Biogeochemistry and dynamics of settling particle fluxes at the Antikythira Strait (Eastern Mediterranean)

Progress in Oceanography 44 (1999) 651–675

Biogeochemistry and dynamics of settlingparticle fluxes at the Antikythira Strait (Eastern

Mediterranean)

Philippe Kerherve´ a,*, Serge Heussnera, Bruno Charrie`re a,Spyros Stavrakakisb, Jean-Luc Ferrandc, Andre Monacoa,

Nicole Delsauta

a Centre de Formation et de Recherche sur l’Environnement Marin (CEFREM), Universite´ dePerpignan, 66 860 Perpignan, France

b National Centre for Marine Research (NCMR), Institute of Oceanography, 166 04 Hellenikon,Athens, Greece

c CEREGE, Europole de l’Arbois BP80, 13 545 Aix-en-Provence, France

Abstract

For the first time, a 12-month trap experiment was conducted on both sides of the straitbetween Crete and Antikythira Island (Eastern Mediterranean Sea) from June 1994 to June1995 as part of the PELAGOS experiment. Analyses of major chemical constituents, includingcarbohydrates and stable lead isotopes and Scanning Electron Microscope studies were perfor-med on the trap samples. Total mass fluxes varied between 1 and 1273 mg m22 d21. Thelowest fluxes observed were in summer and autumn 1994, when stratification of the watercolumn was at its deepest. In general, mass fluxes exhibited very low values throughout thisexperiment confirming the strong oligotrophy of this area. The mean contents of the majorconstituents (carbonates, opal, lithogenic fraction) were quite similar during the survey andbetween traps, with the exception of organic carbon contents, which were highest (7–10%) insummer 1994, i.e. during the period of lowest mass fluxes. During the first 6-month deploy-ment (summer–autumn 1994) there was an important mass flux peak, which was depleted inorganic carbon, at the Ionian near-bottom trap. This event coincided with a violent wind epi-sode, which may have caused the resuspension of particles, which were then transported downthe steep continental slope on the Ionian side of the strait. A smaller peak in mass flux occurredat the Aegean near-bottom trap, coincident with rainfall. Both these events indicate thatenvironmental factors can control flux variations in an oligotrophic environment. During thesecond 6-month deployment (winter–spring 1995) there was another important increase inmass fluxes, which occurred at all three traps, although in the Ionian traps mass flux peaks

* Corresponding author.

0079-6611/99/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(99 )00040-3

652 P. Kerherve´ et al. / Progress in Oceanography 44 (1999) 651–675

were delayed by one to two sampling intervals. The distance between the two mooring sitesgives a rough estimate of a minimum horizontal advection speed of 2 cm s21 for this particulatetransfer from the Aegean to the Ionian area. This estimate is in good agreement with themeasured current velocities. 2000 Elsevier Science Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 652

2. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6532.1. Field work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6532.2. Analytical work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655

3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6563.1. Current meter data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6563.2. Total mass fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6563.3. Composition of settling particles and biogeochemical fluxes . . . . . . . . . . . 6593.3.1. Microscopic examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6593.3.2. Major constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6603.3.3. Organic tracer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6633.3.4. Stable Pb isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665

4. Discussion and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6654.1. Processes controlling particle fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . 6654.2. Water mass circulation and exchanges through the Antikythira Strait . . . . . . 672

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673

1. Introduction

The study of biogeochemical cycles in the marine environment requires the abilityto quantify and chemically assess downward fluxes of settling particles (Monaco,Courp, Heussner, Carbonne, Fowler & Deniaux, 1990; Etcheber, H., Heussner, S.,Weber, O., Dinet, A., Durrieu De Madron, X., Monaco, A. et al., 1996; Heussner,S., Durrieu De Madron, X., Radakovitch, O., Beaufort, L., Biscaye, P., Carbonne,J. et al., 1999). As these fluxes are variable in space and time, as a result of the space-and time-dependency of the underlying processes, use of sequential traps allows thecollection of space- and time-integrated samples to complement the synoptic descrip-tion obtained during classical oceanographic surveys.

The Mediterranean Sea may be regarded as a model of global oceanic evolutionand offers the opportunity of studying many different types of ecosystems within alimited area. The Eastern Mediterranean has unique physiographic and hydrodynam-

653P. Kerherve´ et al. / Progress in Oceanography 44 (1999) 651–675

ical characteristics (Robinson, A.R., Golnaraghi, M., Leslie, W.G., Artegiani, A.,Hecht, A., Lazzoni, E. et al., 1991; Theocharis, Georgopoulos, Karagevrekis, Iona,Perivoliotis & Charalambidis, 1992; Theocharis, Georgopoulos, Lascaratos & Nittis,1993), since it consists of several sub-basins such as the Cretan Sea (South AegeanSea). This particular large and deep sub-basin is delimited to the south by the CretanArc through which numerous straits control the exchanges of water and associatedmaterials with the adjacent basins.

