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The Mediterranean Sea is among the most affected areas of our planet by microplastic (MP) pollution. However, some regions are still underrepresented in the current literature. This work studied the fate of microplastics (MPs) released from major populated areas within the NE Ionian Sea, an area that contains highly significant biodiversity. This was accomplished by incorporating oceanographic data into a Lagrangian particle-tracking numerical model that simulated the transport of MP particles for the interval of 27 months. The findings report a high possibility of beaching within the first weeks of the simulation for most locations, where 63 % of MPs were beached and 37 % were still floating at the end of the simulation. Seaward transport and eddy diffusivity are the controlling mechanisms of the MP transport, with diffusion being the primary force controlling the movement of MP particles in 1/3 of the simulated regions. This is highly significant, because in areas where diffusion is the main mechanism controlling MP transport, accumulation of floating MP particles is occurring, as reported in previous studies. The MPs' transport and beaching behavior, as well as the observed residence times, were used to determine the threat level that MPs pose to the biodiversity of specific areas.

Mesozooplankton vertical pattern was studied down to 2000 m along an east-west transect in the oligotrophic Mediterranean Sea in June 1999. Depth-stratified samples were collected by a WP-2 net (200 µm mesh size). In addition, the copepod grazing impact on autotrophs was assessed in the epipelagic zone by measurement of small and large copepod gut content by day-night sampling. An east-west increasing trend of integrated total biomass and abundance over the 0–1000 m was detected. Similar vertical pattern of standing stock was found along the transect, with subtle differences at stations positioned over cyclonic and anti-cyclonic gyres. No significant differences were found between stations in the epipelagic and upper mesopelagic zone, except at the westernmost station (NW Mediterranean); discrimination of assemblages between the eastern and the western basins were revealed in the lower mesopelagic and bathypelagic zones. Estimated copepod grazing impact on primary production was found overall higher in the western than in the eastern basin. The entire Mediterranean Sea was dominated by copepod functional groups characterized by reproduction strategies (mostly sac spawners), trophic preferences (mainly omnivores and detritivores) and feeding modes (ambush and cruise feeding), facilitating their adaptation and efficiency in the oligotrophic to very oligotrophic environment of the Mediterranean Sea.

The Aegean Sea’s hydrological and circulation characteristics are as complex as its geomorphological setting. All major Eastern Mediterranean (EMed) water masses can be found inside the basin, while the Aegean Sea is a producer of dense intermediate and in some cases deep water masses that are exported to the EMed. The role of the Aegean Sea as a dense/deep water source for the EMed has been revised following the Eastern Mediterranean Transient event of the 1990s decade. The local dense water production has been recognized as having a strong effect over the EMed overturning circulation. In this chapter, up-to-date information on the Aegean Sea is summarized, open questions are highlighted, and major future research aspects are proposed.

Antinioti is a shallow Mediterranean coastal lagoon located on the island of Corfu in West Greece. For several years, the lagoon has been exploited for fish stocks by the local community. Therefore, the lagoon presents regional economic importance. However, studies on the physical, chemical, and biological properties of the lagoon are limited. Our study evaluates the effect of physical and chemical parameters on the zooplankton dynamics of Antinioti lagoon and establishes a reference for the level of biodiversity in terms of zooplankton. This initial biodiversity record will be useful in the case of a natural disaster (i.e. earthquake, tsunami, etc.) for the effective management, protection, and conservation of the lagoon. The lagoon was sampled between October 2020 and March 2021 to evaluate the correlation between physical and chemical parameters and zooplankton. Temperature and Salinity showed a significant positive correlation and BOD 5 a significant negative correlation to zooplankton. The shallow lagoon’s zooplankton community and sustainability are therefore considered to be influenced by the seawater–freshwater fluctuations that are caused by atmospheric forcing (rainfall), tides, and river runoff.

Available temperature and salinity data in the Cretan Sea from 1955 up to 2014 as well as literature sources were revisited in order to trace the appearance of low salinity, temperature, oxygen and nutrient-rich waters inside the basin at depths below the intermediate layer. First appearing as far back as 1961 in literature, these waters were found originating in the layers that separate intermediate and deep waters of the Eastern Mediterranean Sea (EMed) and were named Transitional Mediterranean Water (TMW) in the 1990s. Data analysis showed that the appearance of TMW in the Cretan Sea is a recurrent phenomenon connected to water mass exchanges between the Aegean Sea and the EMed. In particular, the inflow of TMW in the Cretan basin acts as compensation for the outflow of equally dense or denser masses from the Aegean. This export is a result of dense water formation (DWF) events taking place inside the Aegean Sea triggering TMW compensatory inflow into the Cretan Sea through the Cretan Straits. In this context, TMW intrusions in the Cretan basin can be used as a tracer of DWF in the Aegean Sea while the depth of the intrusion can provide valuable information about the intensity of the DWF event. The importance of TMW intrusions is not solely restricted to the tracing and evaluation of DWF events but could additionally expand to the impact on local ecological processes as TMW is a nutrient carrier for the oligotrophic Cretan Sea.

