Thracian Sea Research Articles

Impacts of three severe storms on Aegean Sea oceanography (2018–2020)

AbstractThis study presents a comprehensive analysis of the impacts of severe storms on the oceanography of the Aegean Sea, with specific focus on storms occurring on 15 November 2018, 14 September 2019, and 7 January 2020. Notable changes in oceanographic variables are observed on the storm day and in the subsequent days, gradually returning to pre-storm conditions. During storms, Aegean Sea surface temperatures were 2–3 °C below average, with cooling particularly evident along the west coast and northern part. Salinity increases of 0.5–1 PSU are observed during storms, with the most significant effects noted on 7 January. Storms consistently led to a decrease in sea level in our study area, with the western coast and north Aegean Sea experiencing levels 6–10 cm lower than the monthly average. Surface currents along the west coast and within latitudes 36–40° are 0.3 to 0.4 m/s stronger than average during storms. Storm-induced changes in the Thracian Sea’s anticyclonic gyre on 5 January reveal a speed increase of 0.3–0.4 m/s. Notably, both temperature and salinity fluxes exhibit significant changes during storms, characterized by a shift from east-to-west transport, contrasting with the west-to-east pattern observed before the storms. This shift is mainly influenced by Ekman transport, which predominantly directs water westward and north-westward when the wind comes from the northeast to southwest. Storms contribute to heightened salinity, increased kinetic energy, and enhanced transport, accompanied by a decrease in temperature, with the most pronounced effects near the surface along vertical layers.

Marine Suitability Assessment for Offshore Wind Farms’ Deployment in Thrace, Greece

The exploitation of renewable energy resources is an effective option to respond to climate change challenges. Wind energy can be exploited more efficiently and effectively than any other renewable energy source. By switching from onshore wind energy projects to offshore, the positive aspects of onshore wind energy remain and, at the same time, no valuable onshore area is occupied, while their efficiency (e.g., capacity factor) is increased. Greece has a rich wind potential and the maritime region of Thrace is one of Greece’s maritime regions with the greatest potential for the development of offshore wind energy. The aim of the present paper is to identify the most appropriate sites for the deployment of offshore wind farms in the region of Thrace. The methodology includes (i) the delineation of the study area and the definition of the support structure of the wind turbine, (ii) the identification of seven (7) exclusion and fifteen (15) assessment criteria, (iii) the suitability analysis under five different zoning scenarios (equal weight, environmental, social, techno-economic, and researchers’ subjective), and (iv) the micro siting and qualitative assessment of the most suitable sites based on energy, environmental, social, and economic criteria. The methodology is based on the combined use of Geographical Information Systems (GISs), specifically ArcGIS Desktop version 10.8.1, wind assessment software tools (WaSPs), specifically WaSP version 12.8, and multi-criteria decision-making methods. The results of the paper illustrate that the optimal suitability area that is proposed for offshore wind farm deployment is located at the easternmost end of the Greek part of the Thracian Sea. The planning and the deployment of offshore wind farm projects should follow a holistic and environmentally driven approach to ensure the integrity of all habitats and species affected.

Looking at the Expansion of Three Demersal Lessepsian Fish Immigrants in the Greek Seas: What Can We Get from Spatial Distribution Modeling?

A big number of Red Sea species have entered the Mediterranean Sea since the opening of the Suez Canal. Some of them quickly establish local populations and increase their abundance, forming a potential threat for local biodiversity and fisheries. Here, we use habitat modeling tools to study the expansion of three alien, demersal fish species that entered the Mediterranean basin at different times: Pterois miles, Siganus luridus and Siganus rivulatus. Georeferenced occurrence data from the eastern Mediterranean over the past ten years were compiled using online sources, published scientific literature and questionnaires and were correlated with environmental and topographic variables. The maximum entropy modeling approach was applied to construct habitat suitability maps for the target species over all of the Greek Seas. Results emphasized the three species’ strong coastal nature and their association with the presence of Posidonia oceanica meadows. Probability maps evidenced that for all species there is a higher likelihood of presence along the southeast and central Aegean and Ionian Sea coasts and a lower likelihood throughout the North Aegean Sea. For Siganus spp., predictions in the Thracian Sea were highlighted as highly uncertain, as the environmental conditions in this area partly fall outside the range of values occurring in locations of their current presence.

