (2005) described a link between LNA dis-tribution and the occurrence of mesoscale features,locating its predominance below 100 m. In our study,both LNA and HNA1 abundances followed isopycnalentrainment generated by eddy A1 in the upper 300 m.Inside A1, this entrainment resulted in a 50 m shallow-ing of isolines in HNA1 abundances and also a 100 mshallowing in the distribution of LNA isolines (Fig. 6),with higher abundances inside anticyclonic structures(A1 and A31) compared to westward stations at 19.5°W(Fig. 7). This suggest that the proportion of activeheterotrophic prokaryotes (High Nucleic Acid, seeGasol et al., 1999) increased within eddies A1 andA31, and resulted in enhanced intra-eddy heterotrophicprokaryote activity.

Temperature dependence (Hoppe et al., 2002) as well asincreased nutrient availability compared to general oligo-trophic conditions in the rest of the study area (Mémeryet al., 2005) may help to explain this phenomenon.

Diel variations of heterotrophic prokaryote abun-dance in the upper layer of the water column (Kuiperset al., 2000) cannot be responsible for the observedtrends in our data. During the 60 h period that took thesampling of the 18°W transect, no diel changes wereobserved and high abundances were obtained at earlymorning stations (6 to 7 am) as well as at noon (11 am to13 pm) and night stations (2:35 am).

4.3. Ultraphytoplankton distribution

On a spatial scale, upward and downward transport ofwaterwithin and betweenmesoscale hydrographic featuresin combinationwith advection can have a substantial effecton plankton development. In that context, the springobservations of Karrasch et al. (1996) indicated that themain evolution of the bloom does not follow ahomogeneous development, but is rather formed by apatchwork representing different bloom stages (initiating,culminating and declining) within mesoscale features.These authors linked an anticyclonic structure to aheterotrophic regime, while a cyclonic one was associatedto an autotrophic status. In our study, the presence of Sy-nechococcus, Prochlorococcus, high abundance of smallsize ultraphytoplankton and high Tchl a and nutrients(including silica) in the domain of the anticyclonic eddyA1 (Figs. 9 and 10) seem to follow previous observationsof a general autotrophic–heterotrophic balance in thetrophic status of the POMME area during late summer

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(parallel measurements of net community production andrespiration; Maixandeau et al., 2005).

Our study confirms previous observations of Pro-chlorococcus south of 44°N (Tarran et al., 2001), inspite of classical statements based on temperatureconstraints (SST limit set at 40°N; Partensky et al.,1999a,b; Li and Harrison, 2001) to the development ofsuch organisms. Moreover, Prochlorococcus was dis-tributed deeper than Synechococcus, confirming previ-ous observations of different light adaptation (Partenskyet al., 1999a). We also confirmed previous observationsof Buck et al. (1996) showing that heterotrophicprokaryote biomass can outweigh FCM autotrophicbiomass in the area 25°N–45°N.

4.4. Biological features within an anticyclonic eddystructure

During 2001, biological production within A1 wassustained longer in the year compared to the southern areaand biological consumption of still-available nutrientsremained active during the theoretical oligotrophic season(Fernández et al., 2005b). Based on the gathered data aswell as previous analyses (e.g. Fernández et al., 2005a,b),the general picture for late 2001 in the domain of eddy A1could be therefore described as follows. Autotrophicabundance was sustained during the entire year within thestudy area (Fernández et al., 2005b) as well as within thedomain of eddy A1. After the seasonal shallowing of themixed layer, a bloom of ultraphytoplanktonic organisms(Pseudo-nitzschia) in the then nutrient-rich waters of theeddy ensued as reported byLeblanc et al. (2005). Isolationof the eddy was high and exchange with surroundingwaters minimal (LeCann et al., 2005). Later in the year,silica drawdown reinforced the shift from a diatom-dominated population in spring (N10 μm, Leblanc et al.,2005) to a community dominated by small size cells inlate summer, typical of a regenerative environment.Nevertheless, TChla concentrations in the domain of A1(Fig. 4) remained higher than in surrounding waters,which combined with detectable levels of silica (Fig. 3),suggest a constant fraction of large phytoplankton inbiological production. Higher numbers of heterotrophicprokaryotes in the surface layer of the eddy's corecompared to surrounding waters suggest that duringPOMME 3 the community remained active, supportingregenerative processes that maintained a significantfraction of nutrient cycling inside the lens of eddies A1(NO3, possibly derived from nitrification) and A31 (highnitrite concentration that matched with high levels ofheterotrophic prokaryotes abundance compared withsurrounding waters).

It is known that Prochlorococcus can competesuccessfully in low light levels (Moore et al., 1995;DuRand et al., 2001) and cannot use nitrate for growthin culture (Rippka et al., 2000). This is consistent withthe lack of relationship between Prochlorococcusabundance and the depth of the euphotic layer observedin our study (Fig. 11). In addition, Synechococcus canuse nitrate as a nitrogen source and has been describedas having physiological advantages with respect toProchlorococcus under P limitation in the Mediterra-nean Sea (Moutin et al., 2002). However, Prochloro-coccus in the NE Atlantic showed the highestabundances with increasing nitrate concentration, espe-cially inside A1. More over, in spite of its high abun-dances inside the domain of eddyA1, Synechococcuswaslargely outnumbered by Prochlorococcus. The use oforganic forms of N and P may be an advantage to explainthe high levels of Prochlorococcus concentration withrespect to other groups. The inverse relationship of Sy-nechococcus with Ze may also explain this phenomenonby pointing towards light and temperature as moresuitable control factors than nutrient availability forthese microorganisms.

5. Conclusions

The community structure of eddy A1 evolvedsignificantly during 2001, but remained isolated fromsurrounding waters. Autotrophic activity was extremelyhigh during late spring and continued to be significant inlate summer, with high cyanobacteria abundances.Abiotic conditions inside eddy A1 also enhanced hete-rotrophic prokaryote activity during the year. Indeed,HNA2 were particularly abundant in the core of the eddy,which indicates optimum conditions for heterotrophicprokaryote development during late summer.

Inside A1, ultraphytoplankton was responding not toisopycnal entrainment as heterotrophic prokaryotescould have been (Thyssen et al., 2005), but to tem-perature and light levels. It also responded to substrateavailability for which it partly relied on heterotrophicprokaryote activity. Although microbial communitystructure inside A1 kept unusual characteristics duringthe late summer season, one have to keep in mind thatthe biological characteristics of an eddy are likely tochange at different rates compared to the physical andchemical conservative properties (Lochte and Pfann-kuche, 1987).

Finally, although new highlights concerning thebiogeochemical and microbial environment associatedto mesoscale anticyclonic features have been provided byour study, hydrographic structures such as anticyclonic

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mode water eddies cannot be fully characterized on thebasis of their microbial community structure.

Acknowledgements

We thank the captain and crew of R/V Thalassa foroutstanding assistance during the POMME 3 cruise. S.Newman collected the seawater samples during thecruise. The POMME program was funded by PATOM/PROOF (CNRS, INSU FRANCE). C. Fernández I.acknowledges the support of a CNRS fellowship.

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