Summer mesozooplankton assemblages inthe north-eastern Aegean Sea: the influenceof Black Sea water and an associatedanticyclonic eddy

stamatina isari1,4

, stylianos somarakis2

, epaminondas d. christou3

and nina fragopoulu1

1Laboratory of Zoology, Department of Biology, University of Patras, 26500 Rio, Patras, Greece, 2Hellenic Centre for MarineResearch, PO Box 2214, 71003 Heraklion, Crete, Greece, 3Hellenic Centre for Marine Research, PO Box 712, 19013 Anavissos,Athens, Greece, 4Present address: Hellenic Centre for Marine Research, PO Box 2214, 71003 Heraklion, Crete, Greece

The north-eastern Aegean Sea (NEA) is a marine system of high hydrological complexity, principally induced by the inflowand subsequent advection of the low salinity (,30) Black Sea water (BSW). This water mass occupies the upper layer (�0–20 m) of the NEA and plays a key role in the determination of circulation patterns and the generation of various frontal andeddy structures. Here we are concerned with the examination of mesozooplankton assemblages in the NEA during the thermalstratification period (July 2004) in two discrete sampling layers: (a) Layer 1 (from the base of halocline to the surface: �0–20 m) which is directly influenced by BSW; and (b) the deeper�20–50 m layer (Layer 2). Our main objective was to assess theresponse of mesozooplankton to the BSW and the associated hydrological structures. In July 2004, the BSW was mainlyrestricted in the eastern part of the NEA where it was entrapped in a �50-km wide anticyclonic gyre (Samothraki gyre).A marked spatial differentiation in mesozooplankton assemblage structure, significantly related to this hydrodynamic parti-tioning, was detected in Layer 1. Sampling sites under the direct influence of low salinity–high temperature gyre waters werecharacterized by a considerably higher mesozooplankton stock than the remaining area, mainly due to the outstandingnumerical dominance of the cladoceran species Penilia avirostris. Copepods displayed notably low densities within thegyre and low species diversity, the calanoid Temora stylifera was the only abundant species. The mesozooplankton commu-nity outside the gyre zone shifted towards lower levels of total abundance, with a lesser contribution of cladocerans and anincrease in the importance of small-sized copepods (e.g. Acartia clausi, Paracalanus parvus, copepodites of Oithona spp. andClausocalanus spp.). In the subsurface layer (Layer 2), the mesozooplankton community also exhibited spatial heterogeneitywhich could be hardly explained by variability in environmental parameters. The periphery of the anticyclone below the halo-cline was distinguished from the remaining neritic area, presenting markedly high mesozooplankton productivity and distinctgroup composition. An inverse pattern in the mesozooplankton stock vertical distribution was observed at the periphery of thegyre (Layer 2. Layer 1) comparative to the remaining sites (Layer 1 . Layer 2), which was mainly due to unusually highconcentrations of surface-living zooplankters below the halocline. The latter could be explained in terms of expected waterflow patterns in an anticyclonic eddy.

Keywords: mesozooplankton, community composition, distribution patterns, eddy, eastern Mediterranean

Submitted 21 September 2009; accepted 16 December 2009; first published online 2 June 2010

I N T R O D U C T I O N

Mesoscale hydrodynamic structures in the marine environ-ment (i.e. fronts and eddies), regardless of the nature oftheir driving forces, have long been acknowledged for theirpotential influence on patterns of biological distributions(e.g. Owen, 1981; Robinson, 1983; Le Fevre, 1986). Such phys-ical features have been suggested to assemble the biotic com-ponents in the water column, either via injecting nutrientsinto the euphotic zone (through diapycnal mixing) and

fuelling a subsequent channel of energy upward the foodweb and/or by enhancing the passive accumulation of individ-uals of several trophic levels. However, these impacts arehighly dependent on the time scale of a given feature as wellas the generation times of the organisms (Kiørboe, 1993).

The response of mesozooplankton to this kind of physicalvariability is of particular interest, mainly considering theirposition in the food web and the possible implications forhigher trophic levels. Nowadays, there is a considerableamount of information with respect to permanentMediterranean frontal structures (e.g. Almeria-Oran front:Gaudy & Youssara, 2003; Catalan front: Alcaraz et al., 2007;Mallorca channel: Fernandez de Puelles et al., 2004;Liguro-Provencal front: Pinca & Dallot, 1995; Molineroet al., 2008; north-eastern Aegean front: Zervoudaki et al.,

Corresponding author:S. IsariEmail: misari@her.hcmr.gr

51

Journal of the Marine Biological Association of the United Kingdom, 2011, 91(1), 51–63. # Marine Biological Association of the United Kingdom, 2010doi:10.1017/S0025315410000123

2006, 2007), revealing their possible strong biological differen-tiation (for instance elevated mesozooplankton stock, distinctspecies assemblages and increased vital rates) compared to thesurrounding waters.

Unlike fronts, the control exerted by eddy-motions on theorganization of zooplankton communities is less studied par-ticularly considering the small-scaled eddies generated in theMediterranean waters. Circulation pattern of these eddieshas been reported to affect local mesozooplankton pro-ductivity levels, i.e. high values in cyclones versus lowvalues in anticyclones (Pancucci-Papadopoulou et al., 1992;Mazzocchi et al., 1997), however, persistence in time maybe also important for the observed biological outcome(Mazzocchi et al., 1997). Riandey et al. (2005) have reporteda rather variable influence of Algerian eddies on the mesozoo-plankton component. These eddies can often result in theincrease of the generally poor zooplankton biomass of thearea and affect the distribution of species. Eddies have alsobeen shown to be of primary importance in the formationof distinct copepod assemblages (Siokou-Frangou et al.,1997). They may provide a favourable environment for thegrowth of specific zooplanktonic taxa influencing communitystructure and species diversity (Pinca & Dallot, 1995).

This study was carried out in the north-eastern part of theAegean Sea (NEA) which is a complex and dynamic marinesystem situated at the north-eastern most part of theMediterranean (Figure 1). The hydrodynamic complexity ofthis system has been considered closely linked, due to the geo-graphical vicinity with the Dardanelles Strait, to the surfaceoutflow of low salinity (,30 psu) waters of Black Sea origin(Black Sea waters (BSW)). Water circulation patterns in thearea have been characterized by a high spatiotemporal varia-bility and result in the generation of various frontal andgyre structures (Zodiatis & Balopoulos 1993; Zervakis &Georgopoulos, 2002). Almost permanently, one branch ofthe BSW follows a northward direction towards theThracian Sea where it is captured by an anticyclonic gyreformed around the island of Samothraki (Samothraki gyre)(Zervakis & Georgopoulos, 2002).

The variation of zooplankton assemblages across the strongfrontal structure generated near the BSW exit into the Aegean

Sea has been lately studied by Zervoudaki et al. (2006). Arecent investigation of mesozooplankton distribution in thesurface layer (0–50 m) over the NEA (Isari et al., 2006) alsorevealed a considerable influence of horizontal oceanographicvariability in the assemblage structure. However, taking intoaccount that BSW are mainly restricted in the upper �0–20 m layer (Zodiatis & Balopoulos 1993; Zervakis &Georgopoulos, 2002), sampling the 0–50 m layer in thestudy of Isari et al. (2006) may have masked the actual BSWinfluence.

