JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C9, 8109, doi: /2002jc001397, 2003

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C9, 8109, doi: /2002jc001397, 2003 Continuing influence of the changed thermohaline circulation in the eastern Mediterranean on the distribution of dissolved oxygen and nutrients: Physical and chemical characterization of the water masses Nurit Kress, 1 Beniamino B. Manca, 2 Birgit Klein, 3 and Davide Deponte 2 Received 20 March 2002; revised 1 July 2002; accepted 19 July 2002; published 25 September [1] Changes in the vertical distribution of the physical and chemical parameters observed during this study point to the continuing spatial and temporal evolution of the Eastern Mediterranean Deep Water (EMDW). These changes influenced primarily the water column below 800 m and confined the old EMDW of Adriatic origin to the m in the Levantine, uplifting the minimum-oxygen/maximum-nutrient (Min Ox /Max Nut ) from 2500 m in 1995 to 1500 m in The denser and younger EMDW of Aegean origin (EMDW Aeg ) appeared at the bottom layer, more evident at the central eastern Mediterranean and moving prevalently into the Levantine basin. Younger water still intruded the central area at intermediate depths. In the Levantine Intermediate Waters (LIW) oxygen decreased and nutrients increased westward in agreement with its pathway across the basin. However, a temporal decrease in oxygen and concurrent increase in nutrient was found in the LIW because of its confinement in the Levantine by anticyclonic features. Temporal changes were also found in the EMDW at the western Ionian and eastern Cretan passage. The differences in the vertical placement of the Min Ox /Max Nut layer among the different areas were explained by the physical processes, but the relative displacement of the extreme points within the same area were probably a result of oxidation of particulate matter poorer in nitrogen and phosphorus in the Levantine than in the Ionian. Maximal concentrations of silicic acid were located deeper than the other nutrients because of the slower chemical dissolution of silicious tests. The N:P molar ratios at the EMDW were high ( ), highest in the younger EMDW Aeg. These differences could not be explained by different dissolved inorganic nutrients concentration at the sources nor by the composition of particulate matter in the euphotic zone. It was suggested that DOM may have a significant role in determining those differences; however, data on DOM concentration and composition across the basin is lacking in order to test this hypothesis. INDEX TERMS: 4223 Oceanography: General: Descriptive and regional oceanography; 4532 Oceanography: Physical: General circulation; 4536 Oceanography: Physical: Hydrography; 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; KEYWORDS: hydrography, circulation change, dissolved oxygen, nutrients, N:P ratio, eastern Mediterranean Citation: Kress, N., B. B. Manca, B. Klein, and D. Deponte, Continuing influence of the changed thermohaline circulation in the eastern Mediterranean on the distribution of dissolved oxygen and nutrients: Physical and chemical characterization of the water masses, J. Geophys. Res., 108(C9), 8109, doi: /2002jc001397, Introduction [2] The eastern Mediterranean Sea (Figure 1a) is divided into two basins (the Ionian and Levantine interconnected by the Cretan Passage), which possess distinct oceanographic characteristics. The surface layer is mainly influenced by the Atlantic Water, which after entering through the Straits of 1 Israel Oceanographic & Limnological Research, Ltd., National Institute of Oceanography, Haifa, Israel. 2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentali, Trieste, Italy. 3 Institut fur Umweltphysik, University of Bremen, Bremen, Germany. Copyright 2003 by the American Geophysical Union /03/2002JC Gibraltar, flows through the western Mediterranean basin, passes through the Sicily Straits and modified spreads eastward to the Levantine basin. Its circulation in the eastern Mediterranean displays a number of features with large variability in extent, shape and timescales [Robinson et al., 1991; Malanotte-Rizzoli et al., 1999]. In the intermediate layer, the Levantine Intermediate Water (LIW) flowing westward through the Cretan Passage enters the Ionian Sea veering cyclonically/anticyclonically around two gyres located close to the southwest of Crete and to the Peloponnesus peninsula, respectively. The major flow of LIW, which spreads over the proper kgm 3 isopycnal [Malanotte-Rizzoli et al., 1997], is directed toward the Sicily Straits, with a portion of the LIW also introducing the Adriatic Sea through the Otranto Strait. The deep layer PBE 10-1

2 PBE 10-2 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 1. (a) Bathymetric map of the eastern Mediterranean, (b) CTD sampling stations grouped in six study areas (WI, western Ionian; CI, central Ionian; WC, western Cretan passage; EC, eastern Cretan passage; SC, southern Cretan passage; and LB, Levantine basin) to construct the potential temperature versus salinity diagrams depicted in Figure 2.