The present study was carried out within the framework of the interdisciplinaryPELAGOS experiment and represents, together with EUROMARGE-NB (NorthBalearic basin) and EUROMARGE-AS (Adriatic Sea), one of the projects conductedwithin the Mediterranean Targeted Project (MTP-I). The field strategy was the samein all three experiments and was aimed at providing the data needed to comparedownward fluxes along a west–east oligotrophic gradient. In this context, sedimenttraps coupled with current meters were deployed for the first time on the CretanArc: on the internal (Aegean Basin) and external (Ionian basin) sides of the Antiky-thira Strait. The primary goal of this paper is to characterise, both quantitatively andqualitatively, the downward particle fluxes in this part of the Eastern MediterraneanSea. We attempt to identify the origin (marine vs. continental and biogenic vs.lithogenic) and the temporal variability of these fluxes, as well as the hydrodynamicaland biological fate of particles. The ultimate goal plans to establish transfer pathwaysand mass budgets of exported material in this oligotrophic environment.

2. Methodology

2.1. Field work

Despite some problems associated with the use of sediment traps (hydrodynamicsand sample integrity), they remain the best available tools for collecting settlingparticles and hence enable the chemical assessment and computation of the vari-ous components.

Two mooring arrays, each including two PPS3 traps (Technicap PPS 3, describedby Heussner, Ratti & Carbonne, 1990) and two Aanderaa current meters weredeployed at the end of May 1994 on either side of the strait between Crete andAntikythira Island (Fig. 1). The experimental site was chosen according to thedetailed bathymetric survey performed during the 29–30 April ’94 PELAGOS cruise.

Mooring P1 was deployed on the northern side of the Antikythira Strait (35°519N; 23°269 E), inside the Aegean Basin where the depth was 1000 m, with the inter-mediate trap at 500 m depth (500 m above bottom, m.a.b.) and the deep trap, at 965m depth (35 m.a.b.);

Mooring P2 was deployed on the southern side of the strait, on the Ionian slope(35°329 N; 23°279 E), in slightly deeper water (1380 m) as the slope was too steepto set a mooring safely at shallower depths; it included a mid-water trap at 880 m(500 m.a.b.) and a near-bottom trap at 1345 m (35 m.a.b.).

The traps were programmed to collect settling particles over 2-week periods


Fig

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(rotation of receiving cups on days 1 and 16 of each month). The first 6-monthdeployment covered the period 01/06/94–30/11/94, and the second deployment, theperiod 16/12/94–15/06/95. The associated current meters sampled current speeds anddirection on a 30 min basis. Electronic malfunction resulted in the upper trap of theAegean site working for only 1 month during the first deployment and it could notbe replaced for the second one. Consequently, only 3 complete 1-year time-serieswere obtained. Current meter data were unavailable for the Aegean site during thesecond period deployment.

2.2. Analytical work

The receiving cups of the traps were filled with a buffered 5% formaldehydesolution (pH=7.5) to prevent organic degradation, and the samples were treated asdescribed in Heussner et al. (1990). The settled particulate matter was wet sievedthrough 1 mm nylon mesh in order to extract the largest organisms (‘swimmers’).Large amorphous aggregates were returned to the sample. The samples were thendivided into subsamples for the subsequent analyses. However, because the quantitiesof particles collected were often very limited, it was necessary to pool several con-secutive samples to obtain sufficient quantities for the different analyses. This wasdone by pooling known precise wet fractions of the original samples so that thecomposite samples were representative. Contents of organic and inorganic carbon,biogenic silica (opal), carbohydrates (monosaccharides) and stable lead isotopes(206Pb and207Pb) were determined. The small quantities of particulate matter in thesamples meant that only a restricted number of subsamples could be prepared anddistributed for these analyses.

Total mass fluxes were determined for all samples. The organic and inorganiccarbon contents were determined by combustion in an LECO-CS 125 analyser. Fororganic carbon (OC) determination, samples were acidified with HC1 2N to removecarbonates. Organic matter (OM) was taken as twice the amount of OC. Calciumcarbonate content was determined from inorganic carbon (IC) using molecular massratio 100/12 (carbonates=100/12×IC). Biogenic silica (opal taken as biogenicsilica×2.4) was determined according to the Mortlock and Fro¨elich (1989) procedure.10 mg of dried sample were treated with HCl 1N and H2O2 10% to remove carbon-ates and organic matter, opal was then extracted with Na2CO3 2M solution for 5 hat 85°C. Biogenic silica content was then determined after a molybdate derivatisationby measuring absorbance at 812 nm with a Spectronic Genesys spectrophotometer.The lithogenic fraction, representing the residual component of settling particles suchas quartz, feldspars, heavy minerals and aluminosilicates was calculated by sub-tracting the concentration of the major constituents (OM+carbonates+opal) from totaldry weight. All these constituents are expressed in percentage (%) of the dry weight.Small subsamples were collected by micropipette in each trap samples from the firstdeployment, filtered, rinsed with Milli-Q water and freeze-dried. These subsampleswere used for SEM (Scanning Electron Microscope) examination.