In this paper, we examine the mesozooplankton commu-nity structure in the NEA in the exact layer of BSW influence(surface down to the base of the halocline: �0–20 m) as wellas in the subsurface layer (�20–50 m). Sampling was carriedout during July 2004 over a sampling grid quite similar to thatof Isari et al. (2006), but with a denser coverage over theThracian Sea shelf where the Samothraki gyre is located.The horizontal distribution patterns of the major mesozoo-plankton groups (e.g. copepods, cladocerans and appendicu-larians) in the surface layer (�0–20 m) have been alreadypresented in Isari et al. (2007) and discussed in relation tothe ambient abiotic parameters and possible biological inter-actions, not only between mesozooplankters but also withthe other (lower or upper) trophic levels. The principal objec-tives of the present paper were to assess: (a) how the hydro-dynamic partitioning of NEA regarding the upper layer ofthe water column may influence the entire mesozooplanktoncommunity structure (including copepod species compo-sition); and (b) how the dynamics of the prominent eddy ofthe area, the Samothraki gyre, may affect the distribution ofmesozooplankton in the upper water column (0–50 m).

M A T E R I A L S A N D M E T H O D S

Sampling procedure and analysisHydrographic and mesozooplankton sampling was carriedout during the summer thermal stratification period, from10 to 17 July 2004. Conductivity–temperature–depth(CTD) deployments were performed in a total of 42 stations(Figure 1) using a SEABIRD SBE 9 profiler equipped with afluorometer, in order to measure fluorescence in the watercolumn. Mesozooplankton data were collected from 30 sites(Figure 1) during daylight hours with vertical stratified towsof a WP-2 closing net (200 mm mesh size, mouth area of0.25 m2).

Two depth intervals (Layer 1 and Layer 2) were sampledaccording to the physical structure of the water column asrevealed by the in situ examination of CTD profiles. Layer 1was extended from the sea surface down to the base of thehalocline (�0–20 m, coinciding with the pycnocline), whileLayer 2 from the base of the halocline down to 50 m depthor near the bottom at shallower stations. Due to malfunctionof the closing mechanism equipment, the second depthstratum (Layer 2) was sampled only in 16 out of the 24stations that permitted sampling of the second layer (stationdepth . 20 m). All samples were preserved in 4% borax–buffered sea water formaldehyde solution.

In the laboratory, all samples were divided using a Folsomsplitter. The first half was filtered onto pre-dried and pre-weighed, glass-fibre filters (Whatman GFC) and dried at608C for 24 hours before the dry weight determination.

Fig. 1. Geographical location of the study area and distribution of samplingsites. A, Hydrographic and mesozooplankton stations; †, hydrographicstations.

52 stamatina isari et al.

Estimations of zooplankton biomass based on samples fixed informalin are affected by formalin-related changes in zoo-plankton biochemical content (Omori & Ikeda, 1984).However, we assumed that this would not bias our con-clusions on the general distribution patterns of zooplanktonbiomass during the survey. The other half of the sample wasused for species composition analysis and the estimation oftotal abundance. Mesozooplankton was identified to thelowest possible taxonomic level. Identification was done tothe species level for cladocerans and copepods (apart fromthe copepodites and males of the genera Clausocalanus,Oithona and Oncaea), while all the other taxa were identifiedto phylum, class or order levels. Assuming 100% filtration effi-ciency of the net, dry weight and abundance values were stan-dardized to cubic metre.

Data analysisTo describe the copepod community structure, species rich-ness (S) as the total numbers of species at each site and log2

Shannon –Wiener diversity index (H′) (Shannon & Weaver,1963) was computed. Data on adult copepods only (bothmales and females) were used for the latter index. Males ofthe genera Clausocalanus, Oithona and Oncaea were notidentified to the species level. For these genera, relativespecies abundance of males was assumed to be equal tofemales.

Multivariate analyses, i.e. hierarchical agglomerativeaverage-linkage clustering and non-metric multidimensionalscaling (NMDS) (Clarke & Warwick, 1994) were applied tothe species-samples data set. Abundance values of all meso-zooplankton taxa identified in the samples were square roottransformed in order to down-weight the contribution ofvery dominant as well as very rare taxa. The transformeddata were subsequently used to produce a sample-to-samplematrix for each separate depth stratum, calculated as Bray–Curtis similarity index. Prior to ordination, ‘outlier’ samplesites identified by cluster analysis were removed from thedata set to obviate problems associated with such outliers,e.g. biasing or dominating the ordination, often compressingthe distribution of the remaining sites (Hosie & Cochran,1994).

Analysis of variance was performed on log(x + 1) trans-formed abundance values to identify significant differencesin total mesozooplankton abundance among the groups ofsamples defined by the cluster analysis. Multiple comparisonsamong means were done using the Student –Newman–Keuls(SNK) test (Zar, 1999); t-tests were also performed on simi-larly transformed mesozooplankton data to test for significantdifferences in mesozooplankton stock or taxa abundance,between the different sampling strata.

In order to relate community structure with environmentalparameters, ordination scores produced by the NMDS wererelated by multiple regression analysis with various environ-mental variables to determine which of these parameterscould best explain the mesozooplankters’ distributions(Kruskal & Wish, 1978; Hosie & Cochran, 1994). In theregression analysis the NMDS scores were treated as the inde-pendent variables and each environmental parameter as thedependent variable. Regression lines and their directionswere plotted in the NMDS graphs according to Kruskal &Wish (1978): the direction of maximum correlation of eachregression line is at an angle wr with the rth MDS axis. The

direction cosine, or regression weight cr, of that angle isgiven by the formula:

cr = br/��������b2

1 + b22

√

where b1 and b2 are the coefficients from the multipleregression a + b1x1 + b2x2 and x1 and x2 are the scores inthe first and second MDS axis, respectively. The parametersexamined were mean temperature, mean salinity and inte-grated fluorescence of the depth stratum at each station.

R E S U L T S

Environmental conditionsThe thermohaline structure of the water column in the studiedarea during the survey has been already described in detail byIsari et al. (2007). In the easternmost part of the Thracian Seathe relatively less saline and warmer waters denoted the pres-ence of an anticyclonic gyre (the Samothraki gyre) wherea large amount of the advected BSW was entrapped(Figure 2A, B). Colder and saltier waters occupied thewestern and offshore part of the surveyed area (Figure 2A,B). Sections of salinity and temperature over a longitudinaltransect (Station 12 to Station 39) across the Thracian Seacontinental shelf revealed the horizontal (�50 km) and verti-cal (�20 m) dimensions of the gyre (Figure 2D, E).

Fluorescence map at 10 m depth indicated a gradient fromwest to east (Figure 2C). The south-western region with highsalinity waters presented lower fluorescence than the coastalsites. Deeper layers exhibited increased fluorescence com-pared to surface layers, indicating near-bottom chlorophyllmaxima (Figure 2F).

Mesozooplankton community

layer 1: from sea surface to the base of

halocline (�0--20 m)

The spatial distribution of mesozooplankton stock and abun-dance of major mesozooplankton groups in Layer 1 hasalready been presented in Isari et al. (2007). In the presentpaper, cluster analysis was applied to the entire mesozoo-plankton biota (including copepod and cladoceran species)(Tables 1 & 2) revealing at the 67% similarity level threemajor groups of samples (Groups I1, II1 and III1; Figure 3A)with a clear distinction in group and species composition(see below). This grouping, being also evident in the NMDSordination (Figure 3B), seemed to be closely associated withhydrographic features and the location of sampling sites inrelation to the Samothraki gyre.

Group I1 comprised seven samples collected from sitesaround the island of Samothraki (gyre area), Group II1

included mainly stations situated inside the continental shelfboundaries, while three stations (Station 26, 28 and 29) inthe western and offshore part of the studied area withbottom depth exceeding 250 m formed Group III1

(Figure 3A). One sample collected from a coastal station(Station 11), was also defined as an outlier.

