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Understanding the processes that control the buoyancy fluxes of the Aegean Sea is important for various reasons. First, the Aegean is directly connected with the Black Sea and acts as a buffer between two opposite thermohaline systems, a concentration and a dilution basin (the Mediterranean vs the Black Sea), receiving and filtering the variability and changes of a much broader geographical area. Second, the Aegean is capable to produce large amounts of very dense water, having temporarily been the major originator of Eastern Mediterranean Bottom Water. These processes are controlled by the buoyancy fluxes, both through oceanic advection and atmospheric exchanges. In this work we examine the characteristics and variability of heat, freshwater and the overall buoyancy air-sea fluxes, focusing on the potential role of the interaction with the Black Sea. A thirty-year-long simulation (1985-2015) of the whole Eastern Mediterranean/Black Sea system, forced by ERA-Interim data, was used to estimate and analyze the seasonal and interannual variability of the buoyancy fluxes in the North Aegean. The climatological mean buoyancy flux over the North Aegean has been estimated to be about –10×10–6 kg m–1 s–3 (loss to the atmosphere). However, in the absence of Black Sea Water (BSW) inflow, the buoyancy loss would correspond to the high values observed over the Eastern Aegean, an area not directly affected by the presence of the BSW, i.e. about –30 ×10–6 kg m–1 s–3. It should be noted that the heat loss of the Aegean Sea to the atmosphere is much higher than all neighboring seas, including the Adriatic, the dominant dense-water formation site for the Eastern Mediterranean. Our analysis reveals that the thin surface layer of modified BSW acts as a moderator of the buoyancy loss from the upper water column. This layer not only absorbs the air-sea fluxes (acting as an effective insulator regarding dense-water formation processes), but also moderates or even reverses the buoyancy fluxes over its path. Thus, in addition to the significant lateral buoyancy input to the basin by the Black Sea inflow, an additional mechanism of reduction of winter heat losses to the atmosphere contributes to the control of dense water formation processes.

The North Aegean Sea is a sub-basin of the Mediterranean which exhibits a range of oceanic processes at various scales. Due to the inflow of very light, mesotrophic Black-Sea waters it is the most productive region of the seas around the Hellenic Peninsula, although the regular seasonal coastal upwelling along its eastern shores does not contribute to its productivity. Despite the continuous buoyancy import by the Black Sea, the North Aegean hosts the densest waters of the Eastern Mediterranean. Finally, three semi-enclosed bays located in two islands of the North Aegean exhibit an alternating behavior as sources or sinks of buoyancy for the basin, while their productivity and natural beauty support a range of coastal activities. For the above reasons, the University of the Aegean has invested over several years in the development of a coastal oceanographic observatory (AEGIS), covering both the open North Aegean Sea and the three main bays of the islands of Lesvos and Lemnos. The Observatory consists of a numerical modeling component and an observational component.The modeling component of the observatory consists of four coastal circulation models (for the three bays and the island of Lesvos) nested within a larger domain circulation model covering the whole Aegean Sea north of 37º N. Data assimilation, employing both satellite (sea-surface temperature and sea-level) and field data (employing mostly ARGO float observations) is used in the model of the extended domain (an implementation/configuration of the ROMS system), while the observations obtained in the coastal regions are currently used for coastal models’ (DELFT-3D FLOW and ROMS) validation. In addition to the above circulation models, SWAN is used to simulate the surface waves and DELFT-3D WAQ is being implemented to simulate the biochemical functioning at the various model domains.The observational component at small geographical scales (in the Bays) comprise of continuous meteorological and oceanographic observations through an oceanographic mooring in the middle of the Bay of Kalloni, sea-level observations at the Bays of Kalloni and Gera, and High-Frequency radar observations of sea-surface currents and waves in a region east of Lemnos island, aiming to monitor the Black Sea outflow into the Aegean. The above continuous measurements are supplemented by periodic hydrographic and biogeochemical measurements in the three Bays, to validate the models and calibrate the in-situ continuous data. A recent addition to the AEGIS’s observational arsenal is an ocean glider aimed to capture the variability of the open North Aegean sea.The AEGIS Observatory provides the necessary background to support strategic planning of human interventions at regional and local scales, such as Marine Spatial Planning or the construction of river dams affecting sensitive coastal basins. The implementation of the Coastal Laboratory has been supported by several projects, the most recent being the project “Coastal Environment Observatory and Risk Management in Island Regions AEGIS+” (MIS 5047038), implemented within the Operational Programme “Competitiveness, Enterpreneurship and Innovation” (NSRF 2014-2020), cofinanced by the Hellenic Government (Ministry of Development and Investments) and the European Union (European Regional Development Fund).