The scientific interest in Eastern Mediterranean (EMed) processes of major importance has been revived in recent years due to the predominance of internal variability manifested in a decadal time scale leading to the alternating activation of the two dense water sources (i.e. the Adriatic and Aegean Seas). Analysis of available hydrographic data during the 2003–2012 period reveals an anticorrelated almost decadal oscillation in the thermohaline properties of the upper and intermediate water masses in both the Ionian and the Levantine/Aegean Seas. This event is the manifestation of an ongoing internal mechanism initially introduced by Theocharis et al. (in press) which periodically disturbs the upper thermohaline EMed conveyor belt and changes the respective water mass pathways, thus driving the alternating activation of the two dense water sources through salinity preconditioning. Since 2004–2005, the salinity of the upper/intermediate layer in the eastern part of the EMed gradually increased up to 2010, while at the same time it decreased in the North Ionian Sea. During the same period we observed the activation of the Aegean Sea as a Dense Water Formation area even though the atmospheric forcing conditions were not favorable. After 2010 the salinity trend reversed in both regions. This suggests that in the near future the salinity preconditioning of the Adriatic Sea will be favored again following the respective water mass pathway changes.

We present a detailed account of the changing hydrography and the large-scale circulation of the deep waters of the Eastern Mediterranean (EMed) that resulted from the unique, high-volume influx of dense waters from the Aegean Sea during the 1990s, and of the changes within the Aegean that initiated the event, the so-called ‘Eastern Mediterranean Transient’ (EMT). The analysis uses repeated hydrographic and transient tracer surveys of the EMed in 1987, 1991, 1995, 1999, and 2001/2002, hydrographic time series in the southern Aegean and southern Adriatic Seas, and further scattered data. Aegean outflow averaged nearly 3 × 106 m3 s−1 between mid-1992 and late 1994, and was largest during 1993, when south and west of Crete Aegean-influenced deep waters extended upwards to 400 m depth. EMT-related Aegean outflow prior to 1992, confined to the region around Crete and to ∼1800 m depth-wise, amounted to about 3% of the total outflow. Outflow after 1994 up to 2001/2002, derived from the increasing inventory of the tracer CFC-12, contributed ∼20% to the total, of ∼2.8 × 1014 m3. Densities in the southern Aegean Sea deep waters rose by 0.2 kg/m3 between 1987 and 1993, and decreased more slowly thereafter. The Aegean waters delivered via the principal exit pathway in Kasos Strait, east of Crete, propagated westward along the Cretan slope, such that in 1995 the highest densities were observed in the Hellenic Trench west of Crete. Aegean-influenced waters also crossed the East Mediterranean Ridge south of Crete and from there expanded eastward into the southeastern Levantine Sea. Transfer into the Ionian mostly followed the Hellenic Trench, largely up to the trench’s northern end at about 37°N. From there the waters spread further west while mixing with the resident waters. Additional transfer occurred through the Herodotus Trough in the south. Levantine waters after 1994 consistently showed temperature–salinity (T–S) inversions in roughly 1000–1700 m depth, with amplitudes decreasing in time. The T–S distributions in the Ionian Sea were more diverse, one cause being added Aegean outflow of relatively lower density through the Antikithira Strait west of Crete. Spreading of the Aegean-influenced waters was quite swift, such that by early 1995 the entire EMed was affected. and strong mixing is indicated by near-linear T–S relationships observed in various places. Referenced to 2000 and 3000 dbar, the highest Aegean-generated densities observed during the event equaled those generated by Adriatic Sea outflow in the northern Ionian Sea prior to the EMT. A precarious balance between the two dense-water source areas is thus indicated. A feedback is proposed which helped triggering the change from a dominating Adriatic source to the Aegean source, but at the same time supported the previous long-year dominance of the Adriatic. The EMed deep waters will remain transient for decades to come.

