The SOMLIT-Portzic time-series data characterize the hydrology of the Bay of Brest – Mer d’Iroise ecosystem, located at 48°N (western Brittany). Weekly measurements have been undertaken since 1998 by the IUEM/UBO/CNRS. The SOMLIT-Portzic observation station is a part of the French monitoring network SOMLIT (https://www.somlit.fr/), accredited by the CNRS as a national Earth Science Observatory (Service National d’Observation : SNO). It aims to detect the long-term evolution of these ecosystems including both natural and anthropogenic forcings. Implemented on the Ste-Anne-du-Portzic location (48°21'32.17'' N, 4°33'07.21'' W), the SOMLIT-Portzic observation station is among the oldest long-term coastal observation stations of the French Research Infrastructure dedicated to coastal ocean observations (known as ILICO, https://www.ir-ilico.fr). The high-frequency hydrological COAST-HF-Iroise time-series and the microphytoplankton PHYTOBS-Portzic time-series are also monitored at this location. SOMLIT-Portzic samples are collected at 2m-depth of a 9m water column, at high tide, during slack water, with an average coefficient of 70 ± 10. Samples collected are analysed for more than 20 core parameters: SST, SSS, dissolved oxygen, pH, ammonia, nitrates, nitrites, phosphates, silicates, suspended matter, particulate organic carbone, particulate organic nitrogen, chlorophyll, delta15N, delta13C; pico- and nano- plankton; CTD-PAR-profile. The SOMLIT network quality management system is in line with the ISO/IEC 17025:2017 standard: “General requirements for the competence of testing and calibration laboratories”. Further information on standard operating procedures for sample collection and data acquisition are available at: https://www.somlit.fr/parametres-et-protocoles. For more information on the quality flagging scheme: https://www.somlit.fr/codes-qualite/.