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Seagrasses are flowering plants, adapted to marine environments, that are highly diverse in the Mediterranean Sea and provide a variety of ecosystem services. It is commonly recognized that light availability sets the lower limit of seagrass bathymetric distribution, while the upper limit depends on the level of bottom disturbance by currents and waves. In this work, detailed distribution of seagrass, obtained through geoacoustic habitat mapping and optical ground truthing, is correlated to wave energy and light on the seafloor of the Marine Protected Area of Laganas Bay, Zakynthos Island, Greece, where the seagrasses Posidonia oceanica and Cymodocea nodosa form extensive meadows. Mean wave energy on the seafloor was estimated through wave propagation modeling, while the photosynthetically active radiation through open-access satellite-derived light parameters, reduced to the seafloor using the detailed acquired bathymetry. A significant correlation of seagrass distribution with wave energy and light was made clear, allowing for performing fine-scale predictive seagrass mapping using a random forest classifier. The predicted distributions exhibited >80% overall accuracy for P. oceanica and >90% for C. nodosa, indicating that fine-scale seagrass predictive mapping in the Mediterranean can be performed robustly through bottom wave energy and light, especially when detailed bathymetric data exist to allow for accurate estimations. [ABSTRACT FROM AUTHOR]

COVID-19 pandemic has led to an increase in certain types of litter, many of which are expected to end up in the marine environment. The present study aimed to monitor the pandemic-related litter pollution along the Greek coastal environment. Overall, 59 beach and 83 underwater clean-ups were conducted. Litter was categorized as: PPE (face masks and gloves), COVID-19-related, single-use plastic (SUP) and takeaway items. PPE, dominated by face masks (86.21 %), accounted for 0.29 % of all litter. The average PPE density was 3.1 × 10−3 items m−2 and 2.59 items/ 100 m. COVID-19-related items represented 1.04 % of the total. Wet wipes showed higher densities (0.67 % of all litter) than in the pre-COVID era, while no increase in SUP and takeaway items was observed. Benthic PPE, dominated by gloves (83.95 %), represented 0.26 % of the total. The mean PPE density was 2.5 × 10−3 items m−2. • Pandemic-related litter pollution was monitored in Greek coastal environments. • PPE, dominated by face masks, accounted for 0.29 % of the total. • COVID-19-related litter accounted for 1.04 % of the total. • PPE and wet wipes are indicators of pandemic-driven plastic pollution. • Benthic PPE, dominated by gloves (83.95 %), represented 0.26 % of the total. [ABSTRACT FROM AUTHOR]

Helike, the Achaean Dodecapolis capital, in the Corinth Gulf, Greece, was, according to historical sources, devastated and disappeared from sight during an earthquake followed by sea invasion on to land in 373/372 B.C. A marine remote sensing survey, which was carried out to examine a landslide following a 6.2 R earthquake in 1995, that affected the coastal and near-shore delta plain zone on which Helike stood, accompanied by onshore borehole data, enabled us to postulate the geological processes leading to the Helike catastrophe. Helike was initially leveled during a 6 to 6.7 R earthquake; it is postulated to have then submerged following a translational landslide caused by liquefaction. This Helike catastrophe model is consistent with historical sources and current views regarding the expected geological hazards magnitude in the Gulf. [ABSTRACT FROM AUTHOR]