A simple hydraulic model is used to estimate the deep water fluxes of Cretan Deep Water (CDW), through the Cretan Arc Straits and into the Eastern Mediterranean Basins. The input to the model consists of the height of the deep water reservoir above sill depth and its density difference from the overlying water masses. Data from four hydrographic cruises, which took place in 1995, 1991 and 1987, are used to estimate the depth of the reservoir above the sill and the density difference. The results show a significant CDW outflow of 0.75×106 m3 s−1 in early 1995. The outflow of CDW through Kassos Strait, in the east, is 0.53×106 m3 s−1, while 0.22×106 m3 s−1 outflows through the Antikithira Strait in the west. The model results agree with fluxes estimated from current meter observations. The CDW outflow has been neither steady nor uniform during the period 1987–95. In the Kassos Strait, the outflow commenced in 1987 and increased rapidly until 1991; since then, it appears to have stabilised. In the Antikithira Strait, in contrast, the outflow has increased steadily since 1987. Such modifications in the CDW outflow are associated with changes in its hydrographic characteristics. The salinity of CDW increased constantly, by approximately 0.1, between 1987 and 1995 while its temperature warmed, between 1987 and 1991, and then cooled.

Internal waves have been detected on ERS-1 SAR images obtained during late summer over the eastern Cretan Straits, an area characterised by complex regional physiography, bottom topography, flow regime and stratification patterns of the upper part of the water column. Analysis of the imaged characteristics of the internal waves has revealed a strong diversity in form, propagation direction and type of sea surface modulation, which indicates various mechanisms of generation. Analysis of the currents recorded over the area shows that, although semi-diurnal tidal currents are present, these are of low magnitude in comparison with the overall flow and, therefore, tidal forcing is unlikely to be a major process in the generation of the imaged internal waves. In addition, a well-defined front has been identified within the Rhodes Strait. This front is considered to be the surficial manifestation of the Asia Minor Current, which is a strong and persistent large-scale circulation feature of the Eastern Mediterranean.

Five research cruises were undertaken incorporating ADCP sections along the Cretan Arc Straits and CTD surveys covering the entire area of the Straits and the Cretan Sea. In addition, six moorings (with 15 current meters) were deployed within the Straits, which monitored flows in the surface (50 m), intermediate (250 m), and deep (50 m from the bottom) layers. The ADCP, CM, and CTD datasets enable the derivation of water transports through the Cretan Arc Straits to be assessed. Flow structure through the Cretan Arc Straits is not the typical flow regime with a surface inflow and deep outflow, instead there is a persistent deep outflow of Cretan Deep Water (CDW) (σ θ >29.2) with an annual mean of ~0.6 Sv, through the Antikithira and Kassos Straits at depths below 400 m and 500 m, respectively. CDW outflowing transports are higher (~0.8 Sv) in April–June, and lower (~0.3 Sv) in October–December. Within the upper water layer (0–~400 m), the transport and the water exchanges through the Straits are controlled by local circulation features, which weaken substantially below 200 m. The Asia Minor Current (AMC) influences the Rhodes and the Karpathos Straits, resulting in a net inflow of water. In contrast, the Mirtoan/West Cretan Cyclone influences the Antikithira and Kithira Straits, where there is a net outflow. In the Kassos Strait, there is a complex interaction between the East Cretan Cyclone, the Ierapetra Anticyclone and the westward extension of the Rhodes Gyre, which results in a variable flow regime. There is a net inflow in autumn and early winter, and a switch to a net outflow in early spring and summer. The total inflow and outflow, throughout all of the Straits, ranged from ~2 to ~3.5 Sv, with higher values in autumn and early winter and lower in summer. The AMC carries ~2 Sv of inflow through the Rhodes and Karpathos Straits, and this accounts for 60–80% of the total inflow. About 10–15% of the total outflow is of CDW, and a further 45–70% occurs through the upper 400 m of the Kithira and Antikithira Straits. The Kassos Strait exhibits a net inflow of ~0.7 Sv in autumn and early winter, with a net outflow of ~0.5 Sv in early spring and summer.

Four seasonal oceanographic cruises were carried out in the Eastern Mediterranean Sea, within the framework of the CEC/MAST-MTP Project PELAGOS, during 1994–1995. The surveys covered the South Aegean Sea and the adjacent open sea regions (southeastern Ionian, northwestern Levantine). Analysis of CTD data revealed that a multiscaled circulation pattern prevails in the area. It differs from the circulations detected during the 1986–87, thus indicating interannual variability. Cyclonic and anticyclonic gyres and eddies are interconnected by currents and jets variable in space and time. Most of the features are persistent, others seem transitional or recurrent. The hydrological structure is also complex and apart from the upper layer does not present basinwide any significant seasonality. Dynamical and hydrological regimes are variable in the upper and intermediate layers at the Straits of the Cretan Arc, while the deep regime seems rather constant. Topographic control is evident on the flows through the straits. The new very dense deep water mass, namely the Cretan Deep Water (CDW) and a well-defined intermediate layer of minimum temperature and salinity, the so-called Transition Mediterranean Water (TMW), consists the new important structural elements of the South Aegean Sea. The CDW outflows towards the deep and bottom layers of the Eastern Mediterranean, thus considerably contributing to the formation of the new, denser Deep and Bottom Water of the Eastern Mediterranean, which sinks and displaces the Eastern Mediterranean Deep Water of Adriatic origin in the adjacent sea regions outside the Aegean Sea.

