temporal evolution of physical and chemical characteristics of the water column in the easternmost...

Journal of Marine Systems xxx (2013) xxx–xxx

MARSYS-02458; No of Pages 8

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Journal of Marine Systems

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Temporal evolution of physical and chemical characteristics of the watercolumn in the Easternmost Levantine basin (Eastern Mediterranean Sea)from 2002 to 2010

Nurit Kress ⁎, Isaac Gertman, Barak HerutIsrael Oceanographic & Limnological Res, The National Institute of Oceanography, P.O. Box 8030, Tel Shikmona, Haifa 31080, Israel

⁎ Corresponding author.E-mail address: [email protected] (N. Kress).

0924-7963/$ – see front matter © 2013 Elsevier B.V. All rhttp://dx.doi.org/10.1016/j.jmarsys.2013.11.016

Please cite this article as: Kress, N., et al., TeLevantine basin (Eastern Mediterranean Sea

a b s t r a c t
a r t i c l e i n f o
Article history:Received 7 February 2012Received in revised form 17 November 2013Accepted 18 November 2013Available online xxxx

Keywords:Levantine basinEastern Mediterranean TransientPhysical parametersDissolved oxygenNutrientsChlorophyll-a

The continuing effect of the Eastern Mediterranean Transient event on the distribution of physical and chemicalparameters in the Easternmost Levantine basin was documented from 2002 to 2010 in the open sea and atthe continental slope. By mid-2002, the deep waters had already changed, exhibiting a mid-depth layer, theold Adriatic Deep Water (ADWo), with minimum salinity, temperature (MinSal/T), minimum dissolvedoxygen (MinOx) and maximum nutrient (MaxNut) concentrations above the younger Cretan Sea Ouflowwater. MinSal/T values at the ADWo increased from 2002 to 2010, the depth range narrowed, shallowedand was eroded. The maximum silicic acid layer (N9 μmol kg−1) was positioned with the MinSal/T, whilethe MinOx and maximum nitrate and phosphate layers (b175, N5.5 and N0.2 μmol kg−1, respectively)were shallower (by up to 500 m) due to chemical oxidation of organic matter at the upper layers. Nutrientconcentrations at the base of the nutricline increased with time, concurrently with a widening andshallowing of the deep chlorophyll-a maximum layer and increase in concentration. Since 2008 at theslope and 2010 at the open sea, chlorophyll-a concentrations increased also at and near the surface, indicatinga possible change in the phytoplankton community. A gradual increase in salinity at the upper layers was detectedsince 2006.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Eastern Mediterranean Transient (EMT) event in the 1990schanged considerably the circulation in the Eastern Mediterraneanand the depth distribution of physical and chemical parameters. Theevent, the circulation changes and the temporal and spatial evolutionof the EMT were thoroughly described (Klein et al., 1999; Lascaratoset al., 1999; Roether et al., 1996, 2007 and references therein). Briefly,until the early 1990s the deep layer of the Eastern Mediterranean wasoccupied exclusively by dense water formed in the southern Adriatic,known asAdriatic DeepWater (ADW). A combination ofmeteorologicaland hydrological factors led to the EMT and to the temporary establish-ment of a new source of deep water in the area, the Cretan Sea Outflowwater (CSOW) of Aegean origin. This new, younger, dense water waswarmer and more saline than the older ADW and therefore TS inver-sions appeared at the bottom layer. The CSOWhad also higher dissolvedoxygen and lower nutrient concentrations than the ADW(Roether et al.,1996). While the EMT began around 1990, the main outflow of CSOW(ca. 75%)was delivered betweenmid-1992 and late 1994. It propagatedpreferentially westwards and more slowly towards the east and theADW was pushed up to shallower depths in the Ionian than in the

ights reserved.

mporal evolution of physical) from 2002 to 2010, J. Mar. S

Levantine. The main influence in the South Eastern (SE) Levantineoccurred between 1995 and 2001 being one of the regions wherethe arrival of the CSOW had been delayed. By 2001, eight yearsafter the massive influx of 1993, the EMT induced TS inversionswere still prominent in the Levantine below 1000 m depth althoughwith reduced property ranges compared to 1995.

In contrast to the physical evolution, the changes in the distributionsof dissolved oxygen and in particular of nutrients across the EasternMediterranean were less described in the literature (Klein et al., 1999,2003; Kress et al., 2003, 2011; Roether et al., 1996; Schlitzer et al.,1991). In 1987, prior to the EMT, the cross-basin distribution of dis-solved oxygen and nutrients in the deep water was essentially uni-form from 1000 m downward, except for the Ionian that exhibiteda mid-depth layer at ca. 1200 mwith minimum oxygen and maximumnutrient concentrations (MinOx/MaxNut) (Schlitzer et al., 1991). East-wards, dissolved oxygen decreased whereas nutrient increased, inagreement with the direction of the deep-water circulation. No mid-depth MinOx/MaxNut was present in the SE Levantine (Kress andHerut, 2001). In 1995, ca. six years after the onset of the EMT event,the deep water was not uniform and a pronounced MinOx/MaxNutlayer was detected at the 500–1500 m depth range, depending on thebasin sampled (Klein et al., 1999; Lascaratos et al., 1999). This layercorresponded to the older ADW that was pushed up by the youngerCSOW. The MinOx/MaxNut layer continued to change as the EMT

and chemical characteristics of the water column in the Easternmostyst. (2013), http://dx.doi.org/10.1016/j.jmarsys.2013.11.016

