Variabilité hydrologique et biologique du golfe du Lion. I. Transports en azote et productivité potentielle

Abstract

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" 12'5 E between 43" 10'N and 42" 50'N) at the entrance of the Gulf of Lions was carried out over two years. The NMC exhibited two distinct structures: a winter structure (from November to May) characterised by strong mesoscale 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-'; and a summer structure (from May to November) which was less sensitive to mesoscale variations but showing large interannual variability. The current was located more offshore, was wider and shallower than the one in winter and velocity remained below 20 cm s-'. Meteorological conditions, frontal structures on both sides of the NMC, coastal upwelling and continental inputs explained the observed mesoscale discrepancies between water flux and nitrogen transport. Nitrogen input from the general circulation across the prospected transect is estimated at 187 + 20 kTa-' in the O-200 m layer, with a distribution of 39 % nitrate, 6 % particulate nitrogen and 55 % dissolved organic nitrogen. Nitrate input (25 + 3 kT a-') in the O-l 00 m upper layer could sustain more than one third of the potential new production in the Gulf of Lions. 0 Elsevier. Paris hydrology I mesoscale variability I nitrogen I flux I budget R&urn4-Un suivi mensuel de l'evolution saisonniere du Courant Nord Mtditerraneen (CNM) et des flux en azote a travers une radiale (5" 12'5 E entre 43" 1 O'N et 42" 50'N) a CtC realise, & l'entrte du golfe du Lion, sur une periode de deux ans. Le CNM est decrit selon deux structures disdnctes. Une structure hivernale (de novembre a mai) caracterisee par de fortes variations a m&o-echelle et interannuelles ; la veine de courant est large de 20 a 30 km et profonde d'environ 400 m, avec des vitesses comprises entre 5 et plus de 40 cm s-'. Une structure estivale (entre mai et novembre) moins sensible aux variations a m&o-echelle mais presentant une forte variabilite interannuelle ; la veine est alors plus hauturiere, plus large et moins profonde qu'en hiver. les vitesses restant generalement inferieures a 20 cm s-'. Les conditions meteorologiyues, la presence de zones frontales sur les bords interne et externe du CNM, les phtnomenes d'upweffing &tier et d'apports continentaux permettent d'expliquer les decouplages observes entre les flux en eau et en azote a travers la radiale. Les apports en azote par la circulation g&r&ale a travers la radiale sont estimes a 187 rt 20 kT a-' dans la couche O-200 m, et se repartissent en 39 % de nitrate, 6 % d'azote particulaire et 55 % d'azote organique dissous. Les apports de nitrate dans la couche superficielle O-100 m (25 + 3 kTa-') pourraient etre a l'origine de pres d'un tiers de la production nouvelle potentielle du golfe du Lion. 0 Elsevier, Paris

Nitrification rates in the NW Mediterranean Sea

“…Corresponding to the spring situation, primary production was relatively high for the NW Mediterranean, even at the oligotrophic site of the Balearic Islands, the pelagic Stn S5, with a production of 187 mg C m-' d-'. The most productive station was Stn 1 in April, with about 1700 mg C m-2 d-l, corresponding to values measured during a bloom period (Conan et al 1998). Simultaneously, the depth of the nitracline was around 10 to 20 m in April and May, then deepened to 60 to 70 m in June, as usually observed in this area of the Mediterranean Sea.…”

“…In addition, in the Pacific Ocean, Raimbault et al (1998) showed a n overestimation of 20 to 100% of the new production when nitrification was not taken into account. In less oligotrophic areas, such as Stn M3, the contribution of nitrate regenerated by nitrification was 27%.…”

“…produced by nitrification in the euphotic layer) leads to regenerated production. So, if nitrification is not considered, new production is drastically overestimated from 20 to l o o % , as shown by Raimbault et al (1998) in the equatorial Pacific. In this case, the nitrification has a huge effect on the calculation of the 'f factor', which corresponds to the ratio of new and total production generally estimated using l i N methods.…”

Impact of phytoplankton and bacterial production on nutrient and DOM uptake in the Rhône River plume (NW Mediterranean)

“…Large quantities of nitrogen, phosphorus, and carbon are stored and transported as organic and inorganic matter (in the dissolved phase) in terrestrial water discharge. The Rhône contributes only 0.1% of the regions total water flux, but contributes 1% of the dissolved organic carbon (DOC) flux (3 × 10 10 mol yr -1 , Yoro et al 1997), and 50% of the nutrient flux (1 × 10 8 mol yr -1 for PO 4 and from 4 × 10 9 to 7 × 10 9 mol yr -1 for NO 3 , Conan et al 1998). Inputs may support 50 to 70% of the new production for the whole Gulf of Lion (Lefèvre et al 1997) and their variations may change the growth and behaviour of phytoplankton.…”

Can Satellite-derived Chlorophyll Imagery Be Used to Trace Surface Dynamics in Coastal Zone? A Case Study in the Northwestern Mediterranean Sea

“…Both observations [43] and modeling results [44] have demonstrated the important role of upwellings on marine productivity in the GoL. However, there is not an exact correspondence between the upwelling events predicted by the model and the occurrence of regions A-D (defined by relatively high SSS and [Chl] values).…”