Mesozooplankton stock differed significantly between thethree Groups of stations (F ¼ 11.98, P , 0.001): It washigher in Group I1 (gyre-Group), intermediate in Group II1

structure of mesozooplankton assemblages 53

and lower in Group III1 (Table 1). Cladocerans, predominatedin the community structure within the gyre-Group, mainlyrepresented by the species Penilia avirostris andPseudoevadne tergestina (Table 2), and reduced graduallyfrom Group I1 to Groups II1 and III1 (Table 1; Figure 3C).Zooplankters with particularly low concentrations withinthe gyre (Group I1) such as copepods and appendicularians,were considerably more abundant in Group II1 (Table 1)and contributed at a higher percentage to the communitystructure (Figure 3C). Doliolids consisted also an importanttaxon all over the study area (Figure 3C), nevertheless theywere found at lower densities in Group III1 (Table 1). Thelatter station group generally depicted the lowest abundancevalues for most of the mesozooplankton taxa (Table 1).

An additional spatial differentiation was observed in thecopepod assemblage over the surveyed area. Among the 61copepod taxa identified in Layer 1 of the entire area(Table 2), only an average of 15 were detected per station in

the gyre area (Group I1) and the Shannon–Weiner (H′) diver-sity index based on adults had a mean of 2.8. Temora styliferaalong with its copepodites was by far the dominant copepod inthis station group representing up to 64% of total copepodsabundance. Other species, the harpacticoid Euterpina acuti-frons, the cyclopoid Oithona plumifera along with total copepo-dites of Oithona and the calanoids Paracalanus parvus andAcartia clausi followed in rank order, with their mean relativeabundance ranging between 5 and 10%. Contrary to thegyre-Group, the two others (Groups II1 and III1) presentedhigher species number and H′ (23 species per station; GroupII1: H′ ¼ 3.3, Group III1: H′ ¼ 3.7). Paracalanus parvus wasthe most important copepod species in Group II1 (24% oftotal copepods) followed by taxa with a mean relative densityof about 10%, i.e. copepodites of Clausocalanus spp., copepo-dites of Oithona spp., Temora stylifera and Acartia clausi.The non-calanoids Oncaea media, Oithona nana andEuterpina acutifrons were also present with a mean relative

Fig. 2. Contour maps of salinity (A), temperature (8C) (B) and fluorescence (arbitrary units) (C) at 10 m depth. Sections of salinity (D), temperature (E) andfluorescence (F) over a longitudinal transect (Station 12 to Station 39) across the Thracian Sea continental shelf (redrawn from Isari et al. (2007) with kindpermission of Springer Science and Business Media).

54 stamatina isari et al.

density around 4%. The dominant copepods in Group III1 inrank order were the copepodites of Clausocalanus spp.,Oithona spp., Temora stylifera and Paracalanus parvus (withtheir relative density ranging between 11 and 21%).

Relating mesozooplankton assemblage structure to hydrol-ogy, we found all the examined parameters to explain anamount of the variation in the NMDS ordination (Table 3).Based on relative angles of intersection, mean salinity andtemperature in the upper sampling layer seemed to berelated to the differentiation of gyre-stations (Group I1)from Group II1 (lower mean salinity and higher mean temp-erature in the stations of the anticyclonic zone) (Figure 3B).Integrated fluorescence appeared to explain the differentiationof the pelagic Group III1 from Groups I1 and II1, (pelagicstations had lower fluorescence values) (Figure 3B). Meanvalues of these parameters per group are presented in Table 4.

layer 2: from the base of halocline down to

50 m depth

Cluster analysis divided the 16 samples collected from thislayer into three groups (Groups I2, II2 and III2) (Figure 3A)at the 56% similarity level. This grouping was also evidentin the respective NMDS ordination (Figure 3B). Group I2

comprised samples from four stations located at the peripheryof the anticyclonic zone (Stations 9, 14, 19 and 16). Group II2

corresponded to the remaining sites with the exception of twostations (Stations 18 and 28) in the south-western part of thesurveyed area, which formed Group III2 (Figure 3A). Mean

mesozooplankton abundance presented a significant decrease(F ¼ 48.24, P , 0.001) from Group I2 to Groups II2 and III2

(Table 1). This gradual decrease was followed by a changein the contribution of the major zooplankters in the assem-blage structure, similar to that described for the respectivestation groups of the surface layer (Groups I1 –III1;Figure 3C). The copepod community in Group III2 showedthe highest mean number of species and diversity perstation (31 species, H′ ¼ 3.9) when compared to Groups I2

and II2 (27 and 26 species respectively, H′ ¼ 3.7). Temora sty-lifera along its copepodites were particularly important inGroup I2, whereas copepodites of the genera Oithona andClausocalanus were more abundant in the other stationgroups. The actual contribution of the rest of the identifiedtaxa is listed in Table 2. Among the parameters examined,only integrated fluorescence in the sampling layer (�20–50 m) showed a significant relationship with ordinationscores (Table 3), with an increasing trend from Group III2

towards Groups I2 and II2 (Table 4; Figure 3B).

vertical distribution of mesozooplankton

within the 0--50 m layer

Considering the 16 stations where sampling was successfullyperformed in both sampling layers, two distinct trends were dis-cerned in the vertical distribution pattern of abundance and dryweight values (hereafter referred to as Pattern I and Pattern II)(Figure 4). The majority of the sites (station Groups II2 and III2

in Figure 3A) presented a decrease in the mesozooplanktonstock in the deeper stratum (base of halocline-50 m) compara-tively to the surface layer (Pattern I). An inverse trend (PatternII) was revealed for the four sites located at the periphery of theanticyclone (station Group I2 in Figure 3A). Layer 1 of stationspresenting Pattern I had significantly higher total abundanceand biomass (Table 5), whereas the differences between Layer1 and Layer 2 were not statistically significant for stationsexhibiting Pattern II.

The mesozooplankton group and copepod species verticaldistribution showed heterogeneity among sites, mostlyrelated to the general pattern of vertical distribution(Patterns I or II) (Table 5; Figure 5). Cladocerans, doliolids,appendicularians and pteropods were found at significantlyhigher concentrations within the halocline layer (Layer 1),with the exception of sites displaying Pattern II (Table 5). Inthe latter case no significant differences were found betweenthe two sampling layers. Chaetognath abundance did notpresent any significant difference for both Patterns I and II.Copepods did not present significant difference between thetwo sampling layers of Pattern I-sites, whereas they were sig-nificantly more abundant in the layer below the halocline(Layer 2) at sites presenting Pattern II (Table 5; Figure 5).

Moreover, many copepod species dominant in the surveyedarea presented different depth distributions for Pattern I andPattern II stations (Table 5; Figure 5). Copepodites of thegenera Oithona and Clausocalanus as well as the speciesOithona plumifera, Acartia clausi and Farranula rostrata weredistributed homogeneously in both depth layers over theentire studied area (Table 5). However, some species withclear preference for the surface layer in the majority ofsites, i.e. Temora stylifera, Euterpina acutifrons, Paracalanusparvus and Clausocalanus furcatus were recorded in relativelyhigher concentrations in Layer 2 for Pattern II-sites(Group I2) (Table 5; Figure 5). Interestingly, the harpacticoid

Table 1. List of major mesozooplankton groups identified in the samplesof the two sampling layers (Layer 1, 0-base of halocline; Layer 2, base ofhalocline-50 m). Mean abundance values (ind. m23) per group ofsamples defined by the cluster analysis (Layer 1, Groups I1, II1 and III1;

Layer 2, Groups I2, II2 and III2) are given.