Monitoring of marine litter at the sea surface, the beaches and the seafloor is essential to understanding their sources, pathways and sinks and design effective clean-up programs or increase public awareness for reducing litter waste. Up until today, seafloor litter is the least exploited component of marine litter. Although the protocols for recording and assessing seafloor litter in the deep-sea environments are currently being actively defined and practiced, shallow seafloor litter survey protocols are still notably under-developed. Moreover, trawling for fishing, which is the main means for collecting seafloor litter data, needs to be phased out in the coming years due to its high environmental footprint and be replaced by less destructive ways based on underwater imagery. In this paper we propose an integrated approach for assessing in detail the spatiotemporal distribution and composition of seafloor litter in shallow coastal environments, using common towed underwater cameras. Effort has been put to correctly estimating spatial litter densities regarding the true coverage of the visualized area, which was efficiently extracted through photogrammetric reconstruction of the seafloor. Interpretation of the spatial distribution of litter was aided by auxiliary bathymetric and swath sonar backscatter datasets, to determine the seabed geomorphological features that control their dispersion and composition. Local geo-morphology, along with any reported coastal anthropogenic activity, are correlated to seafloor litter densities to investigate the temporal and spatial dynamics that control their distribution and temporal trends in Syros Island, Cyclades, Greece. There, in the context of LIFE DEBAG project, monitoring of an urbanized shallow bay for 3 consecutive years has been performed to assess the impact of an intensive local awareness raising campaign to the local environment. A significant reduction of litter densities under the impact of this campaign has been documented, while links between the seafloor litter transport dynamics and the seabed micro- and macro-topography were made evident. Monitoring litter densities on the seafloor of urbanized shallow bays proved to be a prospective way of tracking marine litter pressures on the local marine environment. • A shallow benthic litter monitoring protocol, based on Towed Underwater Camera (TUC), is proposed and validated. • The spatiotemporal benthic litter variations are linked to natural and anthropogenic drivers. • Seabed morphology and seafloor hydrodynamics are shown to control litter accumulation. • Significance of monitoring seafloor litter in urbanized bays: Narrowing sources and dynamics to comprehensible scales. • First Proof of an awareness raising campaign ceasing marine litter pollution. [ABSTRACT FROM AUTHOR]

The documentation of underwater cultural heritage (UCH) is the basis for sustainable maritime development including its protection, preservation, and incorporation in coastal zone management plans. In this study, we present a multidisciplinary, non-intrusive downscale approach for the documentation of UCH implemented on the coastal area of Aegina Island, Greece, where a unique submerged harbour complex is preserved. This approach succeeded in obtaining information that serves both geomorphological and archaeological purposes in a time- and cost-effective way, while obtaining information of centimeters to millimeters scale. The geomorphology of the area was mapped in detail through marine geophysical means while ancient submerged conical rubble structures and breakwaters were documented using automatic seafloor segmentation techniques, revealing previously unknown sites of archaeological interest. The structures' parameters were extracted from the acoustic data to analyze their morphometry, while photogrammetry was realized using a Remotely Operated Vehicle to expose their micro-structure. The spatial distribution of the structures revealed the construction of a well-planned harbour complex with multiple passages and different possible functionalities. Finally, through the structures' morphometric analysis (geometry and terrain statistical parameters) their preservation status was revealed, demonstrating the anthropogenic impact on the submerged ancient structures due to the modern harbor activity. [ABSTRACT FROM AUTHOR]

A series of marine remote sensing and ground-truth surveys were carried out at NW Gulf of Patras (W. Greece). The same area was surveyed in 1971 by Throckmorton, Edgerton and Yalouris, who are among the pioneers in the application of remote sensing techniques to underwater archaeology. The researchers conducted a surface reconnaissance survey to locate the site where the Battle of Lepanto took place on 7 October 1571. Their remote sensing surveying resulted in a map of two "target" areas that showed promise as possible remnants of wrecks from that battle and proposed a ground truth survey for their identification and in the detection of two modern shipwrecks. The ground truth survey was never fulfilled. The objectives of our repeat surveys, which were completed 50 years later, were to relocate the findings of this pioneer survey with higher spatial and vertical resolution, to ground-truth the targets, fulfilling their investigation, and to interpret the newly collected data in the light of modern developments in marine geosciences. Our repeat surveys detected mound clusters and individual mounds referred to "target" areas. These mounds could be interpreted as the surface expression of mud and fluid expulsion from the underlying deformed soft sediments. The ground truth survey demonstrated that the tops of mounds represent biogenic mounds. The ROV survey did not show any indication of wreck remnants of the Battle of Lepanto within the two survey areas. The site formation processes of the two modern shipwrecks were also studied in detail. Two noticeable seafloor morphological features were detected around the wreck sites; field of small-sized pockmarks and seafloor depressions. We would like to dedicate this work to the memory of Peter Throckmorton and Harold E. Edgerton, who are among the pioneers in the formative years of underwater archaeology in Greece. [ABSTRACT FROM AUTHOR]