3 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE 10-3 is occupied by the Eastern Mediterranean Deep Water (EMDW) that until the 1990s originated in the Adriatic Sea, with s q kgm 3 [Schlitzer et al., 1991]. During the last decade, the deep thermohaline circulation changed completely by a major event of massive dense waters formation in the Cretan Sea (southern Aegean). These waters, referred to as EMDW of Aegean origin (EMDW Aeg ) in the text, outflowed through the Cretan Arc Straits. Being denser (s q kgm 3 ) than the EMDW of Adriatic origin (EMDW Adr ), they were added to the bottom layer replacing almost 20% of the deep and bottom water of the eastern Mediterranean [Roether et al., 1996]. This event played an important role in changing the deep thermohaline circulation of the eastern Mediterranean affecting not only the deep layer but also the intermediate one by a redistribution of the water masses and related bio-chemical properties. This was the main result that emerged from the observations from the basin-wide survey conducted in January 1995 by the R/V Meteor in the eastern Mediterranean [Klein et al., 1999; Lascaratos et al., 1999]. [3] The distributions of dissolved oxygen and to a lesser extent of nutrients across the entire eastern Mediterranean were described prior to and after the changes in the deep circulation [Schlitzer et al., 1991; Roether et al., 1996; Klein et al., 1999; Lascaratos et al., 1999]. An attempt was made by Vidussi et al. [2001] to estimate the biochemical cycling in this area by using a combination of nitrate + nitrite and pigment information during winter The predominance of general oligotrophic conditions in the eastern Mediterranean was confirmed and the study also showed the phytoplankton composition to be different from other oligotrophic areas of the world s oceans. Prior to the change, in 1987, the cross-basin distribution of the deep water was essentially uniform from 1000 m and below [Schlitzer et al., 1991]. Dissolved oxygen decreased and nutrient increased eastward, in agreement with the direction of the deep-water circulation. The lowest concentrations of dissolved oxygen and highest concentrations of nutrients were found between 1200 and 2500 m in the Levantine (176, 10.5, 4.9, and 0.21 mmol kg 1 for oxygen, silicic acid, nitrate and phosphate, respectively). In the Ionian, the lowest concentrations of dissolved oxygen and highest concentrations of nutrients were found at 1200 m depth (183, 9.3, 5.0, and 0.16 mmol kg 1 for oxygen, silicic acid, nitrate and phosphate, respectively). After the change, in 1995, the deep water was not uniform and a pronounced layer of minimum oxygen and maximum nutrients was detected at the m depth range depending on the basin sampled. For example, the concentration of silicic acid in the western and central Levantine decreased from 10 to 9 mmol kg 1, and in the Ionian from 9 to 8 mmol kg 1 [Klein et al., 1999; Lascaratos et al., 1999]. This was a result of the intrusion of the younger and denser waters, the EMDW Aeg, with higher dissolved oxygen and lower nutrients that pushed up the older EMDW Adr with lower oxygen and higher nutrients. This uplifting occurred preferentially in the Ionian and was in agreement with the salinity and CFC-12 distributions [Klein et al., 1999; Lascaratos et al., 1999]. [4] In addition to the cross-basin surveys, information on the chemical characteristics of the water masses in the eastern Mediterranean emerge from localized studies. Kress and Herut [2001] described the seasonal variations in the southern Levantine while Krom et al. [1991, 1993] focused on a warm core eddy in the area. The northern Levantine was studied by Salihoglu et al. [1990], Yilmaz et al. [1994], and Yilmaz and Tugrul [1998]. The eastern Ionian was described by Souvermezoglou et al. [1992], along a transect from the Otranto straits to Crete and through the straits of the Cretan Arc [Krasakopoulou et al., 1999]. The western and central Ionian and the Sicily Channel were covered by Rabitti et al. [1994] and Civitarese et al. [1996, 1998]. [5] The aim of this work is to provide a further temporal snapshot of the influence of the changed thermohaline circulation on the distribution of dissolved oxygen and nutrients in the eastern Mediterranean as the transient evolves. This study also characterizes the physical and chemical properties of the different water masses, including the N:P ratios and compares among the basins of the eastern Mediterranean. 2. Sampling and Methods 2.1. Area of Study [6] Figure 1b depicts the area of study and the location of the 39 hydrographic stations where water was sampled for the determination of dissolved oxygen and nutrients. The stations were located mainly on an east-west (E-W) transect across the eastern Mediterranean at ca N latitude. There were two north-south (N-S) transects, one perpendicular to the eastern tip of Crete and one in the mid Ionian. The cruise track crossed three main areas: the Levantine basin (28 34 E, stations ), the Cretan Passage region (22 28 E, stations ) and the Ionian Sea (15 22 E, stations ). Two stations were sampled at the Sicily straits (stations ) CTD and Hydrographic Data [7] Continuous profiles of temperature and salinity were performed with a SBE 911 plus Conductivity-Temperature- Depth (CTD) system manufactured by Sea-Bird Electronics Inc. (SBE), interfaced to a SBE Carousel Water Sampler. The physical data were collected using dual sensors both for temperature and conductivity. For consistency analysis, further discrepancies of data obtained by the calibrated sensors were resolved by independent measurements employing SIS digital reversing thermometers (resolution ±0.001) and water sample analyses by a Guildline 8400 AUTOSAL bench-salinometer (accuracy ±0.001). Final accuracies, which resulted less than ±0.005 units both for temperature and salinity, were determined by intercomparison of data collected with different methods. In the following, all depth, temperature and salinity measurements are from the CTD. The pressure in dbar, the salinity according to the practical salinity scale, the potential temperature (q)in C and the potential density excess (s q )inkgm 3, both referred to the sea surface, are used in this paper to report the hydrographic quantities at the different depths Dissolved Oxygen and Nutrients [8] Water samples for the determination of dissolved oxygen and nutrients were collected at 39 stations (Figure 1) with the carousel equipped with 24 Niskin bottles of L volume. Water was sampled at depths,