The coastal marine ecosystem of Saronikos Gulf, a busy Eastern Mediterranean embayment directly impacted by the greater metropolitan area of Greece’s capital, Athens, is examined through a series of state-of-the-art numerical models that address the hydrodynamics (Delft3D-FLOW), the wave regime (SWAN), the biogeochemistry, and pollution related to species of heavy metals and polyaromatic hydrocarbons (Delft3D-WAQ).The study so far has focused on calibrating model components and on reproducing the seasonal thermohaline conditions, known circulation patterns and the variability of biogeochemical constituents (chlorophyll-a, nutrients, dissolved and particulate matter) and pollutant concentrations, focusing on the vicinity of inner Saronikos.The annual cycle ‘Nov 2009 - Oct 2010’ is simulated, forced with atmospheric data from the ERA5 database. Three sets of open boundary conditions data are tested (Mediterranean Sea Physics Reanalysis dataset by Copernicus and two implementations of the ROMS model covering the Aegean and the Eastern Mediterranean respectively), constituting three classes of numerical experiments aiming to optimize model performance. Freshwater discharges from waste treatment facilities and rivers are considered, the latter drawn from the Swedish Meteorological and Hydrological Institute (platform ‘Hypeweb’).Emphasis has been placed in compiling available information on point sources of pollution from the numerous human activities in the vicinity of the study area. These data are used as forcing in the modelling process.A comprehensive dataset of field measurements collected monthly by the Hellenic Centre for Marine Research from a network of ten stations, as well as satellite derived SST data, are used for model validation.This work is carried out within the context of the EMERGE Horizon 2020 project, that develops methodologies to evaluate, control and mitigate the environmental impacts of shipping emissions. For the scope of the project, next steps currently undertaken include the simulation of the Saronikos Gulf status for the year 2018 considering pollutant mass fluxes (a) from shipping emissions as calculated by the Ship Traffic Emission Assessment Model (STEAM) and (b) from atmospheric depositions as calculated from atmospheric modelling, both components from data provided by consortium partners.

The North Aegean Sea is one of the most interesting seas of the Mediterranean, being under the dominant impact of the Black Sea waters though the so-called Turkish Strait System (TSS – including the Dardanelles and Bosphorus Straits and the Marmara Sea). Moreover, it constitutes a potentially deep water formation site of the Eastern Mediterranean Sea along with the Adriatic Sea. Previous studies for the region focused – rightly – on the crucial role of low salinity Black Sea waters in controlling the overall thermohaline function and dynamics of the North Aegean. None of the previous modeling approaches studied the impact of tides in the mixing processes and the production of extremely dense water, especially during 1987, 1992 and 1993 when major deep water formation events took place in the region. In this work we examine the tidal impact via several long term simulations using a high resolution (1.0 km) ocean model covering the period from 1985 to 2013.The Regional Ocean Model System (ROMS) was used for two 28-year-long hindcasts. A computational grid of approximately 1.0 km horizontal resolution in both directions and 31 vertical sigma (σ) levels was develop to cover the region of interest that extends from 22.5ºE to 27.25º E in longitude and 38.35º N to 41.2º N in latitude. Atmospheric forcing fields from ECMWF ERA5 reanalysis dataset with a spatial resolution of 0.25 degrees and hourly time step were used. The inflow from the Black sea to the North Aegean was treated as an open (east) boundary condition and data from Vladimir Maderich work was used as input. The tidal forcing, in total eight (8) harmonics – four diurnal and four semidiurnal – came from Oregon State University (OSU) inverse global tidal model. Two identical simulation – in terms of model setup and input – were conducted: the first without and the second with tidal forcing.After extensive validation of the model’s results, using all available in situ data from different platforms, a comprehensive analysis was conducted and our findings reveal that model results employing tidal forcing exhibit a closer proximity to observations than non-tidal results, thus validating the necessity to incorporate tidal forcing. Furthermore, the use of barotropic tidal forcing enchanced meridional exchanges of heat, salt and buoyancy between the North and South Aegean, thus increasing the stratification and buoyancy content of the upper water column prior to winter mixing. The most significant and surprising result however is that the dense-water volumes produced using tidal forcing were much higher than the ones without tides, a fact signifying the complexity of processes involved prior to and during dense-water formation.This work was partly covered by the project “Coastal Environment Observatory and Risk Management in Island Regions AEGIS+” (MIS 5047038), implemented within the Operational Program “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014-2020), co-financed by the Hellenic Government (Ministry of Development and Investments) and the European Union (European Regional Development Fund).

This paper presents the state-of-the-art research conducted as part of EN.I.R.I.S.S.T. regarding the emerging impact of shipping and transport to the environment. More specifically, it presents the digital platforms and services that are developed as part of the research infrastructure and are related to environmental sciences. The first is the “EcoMarpol Platform”, which caters for collecting and analyzing pollution-driven data as well as calculating the environmental imprint (atmospheric emissions, waste production, chemical pollution of marine waters and sediments) of maritime activities. This platform additionally includes the online monitoring of in-port emissions from shipping and pollution from ship-to-ship transfer operations or accidents and the environmental risk assessment management. The other platform is the “Passenger Sustainable Travel Platform”, as part of which a tool for calculating aircraft emissions (CO2 and air pollutants) will be developed and applied in the Greek airspace and airports.

Inter-basin water exchanges can be quite important in climatic-scale numerical studies simulating the circulation and hydrographic characteristics of neighboring oceanic basins connected through narrow straits. The crucial role of the interaction between the Mediterranean and the Black Seas is often overseen in simulations, which rely mostly on parameterizations to describe the exchange, essentially decoupling the two basins. In this study, the fully interconnected Eastern Mediterranean–Black Sea system is simulated for the historical period (1985–2015) using realistic boundary conditions (lateral, atmospheric and hydrological), with a hydrodynamic fully three-dimensional ocean modeling system. The setup of such a configuration is thoroughly described and the performance of the 30-year hindcast product is validated exhaustively against observations and model results, by evaluating the representation of surface fields, circulation, three-dimensional hydrographic characteristics, volumetric water exchanges, and the spatio-temporal variability of the above. The comparison shows exceptional performance, minimal drift, and substantial improvement compared to modeling studies that do not include the interaction. Moreover, due to the free-run configuration of the simulation (i.e., absence of assimilation schemes) no additional input is required other than the respective boundary conditions, making it possible to reliably extend the same setup for scenarios where observational data are not available, such as in future projections.

