Phytoplankton bloom phenology studies are fundamental for the understanding of marine ecosystems. Mismatches between fish spawning and plankton peak biomass will become more frequent with climate change, highlighting the need for thorough phenology studies in coastal areas. This study was the first to assess phytoplankton bloom phenology in the Western Iberian Coast (WIC), a complex coastal region in SW Europe, using a multisensor long-term ocean color remote sensing dataset with daily resolution. Using surface chlorophyll a (chl-a) and biogeophysical datasets, five phenoregions (i.e., areas with coherent phenology patterns) were defined. Oceanic phytoplankton communities were seen to form long, low-biomass spring blooms, mainly influenced by atmospheric phenomena and water column conditions. Blooms in northern waters are more akin to the classical spring bloom, while blooms in southern waters typically initiate in late autumn and terminate in late spring. Coastal phytoplankton are characterized by short, high-biomass, highly heterogeneous blooms, as nutrients, sea surface height, and horizontal water transport are essential in shaping phenology. Wind-driven upwelling and riverine input were major factors influencing bloom phenology in the coastal areas. This work is expected to contribute to the management of the WIC and other upwelling systems, particularly under the threat of climate change.

The Western Iberian Coast (WIC) is characterized by a dynamic coastal upwelling system with interactions between oceanographic features and continental river run‐off. The present study investigates the spatio‐temporal variability in the concentrations of Chlorophyll a (Chl a) and turbidity, and their relationships with physical and other biogeochemical properties along the entire coast of Portugal, as well as the response of phytoplankton to coastal upwelling pulses. In situ data were obtained during four oceanographic campaigns conducted in spring (April–May 2019), autumn (October 2018 and 2019), and early spring (February–March 2020). Upwelling conditions were evaluated through an upwelling index and auxiliary data such as sea surface temperature and height. In the WIC, the in situ surface temperature and salinity have a clear spatial pattern, both increasing from North to South. In autumn 2018 the highest temperatures coincided with the weakening of spring–summer upwelling conditions. Overall, high Chl a was associated with lower temperatures and higher dissolved oxygen concentrations (e.g., spring 2019). During spring 2019, an increase in Chl a, especially in the northern area, was associated with previous upwelling conditions. In early spring 2020, ongoing upwelling conditions in the northern area were associated with high turbidity and nutrients close to the shoreline with no immediate biological responses. An increase in Chl a was visible in satellite imagery a few days later. These results highlight the importance of understanding the complexity of coastal processes in the WIC.