4 PBE 10-4 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 2. Potential temperature versus salinity relationships depicted as scatterplots for the CTD data grouping together the stations in the six regions indicated in Figure 1b. The reference region is indicated at the top of the panel. The continuous lines depict the potential density excess s q (kg m 3 ) isopycnals. Note the different scales of the salinity on the x axis. from 10 m above the sea bottom up to the surface, the number of samples depending on the station s water depth. [9] Water samples for dissolved oxygen were sampled and pickled. Duplicate samples for nutrient analysis were collected in 15-mL acid washed plastic scintillation vials and immediately frozen. Dissolved oxygen was 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 end point mode. The precision was 0.3%. Precision was determined by analyzing replicate samples from the same Niskin bottle. In the laboratory, nutrients were determined using a segmented flow Technicon AutoAnalyser II (AA-II) system by the methods described by Krom et al. [1991, 1993]. The precision for nitrate + nitrite, phosphate and silicic acid was 0.02, and 0.06 mm, respectively. The limit of detection (2 times the standard deviation of the blank) for the procedures is mm for nitrate + nitrite, mm for phosphate and 0.03 mm for silicic acid. For simplicity, in the text we will refer to nitrate + nitrite as nitrate. Quality assurance of the nutrient measurements was confirmed by the results of a NOAA/NRC Intercomparison for nutrients in seawater [Willie and Clancy, 2000] and by intercomparison with other laboratories working in the oligotrophic eastern Mediterranean [Kress and Herut, 2001]. [10] T-test and Anova, at the 95% confidence level, were used in statistical comparisons for the chemical characterization of the water masses. 3. Results 3.1. Water Mass Properties Across the Eastern Mediterranean [11] In order to present a complete picture of the observed water masses and their transformation along the west-east section across the eastern Mediterranean, potential temperature versus salinity diagrams were constructed using full depth CTD data, grouping together the stations located in six main regions (Figure 1): (1) the western Ionian, (2) the central Ionian, (3) the western Cretan passage, (4) the eastern Cretan passage, (5) the southern Cretan passage,

5 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE 10-5 Figure 3. Vertical sections of (a) potential temperature, (b) salinity, and (c) density along the west-east cross section of the eastern Mediterranean (see inset map). The positions of hydrological stations are indicated at the top of the panels. The upper 800 m layer are highlighted. The water masses present on the sections are indicated on the salinity sections. and finally (6) the stations located in the Levantine basin (Figure 2). [12] All the diagrams show the relatively fresh Modified Atlantic Water (MAW), which is transformed as it moves from the Sicily Straits (S ffi 37.2) into the Levantine basin (S ffi ). It occupies the lower surface layer above the isopycnal 28.4 kgm 3 corresponding to a depth range from about 150 m in the Ionian to 50 m in the Levantine basin. There (Figure 2f ), the MAW may be still distinguished and interacts with the more saline (S ffi 39.0) Levantine Surface Water (LSW). [13] Below the 28.4 isopycnal, the large zonal variability is mostly associated with a very strong halocline, between the isopycnals 28.4 to 29.0 kgm 3. The halocline indicates mixtures of MAW with the LIW and it evolves with different temperatures in the range from about 14.5 C in the Ionian Sea (Figures 2a and 2b) to about 15.0 C in the western Cretan passage (Figure 2c), C in the eastern Cretan passage (Figure 2d), and 17 C in the Levantine basin (Figure 2f), where it develops above the 28.8 kgm 3 isopycnal. [14] The LIW core has its salinity maximum (S 39.07) in the proper Levantine basin over a wide potential temperature range C. Its density is in between the 28.8 and the kgm 3 isopycnals (Figure 2f ), but moving westward into the Ionian Sea the LIW decreases

6 PBE 10-6 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 3. (continued) both in salinity and temperature resulting in a much dense water mass. All the q-s diagrams constructed with stations in the Ionian (Figures 2a 2c) display a spur of salinity maximum indicating the LIW of about , associated with temperature and density in the ranges of C and kgm 3, respectively. Moreover, a high-salinity core water (S > 39.0) can be noted in the western Cretan passage (eastern Ionian) in the same density range, but it exhibits potential temperatures higher than 15.0 C (Figure 2c). Owing to the high-salinity values found close to the Cretan Arc strait, the source of this water mass is recognised to be in the Cretan Sea, thus confirming the dominant role of the Cretan Intermediate Water (CIW) in filling the intermediate layer of that region adjacent to the western Cretan Arc Straits [Georgopoulos et al., 2000; Theocharis and Lascaratos, 2000]. This water was first observed by Schlitzer et al. [1991] below the LIW layer and in the proper LIW layer by Malanotte-Rizzoli et al. [1999] in October [15] A substantial difference emerges in the deepwater range (s q > kgm 3 ) between the various diagrams constructed with the stations located in the Ionian (Figures 2a and 2b), in the Cretan passage (Figures 2c 2e) and in the Levantine basin (Figure 2f). The profiles in the Levantine basin show the inversion in temperature and salinity at potential density s q > kgm 3, as it occurs in the deepest layers in the vicinity of the Cretan Straits. This inversion is clearly connected with the presence of the EMDW Aeg. In the Ionian Sea the signal is only evident close to the source, the Western Cretan Arc straits. However, the western part of the Ionian Sea is subjected to the lateral advection of the EMDW Aeg and to the mixing