Layer 1 Layer 2

I1 II1 III1 I2 II2 III2

HOLOPLANKTONCopepods 377.0 1266.1 648.7 1815.1 739.0 240.4Cladocerans 11739.0 5120.2 1415.0 9662.4 1145.5 41.9Doliolids 659.2 536.1 135.3 1786.0 258.6 27.8Appendicularians 143.3 755.8 130.2 367.0 111.1 21.5Pteropods 49.3 134.1 60.4 74.6 55.9 34.8Chaetognaths 67.4 65.9 26.4 169.7 53.3 14.7Copepod nauplii 6.6 9.1 4.8 4.7 9.4 –Euphausiids 1.7 5.6 4.2 22.4 15.6 2.1Heteropods – – – 1.2 – –Medusae 3.0 11.3 3.3 12.9 5.3 1.8Ostracods – 2.7 1.4 23.5 6.5 2.5Siphonophores 6.2 4.8 5.8 26.0 7.9 4.4Salps – – – – 0.9 –Tomopteris spp. – – – – 0.3 0.5

MEROPLANKTONEchinoderm larvae 12.8 0.9 1.4 11.1 0.7 0.2Gastropod larvae 20.8 58.1 9.9 35.3 20.9 4.8Lamellibranchia larvae 45.9 36.1 6.4 94.0 21.5 2.5Cirriped larvae 10.8 7.1 3.6 16.9 5.4 1.0Polychaet larvae 2.9 5.3 0.9 20.0 3.4 0.4Decapod larvae 1.8 5.7 0.3 3.5 5.0 –Fish eggs 4.6 29.9 – 147.1 4.2 0.3Fish larvae 4.6 13.2 2.2 23.1 2.9 0.5Total abundance 13157.7 8068.1 2460.3 14316.4 2473.4 402.0

structure of mesozooplankton assemblages 55

Table 2. List of copepod and cladoceran taxa identified in the samples of the two sampling layers (Layer 1, 0-base of halocline; Layer 2: base ofhalocline-50 m). Mean abundance values (ind. m23) per group of samples defined by the cluster analysis (Layer 1, Groups I1, II1 and III1; Layer 2,

Groups I2, II2 and III2) are given.

Layer 1 Layer 2

I1 II1 III1 I2 II2 III2

Calanoid copepodsAcartiidae Acartia clausi ∗ 27.0 143.5 20.2 124.3 41.4 3.5Aetideidae Aetideus armatus∗ – 0.3 – – – –

Aetideus giesbrechti∗ – – 0.8 – 0.3 0.2Calanidae Calanus helgolandicus ∗ – 0.8 – 5.6 0.5 0.3

Calanus spp. cop. 0.2 4.2 3.1 8.4 10.4 1.1Mesocalanus tenuicornis ∗ 0.7 5.0 1.1 8.9 7.3 0.9Nannocalanus minor ∗ – 0.8 – 9.1 0.8 1.4Neocalanus gracilis cop. – – – – 0.3 –

Candaciidae Candacia armata 0.4 – – 1.2 0.3 –Candacia spp. cop. – 1.1 – 1.2 3.1 0.3

Centropagidae Centropages kroyeri – 0.2 – – 0.2 –Centropages ponticus ∗ 0.4 1.9 – – – –Centropages typicus ∗ 2.7 36.8 4.0 50.0 45.7 7.4Centropages violaceus cop. – 0.5 1.1 – – –

Clausocalanidae Clausocalanus arcuicornis 0.3 5.2 – 3.7 2.9 1.0Clausocalanus furcatus 4.7 14.4 17.0 44.1 1.9 0.7Clausocalanus jobei 0.7 1.4 – 7.2 4.3 2.4Clausocalanus mastigophorus – 0.1 – – – –Clausocalanus parapergens 0.2 0.7 – 1.2 0.3 1.0Clausocalanus paululus – 0.2 – – – 0.3Clausocalanus pergens – – – – 0.3 2.6Clausocalanus spp. cop. 10.0 132.7 145.6 169.2 91.1 44.6Clausocalanus spp. males 1.1 9.4 12.9 5.9 5.4 0.3Ctenocalanus vanus∗ 0.3 5.9 1.6 11.0 12.8 5.2Pseudocalanus spp. – 0.1 – – 3.8 –

Diaixidae Diaxis pygmaea – – – – 1.0 –Eucalanidae Eucalanus attenuatus cop. – 0.1 – – – –

Eucalanus elongatus cop. – – – 1.2 0.1 –Eucalanus monachus cop. – 0.3 – – 0.1 –

Euchaetidae Euchaeta hebes – – – – 0.6 –Euchaeta spp. cop. – 0.3 – 7.9 3.6 0.7

Heterorhabdidae Heterorhabdus papiliger∗ 0.3 0.1 – – – –Lucicutiidae Lucicutia flavicornis∗ – 2.3 – – 0.4 0.3Mecynoceridae Mecynocera clausi∗ 1.1 1.6 1.4 18.3 9.0 8.1Metridinidae Pleuromamma spp. cop. – 0 0.6 – – –Paracalanidae Calocalanus contractus – – 0.6 – – –

Calocalanus neptunus – 0.2 – – – –Calocalanus pavo∗ 1.0 1.9 3.4 7.1 5.1 1.1Calocalanus pavoninus – 2.9 1.6 4.9 2.1 0.5Calocalanus spA 1.3 2.7 7.5 13.4 18.3 7.5Calocalanus spp. cop. 0.7 3.2 4.7 13.4 10.4 6.4Calocalanus styliremis – 0.5 – 1.2 3.2 1.4Calocalanus plumulosus∗ – – – 2.5 0.9 1.9Paracalanus parvus∗ 29.0 300.5 73.8 228.9 85.2 9.0Paracalanus denudatus∗ 0.3 3.0 2.5 8.4 2.5 1.7Paracalanus nanus 1.1 15.5 16.2 12.5 30.0 10.5

Pontellidae Pontella spp. cop. – 0.7 0.3 – – –Spinocalanidae Spinocalanus spp. – 0.2 – – – –Temoridae Temora stylifera∗ 132.4 157.3 100.5 303.6 24.1 6.2

Cyclopoid copepodsCorycaeidae Agetus limbatus – 0.4 – 1.2 0.3 1.1

Agetus typicus – 0.8 – 13.5 3.0 3.9Corycaeidae cop. 0.7 3.5 6.4 2.5 3.2 4.8Corycaeus clausi – 0.1 – 1.2 0.3 –Ditrichocorycaeus brehmi – 1.4 1.4 4.6 0.2 0.2Farranula rostrata∗ 0.5 14.0 19.3 16.1 20.7 21.6Onychocorycaeus giesbrechti 2.2 2.5 5.3 5.5 0.8 0.5Onychocorycaeus ovalis/lattus 0.3 0.7 0.3 0.9 0.2 –Urocorycaeus furcifer – – – – 0.1 –

Continued

56 stamatina isari et al.

Clytemnestra rostrata and the cyclopoid Oncaea media (alongwith the copepodites and males of this genus) were more abun-dant in the deeper layer of the broader anticyclonic zone(Stations 10, 15 and 20 and Group I2) (Table 5; Figure 5).

D I S C U S S I O N

Recent studies on the structure of zooplankton assemblages inthe marine system of NEA have revealed new data regardingthe controlling mechanisms of this system and its biologicalcomplexity. Broad scale investigations, mainly conductedduring the thermal stratification period, have underlined themajor role of the inflow and advection of BSW in the for-mation of mesozooplankton (Isari et al., 2006; Zervoudakiet al., 2006) and ichthyoplankton assemblages (Isari et al.,2008). These studies point out that variability in zooplanktondistribution patterns at different temporal scales (seasonal andinter-annual), is highly related to respective changes in thehorizontal extension of the BSW.

The hydrological data collected during the present studyclearly revealed a strong influence of the BSW in the upperpart of the water column. Low salinity waters of Black Seaorigin, were advected towards the eastern part of theThracian shelf and subsequently captured by an anticycloneof �50 km wide around the island of Samothraki(Samothraki gyre). This feature resulted in the convergenceand downwelling of BSW down to 20 m depth, hence lesssaline and warmer waters were observed around this island.