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2 N. Kress et al. / Journal of Marine Systems xxx (2013) xxx–xxx

evolved, peaking in 1999 (Klein et al., 2003; Kress et al., 2003). In 2001,ca. 11 years after the onset of the EMT, the eastern part of the deepbasin was still being affected by it, although at its relaxation stage.Vertical sections showed the old ADW (ADWo) with Minox/MaxNut

at intermediate depths, its distribution similar to that of 1999 butwas eroded by mixing with surrounding waters (Kress et al., 2011).

In this study, we present a recent snapshot (2008) of the depthdistribution of physical and chemical parameters in the Levantinebasin and follow the temporal evolution of the EMT between 2002and 2010 at the basin's easternmost part. Specifically, we aim toshow that the EMT event changed the circulation from the deeplayers up to the bottom of the euphotic zone and: a) show the tem-poral evolution of salinity, temperature, dissolved oxygen and nutri-ents in the easternmost part of the Levantine basin, and b) assess thepossible influence of the changes in the chemistry and biology of thearea.

2. Sampling and methods

2.1. Area of study and sampling

The study area, the location of the 20hydrographic stations occupiedin 2008 and the six stations occupied 24 times between Aug-02 andDec-10 (SESAME and Haifa Section programs) are depicted in Fig. 1.The cruises were conducted on board the R/V Shikmona. Continuousprofiles of pressure, temperature salinity, dissolved oxygen and fluo-rescence were performed with a Sea-Bird SBE 911 plus CTD system,interfaced to a SBE Carousel equipped with twelve, 8-liter Niskinbottles. Water samples for determining dissolved oxygen, nutrientsand chlorophyll-a (chl-a) were collected with the Niskin bottles atsampling depths determined on the basis of physical properties ofthe water column representing the different water masses. Watersamples for dissolved oxygen were sampled and pickled. Duplicatesamples for nutrient analysis were collected in 15-ml acid washedplastic scintillation vials and immediately frozen. Seawater forchlorophyll-a determination was filtered through GF/F filters thatwere then wrapped in aluminum foil and frozen. Dissolved oxygenwas measured at sea using the Carpenter–Winkler titration procedure(Carpenter, 1965) and a Radiometer automatic titrator (TTT80)equipped with a dual platinum electrode, in the dead-stop endpoint mode. The precision, determined by analyzing replicate sam-ples from the same Niskin bottle was 0.3%. The results were usedto calibrate the continuous profiles of dissolved oxygen of the CTD.

Fig. 1.Map of stations occupied during SESAME's September 2008 surv

Please cite this article as: Kress, N., et al., Temporal evolution of physicalLevantine basin (Eastern Mediterranean Sea) from 2002 to 2010, J. Mar. S

2.2. Laboratory analysis

In the laboratory, nutrients were determined using a segmentedflow Technicon AutoAnalyzer II (AA-II) and Skalar hybrid system bythe methods described previously (Kress and Herut, 2001; Krom et al.,1991). Since 2010, nutrients were measured with a Seal Analytical AA-3 system. The precision for nitrate + nitrite, phosphate and silicic acidwas 0.02, 0.003 and 0.06 μM, respectively. The limit of detection (2times the standard deviation of the blank) for the procedures is0.075 μM for nitrate + nitrite, 0.008 μM for phosphate and 0.03 μMfor silicic acid. For simplicity, in the text we refer to nitrate + nitriteas nitrate. Quality assurance of the nutrient measurements was con-firmed by the results of intercomparison exercises (NOAA/NRC,JAPAN, QUASIMEME). Chl-a was measured fluorimetrically followingextraction with 95% acetone (Holm-Hansen et al., 1965). The resultswere used to calibrate the continuous profiles of fluorescence of theCTD.

2.3. Data base and data analysis

Additional historical data used in this study were obtained fromISRAMAR, Israel's Marine data center, a member of the InternationalOceanographic Data and Information Exchange (IODE) network.(http://www.ocean.org.il/Eng/ISRAMARDataCenter/ProfileDC.asp).Data analysis and display were performed using the Ocean Data Viewsoftware developed by Prof. Reiner Schlitzer of the Alfred WegenerInstitute, Germany.