Amvrakikos Gulf is a Mediterranean landlocked, fjord-like embayment and marine protected area suffering from natural, human-induced hypoxia/anoxia and massive fish mortality events. Seasonal marine geophysical and oceanographic surveys were conducted focusing on the water-circulation patterns at the sill and the spatial-seasonal distribution of dissolved oxygen (DO) in the gulf. Detailed surveys at the sill, the only communication route between the gulf and the open sea, revealed a two-layer water circulation pattern (top brackish outflow–bottom seawater inflow) and the role of the tide in the daily water exchange. Statistical analysis of the known natural drivers of DO distribution (density difference between the Ionian Sea and Amvrakikos, river inflow, wind) revealed that horizontal density gradients strongly affect anoxia reduction and seafloor oxygenation, while river inflow and wind mainly oxygenate volume/areas located above or within the pycnocline range, with DO concentrations > 2 mg/L. Complex geomorphology with well-formed internal basins contributes to the development and preservation of low DO conditions below the pycnocline. Finally, 43% of the seafloor and 36% of the gulf's total water volume are permanently hypoxic, and reach a maximum of 70% and 62%, respectively, in September and July. This work is tailored to future ecosystem management plans, decisions, and future research on coastal ecosystems. [ABSTRACT FROM AUTHOR]

Amfilochia Bay (Eastern Amvrakikos Gulf, Western Greece), a complex marine area affected by tectonism, was investigated for seabed seepage manifestations and for possible inter-relationships between shallow gas accumulations and hypoxia. For this purpose, an integrated research methodology that combined geophysical, geochemical, and hydrographic surveys was applied. Marine geophysical and bathymetric surveys led to the discovery of a gas-induced pockmark group in the study area. Oceanographic surveying confirmed that the bay is hypoxic/anoxic below a depth of 15 m (dissolved oxygen from ~4 to 60 μM). Very weak CH4 seepage was detected in correspondence to the pockmark group that seemed to slightly enhance the hypoxic environment locally and close to the seabed, with no effect in shallower waters and the oxycline. Methane isotopic analysis showed variable carbon isotopic composition (from −41‰ to −86‰) which could be either related to differential CH4 oxidation or mixing between microbial and thermogenic gas. However, the pathway of degassing is clearly related to the fault-controlled pockmark group. A protrusion mound, which has erroneously been reported as a submarine "volcano" since 1876, could be the result of mud volcanism based on the geophysical data of this study. [ABSTRACT FROM AUTHOR]

Marine geophysical data collected during an underwater natural and cultural heritage assessment survey along the coastal zone of Kefallinia Island in the Ionian Sea, Greece, showed among other seafloor features, the presence of a Roman shipwreck and its amphorae cargo on the seafloor. The study and analysis of the collected data demonstrated that: (i) sidescan sonar and chirp sub-bottom profiling systems, can successfully detect ancient shipwrecks and their amphorae cargo on the seafloor, (ii) the use of objective computer vision techniques in processing sidescan sonar seafloor images, is a valuable tool for the separation of potential ancient shipwreck targets from other seafloor features with similar acoustic signatures. Furthermore, a guideline for the data acquisition parameters that should be used to obtain optimal seafloor sonar images to maximize the separation of potential shipwreck targets from other seafloor features, is provided. The underwater sonar remote sensing techniques may also provide adequate indication regarding the amphorae hull stowage and its vulnerability to human activity in the area. The shipwreck is dated between 1st century BC and 1st century AD and is one of the largest found so far in the Mediterranean Sea, for that period. It is estimated that it was carrying about 6,000 amphorae. The amphorae cargo, visible on the seafloor, is in very good state of preservation and the shipwreck has the potential to yield a wealth of information about the shipping routes, trading, amphorae hull stowage and ship construction during the relevant period and is therefore considered to be of significant archaeological importance. • Ancient shipwreck underwater sonar remote sensing detection. • Automated target detection. • Acquisition parameters to obtain seafloor images to maximize the separation of wreck targets among other seafloor features. • Roman wrecks 1st century BC to 1st century AD, Mediterranean Sea. [ABSTRACT FROM AUTHOR]