7 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE 10-7 Figure 3. (continued) process with the resident EMDW. In fact, a general westward decrease of temperature, salinity and density in the deep layer may be observed Vertical Sections of Temperature, Salinity, and Density [16] The vertical sections of temperature, salinity and density across the eastern Mediterranean (19 stations 1700 km), from the western Ionian to the eastern Levantine are shown in Figure 3. The section runs basically west/east along ca N. The distributions in the upper layer down to 800 dbar provide evidence of the intense dynamics and complexity of the regional hydrography. The main dynamical features (for nomenclature, see POEM Group [1992]), mostly evident in the temperature (Figure 3a) and density (Figure 3b) sections from east to west are (1) the dome bordering the west Cyprus cyclonic Gyre between the stations ; (2) the large anticyclone dominating the southern part of the Levantine basin merging in the Ierapetra anticyclone located in correspondence to stations ; (3) the doming in correspondence of the western tip of Crete (station ) indicating the Cretan cyclone; and (4) the very strong Pelops anticyclone which was detected at station 294 in the eastern Ionian. In the upper layer, the temperature section (Figure 3a, top panel) manifests the different heat storage content in the water column between the Levantine and Ionian Seas. The 14.0 C isotherm develops at about 500 m in the Levantine basin, whereas it rises at about 300 m in the Ionian Sea. The deepening of the isotherms (and isopycnals) from west to east traces the main character of the upper dynamics in the two basins: an anticyclonic regime prevails in the southern part of the

8 PBE 10-8 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 4. Vertical sections of (a) oxygen, (b) silicic acid, (c) nitrate, and (d) phosphate (mmol kg 1 ) along the west-east cross section of the eastern Mediterranean (see inset map). The positions of hydrological stations are indicated at the top of the panels, and the solid points indicate the data points. Levantine basin while a cyclonic one characterises mainly the Ionian Sea and the northern Levantine (e.g., Rhodes Gyre). The contoured lines in the vertical section of salinity in the upper 800 m layer (Figure 3c, top panel) clearly show the frontal system established between the eastward spreading of the MAW in the upper 200 m layer and the LIW propagation to the west in the intermediate layer. The front formed by the fresh MAW at the west of the section seems to extend deeply until the eastern part of the Cretan passage (station 287). Transformed patches of MAW are seen in the central part of the Levantine basin (station 238). These subbasin scale dynamical features play an important role in establishing the dispersion path of the main water masses. In fact, the westward spreading core (S > 39.0) of the LIW seems mostly entrained in the Ierapetra anticyclone limiting its penetration into the eastern Ionian. There a secondary core of salinity maximum is located in correspondence of the Western Cretan Arc Straits (station 294), also entrained in the Pelops anticyclone. [17] Finally, the vertical sections of temperature, salinity and density down to the bottom (Figures 3a, 3b, and 3c, bottom panels) are mostly an expression of the climatic change that had occurred in the deep thermohaline circulation of the eastern Mediterranean at the beginning of

9 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE 10-9 Figure 4. (continued) 90s. It is evident that this change has primarily influenced the water column below the kgm 3, i.e., below the 1000 m. However, it is worthy to remark that this isopycnal was found close to the bottom layer during the previous regime [Malanotte-Rizzoli et al., 1997]. The portion of the transect commonly occupied by the EMDW Adr (core properties of q 13.5 C, S and s q kgm 3 ) has been confined into the layer between the 1000 and the 2000 m in the Levantine basin and between the 1000 m to the bottom into the western Ionian Sea. In the bottom layer the much more saline and dense EMDW Aeg (core properties q 13.7 C, S and s q > kgm 3 ) appears. However, the latter is much more evident close to the deep layer in the central region of the eastern Mediterranean and it moved prevalently into the Levantine basin along the zonal section crossing the entire eastern Mediterranean Vertical Sections of Dissolved Oxygen and Nutrients [18] The vertical sections of dissolved oxygen and nutrients across the eastern Mediterranean are shown in Figure 4. The upper layer (0 150 m) was quite homogeneous in dissolved oxygen across the eastern Mediterranean (Figure 4a). There were indications of the main oceanographic features as seen in the vertical sections of the physical parameters (Figure 3). Some of the features were pronounced even down to 500 m depth. [19] In the m layer the dissolved oxygen isolines sloped down from the Ionian to the Levantine (Figure 4a),

10 PBE KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 4. (continued) in agreement with the cyclonic and anticyclonic regimes prevailing in the Ionian and southern Levantine, respectively. For example, the 180 mmol kg 1 isoline was located at circa 250 m in the Ionian (stations ) and at circa 400 m in the Levantine (stations ). Minimum in dissolved oxygen (Min Ox ) was found across the whole W-E transect in the m depth interval. The Min Ox layer got thicker and deepened eastward. In the Levantine this layer was located between 600 and 1500 m, with minimum concentration of 175 mmol kg 1, while in the western Ionian the Min Ox was between m, with similar dissolved oxygen concentration. The minimal concentration at the Cretan Passage was slightly higher ( mmol kg 1 ) indicating introduction of younger water. Below 1700 m, the dissolved oxygen concentration in the Ionian was higher than in the Levantine at the same water levels. Maximal concentrations of 185, 190, and mmol kg 1 were found in the bottom layer of the Levantine, Cretan Passage, and western Ionian, respectively. [20] The upper layer was also quite homogeneous in nutrients across the eastern Mediterranean (Figures 4b 4d), with indications of the main oceanographic features (cyclones and anticyclones) as seen for the physical properties and dissolved oxygen. The influence of the Pelops anticyclone (station 294) could be traced down to 1000 m by silicic acid (Figure 4b) but only to circa m by dissolved oxygen, nitrate, and phosphate (Figures 4a, 4c, and 4d). These differences may be due to the fact that N and P recycle more easily, in correspondence to dissolved oxygen while silicic acid is regenerated by slower chemical dissolution [Stumm and Morgan, 1981;