The CEC/MAST-MTP PELAGOS Project, which was conducted in 1994 and 1995, found a new dynamic hydrological regime in the Cretan Sea–Cretan Straits region that is interacting with and affecting the whole basin of the Eastern Mediterranean. In the Cretan Sea, the upper thermocline general circulation shows no significant seasonal signal, but the circulation patterns observed were significantly different from those observed in 1986–87, suggesting that there is substantial interannual variability. There was a persistent outflow of Cretan Deep Water (CDW) with an annual mean flow of 0.6 Sv, which was making a substantial contribution to the formation of a new warmer, saltier and denser Deep Water of the Eastern Mediterranean, that was displacing upwards the Eastern Mediterranean Deep Water (EMDW) of Adriatic origin. Thus, the Cretan Sea is the unique source of a new type of the EMDW. There is an inflow of Transition Mediterranean Water (TMW) of low salinity, temperature and oxygen, but rich in nutrients into the South Aegean Sea (Cretan Sea) at intermediate depths (200–600 m) from the open sea areas of the Eastern Mediterranean, which is drastically changing the chemical conditions of the region. Biomass concentrations, primary production rates, the chemical constituents of the suspended particulate matter and the total mass fluxes of settling particles confirm that the South Aegean is one of the most oligotrophic regions of the world. Dissolved element concentrations are comparable to those reported for other straits of the Mediterranean. Particulate Fe and Mn occur in unusually high concentrations, probably originating from hydrothermal sources in the South Aegean Sea. It is expected that the nutrient enrichment of the intermediate layers in the Cretan Sea will influence the extremely oligotrophic character of the South Aegean and is having a direct effect on the biology of the region.

We present definitive observational evidence that the startling change of the Eastern Mediterranean deep circulation observed in winter 1995 and documented by [Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevich, V., Luchetta, A., 1996. Recent changes in the Eastern Mediterranean deep water. Science 271, 333–335.] actually started before October 1991. This change involved not only the deep water mass pathways but also the origin and pathways of the water mass spreading in the intermediate layer. We carry out the first unified analysis of the POEMBC-O91 data set, which shows that, differently from the previous decade of the 80s, the Cretan/Aegean Sea was in 1991 the `driving' engine of the intermediate, transitional and deep layer circulations, with Cretan Intermediate Water (CIW), transitional water and Cretan Deep Water (CDW) spreading out from the Cretan Sea into the basin interior. The most important new results are: (a) the Levantine Intermediate Water (LIW) formed inside or at the periphery of the Rhodes gyre is blocked in its traditional westbound route on its density horizons σθ=29.05 and 29.10 kg/m3 by a three-lobe strong anticyclonic structure in the Southern Levantine, which induces a substantial LIW recirculation in the Levantine basin itself; (b) the CIW exiting from the Western Cretan Arc Straits spreads into the Ionian interior on the σθ=29.05–29.10 kg/m3 isopycnal surfaces, thus replacing the LIW confined in the Levantine basin. A branch of CIW flows eastward in the Cretan passage and is entrained by the Ierapetra anticyclone to flow again into the Cretan Sea through the Eastern Cretan Arc Straits; (c) on the horizons σθ=29.15 and 29.18 kg/m3 a transitional water mass of Cretan origin, denser than CIW, and CDW are observed to spread out massively from the Cretan Arc Straits both into the Ionian and Levantine interiors. These isopycnal surfaces rise to much shallower depths in 1991 than in 1987, increasing the salt content of the intermediate, transitional and deep layers. This leads to a massive salt increase in the Ionian below 1200 m, clearly related to lateral advection of the new denser waters of Cretan/Aegean origin, thus contradicting the hypothesis of a vertical salt redistribution proposed by Roether et al.

Recent changes of water mass characteristics in the south Aegean Sea have considerably influenced the Eastern Mediterranean thermohaline circulation. A combination of salinity increase (1987–92) and temperature drop (1992–94) caused massive dense water formation and strong outflow towards the deep and bottom parts of the Eastern Mediterranean. This climatic shift, the most important since the existence of observations in the basin, is a combined effect of extreme meteorological events superimposed on large-scale trends. It has initiated a series of modifications in the hydrology and dynamics of the entire Eastern Mediterranean, with possible influence on the Mediterranean outflow in the North Atlantic Ocean.

