Seasonal and Interannual Variability of Coastal Black Sea Waters off Bulgaria (2022–2025): A Copernicus Marine Reanalysis Assessment

Abstract

This study assessed spatial, seasonal, and interannual variability in physical and biogeochemical indicators in surface waters along the Bulgarian Black Sea coast during 2022–2025 to provide an integrated characterization of coastal marine conditions. Copernicus Marine Level 4 multiyear reanalysis products at 0.025° × 0.025° resolution were aggregated into four climatological seasons and summarized for a 25 km coastal strip divided at Cape Emine into northern and southern sectors. Sea-water potential temperature increased from a four-year winter mean of 9.02 °C to 24.70 °C in summer, with the southern sector warmer in all 16 season–year comparisons. Salinity peaked in winter (18.276 × 10−3) and was lowest in summer (17.684 × 10−3), with consistently higher values in the south. Current-vector magnitude was highest in summer (0.0614 m s−1), without persistent sectoral predominance. Chlorophyll-a, net primary production, phytoplankton carbon biomass, and dissolved molecular oxygen peaked in spring and were consistently higher in the northern sector; nitrate was also higher there in 15 of 16 comparisons. Biological productivity peaked in spring 2024, while 2025 showed lower biological indicators than 2022 across all season–sector combinations. The four-year series does not support long-term trend inference but provides a reproducible framework for integrated coastal assessment.