Nutrients dynamics in the southern Bay of Biscay (1993–2003): Winter supply, stoichiometry, long‐term trends, and their effects on the phytoplankton community

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The southern Bay of Biscay is a very active region in terms of hydrography due to the interannual variations of its Central Waters, the recurrence of mesoscale features such as slope currents and upwellings, and the freshwater discharges from land. This highly dynamic physical environment influences to a great extent the biogeochemical cycles of nutrients beyond the seasonal cycle typical of middle latitudes. By using a monthly time series (1993–2003) of nitrate, nitrite, phosphate, and silicate consisting of three stations placed along a cross‐shelf transect, we assess the role of the physical forcing on nutrient seasonal and interannual dynamics within the upper 200 m, as well as the interactions with the biological component. The seasonal cycles of all nutrients and the stoichiometric balances (N:P and Si:N) are characterized along this coastal‐oceanic gradient. The year‐to‐year variations in the extent of the winter replenishment are analyzed in relation to the background Central Waters and presence/absence of the Iberian Poleward Current. In the long term we report decreasing linear trends of nitrate, nitrite, and silicate as well as an uncoupled nonlinear variation (i.e., cyclical) for all nutrients. Furthermore, we investigate the effect of this complex long‐term forcing on the phytoplankton: the linear trends are probably related to a decreasing primary production rate, while the nonlinear forcing may be responsible for controlling the community structure of phytoplankton.