Deep Sea Research I

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Deep-Sea Research I ] (]]]]) ]]]–]]] ARTICLE IN PRESS Corresponding author. Tel.: ; fax: . 0967-0637/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2005.06.004 E-mail address: (V. Riandey). 1Present address: UR 098, IRD, Centre de Bel Air, BP 1386, 18524 Dakar, Senegal. Abstract This work represents a first step in understanding the impact of hydrodynamic features on the zooplankton dynamics in the Algerian Basin (southwestern Mediterranean Sea). The mesoscale distribution of mesozooplankton abundance, biomass, specific composition and size structure was investigated during ELISA-1 campaign (1997) in the framework of the program ELISA (Eddies and Leddies Interdisciplinary Study off Algeria, 1997–1998), partly dedicated to study the mesoscale features during 1997. Physical, biogeochemical and biological measurements were made on transects through two hydrodynamic features, one anticyclonic eddy and a small secondary shear cyclonic eddy. The use of combined zooplankton descriptors, i.e. biomass, abundance, size structure (e.g. NB-SS slope) and taxonomic structure, and of cumulative function allowed us to extract spatial trends in the anticyclonic eddy (AE 96-1).

  • mesoscale dynamical

  • distance between successive

  • eddies

  • station

  • mediterranean sea

  • algerian current

  • western mediterranean

  • ctd-rosette

  • created between

  • size structures


Publié le : mardi 19 juin 2012
Lecture(s) : 54
Source : com.univ-mrs.fr
Nombre de pages : 20
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ARTICLE IN PRESS
Deep-SeaResearchI ](]]]]) ]]]–]]]
www.elsevier.com/locate/dsr
Zooplanktondistributionrelatedtothehydrodynamic
featuresintheAlgerianBasin(westernMediterraneanSea)
insummer1997
a, a,1 aVirginieRiandey ,GiseleChampalbert ,Franc -oisCarlotti ,
b cIsabelleTaupier-Letage ,DelphineThibault-Botha
aLaboratoire d’Oce ´anographie et de Bioge ´ochimie, UMR 6535 CNRS/Universite ´ de la Me ´diterrane ´e,
OSU/centre d’Oce ´anologie de Marseille, Station marine d’Endoume, rue de la batterie des lions, F-13007, France
bLaboratoire d’Oce ´anographie et de Bioge ´ochimie, UMR 6535 CNRS/Universite ´ de la Me ´diterrane ´e, d’Oce ´anologie de Marseille, Antenne de Toulon, c/o IFREMER, BP 330, F-83507 La Seyne, France
cLaboratoire d’Oce ´anographie et de Bioge ´ochimie, UMR 6535 CNRS/Universite ´ de la Me ´diterrane ´e,
OSU/centre d’Oce ´anologie de Marseille, Campus de Luminy, Case 901, F-13288 Marseille Cedex 09, France
Received10December2004;receivedinrevisedform6June2005;accepted15June2005
Abstract
Thisworkrepresentsafirststepinunderstandingtheimpactofhydrodynamicfeaturesonthezooplanktondynamics
intheAlgerianBasin(southwesternMediterraneanSea).Themesoscaledistributionofmesozooplanktonabundance,
biomass,specificcompositionandsizestructurewasinvestigatedduringELISA-1campaign(1997)intheframeworkof
theprogramELISA(EddiesandLeddiesInterdisciplinaryStudyoffAlgeria,1997–1998),partlydedicatedtostudythe
mesoscalefeaturesduring1997.Physical,biogeochemicalandbiologicalmeasurementsweremadeontransectsthrough
two hydrodynamic features, one anticyclonic eddy and a small secondary shearcyclonic eddy. The use of combined
zooplankton descriptors, i.e. biomass, abundance, size structure (e.g. NB-SS slope) and taxonomic structure, and of
cumulativefunctionallowedustoextractspatialtrendsintheanticycloniceddy(AE96-1).Itishypothesizedherethat
theeasternedgeofAE96-1,characterizedbythicklayerofchlorophyll(between100and150m)duetothedownward
entrainment of chlorophyll down to 200m, was favorable for small organisms (Paracalanus/Clausocalanus,