In order to refine the characteristics and sources of organic matter, monosacchar-ides were analysed on selected trap samples that contained enough material. They


were determined by anion-exchange chromatography coupled with an amperometricdetector (HPAEC-PAD) (Kerherve´, Charriere & Gadel, 1995). Samples were rinsedwith Milli-Q water and centrifuged to remove salts and formalin. The centrifugationresidues were frozen, freeze-dried and re-weighed. About 15–20 mg of freeze-driedmaterial were hydrolysed in 5 ml H2SO4 1 M at 95°C for 4 h in vials under nitrogenatmosphere in order to release monosaccharides from carbohydrate molecules.CaCO3 powder was added to neutralise the acid and precipitate sulphate (Mopper,Schultz, Chevolot, Germain, Revuelta & Dawson, 1992). The precipitate was spundown at 3000 rpm for 5 min and the supernatant injected without derivatisation intothe chromatographic system. Individual monosaccharides were expressed as percent-age by weight of the total carbohydrate yields (TCHO). C-TCHO/OC is the carbonnormalised total carbohydrate in organic carbon (OC) pool where C-TCHO=0.4TCHO.

Stable lead isotopes (206Pb, 207Pb) were analysed after HF–HNO3–HCl digestionby Thermal Ionisation Mass Spectrometry (Hamelin, Grousset & Sholkovitz, 1990).Sampling and analytical blanks represented less than 5% of total lead analysed.

3. Results

3.1. Current meter data

Examples of hydrodynamical conditions are presented in Figs. 2 and 3 in the formof progressive vector diagrams and 48 h-averaged velocity stick plots. Current meterdata recorded at the depths of the sediment traps showed the following.

On the Antikythira Slope towards the Ionian Sea, currents constantly flowed north-westwards at both depths, 880 and 1345 m, and mean speeds were,2 cm s21 forboth deployment periods. At 1345 m the near-bottom currents occasionally speededup to close to 25 cm s21 (30 min averaged current), in a southerly direction(orientation of the canyon axis) during the first deployment (summer). These stronglocalised flows were not observed during winter.

During the first deployment at the Aegean Sea site, currents within the intermediatewater layer at 500 m flowed continuously southwards (average direction 181°) at anaverage speed of 2.5 cm s21 and a maximum speed of 6.6 cm s21. At 965 m, currentdirections alternated between north-eastwards and south-eastwards (average 95°); theaverage speed was 0.5 cm s21 but the maximum speed was 5.2 cm s21.

More than 95% of the individual speed values were below 8 cm s21, which indi-cates that traps were placed in environments that were calm enough for downwardflux measurements to be considered as representative.

3.2. Total mass fluxes

Throughout the study period, the mean total mass fluxes at the Ionian site were96 and 299 mg m22 d21 at 880 and 1345 m depth, respectively. At the Aegean site(965 m depth) a mean value of 203 mg m22 d21 was recorded. The lowest mean


Fig. 2. Hydrodynamical conditions in the Antikythira Strait demonstrated by progressive vector diagramsrecorded during the first period deployment of the PELAGOS experiment (June 1994–November 1994).Abbreviations: m.a.b.=meter above bottom;I=start.

mass flux values were observed during the first period of deployment: 11 and 209mg m22 d21 at 880 and 1345 m depth, respectively, at the Ionian site and 39 mgm22 d21 at 965 m depth at the Aegean site. During the second period of deploymentmean total mass fluxes were higher, 183 and 390 mg m22 d21 at 880 and 1345 m,respectively, at the Ionian site, and 361 mg m22 d21 at the Aegean site.

At both sites the daily total mass fluxes registered spanned over several orders ofmagnitude (Fig. 4). In the Aegean Sea, minimum fluxes were recorded at the end


Fig

.3.

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aged

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Fig. 4. Temporal variability of the total mass fluxes at the Ionian and Aegean sites.

of June ’94 at 500 m (1 mg m22 d21) and early July at 965 m depth (2 mg m22

d21). Flux rates started to increase in February ’95, reaching a maximum of 1065mg m22 d21 during the second fortnight of March ’95, thereafter they slowlydecreased to the end of the survey.

On the Ionian side at 880 m depth, during the first deployment, a period of mini-mum mass flux extended from mid-June to mid-August ’94, when values werebetween 1 and 2 mg m22 d21. The trap recorded a moderate flux increase in Nov-ember, concomitant with, but less intense than the peak recorded in the Aegean near-bottom trap. At 1345 m depth, a distinct flux event occurred in September–October,which peaked at 781 mg m22 d21. During the second deployment, both trapsrecorded a first period of low fluxes between December ’94 and January ’95, fol-lowed by a strong increase in the flux, from February to May ’95. The flux maximumpersisted at around 400 mg m22 d21. For almost 2 months (from mid-March to mid-May ’95) in the 880 m trap, and reached 1273 mg m22 d21 in the deepest trap inlate April ’95 (the highest value of the entire experiment). At the Ionian site, fluxesincreased greatly with depth by a factor of 7–80 depending on the period, indicatingparticulate inputs were occurring at depth, especially during the flux event from mid-September to mid-October ’94. Another interesting feature was that during thesecond deployment there was a time lag between the incidence of the flux maximumat the Aegean site and its occurrence at the Ionian site.