Mesozooplankton sampling in two discrete layers (withinand below the halocline) facilitated the better understandingof the influence of BSW on mesozooplankton distributionand assemblage structure, but also provided some preliminaryinformation for the dynamics of the Samothraki gyre.

Layer 1: from sea surface to the base ofhalocline (∼0–20 m)Sampling in the actual layer of BSW influence (Layer 1)revealed a marked spatial differentiation in mesozooplanktonabundance and taxonomic group composition throughout theNEA. Stations around the island of Samothraki (Group I1),which were highly influenced by the low salinity–high temp-erature gyre waters, presented increased mesozooplanktonstock as well as a distinctive species composition andabundance (striking dominance of Penilia avirostris, andremarkably low densities of appendicularians and copepods).A prominent shift in the assemblage structure was detectedtowards the western and offshore part of the NEA. Thischange in the community organization was found significantlyrelated to the spatial variability in temperature, salinity andfluorescence (Figure 3B) and mainly involved a generaldecrease in both total abundance values and cladocerans’ rela-tive importance as well as an opposite trend regarding themean contribution of copepods.

Our results clearly showed that the 50-km anticyclonearound the island of Samothraki, despite its rather smallsize, provided distinct boundaries for the meso-sized

Table 2. Continued

Layer 1 Layer 2

I1 II1 III1 I2 II2 III2

Lubbockiidae Lubbockia squillimana – – 0.8 2.1 0.6 –Oithonidae Oithona nana 4.7 47.7 8.2 11.3 8.1 0.7

Oithona plumifera 28.0 29.4 14.7 95.1 23.8 5.2Oithona setigera – 0.6 – – 0.8 0.3Oithona similis 1.6 8.9 – 16.0 11.3 –Oithona spp. cop. 38.8 130.6 109.2 184.6 124.7 44.3Oithona spp. males 1.1 5.7 0.6 3.7 7.4 0.2

Oncaeidae Oncaea media1 19.6 53.1 21.7 147.5 46.7 14.1Oncaea mediterranea 0.7 1.4 0.3 25.3 2.3 1.6Oncaea spp. cop. 4.2 24.7 10.1 60.0 11.4 4.1Oncaea spp. males 8.3 17.1 8.1 58.1 14.3 5.8Triconia conifera – 3.6 – – 0.2 –

Sapphirinidae Copilia mediterranea∗ 0.6 1.0 1.1 3.7 1.7 –Vettoria spp. – – – – 0.2 –Sapphirina spp. – – – – 0.3 –

Harpacticoid copepodsHarpacticoids spp. 0.2 1.0 0.3 – 1.0 0.3

Clytemnestridae Clytemnestra rostrata∗ 9.2 5.3 2.8 43.0 8.5 0.2Ectinonosomatidae Microsetella rosea 2.4 1.4 2.2 – – 0.2Euterpinidae Euterpina acutifrons 38.1 52.8 15.2 44.8 16.6 2.7Macrosetellidae Macrosetella gracilis – – – – 0.2 –

CladoceransEvadne nordmanni – 0.1 – – 0.2 6.5Evadne spinifera 281.7 270.7 84.1 427.6 14.4 8.6Pseudoevadne tergestina 1103.3 627.1 89.2 1804.2 194.6 26.8Penilia avirostris 10354.1 4221.2 1241.7 7429.4 935.0 –Podon intermedius – 0.7 – 1.2 1.3 –Pleopis polyphemoides – 0.4 – – – –

cop., copepodites; ∗, copepodites + adults; 1, includes the triplet of the cryptic species O. media, O. scottodicarloi and O. waldemari.

structure of mesozooplankton assemblages 57

heterotrophic plankton component. Similarly, other meso-scale anticyclonic eddies, but usually much larger, exhibit dis-tinct mesozooplankton assemblages compared to adjacentwaters (e.g. Ligurian Sea: Pinca & Dallot, 1995; CanaryIslands: Hernandez-Leon et al., 2001; Bay of Biscay: Islaet al., 2004; Algerian basin: Riandey et al., 2005; NorthPacific: Mackas et al., 2005). Hence, the ecological effects ofsuch physical features seem to be not necessarily dependenton their spatial scale (Owen, 1981). It is rather the relativesize, i.e. the eddy-size in relation to the dimension of the

area where it forms that determines its effects (Weikert &Koppelmann, 1993).

The increased productivity in terms of mesozooplanktonstock within the Samothraki gyre appears rather exceptionalto what is generally expected for anticyclones. The convergenttendency within an anticyclone (downward motion in theinterior of the eddy) theoretically prevents nutrient-pumpingfrom deeper layers, which would negatively affect the popu-lations of organisms at higher trophic levels (Lalli & Parsons,2002). Indeed, low mesozooplankton concentrations comparedto surrounding waters have been reported for anticyclonic

Fig. 3. (A) Maps showing group of stations (corresponding to group of samples) defined by the cluster analysis in each sampling layer (Layer 1, 0-base of halocline;Layer 2, base of halocline-50 m); (B) ordination plots of the comparison between samples of each layer using non-metric multidimensional scaling and Bray–Curtis similarity index. Respective cluster groups are indicated. Significant multiple regressions between ordination scores and environmental parameters (S,salinity; T, temperature; F, integrated fluorescence) are shown, as well as the fraction (%) of variance in the mesozooplankton data explained by theparameters (O, Groups I1 and I2; A, Groups II1 and II2; †, Groups III1 and III2; , outlier station); (C) mean relative density (%) of the majormesozooplankton groups for each group of samples in the two discrete layers.

Table 3. Multiple regression analysis between various parameters (temp-erature, salinity and fluorescence) and the non-metric multidimensionalscaling scores for two-axis ordination of samples (∗, P,0.05; ∗∗, P ,

0.01; n.s., not significant).

Direction cosines

X Y Adj.R2 F

Layer 1: 0-base of halocline

Temperature 0.549 0.836 18.47 4.29∗

Salinity 0.948 20.318 65.39 28.40∗∗

Fluorescence 20.626 20.779 18.94 4.39∗

Layer 2: base of halocline-50 m

Temperature 0.00n.s.

Salinity 1.12n.s.

Fluorescence 20.544 20.839 33.64 4.80∗

Table 4. Mean values for temperature (T, 8C), salinity (S, psu) and inte-grated fluorescence (F, arbitrary units) for the sample groups defined bythe cluster analysis in the two distinct sampling layers (Layer 1, 0-base

of halocline, Layer 2, base of halocline-50 m). N, number of samples.

I1 II1 III1

Layer 1 T 23.3 21.8 21.8S 33.0 35.7 37.9F 0.029 0.029 0.017N 7 19 3

I2 II2 III2

Layer 2 T 17.8 17.9 17.6S 38.6 38.7 39.0F 0.056 0.037 0.018N 4 10 2

58 stamatina isari et al.

eddies in the Levantine Basin (Pancucci-Papadopoulou et al.,1992; Mazzocchi et al., 1997). Nevertheless, a site intensivestudy over a complex eddy-field in the Algerian basin showedrecently that the impact of mesoscale eddies on productivitylevels could be depended, not only on their intrinsic dynamics(cyclonic or anticyclonic circulation pattern), but also on theiractual location and season (Riandey et al., 2005).

