Biological response to mesoscale eddies in the Algerian Basin

22 Pages·2003·7.65 MB·English

JOURNAL OF GEOPHYSICAL RESEARCH, VOL.108, NO.C8, 3245,doi:10.1029/1999JC000117,2003
Biological response to mesoscale eddies in the Algerian Basin
I. Taupier-Letage, I. Puillat, and C. Millot
Laboratoired’Oce´anographieetBioge´ochimie(LOB),CNRSUMR6535,Universite´delaMe´diterrane´e,AntennedeToulon,
LaSeynesurMer,France
P. Raimbault
Laboratoired’Oce´anographieetBioge´ochimie(LOB),CNRSUMR6535,Universite´ delaMe´diterrane´e,Campusde
Luminy,Marseille,France
Received1November1999;revised14March2001;accepted29June2001;published1August2003.
[1] The Eddies and Leddies Interdisciplinary Study in the Algerian Basin (ELISA)
experiment (1997–1998, MAST-3/MTP2/MATER program) was a multidisciplinary and
multiplatform experiment designed to study the anticyclonic Algerian Eddies (AEs)
generated by the instability of the Algerian Current and their influence on the general
circulation and biological phenomena. This paper presents preliminary results of the data
obtained over the four year-round cruises ELISA-1 to 4. Two AEs (called 96-1 and 97-1)
were tracked with satellite images over their counterclockwise circuit in the eastern
Algerian Basin, from the Algerian to the Sardinian slope, and then to the open sea. They
have been sampled over different periods and positions. Associated biological response
wasanalyzedconsideringthehydrodynamicalstructureandthedistributionofchlorophyll
and nitrate concentration in the upper 300 m. In summer both AEs (96-1 located along
slope and 97-1 located offshore) corresponded to highly oligotrophic areas. The deep
chlorophyllmaximumwas(cid:1)90–110m,withconcentrationsupto(cid:1)1mgm(cid:2)3,thenitrate
concentrations were low down to (cid:1)250 m in the AEs’ central zone. The downward
entrainment of chlorophyll along isopycnals in the AEs’ peripheral zone was well
observed down to (cid:1)250 m. In spring the maximum integrated chlorophyll concentrations
were found offshore, in 96-1, where the upper (cid:1)150 m were quasi-homogeneous. Lower
integrated chlorophyll concentrations were found inshore in 97-1, which was embedded
in an Algerian Current meander and remained stratified throughout wintertime. AEs
generate secondary phenomena such as small-scale cyclonic shear eddies, where the
highest chlorophyll concentrations ((cid:1)4 mg m(cid:2)3) were found. We show that through the
AEs it generates, the Algerian Current can be responsible for producing areas in the
coastal zone that are at least as oligotrophic as the eastern Mediterranean and, alternately,
for productive areas offshore. As AEs generally follow a counterclockwise circuit in
the Algerian Basin, it is inferred that they play an important role in the redistribution of
matter on a basin scale. However, the biological response associated with AEs varies
according to their history, a combination of trajectory, location, and season, in ways that
are not yet clear. INDEXTERMS:4223 Oceanography: General: Descriptiveandregional
oceanography;4520Oceanography:Physical:Eddiesandmesoscale processes;4845Oceanography:
BiologicalandChemical: Nutrientsandnutrientcycling; 4855Oceanography: BiologicalandChemical:
Plankton;4275Oceanography:General:Remotesensingandelectromagneticprocesses(0689);KEYWORDS:
Mediterranean,Algerian Current, mesoscaleeddies,chlorophyll,nitrate, ELISA/MATER
Citation: Taupier-Letage,I.,I.Puillat,C.Millot,andP.Raimbault,BiologicalresponsetomesoscaleeddiesintheAlgerianBasin,
J.Geophys. Res.,108(C8),3245,doi:10.1029/1999JC000117, 2003.
1. Introduction Atlantic Water (AW), which constitutes the surface cir-
culation in the western Mediterranean (Figure 1, see
[2] Water from the Atlantic enters the Mediterranean Millot [1999] and Send et al. [1999] for a review). AW
Sea at Gibraltar mainly to compensate for water lost by
flows along the Algerian slope, forming the Algerian
evaporation. Because of mixing, it rapidly becomes
Current. This current is unstable, meanders and generates
mesoscale eddies [e.g., Millot, 1985; Font et al., 1998].
