To characterize the hydrology of the Western English Channel, the Station Biologique de Roscoff, Sorbonne Université/CNRS, performs the SOMLIT-Roscoff time series. Since 1997, samples are collected at two fixed stations: a/ the near shore station SOMLIT-Estacade, close to the pier of Roscoff strongly influenced by coastal waters and b/ the offshore station SOMLIT-Astan, at 2 nautical miles off Roscoff with a permanently mixed water column (around 60 m depth). Those stations are part of the French coastal environment monitoring network SOMLIT (https://www.somlit.fr/) accredited by the CNRS as a national Earth Science Observatory (Service National d’Observation: SNO). The objective of the SOMLIT network is to characterize coastal and nearshore ecosystems in a context of natural and anthropic climate change. The SOMLIT network is part of the IR ILICO France’s multi-agency Research Infrastructure for coastal ocean observation (https://www.ir-ilico.fr/). Twice a month during the highest neap tide samples are collected at SOMLIT-Estacade and SOMLIT-Astan, at 2m depth for both stations and 60m depth for SOMLIT-Astan. Water samples are analysed for more than 20 core parameters: SST, SSS, dissolved oxygen, pH (spectroscopic method since 01/01/2018), ammonia, nitrates, nitrites, phosphates, silicates, suspended matter, particulate organic carbon, 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/. In the frame of the IR ILICO, additional time series are also performed by the Station Biologique de Roscoff: a/ COAST-HF, high frequency hydrological monitoring national network close to SOMLIT-Astan station (https://coast-hf.fr/); b/ PHYTOBS-Network, national network of microphytoplankton monitoring at SOMLIT-Astan station (https://www.phytobs.fr/) and c/ BENTHOBS national network of macrozoobenthos at two stations in the bay of Morlaix (https://www.benthobs.fr/).