Calocalanus, and Calanus) while higher abundance of large active swimmer such as chaetognaths was observed in
thecenter.Inthecycloniceddy,thehighestoffilter-feeders(ostracods,cladocerans,doliolidsandsalps)was
3related to enhance trophic conditions, i.e. highest chlorophyll concentration (4–8mgm ).Theseresultsshowthat
cyclonicandanticycloniceddiesstronglyinfluencethemesoscalecharacteristicsofzooplanktonintheAlgerianbasin
Correspondingauthor.Tel.:+33491041657;fax:+33491041635.
E-mail address:riandey@com.univ-mrs.fr(V.Riandey).
1PresentUR098,IRD,CentredeBelAir,BP1386,18524Dakar,Senegal.
0967-0637/$-seefrontmatterr2005ElsevierLtd.Allrightsreserved.
doi:10.1016/j.dsr.2005.06.004
`´´ARTICLE IN PRESS
2 V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]
duringsummer.Higherchlorophyllconcentrationsobservedinspringsuggestthatsucheddiescanhaveanevenmore
pronouncedimpactonthestructuringoftheecosystemduringhighproductiveseason.
r2005ElsevierLtd.Allrightsreserved.
Keywords: Mesoscale eddies; Zooplankton; Size distribution; Optical plankton counter; Pelagic environment; Cumulative sums
method;MediterraneanSea;AlgerianBasin
1. Introduction filamentsandsubmesoscale cyclonicsheareddies.
Suchsheareddieshavediametersofafewtensof
Numerousstudiesshowtheimpactofmesoscale kilometers, vertical extents of 100m, and life-
physical oceanic processes such as fronts, rings timesofafewweeks.TheAEcorrespondtolarge,
andeddiesonplanktonicecosystems(Owen,1981; highly oligotrophic areas during the summer-
LegendreandDemers,1984;WishnerandAllison, stratified conditions, because their anticyclonic
1986; Huntley et al., 1995; Huskin et al., 2001). structure maintains the nutricline at depths close
Studies based onthe warm-corerings intheGulf tothatoftheeuphoticzoneinthecentralpartof
Streamhighlightedthatzooplanktonbiomasswas the AE. On the other hand, the cyclonic plumes
higherintheircenters(Wiebeetal.,1985;Cowles sometimes associated with AE and shear eddies
et al., 1987). Fronts (Seguin et al., 1993, 1994; correspondtoricher,ephemeralspotsduetotheir
Thibaultetal.,1994;YoussaraandGaudy,2001) cyclonic doming structure (Moran et al., 2001;
and eddies (Pinca and Dallot, 1995; Pakhomov Taupier-Letageetal.,2003).
and Perissinotto, 1997) have been also known to The ELISA operation (Eddies and Leddies
enhance zooplankton biomass and to influence Interdisciplinary Study off Algeria, 1997–1998—
zooplanktonhorizontaldistribution. see Taupier-Letage et al., 2000, part of the
The Algerian Basin is a place where the MATER /MAST3-MTP2 European program)
mesoscale dynamical activity is intense. The was dedicated to the study of: (i) the general
instability of the Algerian Current generates circulation of the water masses, (ii) the origin,
meanders enclosing anticyclonic eddies (for a structure and trajectories of the eddies, (iii) the
reviewsee Millot and Taupier-Letage, 2005a), biological response associated with the mesoscale
which propagate downstream (i.e. eastward) at a dynamic phenomena, and (iv) the biological
fewkilometerperday.Attheentranceofthe consequences of the mesoscale dynamics for the
Channel of Sardinia the bathymetry narrows and functioning of the Algerian Basin. Throughout
shallows, so that the deeper and larger eddies that experimental year, 4 main campaigns were
cannot go through. They are guided by the organizedinordertotracktwomainAE(96-1and
bathymetry northward along the western Sardi- 97-1)andtosamplethematdifferenttimesofthe
nian slope, and thus detach from their parent year. Their general hydrodynamic characteristics
current.Thentheyfollowacounter-clockwise and associated chlorophyll, and nitrate distribu-