The Maroneia cave in Aegean Thrace. Byzantine period transport amphoras and coarse ware

In western Aegean Thrace, on Mt. Ismaros, the Maroneia cave is located. Its southeastern part was in use from the Late Neolithic period onward, until the verge of the Middle and Late Byzantine periods. The cave lies at a short distance from the homonymous, prosperous, coastal, fortified city. Albeit its southern entrance is well-hidden, the plateau in front of it provides the opportunity for wide observation of the surrounding area. Artifacts deposited in the interior attest that the citizens of Maroneia were hosted here occasionally. The present paper presents the coarse ware, attempting to shed light on: a. the commercial and cultural connections of Maroneia and Aegean Thrace in general with Balkan and Mediterranean sites and b. the various uses of the cave through time; transport amphoras and pithoi reflect its use as a storage area for agricultural and dairy commodities, whereas table- and kitchenware reveals its use as a shelter during the tumultuous times of Aegean Thrace. Invasions of several tribes of the north, e.g., the Goths, Huns, Avars, Normans, Catalans, etc.; the rivalry between the Latin kings; the civil wars of the late Byzantine emperors; and seaward piracy on the coastal region of the Thracian Sea led the Maroneitans to abandon their city occasionally and seek refuge in the nearby cave.

Hydrography and deep chlorophyll maximum patterns of the Athos Basin and the Thracian Sea continental shelf (North Aegean Sea) combining glider and satellite observations

This work focuses on the North Aegean Sea deep chlorophyll maximum (DCM) dynamics, as revealed during the first glider mission over the continental shelf and slope of the Thracian Sea and along the Athos basin, and presents a conceptual model on the response of the DCM in relation to the prevailing stratification/mixing of the water column. Water temperature, conductivity, salinity, chlorophyll-a, CDOM, DO and particle backscatter data (bb700), were recorded during this underwater glider mission in summer 2019. Daily satellite data from Sentinel 3 complemented the glider collected data to validate the satellite-derived estimates of surface Chlorophyll-a levels. The high-resolution physical and chemical observations were analyzed emphasizing on thermal stratification, pycnocline strength, CDOM and dissolved oxygen vertical distribution and the presence of deep chlorophyll maximum (DCM). DCM statistical characteristics were derived by fitting a Gaussian model on the chlorophyll-a profiles. The mean DCM was observed at 76 ± 14.5 m depth, located below the euphotic layer, estimated at 64 ± 2.1 m, with the water column dynamics affecting directly its depth, thickness and peak prominence. The satellite-derived chlorophyll-a, integrated within the euphotic layer, was found in fair agreement to the integrated chlorophyll-a values from surface to optical depth recorded by the glider. Chl-a at the DCM were found up to 0.60 mg m−3, the vertically-integrated pigment content was estimated at 45 ± 14 mg m−2 and the values of Optical Community Index (OCI) illustrate the dominance of small-sized plankton. The increased variability in the particulate backscattering coefficient bb700 in Thracian Sea shelf and slope deep waters seems related to sediment resuspension. CDOM levels increased from 0.76 ppb at the sea surface to 1.11 ppb at 40 m depth, being positively correlated to Chl-a within the DCM. CDOM concentration increased steadily at intermediate and deeper waters up to 1.34 ± 0.07 ppb. DO profiles revealed the presence of an oxycline in the DCM layer, capable to oxygenate the intermediate and deeper waters (+3 × 10−3 ml/L/m). Bottom DO in Athos basin was measured at 5.0 ml L−1, indicating that the apparent oxygen utilization in Athos Basin is approximately 50 μmol L−1.

