NW Aegean Sea Research Articles

Microplastic Particles in Sandy Beaches of Thessaloniki Gulf, Greece

Microplastic pollution is a major issue for aquatic ecosystems. In this study, an assessment of microplastic pollution in beach sediments of Thessaloniki Gulf is presented. Sand sampling was performed in October 2021, in three different coastal areas along the seashore of Thessaloniki Gulf, in NW Aegean Sea. In total, nine sand samples were collected and examined. The morphology of the detected microplastics was recorded: they were fragments and microfibers. Their abundance ranged from 0-32 items/m3. Higher concentrations of microplastics were observed in Karabournaki Beach, located in the city of Thessaloniki. It is the second largest city in Greece, inhabited by about 1,5 million people. Regarding the color, black, red, blue, green, and transparent microplastic fragments were found. Black and blue particles were the most common. Chemical detection and identification were performed with FT-IR spectroscopy. Four polymers were detected: polyethylene (PE), terephthalate (PET), high-density polyethylene (HDPE), polypropylene (PP), and polytetrafluoroethylene (PTFE). Polyethylene terephthalate was the dominant identified polymer (50%). The need for plastic pollution management in coastal areas, as well as the need to communicate the issue of plastic marine pollution and enhance public awareness, are discussed.

Stretched Thin: Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece

AbstractWe present field‐based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the X and Y directions under ductile and brittle‐ductile conditions, indicative of a component of oblate coaxial strain during overall top‐to‐E general shear. New white mica 40Ar/39Ar geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 ± 15°C and 7 ± 1 kbar. Zircon (U‐Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low‐temperature thermochronology for the HP‐LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn‐orogenic processes persisting until at least the early Miocene, facilitated in part by normal‐sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region.

MPs and NPs intake and heavy metals accumulation in tissues of Palinurus elephas (J.C. Fabricius, 1787), from NW Aegean sea, Greece

European marine waters are infested with plastic, with an average density of 1 plastic item in every four square-meters. Research relevant to MPs-NPs ingestion by wild decapods in European waters is limited, none of which regards the European spiny lobster. Totally 4102 plastic particles were extracted from the spiny lobster stomach tissues of both sexes. Out of the 63 samples analysed only three (4.8%) of them were found with no plastic particles. The range of number of MPs in stomachs was from 20 to 273 MPs individual−1. The 98.3% were fragments. In total 3833 plastic particles were extracted from the gill tissues of both sexes. MPs were found in all samples (n = 50),99.2% of the detected particles were fragments. The MPs detected in gills ranged from 11 to 339 MPs individual−1. The DLS method was used in order to evaluate the NPs presence. Nanoplastics were detected in 22.6% of stomachs and in the 48.1% of gills. A total of 43 polymer types were identified in both tissues. Also, our study assessed the accumulation of heavy metals at the edible tail muscle. Certain elements were detected above the EU's Maximum Residue Level, including arsenic. The present results are alarming and the potential human health implications could be serious.

Impact of a Dinophysis acuminata Bloom on the Copepod Acartia clausi: First Indications

Faecal pellet production and content along with egg production of the dominant copepod species Acartia clausi were studied in the Thermaikos Gulf (NW Aegean Sea) during a pre-bloom and a bloom of the toxic dinoflagellate Dinophysis acuminata. Both faecal pellet production (6.8–8.6 ind−1 d−1) and egg production (15.8–47.6 ind−1 d−1) appeared unrelated to the D. acuminata bloom. Less than 11% of the copepod faecal pellets contained one or two D. acuminata cells, almost intact, whereas the other material in the pellets was broken into small pieces or amorphous shapes. The toxin outflux seemed to be insignificant when compared to the mean toxin concentration from the whole D. acuminata population. Finally, the potential grazing impact of A. clausi on D. acuminata during the study period was low.

Seasonal living coccolithophore distribution in the enclosed coastal environments of the Thessaloniki Bay (Thermaikos Gulf, NW Aegean Sea)