circuit in the eastern part of the Algerian basin, tionsarepresentedinTaupier-Letageetal.(2003).
possibly completing up to three loops. The Mesoscale hydrodynamic features play an im-
Algerian Eddies (hereafter AE) are 50–250km in portant role on the structuring of the organic
diameter, can have a vertical extent down to the matter compartment in oceanic regions. The
bottom (3000m; Ruiz et al., 2002; Millot and impact of such structures is clearly shown on
Taupier-Letage,2005b),andappeartolastupto3 phytoplanktonbyremotesensing,butthestudyof
years (Puillat et al., 2002). When returning in the their influence on the zooplankton compartment
Algerian coastal zone (CZ) AE interact with the still need to be assessed through mesoscalen Current, and the intense shear created sampling. Here, the distribution of zooplankton
between the eastward Algerian Current and the related to mesoscale hydrodynamic features was
westward current south of the eddy generates investigatedforthefirsttimeintheAlgerianBasinARTICLE IN PRESS
V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 3
1during the cruise ELISA-1 (summer 1997), using 200m to the surface at 1ms . No flowmeters
both microscope counts and bench-top optical were available but special care was taken while
planktoncounter(OPC).ELISA-1focusedontwo sampling to keep the cable vertical. Intervals
features, the (anticyclonic) eddy AE 96-1 and its between stations ranged between 2 and 13.5
associated(cyclonic)sheareddyC.Moststudiesof nautical miles (Table 1). Samples were preserved
zooplanktonresponsetophysicalchangeslookat directly upon collection (5% buffered formalin
a couple of indexes, i.e. biomass and abundance. solution) and quantitatively split with a Motoda
Inthispaper,weproposedanintegratedapproach boxoncebackinthelaboratory.
by considering together zooplankton biomass,
abundance, population structure (e.g. slope of 2.2. Processing of zooplankton samples: abundance
normalized biomass size spectra (NB-SS)) and and biomass
taxonomicstructure.
Mesozooplankton community characteristics
weredescribedfollowingtwodifferentapproaches,
2. Methods i.e. studies based on traditional taxonomic deter-
minations and size structures. Taxonomic
2.1. Sampling mination was done using a dissecting microscope
(Leica); three subsamples were counted per net
ThesamplingstrategyduringtheELISAexperi- and at least 600 individuals were enumerated per
ment was based on the near real time analysis of subsamples. Identifications were done for cope-
satellite thermal images, which were received on pods to genus level and to taxa level for other
board the RV Le Suroıˆt in order to locate the majorzooplankton(e.g.chaetognaths,ostracods).
mesoscale features. Then transect locations were Microscopecounts(MC)werenotdoneforstation
determined and run using a fine sampling interval 110duetohandlingproblems.Sizestructureofthe
o5 nautical miles (10km) for the CTD stations. same subsamples was studied using a bench-top
The ELISA-1 3rd leg (23/07–04/08/1997, in the versionoftheOPC-1L.
region 37–401N and 4–91E) was dedicated to the TheOPCsetupwassimilartotheonedescribed
time-demanding biological and biogeochemical by Beaulieu et al. (1999). Organisms were gently
sampling(Fig.1andTable1).DuringtheELISA- introduced into the water circulation system. To
1 cruise, the AE 96-1 (diameter 150km) was avoidcoincidence,weimposedamaximumcount
1
sampled along its southern (‘‘S-96-1’’, stations rateat20particlesmin andaconstantflowrate
1
69–82) and eastern parts (‘‘E-96-1’’, stations at18Lmin .Theshapeofthesizespectrumwas
100–121). In the CZ (transect ‘‘CZ’’, stations obtained by counting at least 1000 particles