3. Results

3.1. The Levantine basin in 2008

A recent snapshot of the CSOW in the Levantine basin taken inSeptember 2008 (Fig. 2) showed the structure of the deep water18 years after the EMT event onset. The old Adriatic Deep Water(ADWo) uplifted by the CSOW and identified by the mid-depthminimum salinity and temperature (MinSal/T) was centered at1000–1200 m depth (Fig. 2). The minimum potential temperatureand salinity (b13.60 °C and b38.75, respectively) were higher than inprevious years (Table 1). From 27 °E eastwards, potential temperatureand salinity increased from the ADWo towards the bottom due to theCSOW, while close to Crete, a new deep water, slightly less saline andcolder compared to the CSOW was present (Table 1). This new watermass (ADWn)was attributed to deepwater egressing from the Adriatic

ey and the stations occupied at the Haifa Section Program (inset).


http://www.ocean.org.il/Eng/ISRAMARDataCenter/ProfileDC.asp


Longitude Longitude

Fig. 2. Vertical sections of potential temperature, salinity, dissolved oxygen, silicic acid, nitrate, phosphate and chlorophyll-a along the west–east cross section of the Levantine basin inSeptember 2008. Stations and data points are indicated by filled dots (see inset map). A color version of the figure is available online.

3N. Kress et al. / Journal of Marine Systems xxx (2013) xxx–xxx

Sea probably replacing the CSOW, as has been seen since 2001 in theIonian basin (Manca et al., 2002, 2006; Rubino and Hainbucher,2007).

The MinOx layer was centered at ca. 900 m, more emphasizedand shallow in the eastern part of the transect (b175 μmol kg−1

up to 27 °E) and eroded towards the west (minimum concentrationsof 180–175 μmol kg−1). The concentrations increased towards thebottom, in particular westwards of 27 °E, indicatingmixing and erosion

Table 1Potential temperature and salinity at the deep water masses in the Levantine basin during diff

Water mass 19871 19952

Pot. T (°C) Salinity Pot. T (°C) Salinity

ADWo 13.3–13.4 38.66–38.68 13.4–13.6 38.69–38.CSOW – – 13.6–13.7 38.79–38.ADWn – – – –

1 Schlitzer et al. (1991).2 Roether et al. (1996).3 Manca et al. (2006).4 This study.


of the CSOW in the east and penetration of younger waters in the west(Fig. 2). The MaxNut of nitrate and phosphate (MaxNO3,PO4) was locatedat 400–1500 m depth interval with maximal concentrations higherthan 5 and 0.2 μmol kg−1, respectively, the upper isolines shoalingeastwards up to 400 m depth. The concentrations decreased towardsthe bottom. The maximum in silicic acid (MaxSi) was positioned atthe 700–1750 m depth interval, centered at 1200 m, similar to theposition of the MinSal/T. This layer shoaled eastwards with the

erent years.

20013 20084

Pot. T (°C) Salinity Pot. T (°C) Salinity

72 13.5–13.6 38.73–38.76 13.55–13.65 38.73–38.7789 13.6–13.7 38.79–38.81 13.6–13.7 38.77–39.79

13.5–13.6 38.77–38.79 13.45–13.55 38.74–38.77




upper 9 μmol kg−1 isoline reaching 700 m depth. There were indica-tions of deep intrusion of seawater with lower silicic acid concentrationnear Crete. Chlorophyll-a (chl-a) exhibited a pronounced deep chloro-phyll maximum (DCM) across the Levantine (Fig. 2). As expected, theconcentrations at the DCM decreased eastwards due to increasedoligotrophy and the depth range defined by the 0.15 μg l−1 isolinedeepened from 75–140 m in thewest to 100–150 m at the easternmoststation (Fig. 2).

3.2. The easternmost part of the Levantine basin, 2002–2010

The description of the temporal changes in temperature, salinity,dissolved oxygen and nutrients (nitrate, phosphate, silicic acid) andchlorophyll-a (chl-a) focused on the open sea (stations H06 and H05,bottom depth of 1700 and 1480 m, respectively) and on the slopearea (station H03, 600 m bottom depth). In the open sea, time seriesof temperature, salinity, dissolved oxygen and chl-a parameters mea-suredwith the CTDwere depicted for stationH05, a station thatwas oc-cupied historically since the 1960s. The time series of the nutrients inthe open sea combined the data acquired at stations H06 and H05, inorder to increase the number of discrete data points used in the analysisprocedure.

3.2.1. The open sea stationsSince Aug-02, a minimum salinity and potential temperature layer

(MinSal/T) (≤38.74, min 38.73; ≤13.55 °C) corresponding to theADWowas present at 900–1250 mdepth, that narrowed andwas erod-edwith time (Fig. 3).With time, theminimumvalues of salinity and po-tential temperature at this layer increased as well. A deep layer withmaximum salinity and potential temperature (MaxSal/T) (38.75–38.80;13.55–13.60 °C) corresponding to the CSOW has also been presentsince Aug-02. This layer widened and shallowed by ca. 250 m, from

Fig. 3.Temporal evolution of potential temperature, salinity and dissolvedoxygen at stationH05is available online.


1320 m in Aug-02 to 1150 m inDec10. The temporal increase in salinityat the ADWo and the widening of the CSOW layer indicated mixing ofthe two water masses. In the upper 300 m there was a marked increasein salinity, exemplified by the 39.1 isoline located near the surface inSep05 whereas in Dec10 it reached down to 250 m depth (Fig. 4). Nosimilar temporal trend was evident in the temperature at the upperlayers maybe due to greater seasonal variability.