Mediterranean marine heat waves (MHW) can be defined as abrupt but prolonged, discrete and anomalously warm water events that last for five or more days and exceed temperatures warmer than the 99th percentile (Darmaraki et al. 2019). Like their atmospheric counterpart, Mediterranean MHW have already increased in intensity, frequency and duration - a trend projected to continue under anthropogenic climate change. Recent observations of MHW demonstrated a strong influence of these extreme climatic events on marine organisms, including mass mortalities and shifts in species ranges but also economic impacts on fisheries and aquaculture. MHW can be caused by a combination of atmospheric and oceanic processes and depend on the specific season and location of occurrence. However, the main triggers are generally still not well understood and the current knowledge is largely based on these reported regional impacts. This work focuses on historical (1985 – 2014) atmospheric and marine heat waves in a high resolution CMIP6 model as well as a fully three-dimensional oceanographic hindcast of the interconnected Eastern Mediterranean – Black Sea system. We detect the atmospheric and marine heatwaves and investigate the triggering, compound/concurrent effect of the atmosphere on marine heat waves in the Eastern Mediterranean. For the analysis of atmospheric heat waves, we follow the methodology of Kuglitsch et al. (2010). We use Eastern Mediterranean atmospheric model and ERA-Interim reanalysis to calculate daily maximum (TX) and minimum (TN) air temperatures as well as to set temperature thresholds to estimate the beginning and end of the heat wave events. We identify MHWs from daily sea surface temperatures, applying the approach of Darmaraki et al. (2019). Furthermore, we calculate the heat wave frequency, duration and intensity. The two pairs of datasets are then compared with respect to the spatio-temporal occurrence of heat waves in the atmosphere and ocean, in an effort to reveal feedbacks between the two spheres which would characterize the events as compound. Finally, we estimate a threshold at which an atmospheric heat wave triggers a marine heat wave, and thus a compound event.

Even before the introduction of the term “Marine Heat Wave” (MHW) and its statistical definition in global-scale studies, the scientific community had studied and recorded potentially harmful impacts of persistent conditions of warm surface layers and highly stratified water columns on the marine ecosystem. The main triggers for MHWs are yet not well understood and the current knowledge is mainly based on mass mortalities linked to temperature anomalies. EM-MHeatWaves is an interdisciplinary, collaborative, DAAD/IKYDA funded research project that investigates the atmospheric forcing, oceanic circulation and ecosystem response of MHWs in the Eastern Mediterranean Sea over the past 35 years. Two universities (Justus-Liebig-University Giessen, University of the Aegean) and one research center (Hellenic Centre for Marine Research) re-examine the definition of MHWs with emphasis on the Eastern Mediterranean by applying a holistic approach that includes reverse-engineering using model data and reanalysis covering the period 1985 to 2014. We focus on the Eastern Mediterranean because of the high sensitivity of the basin’s ecosystem to atmospheric and marine warming events, the invasion of tropical alien (Lessepsian) species, the characteristic oceanic circulation with the Eastern Mediterranean Transient events, the exchange with the Black Sea through the Turkish Strait System as well as the coastal upwelling areas. In order to study the spatiotemporal characteristics of Eastern Mediterranean MHWs we work towards a better understanding of the oceanographic processes as well as of the compounding character of the atmospheric contribution. Based on the response of marine biogeochemical cycles (depletion of subsurface oxygen levels, observed changes in the mixed layer and chlorophyll maxima depths, nutrient stoichiometries, carbon uptake and sequestration rates) and their impacts on ecosystems (i.e. shifts in planktonic and benthic community regimes, mass mortality events, disease outbreaks, etc.), triggered by the rise of ocean temperatures, we study the statistical characteristics of the oceanic temperatures and assess the corresponding ocean circulation, the synchronous and lagged contribution of the large scale atmospheric circulation. We further study the signature of these extreme Mediterranean MHW events in future projections from model runs with respect to duration, severity and spatial extent and compare them to reanalysis. EM-MHeatWaves aims at strengthening the partnership between the German and Greek institutions by conducting joint research at a high scientific level.

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.

A review of the evolution of 137Cs activity concentrations in the Aegean Sea is presented almost 34 years after the Chernobyl accident. The data from field measurements are interpreted considering the different water masses present in the Aegean Sea, for better understanding of 137Cs spatial and temporal variation. The last 15 years, a lot of effort is given in the study of the vertical advection of 137Cs and the related processes in the deep basins of the Aegean Sea. These results provided significant information about the average values along with depth of the velocity and diffusion parameters. The role of 137Cs as a circulation and mixing tracer is undergoing a gradual weakening, since the gradient of its activity concentration between the water masses is small. 137Cs is proved to be a valuable radiotracer for identifying the Black Sea Water masses since they are still enriched with higher values of 137Cs compared to the background values in the Mediterranean Sea.

The response of the Aegean Sea to seasonal forcing by the Etesian winds is investigated, based on Copernicus products for weather forecasting (winds) and satellite information (sea-surface temperature (SST) and chlorophyll-α (chl-a) concentration), as well as fisheries records from the Hellenic Statistical Authority. To that end, a site-specific response criterion for upwelling intensity has hereby been developed, based on the zonal SST gradient between eastern and western Aegean. Combined use of the above criterion with lagged cross-correlation between wind, zonal surface temperature gradient, surface chl-a concentration and local fish landings reveals a well-hidden (until now) signal of chl-a increase in response to intensification of wind forcing, as well as a relation between fish landings and upwelling events, challenging the conclusions of previous studies suggesting that the Aegean Sea is immune to summer coastal upwelling, due to the great depth of its nutricline.