3.3. Composition of settling particles and biogeochemical fluxes

3.3.1. Microscopic examinationThe SEM examination of material collected during the first 6-month deployment

revealed the relative importance of the biogenic contribution to all the samples exam-


ined. At both sites, the material in both the upper traps was characterised by thepresence of mucus-like structures (Fig. 5). Well preserved faecal pellets, almostentirely filled by coccoliths and numerous phytoplankton remains were sometimesapparent. In the deeper samples, the composition differed significantly in that therewas a greater abundance of refractory material composed of grains of aluminosil-icates, quartz, calcite, Mg-calcite and some heavy minerals. The proportions ofrefractory material increased during the period of high fluxes in November ’94 atthe Aegean site, very often associated with aggregated and quite well-degradedcoccoliths. Despite the depths of the samples, diatom remains were still present inlarge amounts (Fig. 6). During the period of maximum fluxes at the Ionian site, thebiological contribution was enhanced at 1345 m depth both in summer and in autumn,when small composite aggregates were found composed of clays, detrital grains andcoccoliths, embedded in a fibrillar network (Fig. 6b). During the periods of low massfluxes, fresh diatom remains dominated, even in the Ionian near-bottom trap at 1345m (Fig. 6a, b). From these observations, it appears that in this particular straitenvironment and for low flux periods, aggregation processes (marine snow, grazing;Alldredge & Jackson, 1995) may be the main factor explaining the downward trans-port of fine particles and the presence of ‘fresh’ detritus down to more than 1300m depth.

3.3.2. Major constituentsFlux-weighted mean concentrations of the major constituents were calculated for

each deployment (deployment I: summer–autumn; deployment II: winter–spring) togive a general description of the settling material collected during the experiment(Table 1). The mean contents of the major constituents were quite similar betweentraps, suggesting that the settling particles had a common origin. However, samplesfrom the Ionian site showed a slight trend towards a more marine character ofsamples (highest OM and opal fractions, lowest carbonate and lithogenic fractions).

The temporal variability of these four constituents showed more pronounced fea-tures (Fig. 7). Organic carbon content was maximum during the first part of theexperiment which coincided with the very low mass flux: around 7% org. C for theAegean 965 m samples, and 9–10% for the Ionian samples. Thereafter the percent-ages fell and continued to decline slowly during the course of the second deploymentwith the noticeable exception of a peak of close to 10% in winter in the 880 mIonian trap. These trends are mainly explained by the logarithmic relation betweenOC content and total mass flux (Fig. 8). As already observed in other trap experi-ments (Buscail, Pocklington, Daumas & Guidi, 1990; Kerherve´, 1996) and in estu-aries (Cauwet, Gadel, de Souza Sierra, Donard & Ewald, 1990), OC content of set-tling particles decrease with increasing mass flux tending towards low values similarto those found in resuspended sediment particles.

The temporal evolution of carbonates was quite constant at around 40% throughoutthe first 6 months, but there was a large increase (up to 60%) in the Ionian 880 mtrap when mass fluxes were low. Microscopic examination (SEM and microanalysis)showed that carbonate was related to numerous, poorly-preserved coccoliths, calcite


Fig. 5. Scanning Electron Microscope (SEM) examination. Particles collected by midwater traps,resulting largely from biological activity. Mucus sheets and pieces (a), embedding numerous well-pre-served faecal pellets (b) and phytoplankton, are very common. Faecal pellets are often in a very freshstate, as attested by the presence of an intact peritrophic membrane (c). Generally, they contain largeamounts of phytoplankton structures: here, relatively well-preserved coccoliths (d) are shown.


Fig. 6. Scanning Electron Microscope (SEM) examination. Particles collected by near bottom trapsduring periods of low mass fluxes. Fresh diatoms are observed (Ionian 1345 m, a), as well as fibrillarand composite aggregates (b). During period of higher mass fluxes, these aggregates are dominantlycomposed of fine silicate and carbonate material embedded in fibrillar structures (c, d). Biogenic remainsare still present.


Table 1Flux-weighted seasonal and annual mean concentrations of major chemical constituents in the PELAGOStrap samples. I: first deployment from June to November 94; II: second deployment from December 1994to June 1995

Periods Trap positions

Aegean Sea (965 m) Ionian Sea (880 m) Ionian Sea (1345 m)

Organic matter (%) I 8.3 12.8 9.6II 6.7 5.8 5.9Annual 6.9 6.2 7.2

Carbonate (%) I 41.7 38.4 40.9II 38.8 43.7 39.5Annual 39.1 43.5 40.0

Opal (%) I 11.2 11.8 9.7II 8.5 13.6 10.1Annual 8.8 13.2 10.0

Lithogenic (%) I 38.8 37.0 39.8II 46.0 36.9 44.5Annual 45.2 37.1 42.8

and Mg-calcite grains. The calcite particles were less numerous in the upper trapsthan in the deeper ones.

Biogenic silica (opal) content remained more or less constant (around 10–12%)during the first deployment. But then there were large fluctuations in both Ioniantraps: the highest opal contents (29 and 17%) were observed in late February ’95when mass fluxes were low.