Amid depthminimumoxygen layer (MinOx) (O2 b 175 μmol kg−1),a result of the EMT induced changes in circulation, was already in placeat the start of the time series in Aug-02. The layer, positioned from 370to 720 mdepth,was shallower than theMinSal/T layer (Fig. 3). Dissolvedoxygen is not a conservative property and thedepth distributionwas in-fluenced in this case by the EMT event that raised the older ADWo fromthe bottom layers to mid depths, and by its utilization in the decompo-sition of organicmatter by bacteria. The depth range of theMinOx variedwith time and was between 420 and 1000 m in Jun-04 and 400–760 minDec-10. Theminimumconcentration at this layer remained essential-ly constant at 171–173 μmol kg−1. No trendwas apparent in the upper180 μmol kg−1 isoline (at ca. 350 m). More oxygenated waters weredetected at the bottom layer during the whole study as seen by the po-sition of the deep 180 μmol kg−1 isoline. The bottom layer was widerbetween since Sep-06 indicating, perhaps, an increased supply ofCSOW or new ADWn.

3.2.1.1. Nutrients and chlorophyll-a. Since 2004, there has been a mid-depth range (500–750 m in Sep-04 to 400–1000 m in Dec-10) withmaximumnitrate concentration (NO3 ≥ 5.5 μmol kg−1). The concentra-tions increasedwith time at this layer to higher than 5.8 μmol kg−1, indi-cating an increase in water age. Below the MaxNO3 layer, lower nitrateconcentrations (b5.5 μmol kg−1), corresponding to the CSOW, weremeasured during all surveys. From Jan-03 to Jun-05 and from Sep-08 toMar-10 concentrations lower than 5 μmol kg−1 were measured at the

and silicic acid, nitrate and phosphate at stationsH05andH06. A color version of thefigure



a b

c d

Fig. 4. Temporal evolution of salinity and chlorophyll-a at the upper 300 m at the open sea (station H05, panels a and c) and at the continental slope (station H03, panels b and d). A colorversion of the figure is available online.


deeper layers. There were no indications of shallowing of the upperisolines.

The concentration of phosphate below 500 m was relatively low(b0.20 μmol kg−1) from Aug-02 to Mar-04 compared to data from thefollowing years (Fig. 3). From Sep-04 onwards there was a very distinctmid depth MaxPO4 layer (≥0.21 μmol kg−1) with varying depth range,similar to the MaxNO3/MinOx. Concentrations at the MaxPO4 layer in-creased from 0.23 μmol kg−1 in Jul-07 to 0.26 μmol kg−1 in Dec-10.The lowest phosphate bottom concentrations were measured betweenJuly-06 and Feb-07, similar to the higher oxygen concentrations mea-sured at the same time, and to a lesser degree, to the lower nitrate.There were essentially no changes in the depth of the 0.2 μmol kg−1

isoline, while the 0.15 and 0.1 μmol kg−1 isolines seemed to shallowby ca. 100 m from Aug-02 to Jul-07.

Concentrations of silicic acid higher than 9 μmol kg−1 were mea-sured from 600 m down to 1500 m in Aug-02, at the depth range of700–1400 m from Jan-03 to Jan-06, narrowed to 650–1300 m up toAug-09, further narrowing in 2010. The position of the MaxSi in thewater column was deeper than the MinOx/MaxNO3,PO4 layer and essen-tially at the same depth interval as the MinSal/T layer, corresponding tothe ADWo. The highest concentrations (≥9.5 μmol kg−1) were mea-sured from Aug-02 to Jun-05 centered at ca. 1100 m. Since Jul-06there has been an intrusion of bottom waters (below 1250–1500 m)with lower silicic acid concentration. This water probably mixed withthe older water, decreasing the concentrations at mid depths. Therewere no indications of shallowing of the isolineswith time, only tempo-ral fluctuations. The concentrations at the upper 200 m seemed to havedecreased to less than 1 μmol kg−1 since Jul-07.

A deep chlorophyll-a maximum (DCM) was centered at ca. 110 mduring the spring–summer months and shallower (75 m) or absent inthe winter (Fig. 4). The concentrations at the DCM ranged from 0.1 to0.3 mg m−3. Since Jul-Sep-07, the DCM depth range has seemed towiden and shallow, with a slight increase in concentrations. The con-centrations at the surface seemed to have increased since 2010.

3.2.2. Station H03 at the continental slopeStation H03 is shallower (600 mdepth) and located at the continen-

tal slope (Fig. 1). Potential temperature and salinity decreased with in-crease in water depth, and no MinSal/T layer was present (Fig. 5). Insome surveys, modified Atlantic Water (MAW) was detected by itslower salinity at ca. 80 m depth. In the upper 300 m there has been a


marked increase in salinity since Sep-05, similar to the open sea stations(Fig. 4). No temporal trend was detected in the temperature.