A typical Algerian Current instability can be described as
a meander (AW layer, (cid:1)200 m thick) embedding a
Copyright2003bytheAmericanGeophysicalUnion.
0148-0227/03/1999JC000117 (coastal) anticyclonic eddy, associated with an upwelling
1 - 1
1- 2 TAUPIER-LETAGEETAL.:ALGERIAN EDDIES
Figure 1. The general circulation of the Atlantic Water (AW) in the western Mediterranean [adapted
from Millot, 1999]. The dashed gray arrow represents the general counterclockwise circuit of AEs. The
gray rectangle indicates the ELISA area.
cell on its southwestern side, and often with a short-lived oftengeneratesecondaryphenomena,suchassmallcyclonic
shallow cyclonic circulation on the meander crest [e.g., shear eddies ((cid:1)20–30 km) and filaments [Taupier-Letage
Obaton et al., 2000; Moran et al., 2001]. Only the andMillot,1988].
anticyclonic eddies develop (hereafter Algerian Eddies, [3] This dynamical mesoscale activity modulates the
AEs; see Figure 2 for illustration), so as to reach biological activity, as shown by the close correlation be-
diameters of 50–100 km (up to (cid:1)280 km), and vertical tweenthermalandoceancolorsatellitesignatures[Taupier-
extents of thousands of meters down to the bottom Letage, 1988; Arnone and La Violette, 1986; Arnone et al.,
((cid:1)2800 m [Ruiz et al., 2002; Millot and Taupier-Letage, 1990]. This implies that the biological activity is also
2003]). AEs propagate downstream along slope at a few affected by large spatial and temporal variability on the
km/day (e.g., 97-1 between Figures 2a and 2e), and are mesoscale [e.g., Robinson, 1983; Morel and Andre´, 1991].
likely to detach from the coast after pinching off from the Thiscanlead,locallyandepisodically,tounexpectedlyhigh
Algerian Current when approaching the Channel of chlorophyllorprimaryproductionvaluesfortheMediterra-
Sardinia, because of the shallowing and narrowing ba- nean [Lohrenz et al., 1988a, 1988b; Arnone et al., 1990;
thymetry. AEs then skirt the Sardinian slope (e.g., 97-1 Martinezetal.,1990;Raimbaultetal.,1993].Toaccurately
on Figure 2g) where they interact with the Levantine describe the biological activity in such areas, it is therefore
Intermediate Water (LIW) vein [Millot, 1987; Millot and importanttodesignamesoscale-dedicatedsamplingstrategy
Taupier-Letage, 2003], and propagate westward offshore [e.g.,Aikenetal.,1993].Thisimpliesmultidisciplinaryand
in the central part of the basin (open sea AE). The multiplatform experiments, with data collection at small
resulting trajectory is a counterclockwise circuit in the sampling intervals (few km) within a short time (a few
eastern Algerian Basin [Fuda et al., 2000]. This circuit days),preferablyoverlongperiods((cid:1)1year)andoverlarge
can be closed, as some AEs come back in the Algerian areas (up to basin scale). Moreover, efficient shipboard
coastal zone, and can include several loops, up to three sampling requires one to be aware of the mesoscale field.
loops for 96-1 [Puillat et al., 2002]. The lifetime of most This can be achieved with the (near) real time reception of
AEs is of the order of several months to 1 year [e.g., satellite images (easily accessed nowadays) on board, as
Millot et al., 1997], and one AE (96-1, see section 1.3) satellite observations of mesoscale phenomena are well
has been tracked for nearly 3 years [Puillat et al., 2002]. correlated to in situ ones in the Algerian Basin [Millot et
When located along the Algerian slope, AEs in turn can al.,1994,1997;Fudaetal.,2000].Giventheseconstraints,
interact with the Algerian Current (as 97-1, relayed by such experiments are few. Between 1986 and 1987, the
96-1, on Figure 2e) and dramatically alter its circulation Mediprod-5experiment[Mediprod-5Group,1990]explored
[e.g., Millot, 1991; Benzohra and Millot, 1995a]. AEs the western part of the Algerian Basin on a mesoscale.
Figure2. (opposite)MesoscalelocationandsamplingsitesduringtheELISAcruises(onlythebesttimecoherentsubsets
ofsatelliteandinsituobservationsarepresented).PlusesareCTDtransectsdiscussedintext,identifiedbytheirendstation
numbers. Solid triangles are moorings. Temperature on NOAA/AVHRR images increases from blue to red. Arrows are
shipboardADCPcurrentsinthe16–50-mlayer.(a)ELISA-1CTDtransectsoverNOAA/AVHRRimageof28July1997.