Sardine and anchovy as bioindicators of metal content in Greek coastal waters

Metal and element concentrations (Li, Na, Mg, P, Ca, V, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Mo, Pd, Cd, Cs, Ba, Hg, Tl, Pb, U) were monitored in the tissues of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) from six Greek coastal areas subjected to different natural and anthropogenic forcings in order to be assessed as bioindicator species of marine metal pollution. Sardine and anchovy provided a thorough view of the metal pollution load of each site in accordance with local pollution sources. The wide range of pressures applied in Elefsina Gulf and Thermaikos Gulf are depicted in the large number and the kind of elements reaching maximum concentrations among the sites (e.g. Pb, Cu, Hg, As in Elefsina Gulf, and Cd, Ni, P in Thermaikos Gulf) while in the rest of the sites (Amvrakikos Gulf, Strymonian Sea, Thracian Sea, Artemisium Straits), few elements were found at maximum levels. Statistically significant site-specific differences in metal content were detected. The differentiation in metal content of the fish tissues among the sites could be attributed to anthropogenic pressures, different background levels and environmental conditions. Sardine and anchovy can be evaluated as appropriate, reliable and useful bioindicator species of marine metal pollution.

Dynamics of Water, Salt, and Nutrients Exchange at the Inlets of Three Coastal Lagoons

The intertidal patterns at the inlet of three coastal lagoons (Agiasma, Porto Lagos, and Xirolimni) in Northern Greece were investigated by combining in situ samplings and computational efforts. These lagoons are Mediterranean, microtidal coastal systems, connected with the adjacent open sea (Thracian Sea) through their inlet canals and are highly affected by the lagoon–sea exchange processes. Limited freshwater enters their basins, mostly due to precipitation and agricultural drainage. An intense monitoring program of water flow and quality at the mouth of the three lagoons was carried out, aiming to quantify the transport mechanisms of water, salt, and nutrients across the inlet canal under different tidal/meteorological conditions. Ebb currents were recorded higher than flood currents, and the temporal variability of the longitudinal velocity was characterized by asymmetries. Residual currents were important to the water exchange, with the Eulerian water, salt, and nutrient fluxes being an order of magnitude larger than the Stokes drift. Eulerian transport and tidal pumping are considered as important mechanisms for salt and nutrients exchange through the inlets. The return flow factor varied from 1 to 17.5% of the water exiting the lagoons in ebb, while the residence time ranged from 0.7 days to 4.2 days.

Wind Climate and Wind Power Resource Assessment Based on Gridded Scatterometer Data: A Thracian Sea Case Study

The present analysis utilized the 6-hourly data of wind speed (zonal and meridional) for the period between 2011 and 2019, as retrieved from the Copernicus Marine Environmental Service (CMEMS), covering the Thracian Sea (the northern part of the Aegean Sea). Data were estimated from the global wind fields derived from the Advanced Scatterometer (ASCAT) L2b scatterometer on-board Meteorological Operational (METOP) satellites, and then processed towards the equivalent neutral-stability 10 m winds with a spatial resolution of 0.25° × 0.25°. The analysis involved: (a) descriptive statistics on wind speed and direction data; (b) frequency distributions of daily-mean wind speeds per wind direction sector; (c) total wind energy content assessment per wind speed increment and per sector; (d) total annual wind energy production (in MWh/yr); and (e) wind power density, probability density function, and Weibull wind speed distribution, together with the relevant dimensionless shape and scale parameters. Our results show that the Lemnos Plateau has the highest total wind energy content (4455 kWh/m2/yr). At the same time, the area to the SW of the Dardanelles exhibits the highest wind energy capacity factor (~37.44%), producing 7546 MWh/yr. This indicates that this zone could harvest wind energy through wind turbines, having an efficiency in energy production of 37%. Lower capacity factors of 24–28% were computed at the nearshore Thracian Sea zone, producing between 3000 and 5600 MWh/yr.

Assessing multiple sources of data to detect illegal fishing, trade and mislabelling of elasmobranchs in Greek markets

Elasmobranchs, extremely charismatic and threatened animals, still are an important economic source for fishers in many parts of the world, providing significant income through trade. Even though Greek seas host at least 67 elasmobranch species, our knowledge about their biology and ecology is to a large extent unknown. In the present study the integration of conventional (legislation, official data from fisheries landings and fish market value and import/export data) and unconventional (social media) sources of data, accompanied with the use of genetics, aim at outlining the elasmobranch fisheries and trade in Greece and identifying “weak spots” that sabotage their conservation. Results revealed that: (a) about 60% of the 68 specimens collected in fish markets were mislabelled, with that being very common for Prionace glauca and Mustelus spp., (b) Illegal fishing is a reality, c) Greece represents one of the top-three European Union southern countries in terms of elasmobranch market size, (d) Aegean Sea and especially its Northern part (Thermaikos Gulf and Thracian Sea) contributed to more than half of the M. mustelus Greek fisheries landings and (e) wholesale prices of elasmobranchs have remained stable during the last decade. Mislabelling and illegal trade of elasmobranchs are common ground in Greece. This context stems from incoherent and complex fisheries legislative framework due to institutional decoupling, discrepancies in the collection and analysis of fisheries-related data, thus substantially reducing the efficiency of the fisheries management in Greek seas.