In this paper we revisit, with a thorough in-depth analysis, the dataset collected in the hydrographic surveys of the international collaborative programme POEM (Physical Oceanography of the Eastern Mediterranean) in the period 1986–1987. The work has two major objectives. The first is to refine the dynamic picture of the Ionian upper thermocline sub-basin scale circulation, rather less definitive than the dynamic picture of the Levantine Sea circulation. The second is to identify the pathways of the major water masses of the basin not only in the near-surface, but also in the intermediate and deep layers. To our knowledge, this is the first work defining the pathways of the Levantine Intermediate Water (LIW) and of the Adriatic Deep Water (ADW)/Eastern Mediterranean Deep Water (EMDW) that characterize the intermediate and deep circulations. The major novel results can be summarized as follows. In the upper thermocline: (1) The Atlantic Ionian Stream (AIS) jet entering the Sicily Straits bifurcates into two main branches at 37°N, ∼ 17°E. It advects the Modified Atlantic Water (MAW) into the Ionian Sea interior. The first branch turning directly southward encloses an overall anticyclonic area comprising multiple centers around which the MAW is advected. (2) The second AIS branch extends further into the northeastern Ionian, where it too turns southward before crossing the entire Ionian Sea meridionally, advecting MAW on its left side and Ionian Surface Water (ISW) on its right. This branch of the jet is confined to the Ionian margin and does not pass around the Pelops gyre. (3) A new water mass, the LSW, is formed in the Levantine basin and enters the Cretan passage, then is first veered cyclonically south of Crete by the Cretan gyre and successively is entrained anticyclonically around the Pelops gyre, and then enters the Aegean Sea. (4) A permanent cyclone located in the northeastern Ionian determines the pathway of mixed Adriatic Surface Water/Ionian Surface Water (ASW/ISW). (5) A permanent cyclone is found in all the surveys near the tip of the Italian boot. This novel analysis of the LIW pathways shows that: (1) The major source of intermediate LIW during the period 1986–1987 was actually in the Levantine Sea. LIW formed there entered the Cretan passage, was veered cyclonically by the Cretan gyre south of Crete and then entered the southern Ionian Sea. The major LIW pathway was westward directly to the Sicilian Straits. (2) Secondary important LIW pathways were determined by the interior structures. The strong Pelops anticyclone was entraining LIW around its periphery and was determining the LIW northward pathway that closely followed the eastern Greek coastline. It was along this pathway that LIW entered the Otranto Strait. A further branch of LIW was entrained and recirculated around the multiple Ionian Anticyclones (IA) of the western Ionian Sea. (3) The Cretan cyclone is a feature confined to the upper thermocline-intermediate layer. It disappears at ∼ 400 dbar while the Pelops anticyclone is strongly barotropic below the upper 100 dbar and penetrates quite intense down to 800 dbar. (4) A further completely novel result concerns the new water mass found in the deep layer that spreads on the 29.15 kg/m3 isopycnal surface. This water mass, characterized by high salinity and high oxygen content, is formed inside the Aegean Sea and is observed to spread out all around the Cretan Arc Straits. The final fully novel result is the demonstration of a second pathway for the ADW exiting from the Otranto Strait that is transformed into EMDW and occupies the abyssal layers of the Ionian Sea interior. The traditional pathway for EMDW is along the isobaths along the western side of Italy but ADW was observed to be exiting from the Otranto Strait in the eastern Hellenic trench at 39.5°N, both during POEM-ON86 and POEM-AS87. This second pathway for EMDW follows isobath contours along the western side of Greece. The two EMDW routes converge and merge between 36°N and 35°N, so producing a deep layer of EMDW that occupies uniformly the abyssal plain of the interior of the Ionian Sea.