82–92)SSEofAE96-1,theshearwasintense,and (Sourisseau, 2002). The OPC provides several
resultedinthecreationofthesmallcycloniceddyC, characteristics of the zooplankton community. It
sampledalongthetransect‘‘C’’(stations93–99). gives an estimate of the zooplankton density
3Continuous current speed and direction, in the (indm ),aswellasameasureofthecross-section
upper300–350mwaterlayer,wereobtainedwitha of the individual (digital size) which is converted
shipboard ADCP. Hydrological and chlorophyll intoequivalentsphericaldiameter(ESD)following
fluorescence profiles from the upper 1000m were a semi-empirical formula (Focal Technologies
recorded at each station using a CTD-rosette. Inc.,1997).Biovolumeswerecalculatedbyassim-
Time permitting, additional CTD casts down to ilating each particle to a sphere; particle biovo-
250m were interleaved (an exhaustive list of lumes were summed and then normalized to the
observations carried out during the cruise is volume of water filtered in situ. In order to
available on the ELISA web site; Taupier-Letage estimate biomass of a sample from OPC biovo-
etal.,2000). lumes the following relationship was used log
Zooplankton was sampled using a WP-2 net (W)¼0.87log (BV)0.89 (N¼187, r¼0:83,
3fittedwith200mmmesh-sizetowedverticallyfrom po0:01)with Wdryweight(mgDWm )andBVARTICLE IN PRESS
4 V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]
Fig. 1. Location and characterization of the sampling area. (a) Detail of the zooplankton sampling: stations related to AE 96-1:
southerntransect(stations69–82)andeasterntransect(stations100–119);totheshearcycloniceddyC(stations93–99)andtocoastal
zone(CZ)(stations83–92).(b)Seasurfacetemperatureimage(NOAA/AVHRR)on27July1997(thetemperatureincreasesfromlight
todarkgray),withthesurfacecurrent(fromADCP)overlaid.ARTICLE IN PRESS
V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 5
Table1
Stationcharacteristics(date,samplingtime,latitude,longitude,stationID,anddistancebetweensuccessivestations),zooplanktonabundance(microscopicandOPC
counts)andbiomassranges(287–500,500–1000,1000–2000,42000mmandtotal)
3 3
Date Sampling Longitude Latitude Stations Distance Abundance(indm ) OPC-derivedbiomass(mgDWm )
time
(GMT) (1N) (1E) (Nautical Microscopic OPC 287–500mm 500–1000mm 1000–2000mm 42000mm Total
miles) counts counts
Cycloniceddy(C)
28.07.97 17:15 37109 03155 99 0.0 467 267 0.4 2.2 4.6 2.1 9.3
29.07.97 14:00 37113 03158 98 4.7 520 312 0.6 2.0 2.4 0.8 5.8 7:35 37116 04100 97 4.6 243 150 0.3 1.2 2.4 3.1 7.0
29.07.97 19:00 37125 04110 93 12.1 618 269 0.5 1.9 2.3 1.3 6.0
CoastalZone(CZ)
28.07.97 14:00 37108 04150 92 0.0 270 212 0.4 1.6 1.6 0.4 4.0 7:20 37104 05106 90 13.5 423 270 0.5 1.7 2.6 1.6 6.4
28.07.97 2:00 37103 05112 89 4.9 470 343 0.6 2.4 3.0 3.6 9.6
27.07.97 19:00 37100 05124 86 10.3 436 245 0.4 1.7 2.0 1.3 5.4 14:30 37103 05137 83 11.0 301 156 0.3 1.2 3.0 3.1 7.6
Southerntransect(S-96-1)
27.07.97 11:00 37105 05152 82 0.0 425 208 0.4 1.4 1.9 1.5 5.2 7:30 37107 05152 81 2.2 313 227 0.4 1.7 2.3 2.0 6.4
27.07.97 1:55 37110 05152 80 3.0 496 399 0.6 2.9 4.2 7.8 15.5 1:00 37112 05152 79 2.0 484 364 0.6 2.6 4.0 3.0 10.2
26.07.97 13:30 37119 05152 77 7.5 36 28 0.1 0.3 0.5 1.2 2.1 10:10 37124 05151 76 5.2 170 114 0.2 0.9 1.2 1.4 3.7
26.07.97 1:30 37134 05151 75 10.1 288 186 0.3 1.4 2.1 3.4 7.2
25.07.97 19:30 37144 05152 74 10.0 213 140 0.3 1.1 1.5 1.3 4.2 14:22 37155 05152 73 10.0 196 68 0.1 0.6 1.5 3.3 5.5
25.07.97 8:00 38105 05152 72 11.0 337 248 0.4 1.8 3.1 4.1 9.4
24.07.97 17:50 38114 05152 69 9.0 360 297 0.3 1.9 1.8 1.7 5.7 20:15 38120 05152 70 5.0 274 205 0.5 2.2 3.1 2.0 7.8
Easterntransect(E-96-1)
30.07.97 7:25 38104 05152 100 0.0 467 266 0.5 2.0 2.8 1.2 6.5 13:54 38105 06101 101 6.3 220 170 0.3 1.2 2.3 2.1 5.9
