The role of phosphorus on planktonic production of the Gironde plume waters in the Bay of Biscay

Abstract

More and more studies emphasize the status of phosphorus (P) as the principal limiting nutrient of phytoplankton growth, especially in coastal waters under the influence of freshwater discharges. The purpose of the present paper is to investigate the role of P on planktonic production in the waters influenced by the Gironde discharges; the Gironde being one of the two largest rivers on the French Atlantic coast. The survey is based on several cruises made in 1998 and 1999. Two different patterns were observed for waters with salinity below and above 34.5. For waters with salinity 34.5. P was found to be the first limiting nutrient of winter and spring phytoplankton blooms, based on undetectable phosphate ( 20 nM), high NO3 : PO4 ratios, typically 100:1, short phosphate turnover time (1 to 2 h), high alkaline phosphatase activities (mean of 176 nM h(-1) in late May 1999) and ultimately great increases of chlorophyll a (Chl a) and primary production in phosphate-enriched samples relative to controls. This limitation could be partly explained by the Gironde nutrient supplies, which were phosphate deficient compared with the mineral nitrogen (N-min : PO4 was 40 within a salinity range 16-33). In summer, corresponding to the period of low influence of Gironde supplies (low runoff and a spreading effect of the plume), phytoplankton growth would be controlled by both P and nitrogen (N), according to low nitrate and the major effect of combined P+N (NH4) enrichment on Chl a and primary production compared with the addition of Nor P singly. In early October, after the first autumn gales, the mixed layer was enriched with a sufficient supply of nutrients to support exponential phytoplankton growth for 4 days in enclosures. The pattern was different for waters at the limit of the Gironde plume and Atlantic oceanic waters (within salinity range 34.5-35.4)...