11 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE Figure 4. (continued) Hurd, 1983]. In the m layer of the Ionian higher nutrient concentrations appeared at shallower depths than in the Levantine. [21] Maximal concentrations of nutrients were found at the m layer across the whole W-E transect. The depth at which the maxima appeared was deeper in the Levantine (circa 1500 m) than in the Ionian (circa 500 m) and with a larger vertical extent. The maximal concentration of phosphate and nitrate were similar in the Levantine and Ionian with lower concentrations at the Cretan Passage. The maximal concentration of silicic acid was higher in the Levantine, and decreased westward. At the bottom layer the concentrations decreased from the maximal values and reached concentrations that were higher in the Levantine than in the Cretan Passage and the Ionian Depth Profiles and Water Mass Characterization [22] Figures 5 7 depict the composite depth profiles of bottle salinity, dissolved oxygen and nutrients for the study areas. The basic shapes of the depth profiles were similar among the areas. Dissolved oxygen concentrations were high at the surface, decreased to minimal values at the m layer and increased toward the bottom. In the Levantine and the Cretan passage, the increase continued down to the bottom while in the Ionian the increase continued down to 2500, the concentrations remaining then

12 PBE KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 5. Composite depth profiles of bottle salinity, oxygen, silicic acid, nitrate, and phosphate in the Levantine basin. essentially constant down to the bottom. The nutrient s depth profiles were a mirror image of the dissolved oxygen profile: low concentrations at the surface, an increase to maximal values at the m layer and a decrease toward the bottom. As for dissolved oxygen, in the Levantine and the Cretan passage the decrease in nutrient concentration continued down to the bottom while in the Ionian the decrease continued down to 2500, the concentrations remaining then essentially constant down to the bottom. In the Levantine and the Cretan arc, the silicic acid depth distribution mirrored that of the salinity from 200 m and down. The composite depth profiles (>2000 m) in the Ionian were less variable then in the Levantine and in the Cretan Passage, probably because of homogenization of the water masses of Aegean and Adriatic origin that recirculate in the dominant anticyclonic motion installed in the Ionian abyssal layer [Manca et al., 2002]. In the Ionian, the composite depth profiles of silicic acid and salinity were similar. [23] Depth profiles as such, with extreme values at mid levels, were not seen in the Levantine in 1991 or prior to it. Until 1991, the concentrations stayed essentially constant from the base of the oxycline and the nutricline down to the bottom [Schlitzer et al., 1991; Kress and Herut, 2001]. In 1995, the new typical depth distribution was already present in the central Levantine but not in the eastern Levantine [Klein et al., 1999; Kress and Herut, 2001]. At the Cretan Passage in 1987, there was no evidence of the transient in the dissolved oxygen and silicic acid depth profiles. In 1991, the beginning of the transition between the two circulation patterns was detected by an increase in oxygen and decrease in nutrients below 1200 m depth [Kress and Herut, 2001]. Malanotte-Rizzoli et al. [1999] have shown that the changes in the deep circulation actually started before In the Ionian, mid depth extremes were present already in 1987 as shown by the cross-basin depth distribution of dissolved oxygen (minimum of 185 mmole kg 1 ) and silicic acid (maximum of 9 mmole kg 1 )[Schlitzer et al., 1991; Lascaratos et al., 1999]. In 1991, the uplift of the old EMDW Adr in the eastern Ionian was depicted by a tongue of low (190 mm) dissolved oxygen concentration and high nutrients concentrations (8.9, 5.5, and 0.21 mm silicic acid, nitrate and phosphate, respectively) at the m depth level [Rabitti et al., 1994; Civitarese et al., 1996]. In 1995 the extreme layer was located at depths of m in the western Ionian, showing the continuing evolution of the transient [Klein et al., 1999; Lascaratos et al., 1999]. [24] It was possible to define four different water masses in the eastern Mediterranean on the basis of the physical parameters: MAW in the upper layer (with MAW and LSW in the Levantine), LIW, EMDW Adr, and EMDW Aeg. For the