30.07.97 19:20 38104 06110 103 9.1 511 312 0.5 2.6 3.3 2.6 9.0
31.07.97 1:00 38104 06119 105 9.7 383 245 0.5 1.6 2.7 16.6 21.4 7:45 38104 06128 107 7.2 88 58 0.1 0.5 0.9 2.3 3.8
31.07.97 14:05 38104 06138 109 8.0 469 256 0.5 1.8 1.9 2.1 6.3 19:20 38104 06142 110 3.3 — 157 0.3 1.2 2.6 2.6 6.7
01.08.97 1:15 38104 06147 112 4.2 377 138 0.2 1.1 3.8 12 17.1 7:55 38104 06153 114 7.7 527 330 0.6 2.2 2.3 1.1 6.2
01.08.97 14:40 38104 07101 115 5.9 216 199 0.4 1.4 1.1 0.3 3.2 19:30 38104 07106 117 4.9 844 493 0.9 2.6 3.5 1.2 8.2
01.08.97 1:00 38104 07112 119 4.9 411 273 0.5 1.8 2.4 29.9 34.6ARTICLE IN PRESS
6 V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]
3 3total biovolume (mm m ). This relation was lative function (CF) as defined by Ibanez et al.
based on values from different regions (Gulf of (1993) was applied on NB-SS slopes and on
LionsandGulfofGuinea)considering thewhole zooplankton species, genus or group abundance.
zooplanktoncommunity(Riandey,2005). The CF was only applied on data from the
Methodological bias exists when dealing with anticyclonic eddy transects (S-96-1 and E-96-1).
gelatinous zooplankton; on one hand their high Thecalculationconsistsofsubtractingareference
transparencywillunderestimatetheirsize,andon value (here the mean of each transect) from each
the other hand preservation with formalin solu- data. Then the differences are successively added
tion, although increasing their opacity, will sig- in order to form a CF (Ibanez et al., 1993). This
nificantly reduce their biovolume (Beaulieu et al., method will permit to buffer high frequency
1999).TheOPC-derivedbiomasswasdividedinto variations due to patchiness distribution for
ranges following the Joint Global Ocean Flux exampleandwillhighlightgeneraltrendsbetween
Study (JGOFS) mesozooplankton protocol successive samplings: a negative slope of the
(287–500, 500–1000, 1000–2000 and 42000mm). cumulatedfunctioncurveindicatesthatthevalues
IntheremainderofthetextOPC-derivedbiomass of the studied parameter are lower than their
willbereferredasbiomass. mean; a positive slope of the CF shows that the
values of the studied parameter are higher than
2.3. Analysis of zooplankton size-spectrum their mean. A large peak or dip shows a marked
anomalyonthecurveofcumulativesums.
FollowingPlattandDenman’s(1977,1978)size Inordertoinvestigatetemporalvariations(day,
spectra analysis, NB-SS were estimated by divid- sunset and night), One Way Anovas were
ing the biomass by the weight interval in each performedusingthestatisticalsoftwareSigmaStat.
weight class. For these analyses OPC-derived OneWayAnovawasalsousedtoexaminespatial
biomass was divided into 10 weight classes using variations (cyclonic eddy, CZ and AE 96-1) and
na geometric scale (a geometric 1.6 series) as student t-test was applied to look at spatial
performed by Blanco et al. (1994). A linear variations within the AE 96-1 (edges and other
regression was then applied to log transformed stations).2
NB-SS data; the slope of the regression will
characterize the size structure of the community.
The NB-SS slopes were calculated only on 3. Results
particles ranging from 287 to 1345mm (ESD) for
the upper limit. The lower limit of the OPC 3.1. Hydrological situation in the mesoscale
detectionwasfixedat287mm(formoredetailssee hydrodynamic features
Sourisseau,2002);andtheupperlimitat1345mm.
Thisupperlimitwaschoseninordertoeliminate Isopycnals clearly delimited the different dyna-
empty classes, i.e. no organisms being present in micalfeatures(Fig.3).Isopycnalsweredomingby
larger size classes in some samples (for more 40mineddyC,werehorizontalintheCZ,and