(b)ADCPcurrentsfrom24Julyto2August1997overNOAA/AVHRRimageof27July1997.(c)ELISA-2CTDtransect
over NOAA/AVHRR image of 19 October 1997. d) NOAA/AVHRR image of 27 October 1997. (e) ELISA-3 CTD
transects over NOAA/AVHRR image of 27 March 1998. (f) ADCP currents from 26 to 28 March 1998, over NOAA/
AVHRRimageof27March1998.(g)ELISA-4CTDtransectoverNOAA/AVHRRimageof6July1998.(h)ADCPfrom
1 to 6 July 1998 over NOAA/AVHRR image of 6 July 1998. See color version of this figure at back of this issue.
TAUPIER-LETAGEETAL.:ALGERIAN EDDIES 1 - 3
1- 4 TAUPIER-LETAGEETAL.:ALGERIAN EDDIES
Table1. Summary of the Cruises andOperationsat Sea Duringthe ELISA Experiment
Name Dates R/V Operations
ELISA-1 1Julyto8Aug.1997 LeSuroˆıt deploymentofmoorings1–8
anddrifters,CTD,XBT,
SEASOAR,LHPR,
EK500,biology,
biogeochemistry
ELISA-1.33 5–8Sept.1997 GeorgesPetit recoveryofmooring6,XBT
ELISA-1.66 3–7Oct.1997 BI‘‘GreenPeace’’ recoveryofmooring8
ELISA-2 20–29Oct.1997 Tethys-2 deploymentofmoorings6,8,9andCTD
ELISA-2.5 18Feb.to9March1998 GeorgesPetit recoveryanddeploymentofsix
mooringstoreplacedefective
acousticreleaserings.
ELISA-3 19–30March1998 LeSuroˆıt moorings,CTD,XBT
ELISA-4 22Juneto8July1998 LeSuroˆıt mooringsrecovery,CTD,XBT
Results confirmed the high variability of chlorophyll and MAST3-MTP2 European program, and involved 44 phys-
nutrient distribution, in relation to mesoscale dynamical icists, biologists and geochemists from 8 countries (http://
structures:thelowestchlorophyllconcentrationswerefound www.com.univ-mrs.fr/ELISA). The major aims of ELISA
inthecentralpartofanopenseaAE,thehighest(>9mgm(cid:2)3) were to study: (1) the general circulation of the water
inasmall-scalecyclonicsheareddy[Taupier-Letage,1988; masses, (2) the origin, structure and trajectories of AEs,
Raimbaultetal.,1988a,1988b,1990a].Inthewesternmost and (3) the biological response associated with mesoscale
part of the basin, biological enhancement related to an dynamicalphenomena,and(4)thebiologicalconsequences
Algerian Current instability was observed, especially in its of the mesoscale dynamics for the functioning of the
shallow and transient cyclonic circulation [Moran et al., Algerian Basin. Work at sea was carried out in the eastern
2001].However,mostinvestigationsfocusedonthebiolog- part of the Algerian Basin from July 1997 to July 1998. It
ical enhancement at the outer edge of the Algerian Current involvedfourmaincruises(Table1),the1-yeardeployment
between0and(cid:1)3(cid:1)E,asitoftendelimitsafront[Lohrenzet of a mooring network (Figure 3) and of 15 subsurface
al.,1988b;Raimbaultetal.,1993].Onaverage,thedynam- Lagrangian drifters, and the continuous monitoring of the
icalactivityoftheAlgerianCurrentiswellmarkedbyamore area by satellite thermal NOAA/AVHRR imagery analysis.
productivecoastalzone,asshownbysatellite-derivedchlo- The four cruises ELISA-1 to 4 were scheduled year-round
rophyll concentrations and primary production maps [e.g., to carry out mooring operations (including the periodic
Antoine et al., 1995]. It is also marked, in a coastal zone maintenance of bio-optical moored sensors to avoid bio-
(cid:1)50kmwide,byanintermediate((cid:1)300–600m)maximum fouling problems) and sampling (Figure 4).