The lithogenic fraction tended to increase slightly throughout the whole samplingperiod and for all traps (from more or less 35% to 45%), though the relative contentsof the upper Ionian trap fell during winter (to around 20%).

3.3.3. Organic tracerCarbohydrate contents (TCHO) ranged between 3.24 and 7.66 mg g21. The

relation between TCHO and OC contents differentiated samples from the first andthe second trap deployments (Fig. 9). This figure seems to indicate that there wasa shift in the nature of the organic matter between these periods, though in bothcases a positive relation between OC and TCHO was observed for each group ofdata. During summer ’94, the organic material was richer in organic carbon butpoorer in TCHO than during the following spring ’95 (high C-TCHO/OC ratio), atypical period of phytoplanktonic blooms (Table 2).

Detailed examination of the monosaccharide composition (Table 2) providedfurther information on the origin of settling particles. This composition was ratherhomogeneous for all three traps. Glucose and galactose were the two major neutralsugars. In such a marine environment, both sugars are present in abundance in cellu-lar storage products of living organisms (Ittekkot, Degens & Brockman, 1982; Lieb-


Fig

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Fig. 8. Relationship between particulate organic carbon (POC) and total mass flux for the samplescollected at both sites in the Antikithyra Strait.

ezeit, 1984; Tanoue & Handa, 1987). The products are labile and therefore can beexpected to be the first compounds to be assimilated by heterotrophic organisms.

The spring period was characterised by quite similar monosaccharide ‘finger-prints’, in all three traps during the period 31 March–15 April 1995. At that time,the relative contribution of glucose reached its maximum values (up to 29%) in eachtrap. The TCHO contents and C-TCHO/OC values also peaked at the same period,especially within the Aegean 965 m trap and the upper Ionian trap.

3.3.4. Stable Pb isotopesAnother biogeochemical tracer (stable Pb isotopes) was associated with this (Table

3). The mean stable Pb concentration was 66±19 µg g21, showing a higher meanand standard deviation than values observed in the Western Mediterranean (59±9 µgg21) (Alleman, 1997). The isotopic composition (206Pb/207Pb) was shifted towards ahigher mean (i.e. less anthropogenic) in the Eastern Mediterranean (1.177) as com-pared to that found in the Western Mediterranean (1.175) (Ferrand, Hamelin & Mon-aco, 1999).

4. Discussion and conclusion

4.1. Processes controlling particle fluxes

An important feature of the PELAGOS experiment concerns the very low massfluxes relative to those obtained during the two other MTP experiments in the North-Balearic and Adriatic margins (Table 4). The low quantity of the settling particles


Fig. 9. Relationship between particulate organic carbon (POC) and total carbohydrates (TCHO) for thesamples collected at both sites in the Antikithyra Strait during the two successive deployment periods.

collected during this year of survey confirms the strong oligotrophy of this region,as previously shown in earlier chlorophyll measurements and phytoplankton studies(Kimor & Wood, 1975; Ignatiades, 1976; Pagou & Gotsis-Skreta, 1990). The lowestmass fluxes recorded during the first deployment (i.e. summer–autumn) coincidedwith the period when stratification of surface waters was most intense (Oszoy,Hecht & Unluata, 1989; Theocharis et al., 1993), and therefore during a period ofminimum particle export. However, a similarly intense summer stratification has alsobeen observed in the Adriatic Sea and Balearic areas (Heussner, Calafat & Palanques,1996; Miserocchi, Faganeli, Balboni, Heussner, Monaco & Kerherve´, 1999). In thisEastern region of the Mediterranean Sea the settling material has a pronounced bio-genic character, though weighted mean annual contents of POC, opal and carbo-hydrates (TCHO) were in the same range as found in the Adriatic Sea (Table 4).Inorganic carbon, assumed to be essentially in the form of carbonates, was a morepronounced component of the settling particles relative to the two other Mediter-ranean study areas. These carbonates are likely to originate from coccolithophoresdescribed as the dominant phytoplanktonic group in the south Cretan Sea (Gotsis-Skretas, Pagou, Moraitou-Apostolopoulou & Ignatiades, 1999).


Tab

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the

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the

orga

nic

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C)

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revi

atio

ns:F

uc=f

ucos

e,R

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se,A

ra=ar

abin

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Glc

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sam

ine,

Gal=

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lc=glu

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an=m

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Xyl=

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Site

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lcX

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CH

OC

-T

CH

O/O

C(%

)(%

)(%

)(%

)(%

)(%

)(%

)(m

gg2

1)

(%)

Aeg

ean

sea

(965

m)

01/0

6–15

/06/

948.

308.

209.

5923

.49

24.2

013

.08

13.1

45.

334.

715

/08–

31/0

8/94

10.0

010

.20

11.9

524

.85

19.9

79.

1613

.87

4.12

4.3

01/1

0–15

/10/

948.

637.

3712

.12

23.0

218

.94

10.4

319

.49

3.88

4.4

01/1

1–15

/11/

949.

568.

1013

.95

24.4

823

.10

9.06

11.7

43.

