Initiation of winter phytoplankton blooms within the Gironde plume waters in the Bay of Biscay

Thermostratification and seasonal Light increase are generally considered t he first causes of phytoplankton spring blooms in temperate waters. The obj ective of this study is to confirm the existence of winter phytoplankton bl ooms, responsible for the early exhaustion of phosphate, within the Gironde plume waters (southeast Bay of Biscay), and to understand what may initiat e them so early. Two cruises, BIOMET 2 and BIOMET 3, were carried out respe ctively in early (8 to 21 January) and late winter 1998 (25 February to 11 March). An increase of phytoplankton biomass (chl a) between the 2 cruises and non-conservative nutrients observed in late winter confirm an early phy toplankton growth both in the Gironde plume and adjacent oceanic waters. Am ong factors that possibly initiate these blooms, light availability seems t o be the best candidate. First, the status of available Light for phytoplan kton changed dramatically between the 2 cruises when comparing the instanta neous depth-averaged irradiance (E-m) received by phytoplankton with the li ght saturation parameter E-k determined from P versus E curves. Light was l imiting for phytoplankton growth in early winter according to systematicall y lower values of E-m than E-k in the daytime. However, light was not limit ing during a large part of the day in late winter since E-m was above E-k d uring 40% of the daylight. The estimated critical depth from the Nelson & S mith (1991; Limnol Oceanogr 36:1650-1661) reformulation of the Sverdrup (19 53; J Cons Perm Int Explor Mer 18:287-295) equation was much shallower (4 t o 13 m) than the mixed layer depth (15 to 50 m) in early winter in the whol e Gironde plume up to oceanic waters. On the contrary, it was much deeper ( 35 m) than the mixed layer depth (9 m) in late winter for the intermediate plume (salinity range 33 to 35) and similar for oceanic waters (S > 35.5) w ith a bottom depth of 60 to 70 m. Therefore, according to the Sverdrup mode l, available light is the triggering factor of winter phytoplankton blooms in the Gironde plume and adjacent oceanic waters. Moreover, Riley's (1957; Limnol Oceanogr 2:252-270) empirical critical irradiance of 20.9 W m(-2) co uld be applied in these waters to predict the onset of blooms since it was reached in late winter but not in early winter. The establishment of halost ratification and/or decrease of the light vertical attenuation coefficient prevail in the initiation process of winter phytoplankton blooms.