Dissolved oxygen decreased and nutrient concentrations increasedwith increasing depth. Dissolved oxygen concentration was lowerthan 175 μmol kg−1 below 350 m depth during most of the study.Since Mar-10 an increase in concentrations has been detected, withthe 175 μmol kg−1 isoline positioned at 400–500 m instead of 300–400 m depth (Fig. 5). Nitrate concentrations higher than 5.5 μmol kg−1 have been present since Jun-05 and the 5.5 μmole kg−1 isolinereached 425 m in Feb-07. Concentrations in this layer increased withtime. There were no noticeable changes in the depths of the otherisolines. A similar behavior was observed for phosphate. Concentra-tions ≥0.21 μmol kg−1 were confined in depths deeper than 500 m atthe beginning of the time series. Since Jun-05 (as for nitrate), the concen-trations have increased to≥0.23 μmol kg−1 and the 0.21 μmol kg−1 iso-line reached up to 400 m in Dec-10. The concentrations increased withtime below 400 m depth. Silicic acid concentrations below 500 m werestable at ≥8.5 μmol kg−1 up to Jul-06 and this isoline shallowed to400 m, increasing the depth range with high concentrations. Since Mar-10 the concentrations have decreased at the deep layers, probably corre-sponding to the decreasemeasured in the open sea, but delayed, in agree-mentwith eastwardswaterflow. A slight decrease in concentration at theupper 200 mhas been observed since Sep-08. TheDCMdepth rangewid-ened, shallowed and the maximal concentration increased with timesince Feb-07, as in the open sea, but with slightly higher concentrations(0.1–0.4 mg m3, Fig. 4). Chl-a has increased by 0.05 mg m3 near and atthe surface since Aug-09.

4. Discussion

4.1. Temporal evolution of potential temperature and salinity in the ADWand CSOW

The EMT event changed the depth distribution of potential temper-ature and salinity in the Levantine basin. Those changes were already inplace in 2002 and still visible in 2010, ca. 20 years after the start of theevent, as illustrated in the T-S diagrams (Fig. 6). Since 2002, the physicalproperties of the ADWo and the CSOW in the easternmost Levantinehave been different from the properties of these water masses close tothe source, and different from the values measured in 1995 and 2001(Table 1). The minimum potential temperature and salinity measured



Fig. 5. Temporal evolution of potential temperature, salinity, dissolved oxygen, silicic acid, nitrate and phosphate at station H03. A color version of the figure is available online.


during this study at the ADWo were 13.50 °C and 38.73, respectively,and at the CSOW 13.60 °C and 38.77, indicating mixing of the originalwater masses. This is in agreement with Roether et al. (2007) analysis,which showed that the temperature and salinity ranges generated bythe EMT shrunkwith time,most evidently in the Levantine.Mixing, hor-izontally, vertically and/or in connection with entrainment intooverflowing waters was probably the reason. The process was quitefast: an increase in potential temperature and salinity was found atthe first time in the bottom layer near the Israeli coast in 1996(Gertman et al., 2010). As a coarse estimation, the propagation rate ofthe CSOW from Cretan Passage to the south-eastern continental slopewas about 280 km year−1, considering that the main intrusion ofCSOW occurred in 92–93 (Roether et al., 2007), the first time theCSOW was noticed in the Levantine was 1996 (Gertman et al., 2010)and the distance traveled was 1100 km. From 2002 to 2010, the mini-mum potential temperature and salinity at the ADWo in the opensea increased from 13.50 to 13.57 °C and from 38.73 to 38.75, respec-tively. The depth range narrowed, shallowed and was eroded whilethe depth range of the CSOW expanded. Increase in potential tem-perature near the bottom to 13.64 °C since 2009 may indicate thepresence of younger ADWn close to the bottom. This observation isconsistent with the almost concurrent increase in dissolved oxygenand decrease in silicic acid, and to lesser degree, of nitrate and phos-phate. At the slope station, there were essentially no temporalchanges in temperature and salinity below 400 m depth, except forsome seasonal variations, as expected.

4.2. Temporal evolution of salinity at the upper layer

In the upper 300 m, salinity had increased since 2006 in the opensea and at the continental slope, as exemplified by the 39.10 isolinethat deepened from the surface to below 250 m. Average salinity inthe upper mixed layer (from 0 to 10 m depth) in the open sea


increased from 39.10 in Jun-04 to 39.59 in Jul-06 with maximum av-erage surficial salinity of 39.75 in Sept-08 in the open sea (Fig. 4). Sim-ilar increase in surficial salinity was observed in the Levantine basinprior to the EMT. Advection of this abnormally saline Levantine SurfaceWater in the Aegean Sea during 1989–1990 (Gertman et al., 2006)followed by extremely cold winters 1992–1993 (Lascaratos et al.,1999) forced the formation of deepwater with potential density anom-aly (relative 2000 dbar) of about 37.83 kg m−1. Although the excessover the pre-EMT bottom water density was just 0.03 kg m−1, it wasenough to generate the wide spreading of newly formed water(CSOW). Analysis of the upper mixed layer data prior and post EMT atthe open sea (data from the vicinity of station H05, Fig. 7) showed a cy-clic pattern in salinity: lower salinities at the beginning of the 1980s and2000s and higher salinities at the beginning of the 90s (onset of theEMT) and since 2006. Cyclic pattern changes were observed also inthe upper layer circulation at the Ionian basin that changed from cy-clonic to anti-cyclonic circulation and back (Civitarese et al., 2010;Gacic et al., 2010).