Interactions between fisheries and cetaceans in the Thracian sea (Greece) and management proposals

AbstractGreek fisheries (purse seine, trawling and small‐scale coastal, by order of importance) mostly operate in the Thracian Sea, which despite its relatively small size relative to the Aegean Sea (≈4.4%) is the most productive region in Greek waters due to its elevated nutrient concentration. The Thracian Sea is also vital to several species of cetaceans, mainly bottlenose dolphin, Tursiops truncatus (Montagu), and common dolphin, Delphinus delphis L. The area has been proposed as a potential cetacean conservation hotspot. For this reason, the interactions between fishing activities and local cetacean populations have been investigated through reports of stranded dolphins, interviews with fishers, surveys and detailed dolphin stomach content analysis. Reports of stranded cetaceans and interviews with fishers revealed cases of cetacean entanglement in fishing nets, as well as frequent dolphin damage to fishing nets. Necropsy stomach analysis of stranded dolphins revealed a preference for fish and cephalopods of low commercial value, indicating a low trophic overlap between cetaceans and commercial fisheries. Recommendations for the management of fisheries–cetacean interactions in the Thracian Sea are provided.

Arsenic speciation in fish from Greek coastal areas

Arsenic speciation analysis was conducted on fish samples (sardine and anchovy) collected from six areas along the Greek coastline, i.e. Artemisium Straits, Thermaikos Gulf, Amvrakikos Gulf, Strymonian Gulf, Thracian Sea, and Elefsina Gulf. Total arsenic levels ranging from 11.8 to 62.6mg As/kg dry weight were determined. Arsenobetaine, a non-toxic form of arsenic, was found to be the main arsenic species, present at 8.6 to 58.8mg As/kg dry weight, accounting for 67–95% of the total arsenic. Also detected in all fish samples was dimethylarsinic acid, although at considerably lower concentrations, ranging from 0.072–0.956mg As/kg dry weight. Monomethylarsonic acid was detected at low levels in all anchovy samples, and only in sardines from one area. Finally, inorganic arsenic in the form of arsenate was detected only in fish at one area, indicating the possible effect of an environmental parameter on its presence at detectable amounts. Statistical analysis revealed the environmental variables, such as salinity, total organic carbon and nitrogen, ammonium, phosphate, total phosphorus, dissolved oxygen and pressure index, are potentially correlated to As species concentrations. Furthermore, based on factor analysis, the biological parameters, such as fish weight, lipids, protein and ash content, that are correlated to As species concentrations of fish were also identified. The interrelationship of arsenobetaine and dimethylarsinic acid concentrations within each fish species was evaluated.

Harbour porpoises in the Aegean Sea, Eastern Mediterranean: the species’ presence is confirmed

Results are presented from the first systematic visual and acoustic line-transect survey for harbour porpoises in the Thracian Sea, northern Aegean Sea. During the vessel survey, undertaken in summer 2013, porpoises were observed on nine occasions and detected acoustically 16 times, with a total of 21 distinct encounters recorded. Harbour porpoises were encountered in three discrete blocks: north of the Island of Thasos, Greece; south and west of the city of Alexandroupolis, Greece; and in Saros Bay, Turkey. Saros Bay exhibited the highest relative acoustic encounter rate of harbour porpoises, and porpoises were observed visually there on two occasions 14 days apart, in small groups, one of which included a mother-calf pair. A comprehensive review of stranding records is also presented. The three areas identified as harbour porpoise habitat in this study coincide with the highest number of recorded stranded animals. This paper is the first to report free-swimming harbour porpoises in the Aegean Sea since 1993, and the first time ever in Turkish Aegean waters. Now that the presence of harbour porpoises has been documented, international cooperation towards long term monitoring and management measures are urgently required in order to conserve this vulnerable population.