ofnutrients(nitrate>9.0mM,phosphate>4.0mM)indicating
theoccurrenceofintenseremineralization[Raimbaultetal., 2.1. Data Sets
1990b]. 2.1.1. Satellite Imagery
[4] Theneedforanotherexperimentwaspromptedbythe [6] Thesituationonabasinscalewasfirstanalyzedusing
high mesoscale variability found and the limited observa- the Mediterranean daily composited NOAA/AVHRR SST
tions of the seasonal variability, which prevented general-
izationfromthefewsampledAEs.Theaimofthispaperis
topresentthepreliminaryresultsobtainedduringthe1-year
ELISA experiment [ELISA Group, 1998] (http://www.com.
univ-mrs.fr/ELISA) on the biological response to AEs,
represented here by the distribution of chlorophyll and
nitrate concentration. First we present the ELISA experi-
ment, the data sets and the trajectories of the AEs encoun-
tered (section 2). Then we describe the biological response
associated with these eddies (section 3), and the secondary
phenomena they induce (section 4). We discuss the con-
sequencesonamesoscaleandseasonalscale(section5)and
finally putforwardhypothesesfor consequences onabasin
scale (section 6).
2. ELISA Experiment
[5] ELISA stands for Eddies and Leddies Interdiscipli-
nary Study in the Algerian basin (Leddies are hypothetical
eddieswhichcouldbecreatedintheorybytheinstabilityof
the LIW vein as it goes round the southwest corner of Figure 3. The ELISA experiment area. Bathymetry is in
Sardinia [Millot et al., 1997]). This experiment was part meters. ELISA moorings positions (solid diamonds 1–9)
of the Mass Transfer and Ecosystem Response (MATER) and schematics.
TAUPIER-LETAGEETAL.:ALGERIAN EDDIES 1 - 5
Figure 4. AEs’schematic positions and CTD sampling in (a) summer 1997, (b)–(c) spring 1998, and
(d)summer1998.Seetextforeddyidentification;positionsanddiametersofAEsasestimatedfromtheir
NOAA/AVHRRsignaturesonindicateddates.PlusesareCTDstations;thicklinesaretransectsdiscussed
in paper; and triangles 1–9 are mooring positions.
(Sea Surface Temperature) images of the ISIS database presented only in the upper 300 m, are identified by their
(http://isis.dlr.de), starting mid-1996. In order to get ade- end station numbers. A Seabird 12l 12-bottle carrousel was
quate information for tracking mesoscale features using used for water samples.
thermal signatures [Taupier-Letage et al., 1998], the Ger- 2.1.3. Chlorophyll Concentrations
man Aerospace Centre (DFD/DLR) generated AVHRR [8] Chlorophyll a concentrations were estimated fluoro-
channel 4 images (relative temperatures) of all passes on metrically after extraction of pigment in methanol as de-
the eastern Algerian Basin in near real time. These files scribed by Herbland et al. [1985] and modified by
were retrieved viaftp, including fromthe ship while atsea. Raimbaultetal.[1988a],arapidmethodespeciallyadapted
Over 1000 such images (hereafter generically called SST) to dense mesoscale sampling. Water samples of 250 mL
have been analyzed. Images are color-coded in order to werefilteredusingWhatmanGF/Ffiberglassfilters(25mm)
enhance thermal contrast: as a consequence there is no under low vacuum pressure ((cid:3)100 mm Hg). Filters were
standard temperature-to-color scale. placedinaglasstubewith5mLofpuremethanolandkept
2.1.2. CTD Casts in the dark at (cid:1)4(cid:1)C for (cid:1)30 min. The fluorescence of
[7] The shipboard CTDs were Seabird SBE 911 +, extracts was measured with a Turner Designs fluorometer
calibrated within a few months at most from the cruises. before acidification, for chlorophyll concentration = chlo-
Accuracy and precision for potential temperature, salinity rophyll a + phaeophytin a concentration, and after, for
andpotentialdensityareknownbetterthan±0.1%.Foreach chlorophyll a and phaeophytin a respective concentrations.
