Phytoplankton dynamics were assessed in the macrotidal ecosystem of Arcachon Bay through high-frequency surveys over a 5-year period in order to characterize typology of environmental conditions at the onset of the productive period. Temporal variations of hydrological and biological parameters were examined in external and internal waters of the lagoon, during the winter-spring periods from 1999 to 2003. An additional survey was performed during winter-spring 2005 in order to study the vertical structure of the water column. The occurrence of winter phytoplankton blooms between January and March emerged as a recurrent event. The early onset of the productive period is influenced by the biological functioning of adjacent Bay of Biscay oceanic waters. It is hypothesized that under a propitious hydrodynamic regime, phytoplankton inocula from the Bay of Biscay enter in the Arcachon Bay where cells presumably find favourable conditions for their fast development. The timing of the onset of those winter blooms in Arcachon Bay seems to be mainly influenced by the presence of anticyclonic weather conditions (associated with an increase in incident irradiance) during late winter (i.e. by February), while the water column does not show any particular stabilization nor stratification liable to facilitate the onset of these blooms. Moreover, these winter blooms dominated by diatoms led to an early nutrient depletion which could have inevitable consequences on the structuration of the food web during spring and summer.