Seasonal variability and export of dissolved organic nitrogen in the northwestern Mediterranean Sea

Hydrodynamic and biogeochemical monthly time series were carried out in the Gulf of Lions (northwestern Mediterranean) between 1993 and 1994. Detailed analysis of vertical profiles of dissolved organic nitrogen (DON) revealed an accumulation of nitrogen in the Upper Mixed Layer (UML) above the seasonal thermocline. DON was not corrected for ammonia concentration because of negligible concentrations of this element during the whole year in the studied area. In midwinter (January-February), DON concentrations were quasi-homogeneous throughout the water column (about 3.5 muM). Subsequently, surface DON concentrations increased up to autumn, with a subsurface maximum greater than 6.0 muM observed in September. During this time, inorganic nutrients became virtually undetectable in the UML. The total DON content (0-1000 m) increased by about 784 mmol m(-2) from winter through to the end of summer. At this time, labile and semilabile DON represented a maximum of 20% of the bulk DON. Assuming that DON was essentially removed from surface water by diffusive process during the stratified period and by vertical mixing during the rest of the year, 210 to 723 mmol m(-2) a(-1) was exported as DON, which represented 40-90% of the total new production. Following this DON export enabled us to balance the nitrogen budget of the area studied over the duration of the experiment.