Cross-slope variations of organic carbon and bacteria in the Gulf of Lions in relation to water dynamics (northwestern Mediterranean)

Sc Yoro (1) , Richard Sempere (1) , C. Turley (2) , Ma Unanue (3) , X. Durrieu de Madron (4) , M Bianchi (1)

1 Laboratoire de Microbiologie Marine
2 PML - Plymouth Marine Laboratory
3 UPV/EHU - Universidad del Pais Vasco / Euskal Herriko Unibertsitatea [Espagne]
4 CEFREM - Centre de Formation et de Recherche sur les Environnements Méditérranéens

During November 1994,seawater samples were collected in the Gulf of Lions in the north- western Mediterranean Sea. Four stations were chosen to cover a range of environments, from coastal seawater near Marseille, France, to open ocean waters 30 miles off the coast. Samples were studied for dissolved and particulate organic carbon (DOC and POC) as well as bacterial abundance and chloro- phyll a (chl a) In the water column, DOC comprized 93 to 99% of total organic carbon, ranged from 65 to 118pM and was lower in deep waters on the slope. Considering an average 82 ~.IMDOC concen- tration measured In the surface layer (0 to 70 m) of the slope as typical of the core of the northwestern Mediterranean current, we estimated the DOC load carried by the current to range from 82 to 164 X to3 mol C S-', which was ca 100 times higher than the Rhone River input for the same period. Chl a con- centrations were up to 224 ng I-' whereas bacterial concentrations ranged from 0.9 to 7.7 X 10' cells ml-' making up 17 to 24% of the POC in the surface layer (0 to 70 m). Bacterial-C/phytoplankton-C ratios around the slope were higher than offshore and were in good agreement with bacterial production/ prlmary product~onratios. These results indicate a time lag between autotrophic phytoplanktonic and heterotroph~cbacterial activities and/or differences in the food web structure from the slope to the seaward end of the section. Although the bacterial-C/phytoplankton-C ratios were lower at the coastal station, the lowest primary production as well as higher bacterial production/primary production ratios were calculated in this area. This suggests that a part of bacterial production was sustained by terres- trial organic matter on the shelf Variations among stations sampled during comparable climatological conditions revealed the existence of a spatial gradient across the slope.

Mots clés: Dissolved organic carbon; Particulate organic carbon .; Bacteria Chlorophyll a North- western Mediterranean Sea

Domaines: Océan, Atmosphère

Sc Yoro, Richard Sempere, C. Turley, Ma Unanue, X. Durrieu de Madron, et al.. Cross-slope variations of organic carbon and bacteria in the Gulf of Lions in relation to water dynamics (northwestern Mediterranean). Marine Ecology Progress Series, 1997, 161, pp.255-264. ⟨hal-02053059⟩