844.

016

/02–

28/0

2/95

10.1

210

.76

11.6

724

.32

17.9

112

.13

13.0

85.

536.

101

/03–

15/0

3/95

7.22

10.3

611

.31

24.6

320

.73

12.0

013

.75

5.15

6.2

16/0

3–30

/03/

9512

.27

10.6

611

.09

22.1

020

.64

11.0

012

.23

4.91

6.6

31/0

3–15

/04/

957.

009.

6110

.24

19.6

329

.12

11.3

113

.09

6.98

7.9

16/0

4–30

/04/

958.

9011

.80

11.6

525

.61

25.1

312

.51

4.40

6.20

6.7

01/0

6–15

/06/

957.

7810

.26

11.5

221

.44

21.2

112

.14

15.6

54.

616.

6

Ioni

anse

a(8

80m

)16

/03–

30/0

3/95

9.51

11.5

512

.25

21.1

617

.83

13.2

314

.47

4.97

8.1

31/0

3–15

/04/

957.

927.

768.

6620

.25

23.9

513

.89

17.5

77.

669.

816

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30/0

4/95

7.86

8.87

14.6

421

.65

17.5

113

.75

15.7

16.

939.

001

/05–

15/0

5/95

8.38

9.40

10.6

422

.78

17.8

914

.00

16.9

14.

257.

216

/05–

31/0

5/95

8.78

9.60

11.8

723

.61

17.3

414

.49

14.3

13.

977.

001

/06–

15/0

6/95

8.15

8.90

9.62

20.0

822

.78

13.9

016

.57

4.96

8.1

(co

ntin

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do

nn

ext

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ge

)


Tab

le2

(con

tinu

ed)

Site

Sam

plin

gin

terv

alF

ucR

haA

raG

alG

lcX

ylM

anT

CH

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-T

CH

O/O

C(%

)(%

)(%

)(%

)(%

)(%

)(%

)(m

gg2

1)

(%)

Ioni

anse

a(1

345

m)

01/0

9–15

/09/

9410

.40

9.34

13.4

818

.77

26.2

57.

7214

.05

4.91

4.3

15/0

9–30

/09/

9411

.49

10.9

213

.46

19.2

423

.88

9.77

11.2

44.

554.

701

/10–

15/1

0/94

12.8

610

.73

13.3

017

.63

26.8

97.

9510

.63

5.65

5.3

15/1

0–31

/10/

948.

5614

.38

6.54

13.8

424

.60

12.7

419

.34

3.74

3.1

01/1

1–15

/11/

949.

1713

.16

8.61

15.6

525

.68

12.2

715

.47

3.26

3.1

15/1

1–30

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9410

.69

12.4

79.

2015

.14

22.3

412

.32

17.8

33.

243.

401

/03–

15/0

3/95

9.14

10.7

89.

8821

.83

18.2

815

.55

14.5

56.

128.

916

/03–

30/0

3/95

7.98

10.1

612

.94

21.0

416

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15.8

115

.85

4.29

8.2

31/0

3–15

/04/

958.

249.

1710

.33

20.9

923

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14.7

313

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5.37

7.6

16/0

4–30

/04/

957.

549.

3112

.62

20.1

318

.62

13.4

818

.30

4.85

7.5

01/0

5–15

/05/

958.

4912

.33

12.6

021

.39

20.4

811

.54

13.1

84.

166.

3


Table 3Stable lead concentrations, isotopic ratios (206Pb/207Pb) and total mass fluxes (mg m22 j 21) determinedon selected trap samples collected on either sides of the Antikythira Strait (western Crete)

Site Sampling interval Pb concentration Isotopic ratio Total mass flux(µg g21) (206Pb/207Pb) (mg m22 d21)

Aegean Sea (500 m) 01/06–15/06/94 80.8 1.1742 36.0

Aegean Sea (965 m) 16/08–31/08/94 92.9 1.1761 77.201/09–15/09/94 89.3 1.1746 32.96/09–30/09/94 96.1 1.1743 37.51/11–15/11/94 67.6 1.1768 188.41/04–15/04/94 35.0 1.1788 754.16/04–30/04/95 60.7 1.1787 884.31/06–15/06/95 62.9 1.1781 238.9

Ionian Sea (880 m) 16/04–30/04/95 56.5 1.1761 374.101/06–15/06/95 70.7 1.1747 143.4

Ionian Sea (1345 m) 01/10–15/10/94 31.4 1.1838 583.901/11–15/11/94 48.8 1.1782 176.516/11–30/11/94 35.0 1.1795 225.116/04–30/04/95 49.3 1.1786 1272.901/06–15/06/95 53.4 1.1771 291.0