In the present study, the observation of high mesozoo-plankton stock within the Samothraki gyre could be explainedby a combination of parameters such as the sampling date(season) and the physicochemical properties of the gyrewaters. The increased abundance of mesozooplanktonwithin this gyre was mostly due to the outstanding numericaldominance of the thermophilic cladoceran Penilia avirostris.The temporal match between our sampling and the seasonalcycle of P. avirostris in the Mediterranean waters (Atienzaet al., 2008 and references therein) can explain the generallyhigh abundance of this species all over the Thracian shelfduring the studied period. However, the higher temperatureand other biochemical characteristics of the gyre waters mayhave favoured the rapid development of a spatially con-strained population as has been discussed by Isari et al.(2007). Hence, it seems that the high mesozooplankton con-centrations observed within the Samothraki gyre are closelyrelated to some particular characteristics of the waters

entrapped in the anticyclone (of BSW origin). In otherregions, mesoscale anticyclonic structures exhibited elevatedzooplankton biomass and abundance, mainly due to highnumbers of specific taxa (Pinca & Dallot, 1995) or werecharacterized by accumulation and retention of large zoo-plankters (Hernandez-Leon et al., 2001).

Biological responses to a single physical structure (front oreddy) may differ among various zooplankton taxa (Pinca &Dallot, 1995; Mackas et al., 2005; Riandey et al., 2005;Molinero et al., 2008) or even different developmental stagesof a given species (Boucher, 1984). Patterns in the horizontaldistribution of the major mesozooplankton groups differedmarkedly in this study and have been discussed in detail inIsari et al. (2007), especially the striking low numbers ofcopepods and appendicularians in the cladoceran/dolioliddominated Samothraki gyre. Interestingly, high horizontalheterogeneity was also observed in the copepod species com-position as revealed in this paper. Copepods exhibited ahigher contribution to community composition and increasedspecies diversity in the western and offshore part of the NEA(station Group III1). Moreover, within the anticyclonic zone(Group I1), a distinct copepod assemblage was identifiedwith extremely low abundances and limited species richnessand diversity. The calanoid Temora stylifera was the onlyspecies found in Group I1 in numbers similar to the remaining

Fig. 4. Vertical distribution patterns (Layer 1, 0-base of halocline; Layer 2, base of halocline-50 m) of total mesozooplankton abundance (ind. m23) and biomass(mg m23) throughout the study area (Pattern I, Layer 1. Layer 2; Pattern II, Layer 2. Layer 1). Groups of stations identified by cluster analysis in Layer 2 (GroupsI2, II2 and III2) are indicated.

structure of mesozooplankton assemblages 59

neritic sites. Other species, relatively abundant over theThracian shelf (station Group II1), i.e. Acartia clausi,Paracalanus parvus, Oithona spp. and Clausocalanus spp.were quite rare within the gyre area.

The observed distribution patterns of copepod species couldbe interpreted as reflecting interspecific differences in egg pro-duction and recruitment rates, driven by the complex inter-actions of several environmental factors in combination withdifferences in life history, behavioural and physiological traitsof the various species. Temora stylifera constitutes an importantspecies in neritic Mediterranean waters, mostly during thesummer–autumn period (e.g. Siokou-Frangou, 1996; DiCapua & Mazzocchi, 2004), when egg production rates andhatching success are higher (Ianora & Poulet, 1993, but seeHalsband-Lenk et al., 2001). With regard to coastal Greekwaters, this particular species forms, together with the cladocer-ans Penilia avirostris and Pseudoevadne tergestina, a thermo-philic assemblage with preference for the warmest-stratifiedconditions (Siokou-Frangou et al., 1998). Additionally, otherauthors have pointed out a positive association of T. styliferawith saline stratification (Cabal et al., 2008). Hence, warmerand less saline waters within the gyre (Table 4) may explainhigh abundance of this species. However, T. stylifera was alsoabundant outside these waters (Table 2).

Food availability, both in terms of quantity and quality ofprey, is considered to play also a key role in the reproductionof copepods and the subsequent fate of their populations (ashas been also discussed for T. stylifera by Mazzocchi et al.,2006). Within the cladoceran/doliolid dominated gyre of ourstudy, copepods were likely experiencing a strong food limit-ation (Isari et al., 2007). Nevertheless, different copepod

species may present a variable response to a specific nutri-tional regime according to their feeding selection mechanismand prey size selectivity. Temora is considered among themost advanced genera from an evolutionary perspective,characterized by continuous motion and creation of feedingcurrent, even from the last naupliar stage NVI (Paffenhofer,1998). Moreover, recent feeding experiments on naturalmicrobial assemblages in the Mediterranean have shownthat ciliates and .5 mm phytoplankton cells both consistpotential food items for the species T. stylifera (Broglioet al., 2004). Hence, dietary flexibility and continuousfeeding current may have increased competitive ability ofthis species in the gyre environment.

Layer 2: from base of halocline down to50 m depthSpatial differentiation in mesozooplankton structural proper-ties was also observed below the halocline (Layer 2). Samplesfrom sites located over the shelf (Group I2 and Group II2) werehighly differentiated from those collected in the oligotrophicpelagic area (Group III2), mainly due to their increased meso-zooplankton abundances. Integrated fluorescence within the20–50 m layer was the only parameter significantly relatedto the multivariate community structure, displaying highervalues in the neritic compared to the pelagic sites. Thisresult can be explained by the fact that deep fluorescencemaxima (DFMs) were recorded within this actual samplinglayer in the case of shelf-stations, while DFMs were founddeeper and had lower values at the pelagic sites.

Table 5. Mean values of total mesozooplankton stock and abundance of the major taxa (ind. m23) for the two sampling layers (Layer 1, 0-base of halo-cline; Layer 2, base of halocline-50 m) according to the vertical distribution pattern of total mesozooplankton stock of the stations (Patterns I and II).

Results of t-tests are given for each pattern (n.s., not significant; ∗, ,0.05; ∗∗, ,0.01; ∗∗∗, ,0.001).

Pattern I Pattern II

Layer 1 Layer 2 t Layer 1 Layer 2 t

Total abundance (ind m23) 8094.7 5175.2 4.60∗∗∗ 8923.9 14316.4 21.48 n.s.

Biomass (mg m23) 31.7 24.2 4.58∗∗∗ 37.5 56.3 22.07 n.s.

Copepods 825.9 945.7 1.20 n.s. 755.2 1815.1 22.72∗

Acartia clausi 52.1 35.1 1.19 n.s. 59.7 124.3 21.64 n.s.

Centropages typicus 7.8 39.3 23.04∗∗ 7.9 50.0 22.18 n.s.

Clausocalanus spp. cop. 127.1 83.4 0.80 n.s. 49.3 169.2 21.46 n.s.

Clausocalanus furcatus 13.5 1.7 2.65∗ 13.0 44.1 21.00 n.s.

Clytemnestra rostrata 4.8 7.1 0.77n.s. 6.7 38.5 22.61∗

Euterpina acutifrons 33.1 14.3 3.09∗∗ 56.6 44.8 0.64n.s.

Farranula rostrata 14.1 20.9 20.97n.s. 9.7 16.1 21.35 n.s.

Oithona spp. cop. 114.2 111.3 20.02n.s. 68.8 184.6 21.20 n.s.

Oithona nana 25.3 6.9 2.97∗∗ 28.0 11.3 0.70 n.s.

Oithona plumifera 31.0 20.7 1.92 n.s. 23.8 95.1 22.41 n.s.

Oithona similis 3.6 9.4 21.3 n.s. 4.8 16.0 22.31 n.s.

Oncaea spp. cop. 20.9 10.2 1.14 n.s. 15.8 60.0 22.37 n.s.

Oncaea media1 34.4 41.3 20.97 n.s. 46.5 147.5 24.43∗∗

Paracalanus nanus 15.6 26.8 21.45 n.s. 3.3 12.5 22.80∗

Paracalanus parvus 155.2 72.5 2.19∗ 137.4 228.9 21.64 n.s.

