The seasonal proliferations of green seaweeds under the morphological blade-type Ulva have been observed on Brittany coasts since the late 60s-early 70s. These so-called green tides are directly related to the eutrophication of coastal waters, and especially to high terrigenous inputs of nitrogen to the sea. In this context, a management tool, i.e. the ecological Model for Application at Regional Scale MARS3D-Ulve, has specifically been developed to support stakeholders to set realistic targets for nitrogen input reductions. Among key parameters of the model, the maximum nitrate uptake rate (Vmax) and the half-saturation constant for nitrate (Km) of Ulva sp. have initially been calibrated to reproduce the seasonal variation of the internal nitrogen and phosphorus algal contents measured in situ for five sites along the Brittany coastline (La Fresnaye Bay, Saint-Brieuc Bay, Lannion Bay, Guissény Bay and Douarnenez Bay; Perrot et al. (2014, doi:10.1016/j.jmarsys.2013.12.010)). The calibrated values of Vmax and Km require therefore to be experimentally validated.Laboratory experiments were monthly set up to assess the seasonal evolution of nitrate uptake kinetic of Ulva spp. from May to September during two successive seasons (2022 and 2023). Healthy vegetative thalli of the dominant Ulva species were randomly sampled on Lermot Beach, located in Saint-Brieuc Bay (lat 48.5339, long 2.6622). Eventhough the model MARS3D-Ulve does not differentiate Ulva sp. at the species level, morpho-anatomical identification of the sampled thalli was carried out following the updated taxonomic classification of Hughey et al. (2021, doi:10.1080/09670262.2021.1914862). Algae were grown in 5 l polypropylen beakers in 0.2 µm filtered seawater under 7 different nitrate concentrations (0, 5, 10, 20, 50, 100, 200 µmol/l). Phosphate was added (3 µmol/l) in each beaker to avoid limitation. The experiment was run under a light intensity of 150 µmol photons m²/s. Photoperiod and temperature were adjusted to measured in situ conditions. The laboratory experiments consisted in assessing nitrates concentrations in each medium after 30 minute-incubation period. Nitrate uptake rates were calculated from nitrate concentrations and the corresponding kinetic parameters Vmax and Km were estimated by fitting data with the Michaelis-Menten equation, using a nonlinear least-squares regression (Lotze and Schramm, 2000, doi:10.1046/j.1529-8817.2000.99109.x). Each experiment was replicated three times. Before each experiment, physiological and nutritional states of algae were determined from the assessment of their maximum quantum yield (Fv/Fm), and their nitrogen and phosphorus tissue contents respectively. Those assessments were done from four replicated samples.