cruise, the CTD was fitted at least with an in situ Chelsea Weusedtheformerextractedchlorophylltocalibratethein
AquatrackaMK3fluorometer,aSeaTech25cmpathlength situ fluorometer signal. Indeed, the best vertical represen-
transmissometerat660nm,andaSeabirddissolvedoxygen tation of the chlorophyll profile is given by the CTD
probe. Casts were typically made to 1000 m, and the downcasts, as the discrete water samples cannot assess
reference level for the computation of the geostrophic correctly the amplitude of chlorophyll maxima, or its
current was 1000 dbar. However, casts down to 200 m vertical variability. Also, during upcasts the stratification
were sometimes interleaved to improve spatial resolution is disturbed by the CTD before water samples are taken,
without impacting time. Alternately, some casts were ex- whichresultsinsevereunderestimationofpeaks,ifsampled
tended down to 2000 m or down to the bottom when at all. For each cruise, the upcast bottle fluorescence was
prompted by hydrological peculiarities, as well as to check plotted vs. the corresponding extracted chlorophyll concen-
the vertical extent of AEs. Acquisition rate for all sensors trations (abbreviated hereafter to ‘‘ec’’). The derived rela-
was 24 scans s(cid:2)1. Data were processed using Seabird/ tion was then applied to the downcast fluorescence profile
Seasoft software, and averaged over 1 m bins. Transects, to yield instrumentally and seasonally comparable chloro-
1- 6 TAUPIER-LETAGEETAL.:ALGERIAN EDDIES
phyll concentrations, and to compute reliable integrated which (C) was spotted on the SSTimages (Figures 2a–2b)
chlorophyll concentrations. These concentrations are used and sampled. The second AE, 97-1 ((cid:1)120 km diameter),
throughout the text, and sometimes backed up by the ec had been generated near (cid:1)0(cid:1) by late March 1997 and had
ones. beenpropagatingalongslope.97-1wasthusacoastaleddy
2.1.4. Nutrients still embedded in an Algerian Current meander. By early
[9] DuringELISA-1nutrientswereanalyzedimmediately July it was next to 96-1 (Figures 2a–2b). It altered the
on board (third leg only, from stations 78 to 121). During eastward along-slope propagation of AW, which was then
ELISA-2 no sample was taken. During ELISA-3 nutrient deflected offshore, skirting 97-1 and 96-1 successively.
samples werefrozenforeventualanalysisinthelaboratory, Complex eddy interactions led to secondary phenomena
but it seems that a preservation problem occurred and such as filaments and dramatic changes in shape and/or
concentrations were not all reliable. During ELISA-4 sam- diameters for both 96-1 and 97-1.
ples were collected in 25 mL polyethylene flasks and [13] By October 1997 (Figures 2c–2d), the diameter of
immediately poisoned with HgCl (final concentration of 96-1wasreducedto(cid:1)80km.Sampling(ELISA-2)of96-1
2
20 mg mL(cid:2)1) according to Kirkwood [1992]. Samples were was limited to its NE part. By December 1997 both 96-1
then stored in the dark at (cid:1)5(cid:1)C until processing in the and 97-1 resumed their counterclockwise trajectory.
laboratory. Nitrate concentrations were determined on a [14] ByMarch1998(Figure4bandFigures2e–2f),96-1
Technicon AutoAnalyser according to Tre´guer and Le was offshore propagating westward, while 97-1 was prop-
Corre[1975]. Concentrations of nitrite and phosphate were agating eastward along the Algerian slope. An important
also determined (as well as ammonium during ELISA-1), shear zone was created in between. Sampling (ELISA-3)
but are not addressed here. mainly focused on 96-1, reaching the NE border of 97-1.
AE 96-1 went over mooring 7 in late March (Figure 4c).
2.2. Strategy [15] By June 1998 (Figure 4d and Figures 2g–2h), 97-1
[10] As the mesoscale was a focal point of ELISA, an was also offshore. Interactions between the Algerian Cur-
adaptable strategy was implemented. First the AEs were rent, 98-2 and 97-1 (and 96-1 to a lesser extent) notably
continuously tracked with SST images. While at sea, the altered the eastward along-slope AW path. Sampling
images were sent in near real time to the ship. Once the (ELISA-4) mainly focused on 97-1.
structure to sample was determined, the sampling scheme
wasrefinedconsideringcurrentsfromshipboardADCPand
indications from thermosalinograph, as well as from XBT 3. AEs’ Biological Response
sections. Besides rapid reconnaissance, XBTs were also
[16] As only a limited amount of data is presented here,
used to extend CTD transects in less critical areas in order
it must be kept in mind that the analysis actually relies
to save time, and more generally during transits in the
upon a vast database, including in particular (cid:1)1000 SST
Algerian Basin. A mean sampling interval of 5 nautical
images, (cid:1)300 CTD casts over 269 stations, (cid:1)350 XBT
miles ((cid:1)9 km) was used, sometimes reduced to 3 miles casts, (cid:1)1400 determinations of nutrients, (cid:1)40 current
((cid:1)5 km). Care was taken to complete transects across meters, and 4 CTDF 1-year-long time series (analyses are
structures within a few days. The main interdisciplinary
still ongoing).