13 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE Figure 6. Composite depth profiles of bottle salinity, oxygen, silicic acid, nitrate, and phosphate in the Cretan passage. Solid circles, eastern Cretan; open circles, western Cretan. chemical characterization we combined the eastern and southern Cretan passage as the eastern Cretan area. Tables 1 4 summarize the variability of the properties of the LIW, EMDW Adr, and EMDW Aeg observed in the different areas of the eastern Mediterranean (Figure 1). In addition, we also characterized the transitional layer ( m) between the LIW and the EMDW Adr to facilitate the discussion of the results. Because this study emphasized the discussion of the intermediate and deep layers, we chose one single value to characterize each chemical parameter in the upper water mass. Dissolved oxygen was at or close to saturation with concentration of 228 ± 7 mmol kg 1 (n = 150). The concentrations of the nutrients were low: phosphate, ± 0.01 mmol kg 1 (n = 137); nitrate, 0.21 ± 0.36 mmol kg 1 (n = 131); silicic acid, 0.95 ± 0.40 mmol kg 1 (n = 145). Similar values were found in the southern Levantine in the spring of 1995 [Kress and Herut, 2001]. [25] Across the eastern Mediterranean in the LIW, there was a significant progressive decrease in dissolved oxygen and correspondent increases in nutrients, from the Levantine westward (Tables 1 and 5). In the transitional layer, the concentrations in the Levantine were not significantly different from those in the western Ionian, with higher oxygen and lower nutrients in the central Ionian and western and eastern Cretan passage. In the EMDW Adr and EMDW Aeg oxygen increased and nutrients decreased from the Levantine toward the central Ionian, while in the western Ionian there was a reverse in trend (Table 3 5). [26] The depths of maximal concentration (Figure 5 7) varied with region and at times differed among the parameters at the same area (Tables 2 and 3). In the Ionian there was a good correspondence among the depths of phosphate and nitrate maxima and minimum dissolved oxygen (transitional layer), while maximal silicic acid was found at the EMDW Adr. At the Cretan Passage and in the Levantine there was more variability. Minimum oxygen and maximum phosphate were located at the transitional layer and the EMDW Adr, while maximum nitrate was in the transitional layer and maximum silicic acid in the EMDW Adr. 4. Discussion 4.1. Spatial and Temporal Variability [27] The observed changes in the vertical distribution of the physical and chemical parameters during this study points to the continuing spatial and temporal evolution of the EMDW Aeg in the eastern Mediterranean since the 1990s. It was shown before that the changes in the deep thermohaline circulation affected not only the deep layer but the upper water levels as well [Klein et al., 1999]. The most remarkable change was the formation of a pronounced mid depth oxygen minimum (Min Ox ) and nutrients maxima (Max Nut ) corresponding to the older water mass, the EMDW Adr,

14 PBE KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Figure 7. Composite depth profiles of bottle salinity, oxygen, silicic acid, nitrate, and phosphate in the Ionian basin. Solid circles, western Ionian; open circles, central Ionian. uplifted by the deep intrusion of the younger EMDW Aeg.A layer with Min Ox and Max Nut was not present in the Levantine in 1987, while a broad layer was found in the eastern Ionian [Schlitzer et al., 1991]. This may indicate the beginning of the change in circulation as reported by Theocharis et al. [1999] and the preferential westward intrusion of the EMDW Aeg. In 1995, a layer of Min Ox / Max Nut was already present in the Levantine, while in the Ionian it moved westward and upward to shallower depths [Klein et al., 1999; Lascaratos et al., 1999]. In the present study, 4 years later, the Min Ox /Max Nut layer was further uplifted in the Levantine, from 2500 m to about 1500 m, and the EMDW Aeg reached the eastern Levantine. [28] The distribution of dissolved oxygen and nutrients in the surface layer was homogenous across the basin. Their vertical gradients reflect the upper thermocline basin scale circulation features in agreement with a well-developed seasonal pycnocline, which clearly limits the rising of nutrient rich water into the photic layer. The pycnocline extends to a depth of about 200 m in the Ionian Sea, whereas it deepens to about 500 m in the Levantine basin. In its interior the behaviour of the vertical distributions of the chemical substances is mostly related to the sub-basin scale cyclonic and anticyclonic dynamics. [29] In the LIW layer the concentrations of dissolved oxygen were high in the Levantine and decreased westward while nutrients increased (Figure 4, Table 1), indicating youngest LIW in the Levantine. This is in agreement with the well-known formation site and the path of the LIW in the basin, also demonstrated in this study (Figure 3b). LIW is formed in the Levantine and travels westward toward Sicily [Ozsoy et al., 1989; Schlitzer et al., 1991; Ozsoy et al., 1993]; therefore the older LIW with more nutrients and less dissolved oxygen is found in the Ionian while the younger LIW with lower nutrients and higher dissolved oxygen is found in the Levantine. In addition, the EMDW Adr was lifted closer to the LIW in the Ionian than in the Levantine, and therefore one cannot rule out possible mixing between them further increasing nutrient and decreasing oxygen in the Ionian [Klein et al., 1999; Malanotte-Rizzoli et al., 1999]. [30] Moreover, Malanotte-Rizzoli et al. [1999] found that in 1991 the westward pathway of the LIW was blocked in the southern Levantine by a three-lobe anticyclonic structure, which limited the water exchanges between the Levantine and Ionian Seas. This substantially induced the LIW to recirculate in the Levantine, increasing its water age in this basin. The same blockage of the LIW was reported by Klein et al. [1999] and seen also in this study because of its entrapment in the Ierapetra anticyclone. Manca et al. [2002] have shown that the westward transit of the LIW through the Cretan Passage (south of Crete) was strongly

