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The red mullet (Mullus barbatus Linnaeus, 1758) is a keynote species for the Bulgarian Black Sea ecosystem and fisheries; nevertheless, existing knowledge on population status is very scarce. The present study was intended to assess the health status and adaptive potential of M. barbatus populations inhabiting the Bulgarian waters of the Black Sea. Our findings revealed that populations of M. barbatus are exposed to a variety of anthropogenic and environmental stressors. The species’ status was assessed using representative genetic, morphological, biochemical and chemical biomarkers from specimens obtained in the research area’s northern and southern regions. Based on mtDNA markers, genetic analysis revealed low haplotype and nucleotide diversity, typically observed in overexploited or “threatened” populations. Examining the morphology of the specimens revealed no discernible pattern of differentiation. Except for aluminium and chrome, metal and PAH concentrations in fish were below the regulatory thresholds. The specimens from the southern region ingested more microplastics than those from the northern region. The majority of specimens collected from the southern region also exhibited elevated levels of oxidative stress and decreased antioxidant defence, which can be interpreted as an early indication that they had reached the limits of their adaptive potential. Further research on the composite effects of the stressogenic environment on the Black Sea biota are critically needed, as well as the introduction of new indicators and thresholds at molecular and cellular levels for adequate monitoring of both the ecological state of the marine environment and its biota.

Over the past few years, predicting species spatial distributions has been recognized as a powerful tool for studying biological invasions in conservation biology and planning, ecology, and evolutionary biology. Species spatial distribution models (SDMs) are used extensively for assessing the effects of changes in habitat suitability, the impacts of climate change, and the realignment of the existing conservation priorities. SDMs relate known patterns of species occurrences to a specific set of environmental conditions. Accordingly, we have used MaxEnt SDM tool in order to provide habitat suitability models of 5 keynote fish species: European sprat (Sprattus sprattus L.), red mullet (Mullus barbatus, L.), horse mackerel (Trachurus mediterraneus, L.), bluefish (Pomatomus saltatrix, L.) and whiting (Merlangius merlangus, L.), inhabiting the Bulgarian region of the Black Sea. Presence-only (PO) data collected by pelagic surveys performed between 2017 and 2019 was further utilized to link known species occurrence localities with selected abiotic factors, such as surface sea temperature and salinity, dissolved oxygen, and speed of currents. Biotic interactions were also considered for fitting the patterns of habitat suitability models. The SDMs, obtained from the present research study, prove to have satisfactory predictive accuracy to be further implemented for conservation measures and planning, stock management policy-making, or ecological forecasting.

The present study examined the genetic diversity and spatial distribution of turbot (Scophthalmus maximus), an economically important species on the Bulgarian Black Sea coast. Maximum entropy (MaxEnt) modeling software Version 3.4.4. was utilized to develop a habitat suitability model for S. maximus in the Bulgarian Black Sea region. Data collected via demersal and pelagic surveys and genetic sampling from 2017 to 2021 were utilized to link species occurrence localities with selected abiotic factors. Our findings showed that the species’ habitat preferences are strongly influenced by temperature and dissolved oxygen, and projections based on simplified climatic scenarios indicated potential distribution shifts and a substantial reduction in reproduction habitats in the northern region. The assessment of genetic diversity was based on mtDNA COIII sequencing; MtDNA revealed a low level of polymorphism in all analyzed populations. The extensive fishing pressure may have increased the likelihood of genetic and population bottlenecks and a consequent decline in genetic diversity in the Shabla, Nesebar, and Tsarevo populations. The Tajima’s D values for the latter indicated that turbot underwent a bottleneck followed by rapid population expansion. Our findings are essential for the conservation and effective management of S. maximus stocks in the region.

