MEPS 161:255-264 (1997)  -  doi:10.3354/meps161255

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

Séri C. Yoro1, Richard Sempéré1,*, Carol Turley2, Maria A. Unanue3, Xavier Durrieu de Madron4, Micheline Bianchi1

1Laboratoire de Microbiologie Marine, CNRS/INSU UPR 223, Case 907, Université de la Méditerranée, Campus de Luminy, F-13288 Marseille Cedex 9, France
2Plymouth Marine Laboratory, Citadel Hill, Plymouth PL1 2PB, United Kingdom
3Departamento Inmunologia, Microbiologia y Parasitologia, Facultad de Ciencias Universidad del Pais Vasco, Apdo. 644, E-48080 Bilbao, Spain
4Laboratoire de Sédimentologie et Géochimie Marines, CNRS/INSU UA 715, Université de Perpignan, F-66860 Perpignan Cedex, France
*Addressee for correspondence. E-mail:

During November 1994, seawater samples were collected in the Gulf of Lions in the northwestern 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 chlorophyll a (chl a). In the water column, DOC comprized 93 to 99% of total organic carbon, ranged from 65 to 118 µM and was lower in deep waters on the slope. Considering an average 82 µM DOC concentration 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 103 mol C s-1, which was ca 100 times higher than the Rhône River input for the same period. Chl a concentrations were up to 224 ng l-1 whereas bacterial concentrations ranged from 0.9 to 7.7 x 105 cells ml-1 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/primary production ratios. These results indicate a time lag between autotrophic phytoplanktonic and heterotrophic bacterial 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 terrestrial organic matter on the shelf. Variations among stations sampled during comparable climatological conditions revealed the existence of a spatial gradient across the slope.

Dissolved organic carbon · Particulate organic carbon · Bacteria · Chlorophyll a · Northwestern Mediterranean Sea

Full text in pdf format