Temora stylifera 111.2 21.2 5.44∗∗∗ 167.4 303.6 21.16 n.s.

Cladocerans 5969.3 3136.8 4.34∗∗∗ 6802.6 9662.4 20.51 n.s.

Appendicularians 428.8 163.9 4.13∗∗∗ 341.2 367.0 20.65 n.s.

Doliolids 517.4 611.6 2.53∗ 711.4 1786.0 22.01 n.s.

Chaetognaths 59.5 77.6 1.14 n.s. 71.8 169.7 21.44 n.s.

Pteropods 120.6 57.9 2.33∗ 49.9 74.6 21.64 n.s.

1, includes the triplet of the cryptic species O. media, O. scottodicarloi and O. waldemari.

60 stamatina isari et al.

Despite the relative homogeneity of temperature and sal-inity in this sampling layer (Table 4), a strong variability inmesozooplankton community was evident for sites over thecontinental shelf. This variation seemed to be associatedwith the location of sites in relation to the Samothraki gyre.Samples collected at the periphery of the gyre (Group I2) pre-sented higher abundances for the major mesozooplanktongroups compared to the remaining samples of Layer 2 col-lected over the shelf (Group II2) (Table 1). Surprisingly, forstations located at the periphery of the gyre (Group I2), thisdeeper layer was slightly richer in total mesozooplanktoneven when compared to the corresponding surface layer(Layer 1). This vertical pattern (Pattern II: mesozooplanktonstock of Layer 2. mesozooplankton stock of Layer 1,although of no statistical significance) contrasted withthe usual pattern of the remaining stations (Pattern I: meso-zooplankton stock of Layer 1. mesozooplankton stock ofLayer 2).

Pattern II (the increase of mesozooplankton in the subsur-face layer below the halocline) in stations located at the per-iphery of the anticyclone was due to an unusually high

concentration of generally surface-living zooplankters inLayer 2 (e.g. cladocerans, appendicularians, Clausocalanusfurcatus and Temora stylifera). This could be explained byhydrodynamic processes associated with the anticyclonic cir-culation. Bakun (2006) who analysed the physical mechan-isms involved in the formation of eddies highlighted suchprocesses in the case of anticyclones that may elucidate ourresults. The complex interaction of distinct dynamic forces(Coriolis force, centrifugal force and frictional torques)results in the concentration of waters towards the interior ofthe eddy and the subsequent doming in the sea surface.Such a dome, increases the hydrostatic pressure in thedeeper layer (below the centre of the eddy), causing a radialoutward-directed flow in the sub-layer, opposite to theinward-directed flow pattern in the upper layer. Downwardpumping of water of the upper layer results in a continuousthree-dimensional flow configuration, connecting the flowpatterns in the two layers (see figure 3b in Bakun, 2006). Aresult of such a flow configuration would be the advectionof surface-dwelling zooplankters into subsurface layers atthe periphery of the anticyclone. Moreover, the prominent

Fig. 5. Abundance (ind. m23) of major mesozooplankton groups and dominant copepod species per sampling layer (Layer 1, 0-base of halocline; Layer 2, base ofhalocline-50 m) throughout the study area. Distinct patterns in total mesozooplankton vertical distribution (Patterns I and II, see Figure 4) as well as groups ofstations identified by cluster analysis in Layer 2 (Groups I2, II2 and III2) are indicated.

structure of mesozooplankton assemblages 61

presence of the cyclopoid Oncaea media as well as the harpac-ticoid Clytemnestra rostrata in Layer 2 over the anticycloniczone might be indicative of a habitat with special ecologicalcharacteristics. Species of the genus Oncaea, are ratherabundant in depths greater than 20–30 m, where they preyon sinking and aggregating material of zoo- or phytoplank-tonic origin (Paffenhofer & Mazzocchi, 2003). The increasedpresence of appendicularians in deeper waters has also beenrelated with the presence of aggregates of organic material(Mazzocchi et al., 1997). In other studies, the occurrenceof epipelagic organisms (Penilia avirostris, siphonophoresand appendicularians) down to the depth of 1000 m(Pancucci-Papadopoulou et et al., 1992) or the deepening ofthe deep chlorophyll maximum (Mazzocchi et al., 1997)have been related to the downward flow and deepening ofisopycnals induced by the anticyclonic circulation patterns.

C O N C L U S I O N S

The present study implies that, during the period of thermalstratification, the Samothraki gyre may have a considerableimpact on the physicochemical and biological properties ofthe NEA system. The anticyclonic zone, presented a distinctcopepod community structure, characterized by low abun-dance numbers and limited species diversity. On the contrary,this habitat was suitable for the survival and reproduction ofcertain taxa (cladocerans and doliolids) that can exploit themicrobial food web. Although the influence of BSW wasmainly restricted to the upper 20 m layer (Layer 1), complexhydrodynamic processes related to the anticyclonic circula-tion pattern, might further affect the mesozooplankton com-munity structure of the deeper layer (Layer 2). Consideringthe potential role of this physical structure as a spawningground for small pelagic fish (i.e. anchovy) and a retentionmechanism for fish larvae (Isari et al., 2008), the dynamicsand food web processes associated with this feature deservedetailed future investigations.

A C K N O W L E D G E M E N T S

This work was supported by the EU project ANREC(QLRT-2001-01216). The authors wish to thank the captainand the crew of RV ‘Aegaeo’ as well as all participant scientistsfor their support at sea. Special thanks go to two anonymousreferees for their helpful comments and suggestions on themanuscript.

R E F E R E N C E S

Alcaraz M., Calbet A., Estrada M., Marrase C., Saiz E. and Trepat I.(2007) Physical control of zooplankton communities in the CatalanSea. Progress in Oceanography 74, 294–312.

Atienza D., Saiz E., Skovgaard A., Trepat I. and Calbet A. (2008) Lifehistory and population dynamics of the marine cladoceran Peniliaavirostris (Branchiopoda: Cladocera) in the Catalan Sea (NWMediterranean). Journal of Plankton Research 30, 345–357.

Bakun A. (2006) Fronts and eddies as key structures in the habitat ofmarine fish larvae: opportunity, adaptive response and competitiveadvantage. Scientia Marina 70, 105–122.

Boucher J. (1984) Localization of zooplankton populations in the Ligurianmarine front: role of ontogenic migration. Deep-Sea Research 31,469–484.

Broglio E., Saiz E., Calbet A., Trepat I. and Alcaraz M. (2004) Trophicimpact and prey selection by crustacean zooplankton on the microbialcommunities of an oligotrophic coastal area (NW Mediterranean Sea).Aquatic Microbial Ecology 35, 65–78.

Cabal J., Gonzales-Nuevo G. and Nogueira E. (2008) Mesozooplanktonspecies distribution in the NW and N Iberian shelf during spring 2004:relationship with frontal structures. Journal of Marine Systems 72,282–297.

Clarke K.R. and Warwick R.M. (1994) Change in marine communities:an approach to statistical analysis and interpretation. Plymouth:Natural Environment Research Council, UK.

Di Capua I. and Mazzocchi M.G. (2004) Population structure of thecopepods Centropages typicus and Temora stylifera in differentenvironmental conditions. ICES Journal of Marine Science 61,632–644.

Fernandez de Puelles M.L., Valencia J., Jansa J. and Morillas A. (2004)Hydrographical characteristics and zooplankton distribution in theMallorca channel (Western Mediterranean): spring 2001. ICESJournal of Marine Science 61, 654–666.

Gaudy R. and Youssara F. (2003) Variations of zooplankton metabolismand feeding in the frontal area of the Alboran Sea (westernMediterranean) in winter. Oceanologica Acta 26, 179–189.