[1] Aiming at portraying the Aegean's water mass structure and identifying Dense Water Formation processes, two winter cruises were conducted in 2005–2006, across the plateaus and depressions of the Aegean Sea. The most prominent feature of the water mass distribution in the basin is a distinct “X-shape” of the Θ-S characteristics, suggesting a complicated coupling of the major Aegean sub-basins. The surface and deep waters are relatively decoupled with diverse origin characteristics, while the intermediate layers act as connectors of the main thermohaline cells. The Central Aegean seems to play a key role due to formation processes of water masses with densities equal and/or higher than 29.2 kg/m3, that take place in the sub-basin and disperses in the North Aegean. On the other hand, the South Aegean appears greatly influenced by the Eastern Mediterranean circulation and water mass distribution, especially under the Eastern Mediterranean Transient status. The Transitional Mediterranean Water monitored in the post-EMT period and characterized by low temperature at 14.2°C, low salinity at 38.92 and low dissolved oxygen at 3.97 ml/l, with its core around 750 m and above the saline (39.06) Cretan Deep Water, altered significantly the South Aegean structure. The pre-EMT thermohaline pattern of the Central and South Aegean deep layers were similar, while the bottom density of the Central basin was higher than that in the South Aegean. Thus, it is possible that the deep waters of the Central Aegean acted as a dense water reserve supply for the deeper part of the Southern basin.

Ahstraet The North Aegean Sea was visited from 27 February to March 3 1987, during the late winter LIA-5-87 national cruise in the framework of the Open Sea Oceanography Research Program of NCMR. CTD data were collected from 31 stations on board R.V. Aegaio. Strong (velocities up to 17 ms -1) and cold (-2°C) northerlies, dominating during the week before the cruise, contributed to the cooling of the sea waters. A particularly low sea surface temperature (10.83°C) was observed in the north easternmost part of the north Aegean Sea. The distribution of the hydrological characteristics combined with a series of infrared images reveals the presence of thermohaline and density fronts both over the Samothraki and Limnos Plateaux, possibly generated by the confluence of water masses of different characteristics, namely brackish and cold Black Sea waters effluing from the Straits of Dardanelles, fresh waters from rivers outflowing along the northern Greek coasts and warm and saline waters of Levantine origin. A considerable amount of the latter waters shaped as tongues seem to be projected intermittently over the Samothraki Plateau. Additionally, the surface layers are supplied with waters of Levantine characteristics upwelled from deeper layers within the dome of a cyclonic eddy. Moreover, waters of high densities (oo up to 29.20) appear throughout the water column, while the highest values (o0 = 29.37) are detected in near bottom layers over the shelf areas. The newly formed, oxygen rich, dense waters tend to follow the isobaths and finally slide towards the deepest basins of the North Aegean Trough, in agreement with the theoretical prediction of SMITH (1975, Deep-Sea Research, 22, 853-873) and KtLLWORTH (1977, Deep-Sea Research, 25,927-988).

In the framework of the major multinational coordinated POEM-II-86 (March–April 1986) and POEM-V-87 (September–October 1987) cruises in the Eastern Mediterranean, high resolution hydrographic (CTD) data were collected by R.V. Aegaio in the eastern Ionian Sea, south Aegean Sea and northwest Levantine Basin. The intercalibrated data sets were analyzed objectively for an optimal estimation of the basin's circulation in two different seasons. The analysis reveals a rather complex general circulation pattern consisting of basin, sub-basin and mesoscale features during both study periods. The major results concern the advection of the Modified Atlantic Water (MAW) along its eastward route, the spreading of the Levantine Intermediate Water (LIW) and its trapping by the eddy fields; the interaction between the south Aegean Sea and the eastern Mediterranean; and the reverse circulation in the Cretan Sea during winter and summer. From the hydrography and the dynamic height maps, synthetic schemes show the main circulation features in the two seasons.

International audience; Using long-term data of 149 warm alien species since 1924, we show that the introduction of warm and tropical alien species has been exacerbated by the observed warming of the eastern Mediterranean Sea. The phenomenon has accelerated after an abrupt shift in both regional and global temperatures that we detect around 1998, leading to a 150% increase in the annual mean rate of species entry after this date. Abrupt rising temperature since the end of the 1990s has modified the potential thermal habitat available for warm-water species, facilitating their settlement at an unexpectedly rapid rate. The speed of alien species spreading and response to global warming is apparently much faster than temperature increase itself, presenting an important warning for the future of Mediterranean Sea biodiversity. In addition to the sea warming, other factors that enable and enhance biological invasions, such as salinity increase and oceanographic forcing, are also discussed.

