Spatiotemporal patterns of chlorophyll-a concentration in the western part of the Black Sea for the period 1998-2025

Chlorophyll-a (Chl-a) concentration is a key climate variable. It is directly related to phytoplankton biomass and thus, reflects primary production, the foundation of all aquatic ecosystems. Studying the variation and distribution of Chl-a is crucial for understanding ecosystem functioning, monitoring ecosystem health and environmental changes. In this study we evaluate the reliability of satellite observations (level L3 and L4 data) in comparison to in-situ measurements and investigate the spatial and temporal distributions of Chl-a in the western part of the Black Sea (42°–43.7°N and 27°–29°E) for the period 1998-2025. Satellite products for the concentration of Chl-a were obtained from the Copernicus Marine Service, while in-situ data were collected by ship expeditions of the Institute of Oceanology at the Bulgarian Academy of Science.

The comparison between satellite and in-situ data close to the shore, along the shelf and in the open sea showed that the largest deviations are calculated near the coastline and decrease towards the open sea. Satellite L3 values demonstrate smaller value of the RMSD and higher Pearson correlation coefficient than L4. The analysis includes the surface and subsurface values of the Chl-a, thus revealing the importance of data availability in the whole euphotic layer.

A 28-year time series (1998-2025) of spatially averaged L3 daily values of Chl-a is analyzed in order to identify events of anomalously high concentrations of chlorophyll-a, which could signal the development of phytoplankton blooms. The threshold value is determined and the intra- and inter annual distribution of such events is found.

The L4 data are analyzed in order to capture complex spatiotemporal relations and to reveal the significant modes in the variability by applying EOF analyses. The investigation distinguishes four categories of blooms: (1) north-to-south spreading blooms likely influenced by Danube or other rivers’ nutrient input; (2) large-scale blooms along the entire Bulgarian coast linked to northward currents and coastal upwelling; (3) localized events in the Burgas and Varna Bays, possibly from anthropogenic sources; and (4) blooms south of Burgas, associated with mesoscale eddies concentrating phytoplankton.