Hydrological and biological variability in the Gulf of Lions. I. Nitrogen fluxes and potential productivity.

A monthly time-series of the seasonal evolution of the North Mediterranean Current (NMC) and of nitrogen flux across a transect (5 degrees 12'5 E betw een 43 degrees 10'N and 42 degrees 50'N) at the entrance of the Gulf of Lio ns was carried out over two years. The NMC exhibited two distinct structure s: a winter structure (from November to May) characterised by strong mesosc ale and interannual variations in which the current was 20 to 30 km width, about 400 m depth and velocity scaled between 5 and more than 40 cm s(-1); and a summer structure (from May to November) which was less sensitive to m esoscale variations but showing large interannual variability. The current was located more offshore, was wider and shallower than the one in winter a nd velocity remained below 20 cm s(-1). Meteorological conditions, frontal structures on both sides of the NMC, coastal upwelling and continental inpu ts explained the observed mesoscale discrepancies between water flux and ni trogen transport. Nitrogen input from the general circulation across the pr ospected transect is estimated at 187 +/- 20 kT a(-1) in the 0-200 m layer, with a distribution of 39% nitrate, 6% particulate nitrogen and 55% dissol ved organic nitrogen. Nitrate input (25 +/- 3 kT a(-1)) in the 0-100 m uppe r layer could sustain more than one third of the potential new production i n the Gulf of Lions. (C) Elsevier, Paris.