A novel description of the phenomenology of the Eastern Mediterranean is presented based upon a comprehensive pooled hydrographic data base collected during 1985–1987 and analyzed by cooperating scientists from several institutions and nations (the POEM project). Related dynamical process and modeling studies are also overviewed. The circulation and its variabilities consist of three predominant and interacting scales: basin scale, subbasin scale, and mesoscale. Highly resolved and unbiased maps of the basin wide circulation in the thermocline layer are presented which provide a new depiction of the main thermocline general circulation, composed of subbasin scale gyres interconnected by intense jets and meandering currents. Semipermanent features exist but important subbasin scale variabilities also occur on many time scales. Mesoscale variabilities modulate the subbasin scale and small mesoscale eddies populate the open sea, especially the south-eastern Levantine basin. Clear evidence indicates Levantine Intermediate Water (LIW) to be present over most of the Levantine Basin, implying that formation of LIW is not localized but rather is ubiquitous. The Ionian and Levantine basins are confirmed to form one deep thermohaline cell with deep water of Adriatic origin and to have a turnover time of one and a quarter centuries. Prognostic, inverse, box and data assimilative modeling results are presented based on both climatological and POEM data. The subbasin scale elements of the general circulation are stable and robust to the dynamical adjustment process. These findings bear importantly on a broad range of problems in ocean science and marine technology that depend upon knowledge of the general circulation and water mass structure, including biogeochemical fluxes, regional climate, coastal interactions, pollution and environmental management. Of global ocean scientific significance are the fundamental processes of water mass formations, transformations and dispersion which occur in the basin.

Maps are presented for dynamic height and geostrophic flow in the upper thermocline based upon four basin-wide hydrographic surveys during 1985–1987. The data collection was coordinated, intercalibrated and pooled by the international research programme for Physical Oceanography of the Eastern Mediterranean (POEM). Objective analysis mapping was constrained to have no normal flow into the coasts. These maps reveal a new picture of the general circulation in which sub-basin-scale gyres are interconnected by jets and currents. Important variabilities occur in permanent and recurrent features but transient eddies and jets also occur. A schematic synthesis is constructed.

LONG-TERM GOALS. It is the long-term goal of this work to gain a better understanding of the circulation of the Aegean Sea. While the nations bordering the Aegean Sea each have individual programs in scientific exploration and analysis of the Sea, these efforts have only recently begun to be well-coordinated, and the most modern technology has not been used. It is the goal of this work to begin a coordinated effort to study the Aegean Sea with modern profiling floats. OBJECTIVES. The objective of this work was to collect a large number of high quality temperature and salinity profiles from the Aegean Sea during all seasons, over the course of several years, in order to be able to examine the circulation above 1000 m. The deep circulation in the Aegean Sea has not been previously well-observed over long periods, and it is planned to examine this circulation for the first time in this work. APPROACH. In order to carry out this work, 4 profiling floats are being deployed in the Aegean Sea between 2004 and 2006. A parking depth of near 700 m has been chosen for all of the floats, with CTD profiles collected every 5 days.

[1] Using an intercalibrated set of oxygen data for the eastern Mediterranean, 1987–1999, we study the evolution of oxygen concentrations that accompanied the recent changes in the thermohaline circulation of this sea (the so-called Eastern Mediterranean Transient (EMT)). We find that, by way of massively transferring oxygen-rich near-surface waters into the deep layers, the EMT by 1995 had raised oxygen concentrations considerably relative to the pre-EMT situation in 1987. Between 1995 and 1999, however, the oxygen concentrations decreased noticeably, we identify oxygen decreases of about 5 μmol/kg for the layers below 2200 m and between about 500 and 1000 m depth, for which layers previous work found vanishing or little replenishment during 1995–1999. Supporting evidence for absence of replenishment is obtained from the temporal evolution of tritium-3He ages. The oxygen decreases convert into utilization rates of approximately 1.3 μmol/(kg yr). An even higher rate, 2.3 μmol/(kg yr), is obtained for the deep waters of the Cretan Sea below 1000 m. The utilization rate found for the deepest waters significantly exceeds the pre-EMT value of 0.53 μmol/(kg yr). We propose that the massive invasion of near surface waters affected by the EMT made available large amounts of dissolved organic carbon with an unusually high fraction of labile material, which in turn enhanced oxygen consumption. Supporting evidence is obtained from data of dissolved organic carbon, and from mesozooplankton ecology data. The enhanced oxygen utilization represents a further example of EMT-related disturbances in the biogeochemistry of the eastern Mediterranean.

During the CIESM workshop (Monaco, 22–24 April 2002) entitled “Monitoring Hydro- logical Trends in the Mediterranean”, the review of existing data sets and analyses has revealed important variability in the dynamics and hydrological characteristics, in the past century, ranging from interannual to decadal time scales. These variations are related mainly to local forcing and larger scale atmospheric parameters (NAO, ENSO, Indian monsoon). Moreover, abrupt events contribute to modification of the Mediter- ranean conveyor belt, also influencing the biogeochemical environment and the Mediter- ranean outflow in the Atlantic Ocean. The participating scientists have agreed on the need of a sustained multi-component long-term monitoring system based on both oceanographic and atmospheric observations and modelling. The group also proposed an expansion and strengthening of the ongoing relevant large operational programs by a “climatic approach” component.