A medium resolution underwater gamma ray spectrometer was deployed in the submarine groundwater spring of Anavalos and was combined with conductivity and temperature sensors for continuous measurements of natural radioactivity, salinity and water temperature, respectively. The activity concentrations of radon progenies varied from (5–40) kBq m−3, exhibited reverse dependence with salinity and were found above the guideline proposed by Environmental Protection Agency (11 kBq m−3). Their ratio was above unity indicating radon gas escape to the atmosphere. Time-series of the activity concentration together with their ratios were provided enabling the system configuration for long term monitoring applications. The in-situ method proved to be an effective tool to study the radioactivity levels of the spring and the groundwater – seawater interaction process. Furthermore, the proposed tool provides cost-effective measurements in terms of real-time and long-term requirements and, it is characterized by flexibility in terms of sensor integration in different platforms to perform submarine groundwater studies in a temporal and spatial manner.

This work examines the impact of coastal upwelling on the exchanges of a semi-enclosed basin with the open sea. Five oceanographic transects were performed with a cross-shore direction relative to the western coast of Lesvos island of the eastern Aegean Sea, a region where coastal upwelling is regularly observed during the summer Etesian northerly winds. Filaments of highly saline water were observed during upwelling incidents, and were attributed to the enhanced transport of Levantine water from the south due to the baroclinic response to upwelling. All our observations, as well as a 9-year hindcast of the phenomenon revealed that the origin of the upwelled isopycnals usually remained above 40 m depth, and never exceeded the depth of 60 m. On the contrary, the isopycnals hosting the nutricline appear to downwell during upwelling incidents. Our hindcast showed that the secondary geostrophic circulation is comprised both by surface and subsurface longshore currents, which affect the meridional exchanges at the surface and intermediate layers and reduce the residence time of the water in the basin. We conclude that coastal upwelling in a region with longshore temperature and salinity gradients can modify the transport of salt and heat along the coast, possibly affecting the thermohaline functioning of a basin.

High Frequency (HF) radars, if properly located, can prove to be a valuable tool in areas of specific oceanographic interest, like Straits connecting two contrasting Ocean Basins, as in the case of the Turkish Straits connecting the Mediterranean and the Black Sea. The two basins' exchange generates a surface inflow and current into the North Aegean Sea that determine its characteristics and variability. Thus it is of prime importance to monitor and quantify the lateral advection of Black Sea waters (BSW) after their exit through the Dardanelles Straits. Here, we exploit surface velocity maps obtained through a HF radar in an effort to estimate (a) mixed-layer depth and (b) surface layer volume fluxes with unprecedented high spatiotemporal resolution. Comparison of the results of the hereby introduced method to estimates of the Dardanelles inflow into the Aegean Sea, or to independent estimates of the BSW advection volume fluxes in the interior of the basin, reveal consistently smaller values obtained with the new method. The difference is attributed to the fact that the HF radar method corresponds solely to advection within the surface mixed-layer, which is only a fraction of the total volume influx of Black-Sea waters into the Aegean. Testing this hypothesis on hydrographic measurements from a section further down the flow certifies that the quantity of BSW within the mixed-layer corresponds to a fraction of the total inflow of BSW.

This work introduces a method that enables the production of mixed-layer depth estimates from concurrent measurements of surface current velocities and surface wind stress. The method's basic assumption is that the mixed layer responds to wind forcing as a slab-layer. Here, the surface current velocities are provided by a High Frequency (HF) radar installed at the east coast of Lemnos island, monitoring the Dardanelles outflow of modified Black Sea waters into the Aegean Sea, and the wind stress is provided via the ERA-interim data set. The surface mixed layer is assumed to respond to wind-forcing as a slab. The method is applied both using the full EOF-reconstructed surface velocity field as well as only its wind-forced part. In the current implementation, the former method appears to produce better results than the latter. The method is evaluated using mixed-layer depth values estimated via local CTD profiles as reference.

The first stages of the development of a pilot oceanographic observatory for an enclosed basin (Kalloni Gulf, Lesvos, Greece) are described. The focus of the present work is related to the estimation of the water exchange with the open sea, using a numerical model of the region, volume flux measurements based on current velocity, and sea-level measurements. Three different methods of assessment of the exchange are described and evaluated, in order to select a reference method to calibrate submarine telephone cable measurements. The high-resolution coastal circulation model for the enclosed sea, nested in a larger-domain model, is developed and evaluated against in situ data, focusing on the adequate representation of the exchange and the hydrographic structure in the basin. Monitoring the water level is selected as the best-suited method for estimating the water exchange in hourly-to-weekly time scales. The model reproduces adequately the tidal exchange and sea-level response, as well as the hydrographic characteristics of the basin. The cable voltage measurements exhibit tidal signals overwhelmed by low-frequency noise, possibly attributable to circuitry and ground failures, however, the most suitable method for estimating exchanges of the basin with the open sea appears to be sea-level monitoring.

Caesium-137 activity concentrations (137Cs) were measured between 2003 and 2014 in the water column of Lemnos and Cretan deep basins of the Aegean Sea. The purpose was to assess water-mass dynamics and mixing processes by combining typical oceanographic data with the activity concentrations of 137Cs. The radiological data, combined with dissolved oxygen profiles, provided substantially more information than solely temperature and salinity regarding processes such as water-mass identification, formation and mixing. In each basin it was possible to determine stagnant periods, where turbulent mixing was dominant, as well as periods where replenishment of the deep basins had been taken place. At Lemnos basin, the data profile in 2013 proved evidence of water column ventilation down to 800 m and replenishment of the deep basin by less saline water mass. The 137Cs information provided evidence for contribution of water from the Black Sea to the intermediate and deep layers of the South Aegean (Cretan) basin. The profiles of 2013 and 2014 were characterized by the return of 137Cs inventories to pre-Chernobyl levels.