effort was made during the ELISA-1 cruise. It was divided
into three legs to put the moorings in place, to make rapid 3.1. AE 96-1
synoptic surveys of AEs with XBTs, CTD, and Seasoar 3.1.1. Summer (ELISA-1) and Autumn (ELISA-2)
(tow-yo), and, last, to allow for time-demanding biological 1997 Conditions
and biogeochemical sampling, respectively. In addition to [17] AE 96-1 did not propagate during summertime, so
time-limited shipboard sampling, efforts were made to that the images presented on Figures 2a–2b are represen-
obtain the mesoscale structures description on a longer tativeofalltransects(madewithin18days).Itsshapevaried
term. A network of nine mooring lines (cid:1)50 km spaced however, and its diameter ranged from (cid:1)180 to (cid:1)250 km.
(Figure 3) provided 1-year-long time series. More than 40 Associated maximum currents were (cid:1)50 cm s(cid:2)1, up to
current meters sampled in the main water masses down to 80 cm s(cid:2)1 (Figure 2b and Figure 5a). These maxima lay
thebottom.Mooring8wasequippedwithfourautonomous subsurface ((cid:1)50 to 100–150 m): there was a marked
CTDsfittedwithfluorometers(CTDFs)at(cid:1)40,50,60,and discontinuity at (cid:1)50 m. The transect 18–35 along a SE-
80 m (nominal depths), and mooring 7 was equipped with NW diameter (completed on 14–15 July, Figure 6) was
an RDI 75 kHz ADCP sampling the upper 500 m. representative of the general features of 96-1. Edges were
not crossed, as indicated at transect extremities by geo-
2.3. TrajectoriesofAEsDuringtheELISAExperiment strophic current (cid:1)40 cm s(cid:2)1 and sloping isolines. The
[11] Twomain AEs,called 96-1and97-1, dominatedthe thermocline (Figure 6a) was well marked in the central
ELISA eddy field (Figure 4). Their history and trajectory zone(stations30–26at(cid:1)25m)at20–30m,andweakened
are detailed by Puillat et al. [2002]. toward the edges. Hydrological sections displayed the
[12] From June to November 1997, 96-1 remained cen- classical trough of anticyclonic eddies, isolines being de-
tered on (cid:1)6(cid:1)E. Although 96-1 was close to the slope pressed by (cid:1)150 m in the center. Isotherms and isopycnals
(Figure 4a), it was at least 15 months old, and had already (Figure 6c) were doming ((cid:1)25 m) in the upper (cid:1)75 m, a
completed one counterclockwise loop in the basin. 96-1 feature already observed by Benzohra and Millot [1995b].
(diameter (cid:1)180 km) was intensively sampled during AE 96-1 was characterized by a weakly stratified layer
ELISA-1. The shear between 96-1 and coastline breaks approximately 100-m thick in the central zone, between 50
sporadically generated small cyclonic shear eddies, one of and 150 m: temperatures were confined to 15.0(cid:1)–16.0(cid:1)C,
TAUPIER-LETAGEETAL.:ALGERIAN EDDIES 1 - 7
Figure 5. Geostrophic current (cm s(cid:2)1) sections (a) across 96-1 during summer 1997 (ELISA-1) and
(b)spring 1998(ELISA-3) and(c) across 97-1duringsummer 1998(ELISA-4). Orientationisgivenby
respective schema.
salinities were confined to 37.00–37.10 (Figure 6b), and (i.e., most recent/less mixed AW) were strikingly all inside
potentialdensitieswereconfinedto27.20–27.60.Salinities 96-1, between (cid:1)40 and 60 m. Chlorophyll distribution
lower than 37.00 (minimum 36.80) were found in patches, (Figure 6c) was relatively symmetrical at eddy scale. There
down to (cid:1)80 m. When plotting all the CTD profiles (not was no significant chlorophyll concentration in the upper
shown; see distribution on Figure 4a), minimum salinities (cid:1)30 m. The central zone displayed a deep chlorophyll
1- 8 TAUPIER-LETAGEETAL.:ALGERIAN EDDIES
Figure 6. Transect 18–35 inside 96-1 (summer 1997): (a) potential temperature ((cid:1)C), (b) salinity, and
(c) chlorophyll concentration (mg m(cid:2)3) and potential density (thick solid line).