The thermohaline circulation of the Eastern Mediterranean underwent a dramatic change between 1987 and 1995. In 1987 the “engine” of the Eastern Mediterranean “conveyor belt” was the convective cell of the Southern Adriatic, while in 1995 the active convection region moved to the Aegean Sea. This change actually started as early as 1991. The phenomenological evidence of the POEM programme shows that in 1987 the source of Levantine Intermediate Water (LIW) mass was the Levantine basin and the bottom water mass was formed in the Southern Adriatic. In 1991 all the intermediate/deep water masses on the horizons σθ = 29.00 to 29.18 kg/m3 were formed inside the Aegean sea, from which they spread out into the entire Eastern Mediterranean through the Cretan Arc Straits.

Straits in the Mediterranean Sea form an important network from which one can determine the characteristics of the water exchange between all the constituent sub-basins. This includes the definition of water masses and water transport and their time variability. From 1994, all the major straits in the Mediterranean Sea (Gibraltar, Sicily, Otranto, Balearic Sea Straits, Cretan Arc Straits and Corsica) were subject to long term observations as part of various research projects. Besides adding new elements to the knowledge of internal strait conditions, the data sets collected allow us to propose a fairly consistent representation of the Mediterranean circulation and budgets in key points within the basin. The amplitude of the annual water transport measured at these straits was about 1 Sv and it appears to be modulated by a significant low-frequency and seasonal variability. For the first time, a seasonal component was identified at Gibraltar, thus raising new questions on the actual state of the Mediterranean. Also, the very likely existence of a significant interannual component was documented. In the Corsica Channel, this component was found to be related to the interannual variability of the North Atlantic Oscillation. The observations in the Cretan Arc Straits have provided a more comprehensive representation of the recent changes in the Eastern Mediterranean thermohaline cell. It is noteworthy that the effects of these changes have been observed both in the Otranto and Sicily Straits, and are now affecting the adjacent sea regions. The presence of a stream of Modified Atlantic Water in the Balearic Sea Channels indicates that part of the Atlantic inflow may be diverted directly into the northern region of the Western Mediterranean. Finally, data gathered in the Sardinia Channel indicate that the central Mediterranean region plays a critical role in controlling exchanges between the Eastern and the Western Mediterranean, while it is emphasized that the Tyrrhenian Sea area plays a role in strongly modifying some of the water masses that contribute to the large scale basin circulation. Their mixing creates new water types which modify the currently known pattern and composition of the Mediterranean circulation.

The eastern Mediterranean Sea, one of the poorly known sea regions before the ‘80s, became the object of very intensive research since 1985. The synthesis of the results based on the data sets that were collected in the frame of international (POEM, POEM-BC, EU/MAST/MTP) and national research programs revealed significant information about the circulation, hydrography and their variability in this Basin during the decade 1986-1995. The emerged three scales of motion, the basin, sub-basin and mesoscale, interacting within the general circulation pattern, as well as the hydrological regime present interannual to interdecadal variability. This variability plays important role in all upper and lower thermocline processes, as intermediate and deep water mass formation, transformation, spreading, modification of water mass pathways and vertical structure as well as basins’ interaction

In early spring of 1986, 1992 and 1995, newly formed deep water masses were observed in the cyclonic Rhodes Gyre of the Eastern Mediterranean at depths reaching ∼1000 m in 1995 and exceeding 1000 m in 1986 and 2000 m in 1992. Levantine Intermediate Water (LIW) formation was observed at localized source areas south of the East Cretan Straits at the periphery or near the center of the Ierapetra Anticyclone. Adittional massive LIW formation was observed in 1992 in anticyclonic circulation structures north of Cyprus. The lateral scales of the newly formed water masses in cyclonic structures appear roughly proportional to the penetration depth of the convection, so that a large lateral scale indicating a massive production would be associated with a deep rather than an intermediate formation.

During the last decade the oceanography community has focused much attention on the Mediterranean Sea. One reason for the growing interest is that the Mediterranean's impact on the Northern Atlantic Ocean is more significant than previously realized. The warm, salty Mediterranean water tongue exits the Gibraltar Straits and spreads throughout the North Atlantic at all depths between 1000 and 2500 m. The second reason for the surge in interest is the well-recognized role of the Mediterranean Sea as a laboratory for studying ocean processes that are important in global climate dynamics [Malanotte-Rizzoli and Robinson, 1991; Malanotte-Rizzoli and Robinson, 1994].