4.3. Temporal evolution of the MaxSi

Whereas salinity and temperature are influenced by physical pro-cesses alone, the concentrations of dissolved oxygen and nutrientsare influenced by chemical and biological processes as well. Nutri-ents are utilized and dissolved oxygen is produced during photosyn-thesis while bacterial remineralization of organic matter consumesdissolved oxygen and regenerate nutrients. Silicic acid is uniqueamong the nutrients since it is utilized mainly by diatoms for theproduction of biogenic silica for their tests (Ragueneau et al.,2000). Moreover, the regeneration of silicic acid occurs by chemicaldissolution of silicious tests [Hurd, 1983], a process uncoupled tooxygen consumption and slower than bacterial remineralization.Therefore, the maximum in silicic acid concentration in the water



Fig. 6. TS diagrams in the Easternmost Levantine basin prior, at and after the EMT event(1987, 1995 and 2010, respectively). Station positions are presented in the inset.


column is located deeper than the MinOx/MaxNO3,PO4 and more inaccord with the physical parameters.

The depth of the MaxSi layer at the open sea stations (H06, H05),defined as the 9 μmol kg−1 isoline, was in agreement with theMinSal/T position in the water column and hence attributed to theADWo. It was possible to differentiate two different time periods:from Aug-02 to Jul-06, when the maximal concentration increasedslightly but the depth range of the MaxSi widened with time, andsince Jul-06, when the MaxSi concentration decreased with timeand the depth range narrowed. During the first stage of the time se-ries, the CSOW had already been established in the area, the supplyof new CSOW stopped and the deep water reaching the area wasolder (with higher concentrations). Therefore, the MaxSi layer wid-ened and concentrations increased slightly due to aging of thiswater mass. At the second stage (from Jul-06), the decrease inMaxSi concentration and narrowing of depth range can be explainedonly by mixing with water masses with lower concentration: a) withyounger waters intruding from below, and/or b) s with the upperlayers due to the uplift of the ADWo by the CSOW. It is assumedthat both processes were in place. At the slope, silicic acid concentra-tions were essentially constant until Jul-06 and then increased up

Sal

inity

Year

40.0

39.8

39.6

39.4

39.2

39.0

38.8

38.61960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

Fig. 7. Temporal evolution of salinity at the uppermixed layer (average of 0–10 m) of sta-tion H05 and vicinity. Data obtained from ISRAMAR data base.


until Mar-10. This could be attributed to the incoming of ADWo waterto the area and to the shallow bathymetry (station depth ca. 600 m)that prevented the deeper, younger CSOW from reaching the area anddecreasing the concentrations at depth.

4.4. Temporal evolution of the MinOx/MaxNO3,PO4

The positions of the MinOx/MaxNO3,PO4 layer at the open sea definedby the isolines 175, 5.5 and 0.2 μmol kg−1, respectively, were closelyrelated and much shallower (by up to 500 m) than the MinSal/T posi-tion in the water column at a specific time (Table 1, Figs. 3, 5–6). Thedifferent positions in the water columnwere probably a result of twodifferent processes: physical advection andmixing, and chemical ox-idation of organic matter. If only the physical processes were inplace, we would have expected the temporal depth distribution tobe similar to that of salinity, temperature and silicic acid. However,at the upper layers, but below the photic zone, organic matter is oxi-dized, oxygen is consumed and nitrate and phosphate remineralized.This process is enhanced at the upper part of the range since the con-centration of available organic matter decreases with depth, in particu-lar in the oligotrophic Eastern Mediterranean (Krom et al., 2005;Santinelli et al., 2010). Therefore, the reduced oxidationwith increasingdepth in conjunction with mixing with the deeper, younger, moreoxygenated and with less nutrient water (CSOW) caused theMinOx/MaxNO3,PO4 to be shallower than the MinT/Sal. At the slope,maximal concentrations below 400 m increased sinceMar-05 at sta-tion H03 (prior to the silicic acid increase) and later (since Jul-07) atstation H02 (unpublished results).

4.5. Temporal evolution of chlorophyll-a concentrations

The DCM depth range widened and shallowed with time, with aslight increase in concentrations, more evident at the continentalslope (Fig. 4). The concentrations at the surface increased as well sinceMar-10 in the open sea and earlier (Sep-08) at the slope. There wasno concurrent increase in nitrate and phosphate at the DCM, but theirconcentrations increased by 0.5 and 0.04 μmol kg−1, respectively,at the base of the nutricline (400–500 m). Moreover, the timing ofthe changes at the DCM and the nutricline were similar. Therefore,we assume that the increase in concentration at the base of thenutricline increased the supply of nutrients to the ultra-oligotrophicphotic zone, where they were rapidly consumed by the phytoplankton,increasing chl-a concentrations. This may explain the decrease in silicicacid concentrations at the upper layers that occurred at the same time.It is speculated that increased supply of nitrate and phosphate frombelow may have changed the phytoplankton community structure atthe upper layers, increasing the relative contribution of diatoms andhence depleting silicic acid. However, there are no data on the phyto-plankton community at these time series that can support this theory.