Abstract Background The Black Sea is the largest brackish water body in the world, although it is connected to the Mediterranean Sea and presents an upper water layer similar to some regions of the former, albeit with lower salinity and temperature. Despite its well-known hydrology and physicochemical features, this enormous water mass remains poorly studied at the microbial genomics level. Results We have sampled its different water masses and analyzed the microbiome by shotgun and genome-resolved metagenomics, generating a large number of metagenome-assembled genomes (MAGs) from them. We found various similarities with previously described Black Sea metagenomic datasets, that show remarkable stability in its microbiome. Our datasets are also comparable to other marine anoxic water columns like the Cariaco Basin. The oxic zone resembles to standard marine (e.g. Mediterranean) photic zones, with Cyanobacteria (Synechococcus but a conspicuously absent Prochlorococcus), and photoheterotrophs domination (largely again with marine relatives). The chemocline presents very different characteristics from the oxic surface with many examples of chemolithotrophic metabolism (Thioglobus) and facultatively anaerobic microbes. The euxinic anaerobic zone presents, as expected, features in common with the bottom of meromictic lakes with a massive dominance of sulfate reduction as energy-generating metabolism, a few (but detectable) methanogenesis marker genes, and a large number of “dark matter” streamlined genomes of largely unpredictable ecology. Conclusions The Black Sea oxic zone presents many similarities to the global ocean while the redoxcline and euxinic water masses have similarities to other similar aquatic environments of marine (Cariaco Basin or other Black Sea regions) or freshwater (meromictic monimolimnion strata) origin. The MAG collection represents very well the different types of metabolisms expected in this kind of environment. We are adding critical information about this unique and important ecosystem and its microbiome.

Plankton cyst abundance and distribution is controlled by multiple factors. The stress linked to the fluctuations and variations of the environmental conditions in the water column is a major vector of encystment and intraspecific variability is an important adaptive strategy. The present study aims to disclose a link between the spatial distribution and abundance of different cyst morphotypes of Scrippsiella acuminata complex in surface sediments collected in the Black Sea at 34 sites and selected environmental variables. With this purpose, a basin scale data set was analyzed for patterns of intraspecific spatial heterogeneity. Redundancy analysis (RDA) was implemented to identify explanatory environmental variables associated with the cyst morphotypes abundance. Environmental multiyear data were used to ensure better approximation of a model that links environmental gradients with cyst abundance. Our results show that all S. acuminata cysts morphotypes are significantly correlated to one or a combination of the environmental variables, i.e., salinity, temperature and nutrients (nitrates and phosphates). The geographical distribution of Scrippsiella blooms in the Black Sea indicates that the interplay between the planktonic and benthic habitat of the dinoflagellate gives to S. acuminata the advantage to dominate in the plankton communities.

One of the assets, assigned to the phytoplankton resting stages, is that of serving as the “memory” of the aquatic ecosystems and preserved biodiversity in the course of time. However, an accurate cyst identification proves to be a more difficult and extremely challenging process, even today. In order to gain a better taxonomic coverage of cyst assemblages in the Black Sea, an integrated approach of the classical morphological identification with metabarcoding methods (MySeq sequencing of V7-V9 regions of the 18S rDNA) was applied on thirteen surface sediment samples collected from different sites. A total number of 112 dinoflagellate taxa was detected at the species level and ascribed to 51 genera. In general, it is the molecular analysis that yields a higher number of taxa as compared to those obtained through the morphological taxonomy (66 taxa based on the DNA sequences versus 56 morphologically-identified taxa). Besides, it should be pointed out that the integrated dataset includes 14 potentially toxic dinoflagellate species. Discerned, subsequently, was a good dataset consistency for ten species, followed by some discrepancies as to a number of taxa, identified with one of the methods only, due to specific methodological biases. On the whole, it could be concluded that the combination of morphological and molecular methods is likely to increase the potential for a more reliable taxonomic assessment of phytoplankton diversity in marine sediments which, in turn, proves conclusively the utmost importance of the integrated approach.

We propose a new possible earthquake precursor, a coastal water jet originating from shallow water gas seeps and colored by sediments, lifted from increased gas seeps emissions, preceding some earthquakes with offshore epicenters along the Bulgarian Black Sea coast and M ≥ 3. The new possible earthquake precursor is site specific and may be observed in shallow coastal water regions where active faults are accompanied by hydrocarbon gas seeps. We point out to a region where it can be easily detected by direct observation and satellite remote sensing, Zelenka methane seeps (ZMS), near the Bulgarian Black Sea coast. The ZMS activation is related to the tectonic stress and fracturing preceding and accompanying the earthquakes in the Shabla-Kaliakra-Balchik region along the northern part of the Bulgarian Black Sea coast. We also propose an earthquake forecasting experiment, based on the following four hypotheses: 1) The change of the methane emission of shallow horizons is related to crustal strain changes preceding earthquakes in the region; 2) extreme change of the activity of underwater methane seepages appears immediately before nearby earthquakes; 3) shallow water methane seepages activity can be monitored by remote sensing; 4) satellites can register effects from an extreme increase in their emissions. The proposed earthquake forecasting experiment is based on the monitoring of methane seepages activity in the ZMS area by direct observation and remote sensing which may provide indication for preparatory earthquake activity preceding offshore earthquakes in the region.