TAUPIER-LETAGEETAL.:ALGERIAN EDDIES 1 - 9
maximum (DCM), slightly deeper than in the peripheral
zone.Maximareached(cid:1)1.5mgm(cid:2)3atstations26,28,and
29 at 100–110 m. In the peripheral zone maxima reached
(cid:1)2.0 mg m(cid:2)3 at station 34 and (cid:1)1.6 mg m(cid:2)3 at station 23
at 70–80 m. Finally, significant chlorophyll concentrations
were observed at (cid:1)200 m along the 28.50–28.75 isopyc-
nals,atstation34andstation18(ec:0.3mgm(cid:2)3at190m).
No nutrients were available for this transect.
[18] The transect 100–121 (Figure 7) across the eastern
part of 96-1 was completed within 4 days (30 July to 2
August).Theedgewasmarkedatthesurfaceinthevicinity
of stations 116–119 by a sharp drop of the ADCP current
speed(Figure2b),aswellasofthegeostrophiccurrent(not
shown). A sharp salinity front in the vicinity of station 116
(Figure 7b) also marked the peripheral zone in the upper
layer, where surface salinities were >37.60, and the 38.00
isohaline shoaled at (cid:1)70 m. Isohalines and isopycnals
(Figure 7c) slopes were negligible from station 116 east-
ward but only in the upper (cid:1)100 m. This indicates that the
deeperstructurewaswider.Thecenterof96-1waslocateda
fewmileswestofstation100.Thisisinagreementwiththe
flat isolines at 100 m and the 28.00 isopycnal immersion
((cid:1)210 m), as observed in the center of the 18–35 transect.
Surface temperature increased to (cid:1)25.0(cid:1)C ((cid:1)23.5(cid:1)C for
18–35, Figure 6a), and the thermocline was deeper by
(cid:1)10 m. The (cid:1)100-m-thick central homogenous layer was
well marked. The thin subsurface layer of lower salinity
(<37.00) was quasi continuous (stations 100–113). The
upper (cid:1)50 m were chlorophyll-depleted over the whole
transect(Figure7c).InthecentralzonetheDCMlaybetween
100and110mwithmaximumconcentrations(cid:1)0.8mgm(cid:2)3.
TheDCMslowlyrosefrom(cid:1)100matstation106to(cid:1)80m
at station 114 (maximum (cid:1)0.8–1.0 mg m(cid:2)3) toward
the peripheral zone. In the vicinity of station 115 the
0.75mgm(cid:2)3isoplethwasinterrupted.BelowtheDCMthe
chlorophyll signature progressively subsided centerward,
from station 117 to station 113, following the 28.50–28.75
isopycnalsdownto(cid:1)200m.Outside96-1theDCMdidnot
appear significantly different from that in the 96-1 central
zone. The nitrate-depleted upper layer (the 0.1-mM isopleth
representsthedetectionlimit)was(cid:1)60mthick(Figure7d)
over the whole transect, slightly thicker in the 96-1 central
zone and in the vicinity of station 114. The nutricline,
representedbythe0.5-mMisopleth,wasrelativelyhorizontal
too. Below, however, the nitrate distribution markedly
differed in the central and peripheral zones. In the central
zoneof96-1(stations100–107),concentrationswereweak,
rangingfrom(cid:1)1.0to2.5mMfrom(cid:1)100to250m.Concen-
trationswerelessthan7.0mMat300m.Fromstation110to
112 there was a marked nitrate gradient strengthening
eastward, ending up at station 121 with concentrations of
(cid:1)2.5 mM at (cid:1)100 m. Within 96-1 (other transects not
shown) the isohaline 38.00 roughly corresponded to nitrate
concentrations between 2.0 and 3.0 mM. In the peripheral
zone this was no longer true, as the 38.00 isohaline was Figure 7. Transect 100–121 in 96-1 eastern half
well above the 2.0–3.0 mM isopleth. Note that in the (summer 1997): (a) potential temperature ((cid:1)C), (b) salinity,
peripheral zone nitrate concentrations rapidly increased (c) chlorophyll concentration (mg m(cid:2)3) and potential
with depth: at station 121 the concentration reached density (thick solid line), and (d) nitrate concentration
(cid:1)9.0 mM at 300 m. (mM); isohaline 38.00 (thick solid line), and chlorophyll
[19] The autumn transect in the NE peripheral zone of >0.5 mg m(cid:2)3 (cross-hatched).
