Analysis of salinity measurements near islands on the French continental shelf of the Bay of Biscay

Abstract

The Bay of Biscay is characterised by large fresh water inputs from two major rivers, the Loire and the Gironde, with a mean discharge of about 900 m³/s. To describe the spatial and temporal variability of the Loire and Gironde plumes, continuous salinity measurements were made at 4 locations on French coastal islands over periods lasting at least two years. These islands are located at a distance from the large local gradients linked to local inputs. Whereas sea surface salinity in the centre of the Bay of Biscay is 35.6, salinities at the coast vary between about 35 in summer to occasionally less than 30 in late winter or in spring. Minimum salinities do not occur at the same time along the coast, indicating a strong spatial gradient. Near the Loire and Gironde estuaries, a strong temporal correlation between discharges and salinity variation appears. Far from the estuaries, some drops in salinity seem to be correlated with a change of wind regime. In summer, a general decrease from north to south is found along the coast and may be linked to northwesterly wind conditions, which drive surface water to SW of the bay. These data constitute the first set of time series of salinity. They will be pursued to assess the interannual variability which is presumed to be high because of the large variability in river discharges and wind regimes.