The organic carbon content in settling particles in the deep water layers was rela-tively higher during the first deployment in all three traps (means of 4–7% for thefirst deployment vs. 2.5–3.5% in the second) indicating a significant, less-dilutedorganic input to the benthic system at that time. In contrast total carbohydrate con-tents were higher during the second deployment, when the organic carbon contentwas lower. This apparently paradoxical situation may indicate that at the time ofrelatively high primary production (end of winter and beginning of spring), biogenicparticles rich in carbohydrates (glucose and galactose) produced in the upper columncould sweep down suspended inorganic particles and hence dilute the overall organiccarbon content. This mixed material will quickly reach the traps, so the monosacchar-ide composition, which indicates the freshness of the material, will not be affected.In fact, the presence of numerous fresh diatoms and, at times, silicoflagellates mixedwith the refractory material, implies a rapid export from surface waters, which is inaccordance with this hypothesis. The low contents of TCHO and glucose found inthe near-bottom trap during spring indicates there was an erosion of the fresh andbiogenic signal with depth. Thus, we may assume a heterotrophic (microbacteria andzooplankton) use of this carbohydrate pool as the particles settle through the watercolumn (Ittekkot, Deuser & Degens, 1984; Liebezeit, 1987).

Despite the phytoplankton bloom in March–April, which was revealed by the highcontents of carbohydrates and opal during the second deployment, samples from theAntikythira Strait were characterised by having received greater terrigenous inputsat depth, which increased the total mass fluxes, and the lithogenic and carbonate


Tab

le4

Com

paris

onof

annu

alm

ean

fluxe

san

dflu

x-w

eigh

ted

mea

nco

nten

tsof

part

icul

ate

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nic

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on(P

OC

),op

al,

carb

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geni

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n(e

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das

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enta

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tota

ldr

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t)an

dca

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ydra

tes

(TC

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)be

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nth

eth

ree

expe

rimen

tal

zone

sof

the

Med

iterr

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nT

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ted

Pro

ject

I(M

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dan

nual

mea

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ts

MA

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trap

Tra

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epth

Ann

ual

mea

nM

ass

flux

PO

CO

pal

Car

bona

tes

Lith

ogen

icT

CH

Oex

perim

ents

rang

efr

actio

n(m

)(m

gm

22

d2

1)

(mg

m2

2d

21)

(%)

(%)

(%)

(%)

(mg

g21)

PE

LAG

OS

965

203

2–10

653.

58.

839

.145

.25.

5P

rese

ntst

udy

880

961–

396

3.1

13.2

43.5

37.1

5.6

1345

299

6–12

733.

610

.040

.042

.84.

7

EU

RO

MA

RG

E-N

Ba50

086

–205

82–

6003

2.4–

7.9

4.2–

6.3

24.9

–28.

451

.2–6

6.6

3.7

Heu

ssne

ret

al.

(199

6)75

016

0–15

590.

2–39

982.

2–6.

65.

3–6.

425

.2–3

1.7

48.9

–64.

12.

710

0027

2–37

9943

–126

081.

7–5.

03.

1–5.

927

.0–3

6.7

47.7

–67.

12.

11–

453

EU

RO

MA

RG

E-A

S50

012

21–

453

5.1

11.7

16.1

58.0

Mis

eroc

chi

etal

.(1

999)

1000

235

8–15

974.

011

.021

.260

.04.

9

aR

ange

ofob

serv

edan

nual

mea

nsat

seve

ral

loca

tions

inth

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orth

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tern

Med

iterr

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nba

sin.


contents. In addition to the possible ‘sweeping’ mechanism by biogenic particlesmentioned above, these increases may also have resulted from advective transportof particles originating from the sides of the Strait. This hypothesis is in good agree-ment with the distribution and composition of suspended particulate matter (SPM)observed during two hydrological surveys carried out in March and September ’94.Thus, Price, Lindsay and Pates (1999) in noting the high concentrations of particulateAl and Ca between 200 and|700 m suggested that there was a major contributionof particles from sediment resuspension at the shelf break in the form of nepheloidplumes. Another interesting feature, consistent with the measured fluxes and relatedto their spatial evolution, is shown through the geographical distribution of SPM: inthe western Aegean basin, SPM exhibited two distinct peaks at 100 m depth and inthe deep water layer during three different seasonal surveys (June ’94, September’94 and December ’94) (Varnavas, 1995).

Changes in meteorological conditions could be an important factor influencing theevolution of the hydrology, coastal dynamics and terrestrial discharges. In autumn’94, meteorological conditions recorded on Kithira Island showed the occurrence ofsome violent gusts of wind (.10 Beaufort; 20–22 September ’94) and heavy rainfall(up to 94 mm; 13–16 November ’94). In September the violent wind events coincidedwith the isolated peak in mass flux observed in the Ionian deep trap. The winds mayhave forced vigorous vertical convection and resuspension of rebound particles alongthe western Cretan coast, leading the traps moored within the bottom nepheloid layerto collect both the primary particle flux and resuspended sediment (Gardner, Sou-thard & Hollister, 1985; Walsh, Fisher, Murray & Dymond, 1998). Thus, thedepletion of organic carbon during this mass flux event leads us to assume that thetrapped material mainly reflected the composition of the underlying sediment. Thedeep Ionian trap recorded strong inputs of particles flowing down along the steepcontinental slope within a nepheloid layer. Also, during September ’94 there was ahigher Pb content and a stronger anthropogenic signal (206Pb/207Pb) in the deepAegean trap. Unfortunately, no analyses of stable Pb isotopes have been done onsamples of the Ionian side collected in September. However, we can assume thatthe ‘old, fairly contaminated, material’ originally deposited in shallow water alongthe coastal area of western Crete has been resuspended and carried offshore whereit was trapped in the adjacent basins after the storm that blew over the region.