Living coccolithophores collected from a yearly time interval, January to December 2016, at the SP1 station of the Thessaloniki Bay, an enclosed coastal environment located in the inner part of the Thermaikos Gulf (Northwestern Aegean Sea) were analyzed to determine their seasonal variation and evaluate the effects of environmental factors on their density, species composition and distribution. Our results showed that coccolithophores constituted a significant component of the phytoplankton community and were characterized by the dominance of few opportunistic species, as an impact of eutrophic environmental conditions. Temperature seasonal cycle and the variations in nutrient inputs from riverine and/or anthropogenic sources seem to control the seasonal pattern of coccolithophores. Emiliania huxleyi with densities as high as ∼4.0 × 105 cells l−1 predominated during the late spring–early summer, whereas a strong dominance of Gephyrocapsa oceanica with densities ∼2.0 × 105 cells l−1 was observed during the late summer and the autumn. Emiliania huxleyi appeared to dominate the coccolithophore assemblages once more during the winter; while Helicosphaera carteri was continuously present during the wintertime (exceeding ∼2.7 × 103 cells l−1). Emiliania huxleyi was entirely represented by morphotype type A in the studied samples. Morphometric analysis of E. huxleyi coccoliths indicated that the population is dominated by lightly calcified forms, although an increase of more heavily calcified coccoliths was observed during late autumn-early spring in line with the well-documented seasonal succession pattern of E. huxleyi type A forms in the Aegean Sea.

Morphometric Relationships of the Global Invader Callinectes sapidus Rathbun, 1896 (Decapoda, Brachyura, Portunidae) from Papapouli Lagoon, NW Aegean Sea, Greece. with Notes on Its Ecological Preferences

Callinectes sapidus is native to the Atlantic coasts of the Americas. In the Mediterranean, it appeared around 1949 and though that it is established in East Mediterranean waters, relevant studies are limited. The aim of the present study is to report quantitative and qualitative data on the blue crab’s biology and ecology in its non-native range, that are indispensable for management purposes. Papapouli Lagoon is in Thermaikos Gulf and is ecologically impacted by the blue crabs. Fyke nets with a 20 mm mesh opening were soaked for 12 to 14 h during each survey. Abiotic environmental parameters were obtained. The length–weight relationships were expressed by the equation W = aLb. The species’ ecological preferences at Papapouli Lagoon were assessed with PERMANOVA analysis, using the abiotic parameters as factors. Student’s t-tests were used to assess the differences between sexes. The sex ratio of the blue crab’s population was assessed by a Chi-square (χ2) analysis. The sex ratio was found to be 1.28:1, in favour of males. The most dominant group size of male blue crabs was the 61–70 mm of CL and 130–139 mm of CW. Also, the dominant size group of female blue crabs was 60–69 mm of CL and 120–129 mm CW. The maximum abundance of blue crabs was recorded at a salinity range from 24‰ to 25‰ and the water temperature range was from 26 to 28 °C.

Downwelling Events in a Coastal Mussel Farming Area, NW Thessaloniki’s Gulf (NW Aegean Sea)

The coastal area of Chalastra (NW Thessaloniki’s Gulf, Northern Greece) features prominent mussel farming activity as well as fishing activity. These activities are closely related to the hydrodynamics of the basin’s seawaters. The prevailing hydrodynamic force for the circulation of coastal waters of Chalastra is wind stresses. The hydrodynamics of this coastal region may be further influenced by three rivers outflowing to the southwestern part of the basin; however, this runoff has dramatically decreased during the last few decades. Concerning the wind forcing regime, this is distinguished by two characteristic regimes. The first one is realized in winter and the other mainly in summer. Systematic field studies were conducted during one year for current and density measurements. Seawater samples were collected and chemically analyzed for dissolved oxygen, chlorophyll α and nutrients. Data revealed some cases of flow currents of opposite direction between the upper and the lower water layer, and a characteristic captured downwelling which seemed to evolve periodically on a diurnal time scale, especially in summer. Furthermore, a two layer mathematical model was developed and applied for the study of the water circulation and the investigation of downwelling processes under specific conditions; the tracing as well as the temporal and spatial evolution of these hydrodynamic processes in the mussels’ culture study area constituted the main aim of the present work. The formation of such events, which were observed mainly during summer periods, may affect the mussels’ development and has to be considered when designing farming practices in the area.

Vertical distribution of 137Cs and natural radioactivity in core sediments of Thermaikos Gulf, northwestern Aegean Sea

Sub-superficial sediments have been collected from the Thermaikos Gulf, located in the NW Aegean Sea, in order to determine sedimentation rates using vertical distributions of 137Cs and 210Pb. In addition measurements of the characteristic radionuclides of the 238U and 232Th natural series has also been performed to better understand the transport and the sedimentation process in a marine environment, which is strongly influenced by the main North Greece rivers. The sedimentation rates of the five selected cores using the two methods were consistent and varied from 0.20 ± 0.01 till 0.30 ± 0.02 cm y-1. The mean sedimentation rates derived with both methods are enhanced during the last ten years compared to previous values at the studied region by one order of magnitude, due to human implications. The activity concentration of the 238U decay products does not exhibit variation along the depth. The activities of 226Ra are enhanced (~30 %) compared to the activities of radon daughters (214Pb, 214Bi) especially at the surface layer. The results of the correlation between 232Th decay products and 40K indicate that there is positive correlation between their averaged activities. The good proportionality between 232Th and 40K concentrations may be attributed to the fact that thorium is a particularly insoluble element in water.