15 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE Table 1. Water Mass Properties of the LIW/CIW in the Different Areas of the Eastern Mediterranean, April May 1999 a Pressure, dbar q, C S s q, kg m 3 Statistics PO 4, mmol kg 1 NO 3, mmol kg 1 Si, O 2, mmol kg 1 mmol kg 1 WI avg ± ± ± std n b c CI avg ± ± ± std n c WC avg (LIW) ± ± ± std n b WC avg ** (CIW) ± ± ± std n EC avg ± ± ± std n b LB avg ± ± ± std n d b c a LB, Levantine basin; EC, eastern Cretan passage; WC, western Cretan passage; CI, central Ionian; WI, western Ionian. Avg, average; std, standard deviation; n, number of data points. Double asterisk indicates that it was impossible to differentiate chemically the LIW from the CIW in the western Cretan passage. b Water core properties [Klein et al., 1999]. c Schlitzer et al. [1991]. d Spring results [Kress and Herut, 2001]. N:P reduced because of the presence of a large divergence zone in this area (see their Figure 4b). Figure 2 testifies that a similarity of LIW properties, as observed in the Levantine basin (Figure 2f ), can be seen only in the south of the Cretan passage (Figure 2d), whereas the western region (Figure 2c) seems mostly affected by higher-salinity waters (CIW), which emanate from the western Cretan Straits. They have higher core salinities (39.01) and higher temperature than LIW in that region (Table 1). Inspection of the LIW properties in the Levantine basin (Table 1) shows that there was a decrease in dissolved oxygen and an increase in silicic acid and nitrate from 1987 to 1999, in agreement with the increasing water age. There were no differences in phosphate concentrations that were very low. [31] At the transition layer (i.e., below the LIW and above the EMDW) it was possible to discern regions with different chemical characteristics. The central area (eastern and western Cretan and central Ionian) differed from its adjacent areas in the east and west (Levantine and western Ionian, respectively). In the central area the nutrient concentrations were lower and oxygen higher compared to the extreme eastern and western parts of the study area (Figure 4, Table 2). Table 2. Water Mass Properties of the Transition Layer Between the LIW and the EMDW Adr in the Different Areas of the Eastern Mediterranean, April May 1999 a Pressure, dbar q, C S s q,kgm 3 Statistics PO 4, mmol kg 1 NO 3, mmol kg 1 Si, mmol kg 1 O 2, mmol kg 1 N:P WI avg ± ± ± std n CI avg ± ± ± std n WC avg ± ± ± std n EC avg ± ± ± std n LB avg ± ± ± std n a LB, Levantine basin; EC, eastern Cretan passage; WC, western Cretan passage; CI, central Ionian; WI, western Ionian; avg, average; std, standard deviation; n, number of data points.

16 PBE KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION Table 3. Water Mass Properties of the EMDW Adr in the Different Areas of the Eastern Mediterranean, April May 1999 a Pressure, dbar q, C S s q, kg m 3 Statistics PO 4, mmol kg 1 NO 3, mmol kg 1 Si, mmol kg 1 O 2, mmol kg 1 N:P WI avg ± ± ± std n b CI avg ± ± ± std n c WC avg ± ± ± std n b EC avg ± ± ± std n d >700 <13.8 < c LB avg ± ± ± std n b d >700 <13.8 < c a LB, Levantine Basin; EC, Eastern Cretan passage; WC, Western Cretan passage; CI, Central Ionian; WI, Western Ionian; avg, average; std, standard deviation; n, number of data points. b Water core properties [Klein et al., 1999]. c Schlitzer et al. [1991]. d Kress and Herut [2001]. It is reasonable to assume that it is related to the intrusion of Aegean water at intermediate depths (CIW) introducing younger waters and changing the chemical characterization of this layer through mixing. CFC data also show that CIW was added in the transition layer in the Cretan passage and only there. (B. Klein, unpublished results, 2002). A further process that emphasizes the differences between the central and adjacent areas is the uplift of the older EMDW Adr that was pushed further up to the east and to the west at the extremes of the basin. [32] In the EMDW Adr ( m) the trend in the concentration changes was the opposite of that found in the LIW. Dissolved oxygen increased and nutrients decreased westward indicating the presence of older waters in the Levantine than in the Cretan passage and in the Ionian Sea. This trend continued until the central Ionian, while an inversion occurs in the western Ionian at the shelf break and against the Maltese escarpment (Table 3 and Figures 3 and 4). There, the chemical characterization makes it possible to distinguish the older EMDW of Adriatic origin uplifted to m depth, from the more ventilated and deeper ones. During the early stage of the transient, old EMDW Adr was pushed up to shallower depths in the Ionian than in the Levantine by the preferential westward intrusion of EMDW Aeg [Klein et al., 1999; Lascaratos et al., 1999; Malanotte-Rizzoli et al., 1999] and by the cyclonic character of the Ionian (Figure 3) that may reach down to the EMDW Adr. The waters that reside at the base of the Maltese the escarpment may be generated by mixing of waters originating both in the Adriatic and Aegean Seas. In fact, strong dynamics have shown an anticyclonic circulation movement [Manca et al., 2002] enhancing the homogenisation of the waters that reside in the Ionian abyssal layer. [33] Similarly to the EMDW Adr, in the EMDW Aeg dissolved oxygen increased and nutrients decreased westward but only until the Western Cretan Arc. The presence of the EMDW Aeg was clearly shown by the inversion in temperature and salinity at potential density s q > kgm 3, occurring in the deepest layers in the vicinity of the Cretan Straits and in the Levantine basin. In the Ionian Sea the signal is only evident close to the source, the Western Cretan Arc straits, explaining the trend reversal in the chemical characteristics (Table 4, Figure 2 4). [34] From 1987 to 1999 the general temperature and salinity increase in the deep layer of the central and western Ionian regions may by accounted by the mixing of the EMDW Adr with EMDW Aeg (Table 3). In 1995 there was a front in the Ionian, with EMDW Aeg only present in the eastern half of the Ionian. The influence of the colder and less saline EMDW Adr waters was restricted to the western Ionian, with potential temperature and salinity close to the classical values for the entire eastern Mediterranean before the changes in circulation [Klein et al., 1999]. CFCs and oxygen furthermore indicated that the Adriatic core had aged. This was already the case in 1991 when the old and denser EMDW Adr was pushed to the west and still occupied the bottom layer [Malanotte-Rizzoli et al., 1999]. An increase in nutrients and a decrease in dissolved oxygen can be seen in the EMDW at the western Ionian (Table 4) between 1987 and No trend could be seen in the central Ionian properties. In the eastern Cretan passage there was an increase in nitrate and silicic acid and a decrease in dissolved oxygen at the EMDW Adr (Table 3), in agreement with the observed temporal changes in the thermohaline circulation. In the Levantine, the core chemical properties showed no trend for EMDW Adr. Silicic acid and dissolved oxygen were essentially the same between 1987 and 1999, while there