On the other hand, the mass flux peak recorded in the deep Aegean trap whichseems to have been induced by the downpours of rain in mid-November were lowerthan those induced by the wind events. These rainy days were associated with gustsof wind that did not exceed Force 6, suggesting that the wind forcing has an effecton particle fluxes in the Antikythira Strait greater than that of precipitation.

There is high seismicity in the study area so we also considered earthquakes aspossible factors leading to sediment instability and reworking and hence to abruptincreases of particle concentration. Several earthquakes of up to 3.5 in magnitudeoccurred during the trap deployments, but their epicentres were located in areas farfrom North Crete; and, apparently, none coincided with the flux increases recorded.


4.2. Water mass circulation and exchanges through the Antikythira Strait

Mesoscale circulation of the water masses in this region of the Mediterranean isrelatively well known: a cyclonic circulation in the region of Rhodes flows parallelto the eastern straits, an anticyclone forms to the south-east of Crete, and a largecyclonic feature occurs to the south-west of Crete, known as the Cretan Cyclone(POEM Group, 1992). In the Ionian Sea, Theocharis et al. (1992, 1993) havedescribed a large tongue of warmer and more saline Aegean water outflowingthrough the Kythirian straits. Several other authors (e.g. Gertman, Ovchinnikov &Popov, 1989; Roether, Manca, Klein, Bregant, Georgopoulos, Beitzel et al., 1996;Klein, Roether, Manca, Bregant, Beitzel, Kovacevic et al., 1999) have also mentionedthat Cretan Deep Waters (CDW) are detectable in the vicinity of the Cretan ArcStraits.

Taking into account the spatial and temporal evolution of mass fluxes we canassume that, at least occasionally, the Cretan Deep Waters are importing suspendedmaterial into the Ionian Sea through the Antikythira Strait. During the first deploy-ment, the near-bottom trap on the Aegean side and the 880 m trap on the Ionianside showed a coincidental small flux increase (Fig. 4). As discussed previously,this event may reflect the influence of the rainfall which occurred throughout theexperimental area over 6 days, rather than the influence of a particulate-rich currentflowing from the Aegean Sea (the evolution of the near-bottom trap at 1345 m duringthe former appeared to be totally uncoupled). The most convincing argument thatdemonstrates the occurrence of particle export from the Aegean Sea is the time lagbetween the mass flux peaks between the two ends of the Strait during the seconddeployment. During that time, although all three traps were affected, it occurred oneto two sampling intervals later in the Ionian traps, unlike the carbohydrate signalthat reached all the traps simultaneously. Thus, we can assume that during spring,the vertical transfer of carbohydrate-rich particles was coupled with an advectivedeep current supplying traps with advected resuspended sediment particles. As opalcontents were low and lithogenic fractions high at that time, we may suggest thateither the biological particles sinking from the upper layer were being rapidlydegraded during their passage down through the water column and/or were beingdiluted by resuspended sediment advected in laterally by the currents.

The distance between the two mooring sites was 30 nautical miles, so since thespring mass flux peaks in the Ionian Sea were delayed by 2–4 weeks (two samplingintervals), we can roughly estimate the minimum horizontal speed for this particulatetransfer from the Aegean to the Ionian area to have been 2 cm s21. This estimateis in good agreement with the range of average velocities measured from the 4 currentmeters, despite the lack of data during the second deployment in the Aegean side.

The amount of total exported material settling on the Ionian side is hard to evaluateat the present stage. At the most we can assume that it could represent a significantproportion of the annual mass fluxes to the Ionian trap at 880 m given in Table 5.Once outside the Strait and over the Ionian slope, the particles should encounter thecyclonic gyre and so will be eventually carried further to the northwest.


Table 5Annual mass fluxes (mg m22 d21) of major constituents on either side of the Antikythira Strait

Constituents Aegean 965 m Ionian 880 m Ionian 1345 m

Organic matter 13.8 6.0 21.6CaCO3 79.6 41.8 119.9Opal 18.7 19.1 30.0Lithogenic 90.7 29.5 127.9Total mass 202.8 96.4 299.4

Acknowledgements

This research has been undertaken within the framework of the MediterraneanTargeted Project (MTP)–EUROMARGE–PELAGOS project. We acknowledge thesupport from the European Commission’s Marine Science and Technology (MAST)Programme under contract MAS2-CTP-93-0059. We gratefully acknowledge the pro-fessionalism and helpfulness of the officers and crews of the R.V. Aegeao. Specialthanks to Savas Christianides and Jacques Carbonne who contributed significantlyto the overall success of the trap experiment. This work largely benefited from theinputs of several reviewers to whom the authors are especially grateful.

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Documents

Biogeochemistry and dynamics of settling particle fluxes at the Antikythira Strait (Eastern Mediterranean)