<i>Callinectes sapidus</i> Rathbun, 1896 (Brachyura: Portunidae): An assessment on its diet and foraging behaviour, Thermaikos Gulf, NW Aegean Sea, Greece: Evidence for ecological and economic impacts

<i>Callinectes sapidus</i> Rathbun, 1896 (Brachyura: Portunidae): An assessment on its diet and foraging behaviour, Thermaikos Gulf, NW Aegean Sea, Greece: Evidence for ecological and economic impacts

Succession in megafaunal assemblages of an artificial reef during a three-year monitoring in the Eastern Mediterranean basin

Succession in megafaunal assemblages of an artificial reef during a three-year monitoring in the Eastern Mediterranean basin

Machine learning-based retrieval of benthic reflectance and Posidonia oceanica seagrass extent using a semi-analytical inversion of Sentinel-2 satellite data

ABSTRACTIn the epoch of the human-induced climate change, seagrasses can mitigate the resulting negative impacts due to their carbon sequestration ability. The endemic and dominant in the Mediterranean Posidonia oceanica seagrass contains the largest stocks of organic carbon among all seagrass species, yet it undergoes a significant regression in its extent. Therefore, suitable quantitative assessment of its extent and optically shallow environment are required to allow good conservation and management practices. Here, we parameterise a semi-analytical inversion model which employs above-surface remote sensing reflectance of Sentinel-2A to derive water column and bottom properties in the Thermaikos Gulf, NW Aegean Sea, Greece (eastern Mediterranean). In the model, the diffuse attenuation coefficients are expressed as functions of absorption and backscattering coefficients. We apply a comprehensive pre-processing workflow which includes atmospheric correction using C2RCC (Case 2 Regional CoastColour) neural network, resampling of the lower spatial resolution Sentinel-2A bands to 10m/pixel, as well as empirical derivation of water bathymetry and machine learning-based classification of the resulting bottom properties using the Support Vector Machines. SVM-based classification of benthic reflectance reveals ~300 ha of P. oceanica seagrass between 2 and 16 m of depth, and yields very high producer and user accuracies of 95.3% and 99.5%, respectively. Sources of errors and uncertainties are discussed. All in all, recent advances in Earth Observation in terms of optical satellite technology, cloud computing and machine learning algorithms have created the perfect storm which could aid high spatio-temporal, large-scale seagrass habitat mapping and monitoring, allowing for its integration to the Analysis Ready Data era and ultimately enabling more efficient management and conservation in the epoch of climate change.

Investigation of the Magnetic Properties and Trace Elements in Sediments from Thermaikos Gulf, NW Aegean Sea

Investigation of the Magnetic Properties and Trace Elements in Sediments from Thermaikos Gulf, NW Aegean Sea

Interannual Change Detection of Mediterranean Seagrasses Using RapidEye Image Time Series.

Recent research studies have highlighted the decrease in the coverage of Mediterranean seagrasses due to mainly anthropogenic activities. The lack of data on the distribution of these significant aquatic plants complicates the quantification of their decreasing tendency. While Mediterranean seagrasses are declining, satellite remote sensing technology is growing at an unprecedented pace, resulting in a wealth of spaceborne image time series. Here, we exploit recent advances in high spatial resolution sensors and machine learning to study Mediterranean seagrasses. We process a multispectral RapidEye time series between 2011 and 2016 to detect interannual seagrass dynamics in 888 submerged hectares of the Thermaikos Gulf, NW Aegean Sea, Greece (eastern Mediterranean Sea). We assess the extent change of two Mediterranean seagrass species, the dominant Posidonia oceanica and Cymodocea nodosa, following atmospheric and analytical water column correction, as well as machine learning classification, using Random Forests, of the RapidEye time series. Prior corrections are necessary to untangle the initially weak signal of the submerged seagrass habitats from satellite imagery. The central results of this study show that P. oceanica seagrass area has declined by 4.1%, with a trend of −11.2 ha/yr, while C. nodosa seagrass area has increased by 17.7% with a trend of +18 ha/yr throughout the 5-year study period. Trends of change in spatial distribution of seagrasses in the Thermaikos Gulf site are in line with reported trends in the Mediterranean. Our presented methodology could be a time- and cost-effective method toward the quantitative ecological assessment of seagrass dynamics elsewhere in the future. From small meadows to whole coastlines, knowledge of aquatic plant dynamics could resolve decline or growth trends and accurately highlight key units for future restoration, management, and conservation.