Successive CTD profiles of an Argo float trapped within a deep (~1250 m) sub-basin of the North Aegean Sea during 2014–2015 enabled identification of deep-water ventilation episodes and inference of a bulk (budget-based) vertical eddy diffusivity during the period that followed these events. Independent effective eddy diffusivity profiles were computed under the assumption that the turbulent exchange of heat, salt and buoyancy through each isobath below 400 m will balance the rate of change of the corresponding variable's content of the water volume within the basin below that depth. The initially estimated thermal, saline and density diffusivity profiles were significantly different from each other. Assuming turbulence was the dominant mixing process between the intermediate and deep water masses led to identification of conductivity sensor drift rate of 4.6 × 10−6 S m−1 day−1. After correcting the sensor's drift, bulk eddy diffusivities K ρ , K S and K T were found to span (2–30) × 10−3 m2 s−1, in close agreement below 400 m depth. These estimates are at least one order of magnitude higher than eddy diffusivities based on a finescale internal-wave strain parameterization. Enhanced boundary mixing and double diffusion are examined as candidates to explain the high effective diffusivity values. A hypsometric correction appears to provide sufficient diapycnal transport along the boundaries to reproduce bulk buoyancy transports.

The lack of knowledge of the mesopelagic layer inhabitants, especially of those performing strong vertical migration, is an acknowledged challenge as its incomplete representation leads to the exclusion of an active carbon and nutrient pathway from the surface to the deeper layers and reversely. The vertical migration of mesopelagic inhabitants (macro-planktonic and micro-nektonic) was observed by acoustical means in the epi- and mesopelagic layer of the open oligotrophic Cretan Sea (Eastern Mediterranean) for almost 2.5 years at the site of an operational fixed-point observatory located at 1500 m depth. The observed organisms were categorized in four groups according to their migration patterns. The variability of the migration patterns was inspected in relation to the physical and biological environmental conditions of the study area. The stratification of the water column does not act as a barrier for the vertical motion of the strongest migrants, moving up to 400 m every day. Instead, changes of light intensity (lunar cycle, daylight duration, cloudiness) and the presence of prey and predators seem to explain the observed daily, monthly and seasonal variability. The continuous presence of these organisms, yet capable of vertical motion and despite the profound seasonal circulation variability at the site of the observatory, implies their presence in the broader study area. The fundamental implications of the above for biogeochemical processing in oligotrophic seas due to the intimate link of the C and nutrient cycles, are discussed.

International audience; High Frequency Radar (HFR) is a land-based remote sensing instrument offering a unique insight to coastal ocean variability, by providing synoptic, high frequency and high resolution data at the ocean atmosphere interface. HFRs have become invaluable tools in the field of operational oceanography for measuring surface currents, waves and winds, with direct applications in different sectors and an unprecedented potential for the integrated management of the coastal zone. In Europe, the number of HFR networks has been showing a significant growth over the past 10 years, with over 50 HFRs currently deployed and a number in the planning stage. There is also a growing literature concerning the use of this technology in research and operational oceanography. A big effort is made in Europe toward a coordinated development of coastal HFR technology and its products within the framework of different European and international initiatives. One recent initiative has been to make an up-to-date inventory of the existing HFR operational systems in Europe, describing the characteristics of the systems, their operational products and applications. This paper offers a comprehensive review on the present status of European HFR network, and discusses the next steps toward the integration of HFR platforms as operational components of the European Ocean Observing System, designed to align and integrate Europe’s ocean observing capacity for a truly integrated end-to-end observing system for the European coasts.

The identification and examination of ‘complete’ potential density overturns in CTD profiles, within the framework of SESAME project, are employed to assess vertical eddy diffusivities, mostly in the top 100 m of the water column, for a broad area covering the East Mediterranean, the Turkish Straits and the Black Sea. The implementation of this method shows that, mixing induced by mechanical turbulence is enhanced in frontal areas, in the proximity of straits and inside anticyclones; furthermore, that mechanical turbulence is insignificant, down to the scale of CTD resolution, within areas of double diffusive staircases, encountered in deep layers of the water column. Consequently, only laminar theories about double diffusion are applied for assessing diffusivities therein. Susceptibility to different types of double diffusion seems to be related to the interaction of different types of water masses.

137 Cs activity concentration in seawater is an efficient radio-tracing technique to separate and identify the origin of different water masses along with depth. This technique has been applied for the study of deep basins at the North Aegean Sea [1], a marine region which is continuously enriched with 137 Cs originated from the Black Sea, through the Dardanelles Straits. In this work, a second sampling campaign (from 2008 to 2013) was carried out in Lemnos’s deep basin and water quantities were collected from different depths. These quantities were chemically treated at HCMR to pre-concentrate 137 Cs by using the AMP precipitation method [2]. The produced samples were measured by means of γ - ray spectroscopy. The 137 Cs data profile in combination with other oceanographic data (salinity and temperature), provides significant information for the water masses origin with respect to depth. More specifically, at the surface layer, Black Sea water masses (characterized by enriched concentrations of 137 Cs) are observed, with 137 Cs concentrations about 4 Bq/m 3 . At the intermediate layers, Levantine water masses (characterized by low concentrations of 137 Cs) appear with 137 Cs activity reduced to 2 Bq/m 3 , while the deep layer dense water masses exhibit intermediate 137 Cs concentrations of 3 Bq/m 3 . The 137 Cs concentration results are compared with previous measurements performed in the same basin from 1999 to 2013, where the decline of 137 Cs concentrations in the surface layer is observed and similar concentrations about 3 Bq/m 3 (within uncertainties) in the deep layer are obtained.