Picocyanobacteria of the genus Synechococcus are major contributors to global primary production and nutrient cycles due to their oxygenic photoautotrophy, their abundance, and the extensive distribution made possible by their wide-ranging biochemical capabilities. The recent recovery and isolation of strains from the deep euxinic waters of the Black Sea encouraged us to expand our analysis of their adaptability also beyond the photic zone of aquatic environments. To this end, we quantified the total abundance and distribution of Synechococcus along the whole vertical profile of the Black Sea by flow cytometry, and analyzed the data obtained in light of key environmental factors. Furthermore, we designed phylotype-specific primers using the genomes of two new epipelagic coastal strains – first described here – and of two previously described mesopelagic strains, analyzed their presence/abundance by qPCR, and tested this parameter also in metagenomes from two stations at different depths. Together, whole genome sequencing, metagenomics and qPCR techniques provide us with a higher resolution of Synechococcus dynamics in the Black Sea. Both phylotypes analyzed are abundant and successful in epipelagic coastal waters; but while the newly described epipelagic strains are specifically adapted to this environment, the strains previously isolated in mesopelagic waters are able, in low numbers, to withstand the aphotic and oxygen depleted conditions of deep layers. This heterogeneity allows different Synechococcus phylotypes to occupy different niches and underscores the importance of a more detailed characterization of the abundance, distribution, and dynamics of individual populations of these picocyanobacteria.

The clear and reliable detection of effluent plumes using satellite data is especially challenging. The surface signature of such events is of a small scale; it shows a complex interaction with the local environment and depends greatly on the effluent and marine water constitution. In the context of remote sensing techniques for detecting treated wastewater discharges, we study the surface signature of small river plumes, as they share specific characteristics, such as higher turbidity levels and increased nutrient concentration, and are fresh compared to the salty marine water. The Bulgarian Black Sea zone proves to be a challenging study area, with its optically complex waters and positive freshwater balance. Additionally, the Bulgarian Black Sea coast is a known tourist destination with an increased seasonal load; thus, the problem of the identification of wastewater discharges is a topical issue. In this study, we analyze the absorption components of the Inherent Optical Properties (IOPs) for 84 study points that are located at outfall discharging areas, river estuaries and at different distances from the shoreline, reaching the open sea area at a bottom depth of more than 2000 m. The calculations of IOPs take into account all available Sentinel 2 cloudless acquisitions for three years from 2017 until 2019 and are performed using the Case-2 Regional CoastColour (C2RCC) processor, implemented in the Sentinel Application Platform (SNAP). The predominant absorber for each study area and its temporal variation is determined, deriving the specific characteristics of the different areas and tracking their seasonal and annual course. Optical data from the Galata AERONET-OC site are used for validating the absorption coefficient of phytoplankton pigment. A conclusion regarding the possibility of distinguishing riverine, marine and coastal water is derived. The study provides a sound basis for estimating the advantages and drawbacks of optical satellite data for tracking the extent of effluent and fluvial plumes with unknown concentrations of optically significant seawater constituents.

The ultimate goal of modern operational oceanography are end user oriented products with high scientific quality. Beneficiaries are the governmental services, coast and offshore based enterprises and research institutions that make use of the products generated by operational oceanography. Direct users are coastal managers, shipping, search and rescue, oil spill combat, offshore industry, ports, fishing, tourism, and recreation industry. Indirect beneficiaries, through climate forecasting based on ocean observations, are food, energy, water and medical suppliers. Availability of updated information on the actual state as well as forecast of marine environment is essential for the success and safety of maritime operations in the offshore industry. Various systems for the collection and presentation of marine data for the needs of different users have been developed and put in operation in the Black Sea. The systems are located both along the coast and in the open sea and the information they provide is used by both the maritime industry and the widest range of users. The Black Sea Monitoring and Forecasting Center in the frame of the Copernicus Marine Service is providing regular and systematic information about the physical state of the ocean, marine ecosystem and wave conditions in the Black Sea area, assimilating observations, keeping efficient operations, advanced technology and high quality modeling products. Combining and optimizing in situ, remote sensing, modeling and forecasting into a Black Sea observing system is a task that has to be solved, and that will allow to get a more complete and comprehensive picture of the state of the marine environment as well as to forecast future changes of physical and biogeochemical state of the Black Sea and the Black Sea ecosystem.