Until this study, basin-wide oceanographic transects conductedin 1991 (Yacobi et al., 1995), 2001 (Kress et al., 2011) and 2008(Fig. 2) did not show a trend in chl-a concentrations, nor did satelliteimagery analysis up to 2002 (D'Ortenzio et al., 2003). In October1991, chl-a ranged from 0.01 to 0.25 μg l−1 and depth integratedchlorophyll ranged from 17 to 35 mg m−2, with higher values inthe western Levantine. In October 2001, chl-a concentrations rangedfrom 0.01 to 0.28 μg l−1 (integrated values of 16.5–23.1 mg m−2)and in September 2008, from 0.01 to 0.33 μg l−1 (depth integrated10 to 27.1 mg m−2). Therefore, it is evident that the occasionalbasin-wide snapshot is not enough to indicate trends that may be cy-clic and short lived; as with salinity, high frequency surveys wereneeded to identify this trend. Moreover, as surficial chl-a concentra-tions increased only since Sep-08 at the slope andMar-10 in the opensea area, satellite imagery analysis should verify our findings.




5. Summary and conclusions

Bymid-2002, the physical and chemical character of thedeepwatersat the easternmost part of the Levantine basin had already changedas a result of the EMT event: A mid depth layer, the ADWo, withmin-imum salinity, temperature, dissolved oxygen and maximum nutri-ent concentrations, and the younger, deeper CSOW were welldefined at the open sea stations. From 2002 to 2010, the continuingevolution of the CSOW and the uplift of the ADWo influenced alsothe upper water column, increasing the nutrient concentrations atthe base of the nutricline and probably the supply to the photiczone. The depth range of the deep chl-a maximum widened,shallowed and the maximal concentrations increased, more evidentat the continental slope. Since 2008 at the slope and 2010 in theopen sea, chl-a concentrations have increased also at and near thesurface, accompanied by a decrease in silicic acid concentrations.This may indicate a change in the relative contribution of diatomsto the phytoplankton, however, there were no concurrent data onthe community composition to verify this assumption. The most no-ticeable temporal change observed during this study was the in-crease in salinity at the surface that started by mid-2005. Salinitycontinued to increase in the upper layers (down to 300 m) untilthe end of 2010. However, historical data showed that this couldhave been a cyclic process, as are other processes that have startedto being identified in the Eastern Mediterranean. This salinity in-crease, identified at the easternmost part of the basin, should contin-ue to be followed and analyzed not only in the local context butbasin-wide, since it had been shown that a similar increase in salinityin the area may have triggered or conditioned the EMT event in the1990s.

Acknowledgment

The authors thank the captain and crewof the R/V Shikmona and theresearch assistants at theMarine Chemistry, Physical Oceanography andData Center departments at IOLR for their dedicated work at sea and inthe laboratory. We acknowledge the comments of two anonymous re-viewers that helped improve the manuscript. The research was fundedby the Israel Ministry of National Infrastructures and by the SESAMEProject, EC Contract No. GOCE-036949 in the framework of theEuropean Commission's Sixth Framework Program under the priority‘Sustainable Development, Global Change, and Ecosystems’.

References

Carpenter, J.H., 1965. The accuracy of the Winkler method for dissolved oxygen analysis.Limnology and Oceanography 10, 135–140.

Civitarese, G., Gacic, M., Lipizer, M., Borzelli, G.L.E., 2010. On the impact of the BimodalOscillating System (BiOS) on the biogeochemistry and biology of the Adriatic andIonian Seas (Eastern Mediterranean). Biogeosciences 7, 3987–3997.

D'Ortenzio, F., Ragni, M., Marullo, S., Ribera d'Alcalà, M., 2003. Did biological activity in theIonian Sea change after the EasternMediterranean Transient? Results from the analysisof remote sensing observations. J. Geophys. Res. 108, 8113.


Gacic, M., Borzelli, G.L.E., Civitarese, G., Cardin, V., Yari, S., 2010. Can internal processessustain reversals of the ocean upper circulation? The Ionian Sea example. Geophys.Res. Lett. 37, L09608 [doi:09610.01029/02010GL043216].

Gertman, I., Pinardi, N., Popov, Y., Hecht, A., 2006. Aegean Sea water masses during theearly stages of the Eastern Mediterranean climatic transient (1988–90). J. Phys.Oceanogr. 36, 1841–1859.

Gertman, I., Herut, B., Kress, N., 2010. Assessment of post-transient changes in Levantinebasin deep waters. Rapports Commission Internationale pour la Mer Mediterranee39, 113.

Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W., Strickland, J.D.H., 1965. Fluorometric de-termination of chlorophyll. J. Cons. Int. Explor. Mer 30.

Hurd, D.C., Birdwhistell, S., 1983. On producing a more general model for biogenic silicadissolution. American Journal of Science 283, 1–28.