The abundance of marine debris was quantified for a total of sixty-two inaccessible beaches in the western Saronikos Gulf, Greece. High resolution images were obtained through vessel-based photography survey, merged into seamless photomosaics, and manually processed to quantify beach litter abundance. A sample of four selected beaches were subjected to detailed photography followed by beach macro-litter (≥ 2.5 cm) in-situ sampling surveys over a period of one year, to calibrate and validate the proposed method. Regression analysis between photographic and in-situ data showed a significant correlation, hence providing a highly accurate regression model to assess the real number of beach litter stranded on the rest of the investigated beaches, exhibiting clear correlations to the hydrodynamic status of the area and, provide an indication of the main litter sources. The proposed method is an easily applicable and useful tool for fast and low-cost macro-litter monitoring in extended, remote coastlines, when only photographic data are available. • Vessel-based, beach litter monitoring. • High resolution, fast, low cost and easily applicable method for photographic beach litter identification. • Macro-litter monitoring in extended, remote, inaccessible coastlines. • Beach litter assessment for the inaccessible beaches of western Saronikos Gulf, Greece. [ABSTRACT FROM AUTHOR]

Straits are crossed by marine currents that are amplified due to constrictions. These nearshore high-velocity flows are problematic for offshore infrastructures (bridge pillars, cables, pipelines, etc), but constitute an interesting carbon-free energy source. Many modern straits are dominated by tidal currents which flow axially, with reversal directions and phase difference between the two interlinked basins. These tidal currents interplay with: sediment sources (including in situ carbonate production and deltas), tectonic activity, and inherited lowstand features, all shaping the seafloor into complex geomorphologies. Previous studies have highlighted a common tidal strait depositional model with a strait-center zone in erosion and on each side a dune-bedded strait zone with 3D and 2D tidal dunes and tidal ripples. Even if the internal waves associated with and generated by the straits are widely documented, the effects of the internal waves on the seafloor need to be further investigated. The aim of this study is to unravel the combined effects of the tidal currents and the internal tides on current pattern and on the morphosedimentary features. We present a strait example based on an interdisciplinary approach using high-resolution geophysical and oceanographical data to better constrain the hydrodynamics and the processes acting on the seafloor. We focus on the Rion Strait in Greece which controls the connection between the Corinth Gulf and the Mediterranean Sea. Based on high-resolution multibeam bathymetry (MBES) over an area of 211 km2, we identify and quantify different morphologies by extracting bathymetric swath profiles. These results are integrated with currents data (ADCP) and CTD profiles. In addition, we use high-resolution chirp sub-bottom profiles and sparker seismic reflection profiles to document the morphology and internal architectures of the sedimentary deposits and the erosional features in the strait bottom. To complete this dataset, we analyzed Sea Surface Temperatures (SST) from satellite sensors. The Rion Strait displays complex bathymetric features without tidal dunes. At the excepted depositional location of tidal dunes, we identify an erosive area with a pool and crest morphology. This new example completes the tidal strait depositional model by a re-localization of erosion, bypass and deposition in an asymmetric strait swept by baroclinic currents. This example illustrates the key role of internal tides in straits located between a confined deep basin and an open sea. • Rion strait currents are dominated by tides and internal tides from the Corinth Gulf. • The HR bathymetry shows specific pool-and-crest erosive morphology in contrast with the dune fields of tidal strait model. • On the strait margins, the sediments are veneered on the slope and formed plastered and slope drifts. [ABSTRACT FROM AUTHOR]

