The COAST-HF-MAREL-Iroise buoy is a scientific plateform to monitor at high frequency (subhourly) and for long term (since 2000) the coastal ecosystem of the Bay of Brest, which is impacted by both continental and Iroise Sea inputs.This buoy is a part of the national observation network COAST-HF - COAstal ocean observing system-HighFrequency (http://www.coast-hf.fr) from the ILICO research infrastructure (https://www.ir-ilico.fr) and is property of IUEM (Institut Universitaire Européen de la Mer) and Ifremer. The buoy is moored at 200 m of the coast at the outlet of the Bay of Brest where manual sampling is also performed for the weekly observation network SOMLIT-Portzic (https://www.somlit.fr/). Both data base are complementary so that manual data are used to qualify and/or correct buoy’s data (e. g. for temperature, salinity and dissolved oxyge). Corrected data are subsequently called « adjusted-parameter » in the Iroise data base. Manual SOMLIT-Portzic data are also systematically used to convert fluorescence sensor data collected in raw fluorescence unit(FFU) into « eq-µg/L of  chlorophyll » : all fluorescence data are then available as « adjusted-fluorecence » in the data base. Precision estimated of the complete data collection process is : temperature (±0.1°C), conductivity (±0.3mS/cm), dissolved oxygen (±10%), in vivo fluorescence (±10%), and turbidity (±10%). By the mean of two additional sensors, these core parameters are completed by aerial Photosynthetic Activated Radiation (PAR) and FugacityCO2 (±3µatm). The data can be viewed on the website: https://data.coriolis-cotier.org/platform/6200450.