Ž . Journal of Marine Systems 25 2000 119–128 www.elsevier.nlrlocaterjmarsys Diel variations of the DMSP-to-chlorophyll a ratio in Northwestern Mediterranean surface waters S. Belviso a, ) , U. Christaki a,1 , F. Vidussi b,2 , J.-C. Marty b , M. Vila c , M. Delgado c a Laboratoire des Sciences du Climat et de l’EnÕironnement, UMR CEA-CNRS 1572, AÕenue de la Terrasse, 91198 Gif-sur-YÕette, France b ObserÕatoire Oceanologique de Villefranche, L.P.C.M., BP 08, 06230 Villefranche-sur-mer, France ´ c Institut de Ciences del Mar, Passeig Joan de Bordo s r n, 08039 Barcelona, Spain ` Received 6 July 1999; accepted 10 January 2000 Abstract In the framework of the projects DYFAMED and PICASSO, diel variations of particulate dimethylsulfoniopropionate Ž . Ž . DMSPp and of its size fraction higher than 10 mm DMSPp )10 mm were studied in surface waters of the central Ž . Ligurian Sea in May 1990 and May 1995, and in the harbour of the city of Barcelona Spain in July 1998. Time series performed in stratified and nitrate depleted surface waters of the Ligurian Sea revealed that DMSPp )10 mm was undergoing diurnal variations. DMSPp-containing particles in the size range higher than 10 mm also markedly affected the Ž . DMSPp-to-chlorophyll chl a ratio of surface waters on a daily basis. The ratios were 35% to 72% lower at dawn than at Ž . dusk. The fact that the diadinoxanthin DD -to-chl a ratio of surface phytoplanktonic populations did not exhibit diurnal cycles suggests that physiological adaptation of cellular DMSP and chl a to the light conditions was not a likely process to account for the diurnal changes of the DMSPp-to-chl a ratio. It is suggested that such diurnal variability resulted from changes in plankton composition due to vertical migrations of DMSP-containing organisms larger than 10 mm. We have demonstrated from samples collected in the harbour of the city of Barcelona that DMSP-containing dinoflagellates are active diel migrants. However, the results obtained in the open sea in May 1990 suggest that dinoflagellates and also ciliates contribute to the pool of DMSPp in the size range larger than 10 mm. The results of May 1995 are ambiguous as to the role of dinoflagellates because, in the absence of specific cell counts, DMSPp )10 mm and the pigment peridinin, which is Ž . usually present in dinoflagellates but peridinin-free dinoflagellates exist showed very different vertical and temporal patterns. q 2000 Elsevier Science B.V. All rights reserved. Keywords: DMSP; DMS; chlorophyll; pigments; diel cycles; diel migrations; dinoflagellates; ciliates ) Corresponding author. Tel.: q 33-1-6908-8151; fax: q 33-1- 6908-8546. Ž . E-mail address: belviso@lsce.saclay.cea.fr S. Belviso . 1 Present address: National Centre for Marine Research, 16604 Aghios Kosmas, Hellinikon, Greece. 2 Present address: Institut des Sciences de la Mer de Rimouski, 310 allee des Ursulines, G5L 3A1 Rimouski, Quebec, Canada. ´ 1. Introduction Ž . Particulate dimethylsulfoniopropionate DMSPp in the oceanic surface waters has a phytoplanktonic origin and is the major precursor of the atmospheri- Ž cally important gas dimethylsulfide Charlson et al., . Ž . 1987 . Chlorophyll a chl a is the most widely employed measure of phytoplankton biomass and 0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-7963 00 00011-7