Summarization: The aim of this work has been to examine the relationship of steep bathymetry in the coastal areas around the permanent Cal/Val facility of Gavdos, and their influence on the produced calibration values for the Jason-2 satellite altimeter. The paper describes how changes in seafloor topography (from 200 to 3500 m depth over a distance of 10 km) are reflected on the determined altimeter parameters using different reference surfaces for satellite calibration. Finally, it describes the relation between these parameter trends and the region’s local characteristics. Using 3.5 years of Jason-2 calibration data and careful data analysis, certain features and variations, with permanent signatures at some locations south of Gavdos, have been found in the altimeter bias values. Causes of these variations have been interpreted. Some of these features, for example, at 12 km south of the Cal/Val site seem to be related to the general oceanographic circulation, but others of short wavelength (in the order of 1 km) are because of the insufficient geoid model resolution. Along Pass No. 109, the concealed effect of bathymetry on the geoid has produced a slope of 3.1 cm over 14–21 km from Gavdos. Along the other Pass No.18 and in the range 12–21 km, a slope in the geoid model, caused again by the sharp changes in the topography of seafloor (from 300 to 3200 m depth over a distance of 10 km), have been discovered. Those geoid model imperfections of short wavelength do not change the previously reported bias values. Finally, boat campaigns that took place for verifying the previous features, were also used to determine an altimeter bias of B = +19.6 ± 4.5 cm, for cycle = 80, at 15:14:58 UTC, on 7-Sept-2010, along Pass No. 109. Calibration took place at a distance of about 19 km south of Gavdos, in the open sea while Jason-2 was flying over. Presented on: Advances in Space Research

Combined micropaleontological and geochemical analyses of the high-sedimentation gravity core M-4G provided new centennial-scale paleoceanographic data for sapropel S1 deposition in the NE Aegean Sea during the Holocene Climatic Optimum. Sapropel layer S1a (10.2–8.0 ka) was deposited in dysoxic to oxic bottom waters characterized by a high abundance of benthic foraminiferal species tolerating surface sediment and/or pore water oxygen depletion (e.g., Chilostomella mediterranensis, Globobulimina affinis), and the presence of Uvigerina mediterranea, which thrives in oxic mesotrophic-eutrophic environments. Preservation of organic matter (OM) is inferred based on high organic carbon as well as loliolide and isololiolide contents, while the biomarker record and the abundances of eutrophic planktonic foraminifera document enhanced productivity. High inputs of terrigenous OM are attributed to north Aegean borderland riverine inputs. Both alkenone-based sea surface temperatures (SSTs) and δO18G. bulloides records indicate cooling at 8.2 ka (S1a) and ~7.8 ka (S1 interruption). Sapropelic layer S1b (7.7–6.4 ka) is characterized by rather oxic conditions; abundances of foraminiferal species tolerant to oxygen depletion are very low compared with the U. mediterranea rise. Strongly fluctuating SSTs demonstrate repeated cooling and associated dense water formation, with a major event at 7.4 ka followed by cold spells at 7.0, 6.8, and 6.5 ka. The prominent rise of the carbon preference index within the S1b layer indicates the delivery of less degraded terrestrial OM. The increase of algal biomarkers, labile OM-feeding foraminifera and eutrophic planktonic species pinpoints an enhanced in situ marine productivity, promoted by more efficient vertical convection due to repeated cold events. The associated contributions of labile marine OM along with fresher terrestrial OM inputs after ~7.7 ka imply sources alternative/additional to the north Aegean riverine borderland sources for the influx of organic matter in the south Limnos Basin, plausibly related to the inflow of highly productive Marmara/Black Sea waters.

The North Aegean Sea is considered as one of the major dense water formation sites of the Eastern Mediterranean Sea, contributing to the renewal of the Mediterranean deep water masses. We investigate the variability of the deeper water mass characteristics during the period 2002–2008, employing the Hybrid Coordinate Ocean Model for the North Aegean region. Two significant dense water formation events were detected, and the role of the major lateral water inputs, namely waters of Levantine Sea and Black Sea origin, was investigated. Atmospheric conditions were identified as the main factor of dense water formation during 2003, when strong buoyancy loss due to the low winter temperatures produced new dense water masses in the surface and intermediate layers (

Intensive microstructure sampling over the southern slope of the Cycladic Plateau found very weak mixing in the pycnocline, centered on a thin minimum of diapycnal diffusivity with K ρ = 1.5 × 10 − 6 m2 s− 1. Below the pycnocline, K ρ increased exponentially in the bottom 200 m, reaching 1 × 10− 4 m2 s− 1 a few meters above the bottom. Near-bottom mixing was most intense where the bottom slope equaled the characteristic slope of the semi-diurnal internal tide. This suggests internal wave scattering and/or generation at the bottom, a conclusion supported by near-bottom dissipation rates increasing following rising winds and with intensifying internal waves. Several pinnacles on the slope were local mixing hotspots. Signatures included a vertical line of strong mixing in a pinnacle's wake, an hydraulic jump or lee wave over a downstream side of the summit, and a ‘beam’ sloping upward at the near-inertial characteristic slope. Because dissipation rate averages were dominated by strong turbulence, ϵ/νN2 > 100, the effect on K ρ of alternate mixing efficiencies proposed for this range of turbulent intensity is explored.