CUBESAT-DERIVED DETECTION OF SEAGRASSES USING PLANET IMAGERY FOLLOWING UNMIXING-BASED DENOISING: IS SMALL THE NEXT BIG?

Abstract. Seagrasses are one of the most productive and widespread yet threatened coastal ecosystems on Earth. Despite their importance, they are declining due to various threats, which are mainly anthropogenic. Lack of data on their distribution hinders any effort to rectify this decline through effective detection, mapping and monitoring. Remote sensing can mitigate this data gap by allowing retrospective quantitative assessment of seagrass beds over large and remote areas. In this paper, we evaluate the quantitative application of Planet high resolution imagery for the detection of seagrasses in the Thermaikos Gulf, NW Aegean Sea, Greece. The low Signal-to-noise Ratio (SNR), which characterizes spectral bands at shorter wavelengths, prompts the application of the Unmixing-based denoising (UBD) as a pre-processing step for seagrass detection. A total of 15 spectral-temporal patterns is extracted from a Planet image time series to restore the corrupted blue and green band in the processed Planet image. Subsequently, we implement Lyzenga’s empirical water column correction and Support Vector Machines (SVM) to evaluate quantitative benefits of denoising. Denoising aids detection of Posidonia oceanica seagrass species by increasing its producer and user accuracy by 31.7 % and 10.4 %, correspondingly, with a respective increase in its Kappa value from 0.3 to 0.48. In the near future, our objective is to improve accuracies in seagrass detection by applying more sophisticated, analytical water column correction algorithms to Planet imagery, developing time- and cost-effective monitoring of seagrass distribution that will enable in turn the effective management and conservation of these highly valuable and productive ecosystems.

Satellite-based quantitative assessment of seagrass habitats in Thermaikos Gulf, NW Aegean Sea, Greece

Satellite-based quantitative assessment of seagrass habitats in Thermaikos Gulf, NW Aegean Sea, Greece

New gastropod records for the Eastern Mediterranean Sea and one new alien (Emarginula decorata Deshayes, 1863) for the Mediterranean Sea from NW Aegean Sea, Greece.

BackgroundThe NW Aegean Sea has a complex topography, high quality waters, oligotrophic to eutrophic conditions, is connected with estuaries and wetlands, is of high ecological interest, harbours all the types of human activities and yet few researchers work on its marine biodiversity. With this study, the contribution to the knowledge of the Hellenic and Eastern Mediterranean gastropod biodiversity of the studied families is continued, and an expansion of the search in other substrates and deeper waters of the NW Aegean Sea with emphasis on the minor in size species during the period from October 2008 to January 2014.ResultsThirty seven species belonging to seven families (Cerithiopsidae, Fissurellidae, Phasianellidae, Scissurellidae, Siliquariidae, Skeneidae, and Triphoridae) were identified and their biodiversity was compared with the current checklists of marine gastropod molluscs for the Hellenic Seas based on previous surveys. In this collection of gastropods, one species (Emarginula decorata Deshayes, 1863) is a new alien for the Mediterranean Sea, 14 species are new for the Eastern Mediterranean Sea and 16 species are new for the Hellenic fauna (with the one above mentioned alien species included). The main identification characteristics and ecological information such as habitat, distribution, alien expansion paths to the NW Aegean Sea and origin of the species are given and discussed.ConclusionsThe Hellenic gastropod biodiversity of the studied families was enriched with 37 new records for the N Aegean Sea, out of which 16 are new for Greece, 14 are new for the Eastern Mediterranean Sea while one (Emarginula decorata) is a new alien for the Mediterranean Sea.

Interannual variability of the physical characteristics of North Thermaikos Gulf (NW Aegean Sea)

Thermaikos Gulf is a marine ecosystem of major importance, not only environmental, but also due to the various socioeconomic activities associated with the area. The physical characteristics of the gulf's waters were studied, analyzing in situ measurements of oceanographic parameters, collected during 5 oceanographic surveys from 1994 to 2007, on the same grid of 26 sampling stations. Aim of this paper is the detection and description of the main changes (seasonal and interannual) in the water masses' characteristics that are related to the seawater quality of the North Thermaikos. The connection between the main forcing factors and the major circulation patterns is also under investigation. The interannual analysis of the collected data revealed the existence of strong seasonal fluctuations that present significant deviations from a mean seasonal pattern in specific periods. A general decreasing trend of the salinities of the domain is observed during the study period. At the same time, a strong relation between open Aegean Sea waters and riverine freshwater fluxes is identified, factors that significantly influence stratification, circulation and renewal of the gulf. Based on the thermohaline properties, two dense water formation events (February 2000 and 2005), not previously reported, are detected and analyzed for the first time.