detailsseeSourisseau,2002). depressedby150minthecenterofAE96-1.As
The spectra/slope comparison (Fig. 2) shows shownbyADCPcurrent(Fig.1B),AE96-1edges
thatahighslope(withhighnegativevalue)reflects weremarkedbynearlyhorizontalisopycnalsinthe
a high relative abundance of small organisms, upper100mwestofstation117(easternedge)and
whilealowslopereflectsamoreevendistribution south of station 81 (southern edge). The chlor-
ofparticlesamongallclasses. ophyll distribution displayed a typical deep
chlorophyll maximum (DCM, Fig. 3) and was
2.4. Analytical treatments verypatchy.Maximumchlorophyllconcentrations
3(4–8mgm )wereobserved between70and80m
3In order to state relationships between size ineddyC.DCMconcentrationswereX1mgm
structureandtaxonomicalinformation,thecumu- in the CZ between 70 and 100m, but hardlyARTICLE IN PRESS
8 V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]
3Fig.3. Chlorophyllconcentrations(mgm )andisopycnalsdistribution(blacklines)acrosstheshearcycloniceddyC,theCZandAE
96-1(southernandeasterntransectunfolded)(modifiedfromTaupier-Letageetal.(2003)withpermissionofAmericanGeophysical
Union).
3.2. Temporal and spatial variations of zooplankton fortheeasternone(station117,Fig.4andTable
abundance and biomass 1), but no statistical temporal differences were
highlighted. It must be noted that zooplankton
3OPC counts varied from 27 to 494indm abundance were higher at the edges of AE 96-1
(Fig. 4, Table 1) and microscopic counts varied (stations 79, 80, 81, 82, 117 and 119) than at the
3from 35 to 844indm (Table 1). The overall otherstationsofAE96-1(po0:01, N¼24,Fig.4
spatial distribution of zooplankton density was andTable1).
similar when measured by OPC and microscopy. Total biomass ranged from 2.06 to
3This was highlighted by a significative regression 34.51mgDWm (Table 1). The spatial distribu-
between OPC and microscopic counts tion pattern of the 287–1000 and 500–1000mm
(OPC¼0.5538 MC+27.977, r¼0:90, N¼32, biomass showed the same trend that the one of
po0:01), leading to an underestimation of totalzooplanktonmicroscopiccounts(resultsnot
the number of organisms by the OPC (Fig. 5A, shown). On the contrary, it is clear from Fig. 6
Table1). thatorganismslargerthan1000mmhadinAE96-
Highest abundances were observed at night at 1 a pattern presenting high diel variation with
the southern most stations for the North–South lowerbiomassduring daytimeandsunset than at
transect(stations79and80,Table1)andatsunset night(N¼32, po0:001).Intheanticycloniceddy,ARTICLE IN PRESS
V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]] 9
3
Fig.4. Temporalandspatialdistributionofzooplanktonabundance(indm )estimatedbyopticalplanktoncounter(OPC).White
charactersonblackforegroundcorrespondtosunsetandnightstations.(C)Cycloniceddy,(AE96-1)anticycloniceddy.
3Y=X cycloniceddyatstation99(4.58mgDWm )and
600
at the southern most stations of the north–south
500 transect in the anticyclonic eddy (stations 79 and
380;4.23and3.96mgDWm ).400
300
3.3. Temporal and spatial variations of zooplankton
200 compositiony = 0.56x+27.67
r = 0.90100
Distribution patterns of the main genera/taxa
0 abundance are reported in Table 2. A total of 22
0 100 200 300 400 500 600 700 800 900
-3 taxa were identified among which 15 taxa belongMicroscopic counts (ind.m )
to the holoplankton and four to the meroplank-
3Fig.5. Relationshipbetweenmicroscopiccounts(indm )and ton.Thecopepodcommunitymadeup7276%of
3OPCcounts(indm ).
the total zooplankton and was composed of 28
genera/species. Paracalanus/Clausocalanus (no at-
nocleartemporalpatternwasobservedforsmaller tempt was made to separate these two genera)
organisms(287–500and500–1000mm). dominatedthezooplanktonpopulation(2377%).