• The acquisition of high-density bathymetric data up to the water line, resulted in the definition of a high-resolution baseline bathymetric surface of the Methoni bay and strait. • Ultra high-resolution bathymetric products allowed for detailed seafloor mapping of the nearshore zone of Methoni bay, investigation of wrecks or features of archaeological interest and for 3D visualisation of underwater antiquities. • Side-scan sonar mosaics show the spatial distribution of large manmade underwater features such as the submerged ancient breakwater or the submerged prehistoric settlement and the spatial distribution of large natural underwater features such as the underwater mount or the sand basin across the Methoni strait. • The dense grid of seismic profiles collected in the project area, documents the extension of the prehistoric settlement beyond the surficial ruins and support the 3D reconstruction of buried shipwrecks. • The fusion and spatiotemporal analysis of geoarchaeological data support the decision-making processes of stakeholders for maritime spatial planning, site preservation management, towards sustainable growth. The 'Evolved GE.N.ESIS Project' highlights the underwater cultural heritage resources off the coast of Methoni, Greece that could locally drive sustainable socioeconomic growth. An integrated marine geophysical survey, a hydrographic survey, and a GNSS survey were conducted off Methoni, recording six historic wreck sites, artefacts, the ruins of a submerged prehistoric settlement, and the town's ancient harbour/breakwater, as well as the geophysical properties of the underwater environment. The preliminary project results present bathymetric surfaces, backscatter intensity and magnetic maps, drawings, and seismic reflection profiles of the underwater antiquities and of the seabed, all fused in a 3D geographical platform. The results also shed light on the archaeological potential of the site, the nearshore physical processes, and their effect on the underwater archaeological resources. The project outcomes have shown that the establishment of an underwater archaeological park and diving sites at the cultural heritage sites will support cultural tourism development in the area and will have a positive impact on local socioeconomic development. The underwater archaeological park should comply with the basic principles of a site management plan – one that is established in the context of an integrated coastal management plan that identifies the maritime synergies or conflicts among human activities, archaeological resources, and the local environment, and utilises the 3D synthesis of marine knowledge from the project outcomes as a decision-making tool. [ABSTRACT FROM AUTHOR]

Abstract: Oceanographic research in the Amvrakikos Gulf in Western Greece, a semi-enclosed embayment isolated from the Ionian Sea by a narrow, shallow sill, has shown that it is characterised by a fjord-like oceanographic regime. The Gulf is characterised by a well-stratified two layer structure in the water column made up of a surface layer and a bottom layer that are separated by a strong pycnocline. At the entrance over the sill, there is a brackish water outflow in the surface water and a saline water inflow in the near-bed region. This morphology and water circulation pattern makes the Amvrakikos Gulf the only Mediterranean Sea fjord. The investigations have also shown that the surface layer is well oxygenated, whereas in the pycnocline, the dissolved oxygen (DO) declines sharply and finally attains a value of zero, thus dividing the water column into oxic, dysoxic and anoxic environments. At the dysoxic/anoxic interface, at a depth of approximately 35 m, a sharp redox cline develops with Eh values between 0 and 120 mV occurring above and values between 0 and −250 mV occurring below, where oxic and anoxic biochemical processes prevail, respectively. On the seafloor underneath the anoxic waters, a black silt layer and a white mat cover resembling Beggiatoa-like cells are formed. The dysoxic/anoxic conditions appeared during the last 20 to 30 years and have been caused by the excessive use of fertilisers, the increase in animal stocks, intensive fish farming and domestic effluents. The inflicted dysoxia/anoxia has resulted in habitat loss on the seafloor over an area that makes up just over 50% of the total Gulf area and approximately 28% of the total water volume. Furthermore, anoxia is also considered to have been responsible for the sudden fish mortality which occurred in aquaculture rafts in the Gulf in February 2008. Therefore, anoxic conditions can be considered to be a potential hazard to the ecosystem and to the present thriving fishing and mariculture industry in the Gulf. [Copyright &y& Elsevier]