A strange occurrence of Plectorhinchus gaterinus (Actinopterygii: Perciformes: Haemulidae) in the Thracian Sea (eastern Mediterranean)

A juvenile specimen of Plectorhinchus gaterinus (Forsskål, 1775), a fish native to the western Indian Ocean, was found in the stomach of a squid caught in the shallow waters of the Thracian Sea (northern Aegean Sea). Apart from the occurrence of the species which is reported for the first time in the Mediterranean, this case is rather interesting but should be considered with some reservation for two reasons: the unusual detection of the species of 10 cm as a prey item of a medium sized cephalopod, and the place of its detection i.e., a marine region that is considered quite inhospitable to alien fish native to tropical environment.

Modeling the buoyancy-driven Black Sea Water outflow into the North Aegean Sea

Summary A three-dimensional numerical model was applied to simulate the Black Sea Water (BSW) outflux and spreading over the North Aegean Sea, and its impact on circulation and stratification–mixing dynamics. Model results were validated against satellite-derived sea surface temperature and in-situ temperature and salinity profiles. Further, the model results were post-processed in terms of the potential energy anomaly, ϕ , analyzing the factors contributing to its change. It occurs that BSW contributes significantly on the Thracian Sea water column stratification, but its signal reduces in the rest of the North Aegean Sea. The BSW buoyancy flux contributed to the change of ϕ in the Thracian Sea by 1.23 × 10 −3 W m −3 in the winter and 7.9 × 10 −4 W m −3 in the summer, significantly higher than the corresponding solar heat flux contribution (1.41 × 10 −5 W m −3 and 7.4 × 10 −5 W m −3 , respectively). Quantification of the ϕ -advective term crossing the north-western BSW branch (to the north of Lemnos Island), depicted a strong non-linear relation to the relative vorticity of Samothraki Anticyclone. Similar analysis for the south-western branch illustrated a relationship between the ϕ -advective term sign and the relative vorticity in the Sporades system. The ϕ -mixing term increases its significance under strong winds (>15 m s −1 ), tending to destroy surface meso-scale eddies.

Suspended particulate matter estimates using optical and acoustic sensors: application in Nestos River plume (Thracian Sea, North Aegean Sea)

The present study investigates the use of combined methods of optical and acoustic sensors, in collaboration with direct in situ measurements, for the calibration and validation of a model transforming acoustic backscatter intensity series into suspended particulate matter (SPM) concentration datasets. The model follows previously elaborated techniques, placing particular attention to the parameterization of the acoustic absorption index as a function of water physical properties. Results were obtained from the annual deployment (during 2007-2008) of an upward-facing acoustic Doppler current profiler (ADCP) (307 kHz), equipped with a Wave Array, and an optical backscatter sensor (OBS), at the bottom of Thassos Passage near Nestos River plume (Thracian Sea, Northern Greece). The OBS was calibrated through linear regression, using 2007 and 2012 field sampling data, exhibiting an error of 13-14 % due to chlorophyll presence. The ADCP signal was calibrated through simultaneous measurements of backscatter intensity and turbidity profiles. Harmonic analysis on the model-produced SPM concentrations explained the tidal influence on their variability, especially during the summer. Empirical orthogonal functions analysis revealed the impact of waves and wave-induced currents on SPM variability. Finally, Nestos River sediment load was found uncorrelated to the SPM change in Thassos Passage, due to the dispersal and sediment deposition near the river mouth.

Water Quality of Drainage Canals and Assessment of Nutrient Loads Using QUAL2Kw

The water quantity and quality in two of the four main drainage canals located in the plain of Chryssoupolis in Kavala, North Greece, are studied. The area is intensively used for agriculture. Irrigation water is supplied by Nestos River, using a network of irrigation canals. Drainage water and irrigation return flows are collected in the four main drainage canals, which outflow into the Thracian Sea, North Aegean. A monitoring program in the four canals was designed to monitor flow and main constituents of drainage water. Main pollutants included nitrate, phosphorus and suspended solids. Based on the collected data, the trophic state of the canals and its seasonal variation were assessed, and the limiting nutrient was identified. QUAL2Kw was also used to model flow and water quality in two of the four canals. The calibration and verification of QUAL2Kw was undertaken using the collected field data, in an effort to identify the pollution sources and propose future management actions for the improvement of drainage canal water quality. The results showed that QUAL2Kw is a useful tool to simulate water quality in rivers and canals, and to quantify the impacts of non-point source pollution from agricultural areas.