Nostatisticalspatialvariationswereobservedin Oncaea, Oithona, Corycaeus and Calanus corre-
thedifferentbiomassranges.Weobserved,never- spondedto,respectively,1074%,973%,672%
theless, some trends (see Table 1). In the antic- and 572% of the zooplankton population.
yclonic eddy, the highest night total biomass was Chaetognaths, ostracods, and appendicularians
observed at the southern edge (station 79: represented, respectively, 974%, 773% and
3 310.11mgDWm ; 80: 15.62mgDWm ) and 372% of the total zooplankton and meroplank-
3eastern edge (station 119: 34.51mgDWm ). tonindividualsmadeuponlyaweakproportionof
The biomass of the following three size classes, the zooplankton community (271%). Nauplii
287–500, 500–1000 and 42000mm, followed the werenotquantitatively sampled byourcollecting
same trend (Table1).The highestbiomassvalues device,whichexplainsthelowpercentageobserved
forthe1000–2000mmfractionweredetectedinthe (0.670.6%).
-3
OPC counts (ind.m )ARTICLE IN PRESS
10 V. Riandey et al. / Deep-Sea Research I ] (]]]]) ]]]–]]]
3
Fig.6. Temporalandspatialdistributionofzooplanktonbiomass41000mm(mgDWm ).Whitecharactersonblackforeground
correspondtosunsetandnightstations.(C)Cycloniceddy,(AE96-1)anticycloniceddy.
The impact of sampling time on zooplankton 3.4. Size structure in relation to temporal and
species, genus or group abundance was investi- spatial variations of zooplankton
gatedintheanticycloniceddy(notenoughstations
weresampledatsunsetoratnightinthecyclonic Zooplankton NB-SS slopes values varied from
eddyandintheCZtoallowstatisticaltreatments). 0.95 to 0.42. Their distribution pattern fol-
Thistemporalvariationwasclearfor Calocalanus, lowed the general trend of microscopic counts
Pleurommama, ostracods and jellyfishes (Fig. 7). (Fig.9; r¼0:39, N¼32, p¼0:027).
Calocalanus were statistically more abundant at Weanalyzed,intheanticycloniceddyAE96-1,
sunsetandatnightthanduringdaylight(N¼32, the variation of the CF applied to hydrological
po0:02, Fig. 7A); Pleurommama abundance was parameters (28.75 isopycnal immersion and
significantly lower during the day than at night thickness of the water layer where concentration
3
and sunset (N¼32, po0:03, Fig. 7B); ostracods in chlorophyll a was higher than 0.25mgm ),
abundance was statistically higher at night and zooplankton OPC abundance, NB-SS slopes
at sunset than during the day (N¼32, po0:02, and main zooplankton taxa/genus. The whole
Fig. 7C) and jellyfish abundance at sunset was sampling dataset of the anticyclonic eddy was
significantlyhigherthanduringdaylightandnight used here to create Fig. 10, which allowed us to
(N¼32, po0:02,Fig.7D). visualize for which sampling date or place the
Spatial variations were also observed for some general trend of the CF changes (Fig. 10)in
taxa. We limited our analysis to daylight stations buffering high frequency variability due to
in order to remove potential diel variations nycthemeral migrations and tospatialpatchiness.
(Fig. 8). Ostracods were more abundant during For example, the impact of the temporal
daylight in the cyclonic eddy than in the antic- factors was clearly detected by marked positive
yclonic eddy (Fig. 8A); cladoceran, doliolid, fish peaks, such as on the CF of Calocalanus
larva, and salp abundance were higher in the abundance (night stations 80, 79, 75, 105, 112
cycloniceddythanintheCZandtheanticyclonic and 119; Fig. 10G), which pointed out the
eddy(Figs.8B–E).Thosepatternswererelatedto arrival of those organisms in the upper
higherchlorophyllconcentrationsobservedinthe 200m, however, the general trend was not
cycloniceddy. disturbed.

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