Summarization: This paper presents the improvements made on the calibration methodology conducted at the Gavdos calibration/validation facility along with the latest altimeter calibration results for Jason-1 and Jason-2 satellite missions. Calibration results are presented, for the first time, for both ascending and descending passes of Jason satellites. The altimeter bias for Jason-2 has been estimated to be +173 ± 4 mm for Pass No. 109 and +171 ± 5 mm for Pass No. 018 over cycles 1–79. In tandem mission, the difference between Jason-1 and Jason-2 has been determined to be 72mm (Pass No. 109) and 68 mm (pass No. 018) and over cycles 2–20. Presented on: Marine Geodesy

We targeted the warm, subsurface waters of the Eastern Mediterranean Sea (EMS) to investigate processes that are linked to the chemical composition and cycling of dissolved organic carbon (DOC) in seawater. The apparent respiration of semi-labile DOC accounted for 27 +/- 18% of oxygen consumption in EMS mesopelagic and bathypelagic waters; this value is higher than that observed in the bathypelagic open ocean, so the chemical signals that accompany remineralization of DOC may thus be more pronounced in this region. Ultrafiltered dissolved organic matter (UDOM) collected from four deep basins at depths ranging from 2 to 4350 m exhibited bulk chemical ((1)H-NMR) and molecular level (amino acid and monosaccharide) abundances, composition, and spatial distribution that were similar to previous reports, except for a sample collected in the deep waters of the N. Aegean Sea that had been isolated for over a decade. The amino acid component of UDOM was tightly correlated with apparent oxygen utilization and prokaryotic activity, indicating its relationship with remineralization processes that occur over a large range of timescales. Principal component analyses of relative mole percentages of monomers revealed that oxygen consumption and prokaryotic activity were correlated with variability in amino acid distributions but not well correlated with monosaccharide distributions. Taken together, this study elucidates key relationships between the chemical composition of DOM and heterotrophic metabolism. Published by Elsevier Ltd.

The short-time scale evolution of plankton carbon partitioning and downward flux in the modified Black Sea water (BSW) mass entering the northeast Aegean Sea was studied using a Lagrangian approach (6-10 April 2008). The free-drifting sediment trap positioned at the bottom of the BSW layer and the control drifter, followed the same path within the anticyclone that circulates the BSW in the area. Zooplankton biomass increased (from 159 to 292 mg C m -2 ), as did faecal pellet production (from 5 to 8 mg C m -2 day -1 ), whereas a generally decreasing trend was displayed by particulate organic carbon (POC) (from 2099 to 1440 mg C m -2 ), net primary production and biomass of plankton cells >5 μm (from 32 to 11 mg C m -2 day -1 and from 153 to 124 mg C m -2 , respectively). At the same time, the organic carbon flux increased (from 131 to 311 mg C m -2 day -1 ), due to the contribution of zooplankton detritus (from 30 to 165 mg C m -2 day -1 ). Normalized biomass-size spectra slopes suggest an elevated grazing pressure upon microplankton cells and a non-steady-state ecosystem. Moreover, both the overall shallow slope values and their high correlation to organic carbon flux indicate an increased efficiency of energy transfer to higher trophic levels.

A video-based method was used to acquire wave run-up information from 3 microtidal, sediment-starved ‘pocket’ beaches on the island of Lesbos (Greece, NE Mediterranean). The results showed that the investigated beaches exhibit particular characteristics with regard to the wave run-up, compared to the sandy oceanic beaches previously studied. Run-up heights appeared to be characterised by similar frequencies to those of the nearshore waves (studied through the acquisition of high frequency wave data in the nearshore zone). Low frequency motions are introduced via the swash, but their contribution to the total run-up energy appeared to be less significant than in previous studies. Previously-proposed parameterisations were mostly found not to describe adequately the present data, whereas predictions based on previously-proposed expressions were also not satisfactory, with the possible exception of the expression of Stockdon et al. [Stockdon, H.F., Holman, R.A., Howd, P.A., Sallenger, J.A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng., 53(7), 573–588], which has been obtained on the basis of extensive datasets from diverse beaches.

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m − 3 and 14,157 ind m − 3 ) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi , Centropages typicus , Paracalanus parvus . The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera , Oithona copepodites, Oncaea media , Ctenocalanus vanus , Farranula rostrata . During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.

During the pilot phase of the Mediterranean Forecasting System (MFSPP) (2000 2003), a prototype observing system (Mediterranean Moored Multi-sensor Array – M3A) was designed, developed and operated in the Cretan Sea for continuous oceanographic measurements in real time. The main problems encountered were associated with biofouling, underwater and aerial communication and with the design of the surface buoy. In the second phase of the MFS project named Mediterranean Forecasting System Towards Environmental Predictions (MFSTEP) (2003–2005), the aim was to solve those problems and to consolidate the M3A. For the minimisation of biofouling a pilot field study was performed and the techniques of bromine solution and copper shielding were evaluated. Additionally, the oligotrophy of the Aegean Sea dictated the need to ignore factory calibrations and to perform site-specific laboratory calibrations of the sensors. This procedure was proved necessary and produced calibration coefficients that gave results comparable to the measurements obtained with the laboratory analysis method. During the approximately five years of operation, there were 13 scheduled and 15 emergency visits, with a total duration of 65 days. The acquired experience through the maintenance program proved that the continuous observation of such a very important system with a relative low cost is feasible.

