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.

During a survey of benthic diatoms from the Bulgarian Black Sea coast near the historic town of Sozopol, we found a high diversity of Amphora sensu lato taxa at a single site and on a single type of artificial substratum. A total of 40 taxa from the current genera Amphora, Halamphora, Seminavis and Tetramphora were recognized during light microscopy (LM) and scanning electron microscopy (SEM) observations. We were able to identify with a high degree of certainty only 6 taxa based on the literature, whereas for 19 taxa (or almost 50% of all taxa) we could not find even sufficiently similar species, and they remained unidentified to species level. Many of the species had small valve dimensions. The paper presents all taxa with short morphometric data and microphotographs under LM and SEM, where possible. All taxa are discussed and compared with similar known taxa. The illustrative materials in earlier works from the Black Sea and species descriptions, where such were available, were also consulted. Diversity of Amphora sensu lato in the Black Sea coastal waters seems to be underestimated, whereas some of the past records are uncertain. A discussion on the current state of knowledge of Amphora sensu lato species from the Black Sea is given.

Introduction The Institute of Oceanology – Bulgarian Academy of Sciences (IO-BAS) manages the Black Sea macrosite, comprising the Kaliakra, Galata, and Koketrice marine sites, which have been part of the Bulgarian national Long-Term Ecological Research (LTER) network since 2007. Since 2022, a new equipment - WiMo Plus multiparameter platform - has been installed in Varna Bay near Cape Galata, providing in situ data on temperature, conductivity, salinity, pH, dissolved oxygen, chlorophyll-a and other physicochemical indicators. In addition, samples are taken monthly by the Testing Laboratory "Marine Chemistry" for analysis of the main physicochemical parameters, dissolved oxygen and nutrients, as well as chlorophyll-a. The analyses are carried out in the accredited chemical laboratory and the specialized biological laboratory using validated methods for marine waters. The present study aims to investigate the interannual variability of physicochemical quality elements and assess the ecological status of the Galata site, with particular emphasis on trophic status and temporal variations in nutrient ratios. Material and methods The study is based on data from Varna Bay - Galata site, including continuous in situ measurements obtained using the WiMo Plus multiparametric platform and monthly field sampling conducted between January 2022 – December 2025. In situ data were processed statistically (TIBCO Software Inc. (2018) 2023). Water samples were measured by a WTW multimeter on site for temperature (T), conductivity (Cond), salinity (S ‰) and pH. Dissolved oxygen (DO) was fixed for subsequent analysis in the Testing Laboratory “Marine Chemistry” by the Winkler method. The samples were analysed in the Testing Laboratory for the content of ammonium nitrogen (N-NH4), nitrate nitrogen (N-NO3), nitrite nitrogen (N-NO2), total nitrogen (TN), phosphate phosphorus (P-PO4), total phosphorus (TP) and silicon (Si) spectrophotometrically with a CECIL 1021 spectrophotometer by internally validated methods based on Grasshoff et al. (2009)). Chlorophyll-a was prepared in the specialized biological laboratory following the steps: filtration, extraction, photometry at different wavelengths, and calculation of concentrations according to the equations of Jeffrey and Humphrey (1975) using a specialized computer program (Edler 1979). Results and Discussions The laboratory analysis results were evaluated on both a seasonal and annual basis. The coldest winter occurred in 2022, with an average sea surface temperature of 6.1 °C, while the warmest was in 2023, averaging 8.7 °C. Naturally, summer was the warming season overall, peaking in 2024 with the maximum seasonal average of 25.9 °C. Throughout the study period (2022-2025), oxygen conditions remained dynamic and frequently unstable. Dissolved oxygen reached maximum levels during the winters of 2024 (14.9 mg/L) and 2025 (16 mg/L). As expected, DO concentrations decreased toward the summer months, reaching a minimum of 4.4 mg/L in summer 2023. Oxygen saturation (OS%) recorded its minimum in January 2022 (69.1%) and peak levels in March 2023 (159.9%) and March 2025 (152%). Absolute maximum for chlorophyll-a (76.19 mg/m³), five- day biochemical oxygen demand BOD 5 (10.3 mg/L), and dissolved oxygen (15.96 mg/L) coincided with this saturation peak, clearly indicating phytoplankton blooms. Nutrient concentrations varied widely across the study period. Phosphate phosphorus and total phosphorus shared a common pattern, reaching minimum values in autumn 2022 and maximum values in summer 2025. Dissolved silicate peaked in July 2025 before dropping to its minimum in September, whereas inorganic nitrogen species exhibited no distinct seasonal trend. The annual ecological status assessment was conducted using reference values for OS%, DO, BOD 5, N-NH4, N-NO2, N-NO3 and P-PO4 across the seasons and categories specified in Regulation No. H‑4/14.09.2012 (rev. 2023) on the characterization of surface waters. An adapted approach for applying the classification system to assess physicochemical quality elements in the "coastal marine waters" category, developed by IO-BAS, was utilized. The ecological status of the site based on physicochemical quality elements for the studied period 2022–2025 is Moderate. This assessment is confirmed by the chlorophyll-a data, for which categories outside of "good" and "high" account for 55–76% of all measurements. The trophic status assessment was performed using the Trophic Index - TRIX (Vollenweider et al. 1998), whose values range between 2.38 and 8.92 (mean 5 ± 1.23). The results indicate moderate to high anthropogenic nutrient loading, accompanied by oxygen supersaturation and phytoplankton blooms. Over the period under review (2022–2025), an increasing trend in the trophic index is observed, confirming the site's vulnerability to local anthropogenic sources and climate fluctuations, and defining the area as a coastal zone hotspot. The analysis of the in-situ platform data and the trend in laboratory test results correlate, underscoring the appropriateness of selecting the Galata site for monitoring. Beyond contributing to long-term ecological research, the monitoring results emphasize the need for enhanced control of coastal runoff and anthropogenic pressure in the area.