A 15-year (1997–2012) time series of chlorophyll a (Chl a) in the Baltic Sea, based on merged multi-sensor satellite data was analysed. Several available Chl a algorithms were sea-truthed against the largest in situ publicly available Chl a data set ever used for calibration and validation over the Baltic region. To account for the known biogeochemical heterogeneity of the Baltic, matchups were calculated for three separate areas: (1) the Skagerrak and Kattegat, (2) the central Baltic, including the Baltic Proper and the gulfs of Riga and Finland, and (3) the Gulf of Bothnia. Similarly, within the operational context of the Copernicus Marine Environment Monitoring Service (CMEMS) the three areas were also considered as a whole in the analysis. In general, statistics showed low linearity. However, a bootstrapping-like assessment did provide the means for removing the bias from the satellite observations, which were then used to compute basin average time series. Resulting climatologies confirmed that the three regions display completely different Chl a seasonal dynamics. The Gulf of Bothnia displays a single Chl a peak during spring, whereas in the Skagerrak and Kattegat the dynamics are less regular and composed of highs and lows during winter, progressing towards a small bloom in spring and a minimum in summer. In the central Baltic, Chl a follows a dynamics of a mild spring bloom followed by a much stronger bloom in summer. Surface temperature data are able to explain a variable fraction of the intensity of the summer bloom in the central Baltic.<

The North Atlantic Subtropical Gyre (NASTG) has experienced the fastest expansion of oligotrophic waters worldwide in response to ocean warming. We study the trophic regime changes in the NASTG by using 21 years (1998–2018) of satellite chlorophyll‐a (CHL) data, complemented with other variables such as Sea Surface Temperature, optical backscatter coefficients (b_bp), Secchi disk (z_SD), and Mixed Layer Depth. To this aim, we describe the spatial/temporal variability of waters with the lowest CHL concentrations (CHL ≤ 0.04 mg m⁻³) with an inter‐annual variability analysis of key environmental variables. Main results demonstrate that these ultra‐oligotrophic waters are spatially expanding and increasing in frequency, shifting the NASTG to a dominant quasi‐permanent ultra‐oligotrophic condition, thus confirming the ongoing ocean desertification.