96-1 (Figure 8) displayed similar features: a central
homogeneous layer and a subsurface lower-salinity layer
1- 10 TAUPIER-LETAGEETAL.:ALGERIAN EDDIES
3.1.2. Spring 1998 Conditions (ELISA-3)
[20] AE 96-1 was offshore, propagating northwestward
(Figures 4b–4c). The transect 169–179 (Figure 9) was
completed on 28 March, (cid:1)36 hours after a (cid:1)2-day
strong wind event. It cut 96-1 slightly east of its center
(Figures 2e–2f), located between stations 173 and 174.
Diameter estimates ranged from 130 to 160 km. Its
structure had become complex, and was organized in three
parts (Figure 5b): the main one was the upper layer
((cid:1)180–200 m thick) along the whole transect length,
overlaying two secondary cells between (cid:1)200–300 m
(stations 170–173 and 175–179, respectively, identified
by dashed boxes), which were in anticyclonic rotation on
themselves. In the 96-1 upper layer the current was
maximum ((cid:1)25 cm s(cid:2)1) in the peripheral zone, and
homogeneously weak ((cid:1)0–10 cm s(cid:2)1) in the central
one. The 96-1 upper central zone corresponded to a well-
mixed layer: at station 174 the temperature (Figure 9a)
onlyvariedfrom15.49(cid:1)Cat(cid:1)10mto15.46(cid:1)Cat(cid:1)150m,
andindividualized an AWthermostad. In the same way the
salinity was (cid:1)37.07 from the surface down to (cid:1)130 m
(Figure 9b). On either side of station 174, the two cells
were relatively homogeneous too. The southern cell was
also identified on moored ADCP data by the east (north)
velocity intermediate minimum (maximum) in late March
(Figure 10a (Figure 10b)). Betweentheupperlayerandthe
two cells the shear was locally intense, as observed on
shipboardADCP(Figure2fandFigure11),mooredADCP
(Figures 10a and 10b) and geostrophic current (Figure 9a)
datasets.Thepassageof96-1onmooring7wasmarkedby
stronghorizontalcurrentsdownto500m(upto30cms(cid:2)1,
Figures10a–10b;increasealsoclearlyrecordedonthe1700
mcurrentmeter,notshown).Factoryalgorithmsalsoyielded
strongdownwardvelocity,howevertheirvalidityiscurrently
being investigated. The echo signal (Figure 10c) showed
minimumvaluesduringthe96-1passageinthedeeperlayer
((cid:1)350–500 m), whereas it corresponded to a maximum in
theupperone((cid:1)100–300m).
[21] The chlorophyll distribution (Figure 9c) was also
organized in three parts, a major maximum overlying two
secondary ones. From the surface down to (cid:1)100 m, and
reaching almost 150 m in the central zone, chlorophyll
concentrations were (cid:4)0.75 mg m(cid:2)3 (ec: 0.75 mg m(cid:2)3 at
100 m at station 171). Maximum concentrations were
>1.2 mg m(cid:2)3 at (cid:1)20–50 m. The thermostad was also
identified by localized vertical maximum oxygen concen-
trations (Figure 9c) reaching (cid:1)150 m. The major chloro-
phyll maximum lower limit was sharp and matched the
base of the mixed layer. The same feature applied to
dissolved oxygen and light transmission (not shown).
Underneath, the two secondary chlorophyll maxima were
mirrored by the oxygen distribution. The southern one was
well defined between (cid:1)130–230 m, with sharp limits at
Figure8. Transect122–127intheNEof96-1(fall1997):
stations 169 and 173. Concentrations were >0.8 mg m(cid:2)3,
(a) potential temperature ((cid:1)C), (b) salinity, and
as confirmed by the bottle sample: at 150 m at station 171,
(c) chlorophyll concentration (mg m(cid:2)3) and potential
ecwas0.84mgm(cid:2)3,andchlorophyllaalone0.44mgm(cid:2)3.
density (thick solid line).
The chlorophyll distribution in the northern cell did not
exhibit sharp limits, concentrations were homogeneously
(Figure 8b). Seasonal evolution however was marked lower ((cid:1)0.3 mg m(cid:2)3).
by thermocline deepening ((cid:1)50 m, Figure 8a) and mixed [22] The transect 136–144 (Figure 12) also cut the
layer homogenization. The DCM ranged between 50 and southwesternpartof96-1(station136(cid:5)station171)before
100 m, maxima hardly reached 1 mg m(cid:2)3 (Figure 8c). the wind event (on 24 March). It showed that a warmer