Fine sediment transport model for river influenced microtidal shelf seas with application to the Thermaikos Gulf (NW Aegean Sea)

Modelling the transport of fine sediments in the marine environment is a highly complex task with a broad field of application. Various processes take place and interact with one another to determine the movement of fine sedimentary particles in the aquatic domain. Amongst these processes stratification significantly affects the vertical propagation of a sedimentary plume, especially in areas of freshwater influence, with the possibility of matter entrapment and accumulation along the interface; this possibility is higher the finer the sedimentary matter is. The proposed modelling approach describes the physical processes that take place in the water column (advection, dispersion, coagulation, settling, effects of stratification), at the boundary of the seabed (benthic shear) and the mass-exchanges between the two (deposition, consolidation, resuspension, erosion). The Fine Sediment Transport Model was applied in the gulf of Thermaikos (NW Aegean Sea), an area of significant freshwater and riverine sediment inflow, considering the major rivers of the domain as sources of particulate matter. The results were, generally, in agreement with available in-situ measurements of suspended particulate matter. In cases of significant deviations between simulation and measurement, the source of the error did not lie within the modelling approach, but was due to the considered input parameters. The investigation revealed the existence of transport and sedimentation patterns related to the riverine source of the material and the individual parts of the gulf. The sedimentation rates were estimated at 18, 7 and 3mm/yr locally at the outflows of Aliakmon, Axios and Pinios, respectively, while the areas of high sediment accumulation coincided with locations where the benthic material ranges from very fine to fine silt.

Determination of 137Cs activities in surface sediments and derived sediment accumulation rates in Thessaloniki Gulf, Greece

Surficial and core samples collected from the eastern Thessaloniki Gulf, located in the NW Aegean Sea, were analyzed for their sedimentation rate and inventories of 137Cs and 210Pb. The study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels. The sedimentation rates were calculated from the vertical distribution of 137Cs and excess 210Pb in the sediment cores. The spatial distribution of 137Cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content. The activity concentrations were measured by means of gamma ray spectrometry using HPGe detector for 137Cs and gross alpha counting using alpha counter for 210Pb. The average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0.18 ± 0.02 to 0.22 ± 0.03 cm year−1. The 137Cs inventories varied from 205 ± 15 to 602 ± 39 Bq m−2, while the 210Pb inventories of the studied cores exhibited average value of 140 ± 9 Bq m−2. Elevated 137Cs activities were observed compared to certain Mediterranean marine areas; however, they were lower than the reported values in the Black Sea.

Simulating the fate of mechanically eroded masses in the Thermaikos Gulf

Benthic trawlers, through the scraping of the upper layers of the seabed sediment, are responsible for major alterations to the distribution of suspended matter in coastal areas. A major trawling site in Greece is that of the Gulf of Thermaikos, located in the NW Aegean Sea (Eastern Mediterranean). In an attempt to assess, for the first time, the quantities of the mobilized sedimentary masses and the properties of their motion in the aquatic domain of the Gulf of Thermaikos, a mathematical model has been formulated. The modeling approach includes the determination of the mechanical erosion rate, the production of turbulence due to the flow through the mesh of the net and the physical processes that control the propagation of the generated sedimentary plume in the marine environment. In the absence of information regarding the haul paths in the gulf, their determination was made stochastically, based on the existing legislative restrictions and trawlers' fishing practices. The trawling-activated masses thereby defined for the duration of the trawling period in the gulf were very high, indeed several times greater than the related contribution from the major rivers of the area, emphasizing the significance of the activity to the sedimentary input of the local marine environment. The total annual erosion by trawls in the gulf was calculated at 3.38×10 6 t/yr. Τhe mean mechanical erosion rate per trawled surface (parameter free from the bias raised by the stochastic simulation of the trawl trajectories) was estimated at 430 g/m 2, values directly comparable to ones defined for similar coastal areas. Results from a simulation of the 2001–02 trawling period were well correlated with in-situ measurements for the same period, while the investigation of the matter's suspension level and times revealed that the matter exhibits low residence times in the water column and at the same time moves in the vicinity of the bed, contributing to the formation and distribution of benthic nepheloid layers in the gulf.