Lucio Bellomo et al., High-frequency (HF) coastal radars measure current velocity at the ocean surface with a 30-100 km range and 1-3 km resolution, every 0.25-1 h. HF radars are well suited to many applications, such as search and rescue (SaR), oil-spill mitigation and ecosystem management. Here we present a first organized core of 12 HF radars installed in five sites in four countries (Greece, Italy, France and Spain) within the European MED project, the Tracking Oil Spill and Coastal Awareness (TOSCA) network. Dedicated experiments tested radar capabilities to estimate transport driven by currents, which is the key feature for all the above applications. Experiments involved the deployment of drifters, i.e., floating buoys, acting as proxies for substances passively advected by currents. Using HF radars the search range is reduced by a factor of 1.6 to 5.3 after 24 h. The paper also underlines the importance of sharing common tools for HF radar data processing and the need to mitigate radio frequency interference. The effort can be regarded as an initial step toward the creation of a Mediterranean or European HF radar network, crucial for any European integrated ocean observing system (IOOS)., This work was supported by the Tracking Oil Spills and Coastal Awareness (TOSCA) Network project [grant number 2G-MED09-425], co-financed by the European Regional Development Fund in the framework of the MED programme

In the Aegean Sea and Eastern Mediterranean there are large discrepancies between in situ and satellite ocean colour derived chlorophyll concentrations. The quantity that is monitored by ocean colour satellites and that can be used in the estimation of chlorophyll concentration is the remote sensing reflectance, defined as the ratio of the water leaving spectral radiance to the downwelling spectral irradiance. It can be determined in the field, with either above or in-water radiance and irradiance measurements. The complex optical properties of the North-East Aegean Sea, including radiance and irradiance, were studied during the AegeanMarTech project. Chlorophyll concentration estimates were derived from simultaneous above and in-water radiometric measurements. These were validated against chlorophyll concentration field data and compared against concurrent MODIS data from which chlorophyll was derived using two simple empirical algorithms. It was found that the MedOC3 algorithm outperforms the operational OC3M-547 algorithm and produces the least bias when compared against HPLC derived in situ chlorophyll. It is concluded that the greatest uncertainty in the inversion arises due to CDOM absorption below the 488 nm band. The reflectance ratios indicated that there is always an excess of yellow matter present in the study area and the water type could not be characterized optically as ‘’typical open ocean” Case 1.

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Measurements of 234 Th/ 238 U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100mm) organic C and 234 Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m � 2 d � 1 in the Ligurian and Tyrrhenian Seas (2.370.5–14.973.0 mmol m � 2 d � 1 ) and 0.9 mmol m � 2 d � 1 in the Aegean Sea. These results are comparable to previous measurements of 234 Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/ 234 Th ratio suggest two possible controls. First, decreasing POC/ 234 Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/ 234 Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical 234 Th-method was evaluated by quantifying the extent to which the 234 Thbased estimate of POC flux, PPOC, deviates from the true flux, FPOC, defined as the p-ratio (p-ratio ¼ PPOC/FPOC ¼ STh/ SPOC, where S ¼ particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and 234 Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and 234 Th-carrying particles did not bias 234 Th-normalized POC fluxes by more than a factor of two.

The hydrography, circulation and distribution of particulate matter in Thermaikos Gulf (NW Aegean Sea) are described, for September 2001 and October 2001 and February 2002. The observations were obtained to assess the impact of anthropogenic and natural resuspension processes on the coastal ecosystem. The first two periods represent highly stratified conditions and mild weather conditions; in September trawling is prohibited, whilst the October cruise took place following the commencement of the activity. The sampling in February was undertaken to record the contribution of natural resuspension to the system, in a stormy period. The water circulation was mainly baroclinic during autumn and barotropic in winter; this was due to vertical homogenisation of the water column, during the latter period. The distribution of the bottom nepheloid layer (BNL) was very different in the winter than in the autumn. Microstructure measurements were undertaken, which allowed the vertical eddy diffusivity to be estimated, together with the near-bed shear stresses. Computation of the diffusive flux of particulate matter (PM), from the BNL into the intermediate water column in response to turbulent motions, reveals that the PM flux was much higher in October than in September; this is despite the fact that the friction velocities were similar. This observation could explain partly the observed higher PM concentrations (PMC) in the intermediate layer, in October. Likewise suggest that the higher PMC in October could be attributed to trawling. The limited shear stress and friction velocity values obtained during the September and October cruises were always low; these were possibly capable of resuspending only recently deposited, “fluffy” material. Thus, the observed nepheloid layers are likely to be maintained by advection from the shallow water regions and trawled areas.

This work constitutes an attempt to assess the relative importance of natural versus anthropogenic forcing for sediment resuspension on the shelf of the Thermaikos Gulf (NW Aegean) through a 2-year experiment in the framework of the E.U. project INTERPOL. Four periods of different hydrographic characteristics were identified, including two periods of stronger vertical homogenisation and two of stratified conditions. The former periods were characterized by stronger near-bed currents, while relatively weak internal wave motions dominated the periods of stratification. The near-bed currents showed strong coherence with the wind during the period of full homogenisation, whereas during stratification periods the wind provided indirect forcing evident mostly in the internal-wave bands. The site was too deep for the surface waves to cause any sediment resuspension. In conclusion, the observed near-bed currents at the site of interest did not appear to produce significant local resuspension of sediment; notably, the current-turbidity correlation suggested a shoreward transport of suspended material. The large-scale turbidity variability was rather related to the onset of the trawling period in mid-October 2001 and the increased riverine discharge in late summer/autumn 2002.

Evidences of temperature-controlled dense water cascading off the Gulf of Lion shelf, in the northwestern Mediterranean, and several Aegean shelves in the northeastern Mediterranean are reported. Together with the Adiatic shelf, already listed by Ivanov, Shapiro, Huthnance, Aleynik, & Golovin (2004), these zones represent the major coastal areas of the Mediterranean Sea where dense water is produced.