Nineoceanographic cruises were carried out in the Central Cantabrian Sea, with the aim to describe the spatial and seasonal variations in phytoplankton biomass andproduction. Two periods were recognized according to the hydrodynamiccharacteristics of the water column: mixing and stratification periods. Duringthe mixing period, very high chlorophyll-a concentrations 5µgl-l) were measured in March, associated to the existence of ashelfbreak front. However, the highest primary production rates detected duringthis period were measured in April close to the coast, where strong salinestratification was present in the upper layers of the water column. During thestratification period a subsurface chlorophyll maximum associated with thenitracline developed at all the stations. This vertical structure was modifiedwhen advection of deep water at the coastal stations occurred. In thishydrographic scenario, chlorophyll and production maxima were located at thesame depth (10 m) and the highest production rates within the study period weremeasured 10 µgC l-l h-l). Besides thehydrodynamic regulation of the production of phytoplankton biomass on aseasonal scale, other environmental and biological factors modulate thisprocess. Chlorophyll- a concentration and, to a lesser extent, illuminationexplained the highest proportion of the observed variability in primaryproduction, both in mixed and stratified situations. However, when theproduction/chlorophyll ratio (P/CHLA) was considered, clear differences betweenperiods appeared. Illumination (33.25%) during the mixing period, and theproteinlcarbohydrate ratio (27.97%), during the stratification period, were thevariables which mostly contribute to the total variance in PICHLA.