Klein, B., Roether, W., Manca, B.B., Bregant, D., Beitzel, V., Kovacevic, V., Luchetta, A., 1999.The large deep water transient in the Eastern Mediterranean. Deep-Sea Res. IOceanogr. Res. Pap. 46, 371–414.

Klein, B., Roether, W., Kress, N., Manca, B.B., d'Alcala, M.R., Souvermezoglou, E.,Theocharis, A., Civitarese, G., Luchetta, A., 2003. Accelerated oxygen consumption inEastern Mediterranean deep waters following the recent changes in thermohalinecirculation. J. Geophys. Res. Oceans 108. http://dx.doi.org/10.1029/2002JC001454.

Kress, N., Herut, B., 2001. Spatial and seasonal evolution of dissolved oxygen and nutrientsin the Southern Levantine Basin (Eastern Mediterranean Sea): chemical characteriza-tion of the water masses and inferences on the N:P ratios. Deep-Sea Res. I Oceanogr.Res. Pap. 48, 2347–2372.

Kress, N., Manca, B.B., Klein, B., Deponte, D., 2003. Continuing influence of the changedthermohaline circulation in the Eastern Mediterranean on the distribution of dis-solved oxygen and nutrients: physical and chemical characterization of the watermasses. J. Geophys. Res. Oceans 108. http://dx.doi.org/10.1029/2002JC001397.

Kress, N., Herut, B., Gertman, I., 2011. Nutrient distribution in the Eastern Mediterraneanbefore and after the transient event. In: Stambler, N. (Ed.), Life in the MediterraneanSea: A Look at Habitat Changes. Nova Science Publishers, Inc.

Krom, M.D., Kress, N., Brenner, S., Gordon, L.I., 1991. Phosphorus limitation of primaryproductivity in the Eastern Mediterranean Sea. Limnol. Oceanogr. 36, 424–432.

Krom, M.D., Woodward, E.M.S., Herut, B., Kress, N., Carbo, P., Mantoura, R.F.C., Spyres, G.,Thingstad, T.F., Wassmann, P., Wexels-Riser, C., Kitidis, V., Law, C.S., Zodiatis, G., 2005.Nutrient cycling in the south east Levantine basin of the Eastern Mediterranean: re-sults from a phosphorus starved system. Deep-Sea Res. II Top. Stud. Oceanogr. 52,2879–2896.

Lascaratos, A., Roether, W., Nittis, K., Klein, B., 1999. Recent changes in deep water forma-tion and spreading in the Eastern Mediterranean Sea: a review. Prog. Oceanogr. 44,5–36.

Manca, B.B., Kovacevic, V., Gacic, M., Viezzoli, D., 2002. Dense water formation in theSouthern Adriatic Sea and spreading into the Ionian Sea in the period 1997–1999.J. Mar. Syst. 33–34, 133–154.

Manca, B.B., Ibello, V., Pacciaroni, M., Scarazzato, P., Giorgetti, A., 2006. Ventilation of deepwaters in the Adriatic and Ionian Seas following changes in thermohaline circulationof the Eastern Mediterranean. Clim. Res. 31, 239–256.

Ragueneau, O., Tréguer, P., Leynaert, A., Anderson, R.F., Brzezinski, M.A., DeMaster, D.J.,Dugdale, R.C., Dymond, J., Fischer, G., François, R., Heinze, C., Maier-Reimer, E.,Martin-Jézéquel, V., Nelson, D.M., Quéguiner, B., 2000. A review of the Si cycle inthe modern ocean: recent progress and missing gaps in the application of biogenicopal as a paleoproductivity proxy. Glob. Planet. Chang. 26, 317–365.

Roether, W., Manca, B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevic, V.,Luchetta, A., 1996. Recent changes in Eastern Mediterranean deep water. Science271, 333–335.

Roether, W., Klein, B., Manca, B.B., Theocharis, A., Kioroglou, S., 2007. Transient EasternMediterranean deep waters in response to the massive dense-water output of theAegean Sea in the 1990s. Prog. Oceanogr. 74, 540–571.

Rubino, A., Hainbucher, D., 2007. A large abrupt change in the abyssal watermasses of theEastern Mediterranean. Geophys. Res. Lett. 34, L23607.

Santinelli, C., Nannicini, L., Seritti, A., 2010. DOC dynamics in the meso and bathype-lagic layers of the Mediterranean Sea. Deep-Sea Res. II Top. Stud. Oceanogr. 57,1446–1459.

Schlitzer, R., Roether, W., Oster, H., Junghans, H.G., Hausmann, M., Johannsen, H.,Michelato, A., 1991. Chlorofluoromethane and oxygen in the Eastern Mediterranean.Deep-Sea Res. A Oceanogr. Res. Pap. 38, 1531–1551.

Yacobi, Y.Z., Zohary, T., Kress, N., Hecht, A., Robarts, R.D., Waiser, M., Wood, A.M., 1995.Chlorophyll distribution throughout the Southeastern Mediterranean in relation tothe physical structure of the water mass. J. Mar. Syst. 6, 179–190.


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