Halsband-Lenk C., Nival S., Carlotti F. and Hirche H.J. (2001) Seasonalcycles of egg production of two planktonic copepods, Centropagestypicus and Temora stylifera, in the north-western MediterraneanSea. Journal of Plankton Research 23, 597–609.

Hernandez-Leon S., Almeida C., Gomez M., Torres S., Montero I. andPortillo-Hahnfeld A. (2001) Zooplankton biomass and indices offeeding and metabolism in island-generated eddies around GranCanaria. Journal of Marine Systems 30, 51–66.

Hosie G.W. and Cochran T.G. (1994) Mesoscale distribution patterns ofmacrozooplankton communities in Prydz bay, Antarctica—January toFebruary 1991. Marine Ecology Progress Series 106, 21–39.

Ianora A. and Poulet S.A. (1993) Egg viability in the copepod Temora sty-lifera. Limnology and Oceanography 38, 1615–1626.

Isari S., Ramfos A., Somarakis S., Koutsikopoulos C., Kallianiotis A.and Fragopoulu N. (2006) Mesozooplankton distribution in relationto hydrology of the Northeastern Aegean Sea, EasternMediterranean. Journal of Plankton Research 28, 241–255.

Isari S., Psarra S., Pitta P., Mara P., Tomprou M.O., Ramfos A.,Somarakis S., Tselepides A., Koutsikopoulos C. and FragopouluN. (2007) Differential patterns of mesozooplankters’ distribution inrelation to physical and biological variables of the northeasternAegean Sea (eastern Mediterranean). Marine Biology 151, 1035–1050.

Isari S., Fragopoulu N. and Somarakis S. (2008) Interannual variabilityin horizontal patterns of larval fish assemblages in the northeasternAegean Sea (eastern Mediterranean) during early summer.Estuarine, Coastal and Shelf Science 79, 607–619.

Isla J.A., Ceballos S., Huskin I., Anadon R. and Alvarez-Marques F.(2004) Mesozooplankton distribution, metabolism and grazing in ananticyclonic slope water oceanic eddy (SWODDY) in the Bay ofBiscay. Marine Biology 145, 1201–1212.

Kiørboe T. (1993) Turbulence, phytoplankton cell size and the structureof pelagic food webs. Advances in Marine Biology 29, 1–72.

Kruskal J.B. and Wish M. (1978) Multidimensional scaling. Beverly Hills,CA: Sage Publications.

62 stamatina isari et al.

Lalli C.M. and Parsons T. (2002) Biological oceanography: an intro-duction, 2nd edition. Great Britain: The Open University,Butterworth-Heinemann.

Le Fevre J. (1986) Aspects of the biology of frontal systems. Advances inMarine Biology 23, 163–299.

Mackas D.L., Tsurumi M., Galbraith M.D. and Yelland D.R. (2005)Zooplankton distribution and dynamics in a North Pacific Eddy ofcoastal origin: II. Mechanisms of eddy colonization by and retentionof offshore species. Deep-Sea Research Part II: Topical Studies inOceanography 52, 1011–1035.

Mazzocchi M.G., Christou E.D., Fragopoulu N. and Siokou-Frangou I.(1997) Mesozooplankton distribution from Sicily to Cyprus (EasternMediterranean): general aspects. Oceanologica Acta 20, 521–535.

Mazzocchi M.G., Buffoni G., Carotenuto Y., Pasquali S. and Riberad’Alcala M. (2006) Effects of food conditions on the development ofthe population of Temora stylifera: a modelling approach. Journal ofMarine Systems 62, 71–84.

Molinero J.C., Ibanez F., Souissi S., Bosc E. and Nival P. (2008) Surfacepatterns of zooplankton variability detected by high frequencysampling in the NW Mediterranean: role of density fronts. Journalof Marine Systems 69, 271–282

Omori M. and Ikeda T. (1984) Methods in marine zooplankton ecology.New York: John Wiley & Sons.

Owen R. (1981) Fronts and eddies in the sea: mechanisms, interactionsand biological effects. In Longhurst A.R. (ed.) Analysis of marine eco-systems. London: Academic Press, pp. 197–233.

Paffenhofer G.A. (1998) On the relation of structure, perception andactivity in marine planktonic copepods. Journal of Marine Systems15, 457–473.

Paffenhofer G.A. and Mazzocchi M.G. (2003) Vertical distribution ofsubtropical epiplanktonic copepods. Journal of Plankton Research 15,1139–1156.

Pancucci-Papadopoulou M.A., Siokou-Frangou I., Theocharis A. andGeorgopoulos D. (1992) Zooplankton vertical distribution in relationto the hydrology in the NW Levantine and the SE Aegean Seas (spring1986). Oceanologica Acta 15, 365–381.

Pinca S. and Dallot S. (1995) Meso- and macrozooplankton compositionpatterns related to hydrodynamic structures in the Ligurian Sea(Trophos-2 experiment, April–June 1986). Marine Ecology ProgressSeries 126, 49–65.

Riandey V., Champalbert G., Carlotti F., Taupier-Letage I. andThibault-Botha D. (2005) Zooplankton distribution related tothe hydrodynamic features in the Algerian Basin (western

Mediterranean Sea) in summer 1997. Deep-Sea Research Part I:Oceanographic Research Papers 52, 2029–2048.

Robinson A.R. (1983) Overview and summary of eddy science. InRobinson A.R. (ed.) Eddies in marine science. Berlin and Heidelberg:Springer-Verlag, pp. 1–15.

Shannon C.E. and Weaver W. (1963) The mathematical theory of com-munication. Urbana, IL: Urbana University Press.

Siokou-Frangou I. (1996) Zooplankton annual cycle in a Mediterraneancoastal area. Journal of Plankton Research 18, 203–223.

Siokou-Frangou I., Christou E.D., Fragopoulu N. and Mazzocchi M.G.(1997) Mesozooplankton distribution from Sicily to Cyprus (EasternMediterranean): II. Copepod assemblages. Oceanologica Acta 20,537–548.

Siokou-Frangou I., Papathanassiou E., Lepretre A. and Serge F. (1998)Zooplankton assemblages and influence of environmental parameterson them in a Mediterranean coastal area. Journal of Plankton Research20, 847–870.

Weikert H. and Koppelmann R. (1993) Vertical structural patterns ofdeep-living zooplankton in the NE Atlantic, the Levantine Sea andthe Red Sea: a comparison. Oceanologica Acta 16, 163–177.

Zar J.H. (1999) Biostatistical analysis, 4th edition. Upper Saddle River, NJ:Prentice-Hall.

Zervakis V. and Georgopoulos D. (2002) Hydrology and circulation inthe North Aegean (eastern Mediterranean) throughout 1997 and1998. Mediterranean Marine Science 3, 5–19.

Zervoudaki S., Nielsen T.G., Christou E.D. and Siokou-Frangou I.(2006) Zooplankton distribution and diversity in a frontal area ofthe Aegean Sea. Marine Biology Research 2, 149–168.

Zervoudaki S., Christou E.D., Nielsen T.G., Siokou-Frangou I.,Assimakopoulou G., Giannakourou A., Maar M., Pagou K.,Krasakopoulou E., Christaki U. and Moraitou-Apostolopoulou M.(2007) The importance of small-sized copepods in a frontal area ofthe Aegean Sea. Journal of Plankton Research 29, 17–338.

and

Zodiatis G. and Balopoulos E. (1993) Structure and characteristics offronts in the North Aegean Sea. Bolletino Oceanologia Teorica edApplicata 11, 113–124.

Correspondence should be addressed to:S. IsariHellenic Centre for Marine ResearchPO Box 2214, 71003 Heraklion, Crete, Greeceemail: misari@her.hcmr.gr

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