Daimones in the Thracian Sea: Mysteries, Iron, and Metaphors

Abstract The daimones associated with the mystery cult of Samothrace have been interpreted as the distant memory of prehistoric smiths or dismissed from investigation altogether because of their absence from the epigraphic record. Anthropological models of metaphor formation offer a more nuanced approach to understanding their functions in the cult. The daimones’ ethnicity suggests a response to cultural interactions and economic production in the region; their metallurgical skills provided a key metaphor articulating a range of concepts which defined the rites, including secrecy, regional identity, and cosmological speculation.

Gökçeada - a Turkish island on the Thracian Sea

Gokceada is the biggest of all Turkish islands situated on the Thracian Sea about 20 km west of the Gallipoli Peninsula. Archaeological discoveries have so far indicated that colonization on this island dates back to 3000 BC. Currently, about 8300 peo- ple live on Gokceada, including about 250 inhabitants of Greek origin. The diversity of the island's geological structure affects its rich relief and area colouring. Cliffs on the northern coast and beaches on the southern one, high mountains and deep canyons in the central part of the island attract fans of solitude, diving and nature. Monuments from the time of the Roman Empire are rare on the island. One of them is a huge rock with two tombs hewn out in it. It is located in a completely uninhabited and treeless area. There are seven old plate trees on the island registered as natural monuments. The youngest one is 180 years old and the oldest tree is almost 630 years old.

Incidental catches of Acipenseridae in the estuary of the River Evros, Greece

Sturgeons are primitive large-bodied anadromous fish with most of the species living in the sea and reproducing in rivers. Because of their complex life history and their intensive exploitation, sturgeons are considered as globally threatened species and close to extinction (Gessner, 2000). Presently, fishing by-catch, poaching, habitat degradation, and physical obstacles to migration are major threats to the survival of these species (Ludwig, 2008). The River Evros basin, including in its lower drainage the tributaries Arda, Tundja and Ergene, with a total length of 550 km and a total catchment area of 39 000 km is the second largest river system in the Balkans, after the Danube. The river and the sea around its estuary (Thracian Sea) were the last fishing areas for A. sturio in Greece. This fishery sustained a small-scale canning industry, which during the early 1960s gave a production of 90–120 kg of sturgeon caviar annually (Georgacas, 1978). The River Evros population of A. sturio showed signs of decline already in the 1960s with the catches decreasing dramatically in the 1970s. Since 1980 only one catch of a mature female is reported by the Local Fishing Cooperative of Alexandroupolis in 1991. Consequently, A. sturio was considered as extinct from the river, despite the fact that this was never reported officially. During the recent years sporadic capturing of sturgeons has been recorded in the estuary of the River Evros (Koutrakis and Economidis, 2006). Herein, by employing morphological and molecular tools, we describe the taxonomic identification of three specimens captured in the years 2005 and 2006.

Meteorological influences on the surface hydrographic patterns of the North Aegean Sea

Hydrographic data from the North Aegean Sea were used to examine the summer variability of surface water masses during the period 1998–2001. Attention was placed on the surface hydrographic features of the area, such as the Black Sea Water (BSW) plume expansion, the frontal characteristics of the BSW with the Levantine Intermediate Water (LIW) and the variability of submesoscale hydrographic features (such as the Samothraki Anticyclone). Strong southerly wind stresses were found responsible for relaxing the horizontal density gradients across the BSW-LIW frontal zone and displacing this front to the north of Lemnos Island, thus suppressing the Samothraki Anticyclone towards the Thracian Sea continental shelf. Under northerly winds, the BSW-LIW front returns to its regular position (south of Lemnos Island), thus allowing the horizontal expansion of the Samothraki gyre up to the Athos Peninsula. Present results indicate the importance of medium-term wind stress effects on the generation of Samothraki Anticyclone suppression/expansion events.