17 KRESS ET AL.: CHANGED THERMOHALINE CIRCULATION PBE Table 4. Water Mass Properties of the EMDW Aeg in the Different Areas of the Eastern Mediterranean, April May 1999 a Pressure, dbar q, C S s q, kg m 3 Statistics PO 4, mmol kg 1 NO 3, mmol kg 1 Si, mmol kg 1 O 2, mmol kg 1 N:P WI avg ± ± ± std n b > CI avg ± ± ± std n b > WC avg ± ± ± std n c,d EC avg ± ± ± std n c,d LB avg ± ± ± std n c d b > a LB, Levantine Basin; EC, Eastern Cretan passage; WC, Western Cretan passage; CI, Central Ionian; WI, Western Ionian; avg, average; std, standard deviation; n, number of data points. b Schlitzer et al. [1991], before the changes in circulation. c Water core properties, western Levantine [Klein et al., 1999]. d Water core properties, eastern Levantine [Klein et al., 1999]. were variations in the nitrate and phosphate concentrations but without an apparent trend (Table 3). The trends were significant for both dissolved oxygen and nutrients in the Levantine basin for the same period (Table 4), showing the intrusion of EMDW Aeg Depth Profiles [35] In addition to the cross-basin differences in chemical characteristics of the water masses noteworthy of attention are the relative depths at which the Min Ox /Max Nut were located at each of the basins in this study. While the physical processes could explain the differences in the vertical placement of the Min Ox /Max Nut among the different areas, they cannot explain the relative displacement of the extreme points within the same area. Chemical parameters are not conservative and change also as a result of chemical and biological processes. Dissolved oxygen is utilized during respiration (oxidation) of organic matter, while biological production in the photic zone increases its concentration [Stumm and Morgan, 1981]. Nutrients are utilized in the photic zone and nitrate and phosphate are regenerated during oxidation of organic matter, and therefore closely related to dissolved oxygen below the photic zone. Silicic acid is released to the water column by chemical dissolution of silicious tests [Hurd, 1983]. This process is not coupled to oxygen consumption and it is slower than the oxidation of organic matter. Therefore the maximal concentration of silicic acid in all the study areas were located deeper then the maxima for nitrate and phosphate (Tables 2 and 3, Figures 5 7). [36] In the Levantine, the phosphate and nitrate maxima were similar and shallower (600 m) than the Min Ox (800 m), probably as a result of oxygen consumption during the oxidation of organic matter poor in P and N below 600 m, as speculated by Kress and Herut [2001]. In the Cretan passage there was still a relative displacement of the oxygen extreme point compared to the nitrate and phosphate (500 m), but smaller than that in the Levantine. In the Ionian, the extreme points were located at similar depth (400 m) as expected. These differences among the areas of study may indicate increased utilization of recycled nitrate and phosphate eastward, i.e., a more rapid regeneration and utilization of recycled nutrients in the Levantine, decreasing the N and P in the particulate organic matter falling down and oxidized in the water column. Data on the C:N:P composition of particulate organic matter across the basin is lacking to test this hypothesis. Recently, Edinger et al. [1999] published data on the composition of particulate matter in the euphotic zone of the northeastern Mediterranean (down to 160 m depth), but there is still none in the deeper layers (LIW and EMDW) N:P Ratios [37] The exceptionally high N:P molar ratio found in the EMDW interests the researchers of the area and is the basis of the hypothesis that the productivity of the eastern Mediterranean is limited by the availability of phosphorus and not nitrogen as in other oceans. N:P ratios in the deep waters were calculated by Krom et al. [1991] - N:P = 28.1 ± 3; Civitarese et al. [1998] - N:P = 25 27; Yilmaz and Tugrul [1998] - N:P = ; Krasakopoulou et al. [1999] - N:P = 24 36; Kress and Herut [2001] - N:P = 25 ± 3. Examination of the data collected in this study shows indeed that at the deep water masses (EMDW Adr and EMDW Aeg ) the N:P ratios were higher than Redfield s ratio of 16 [Redfield et al., 1963] and ranged between 26.2 and 30.7 (Tables 3 and 4). Comparison of N:P in the EMDW Adr showed that the ratios were significantly higher in the central Ionian than in the Levantine, i.e., higher in the younger waters (Tables 3 and 5). The ratio in the western