ALBERT

Description: During summer 1994, the production regime at 2 sites located in the Indian sector of the Southern Ocean, one in the Permanent Open Ocean Zone (POOZ) at 52° S, and a second in the Seasonal Ice Zone (SIZ) at 63° S, was dominated by regeneration (0.3 〈 f-ratio 〈 0.4). Two time series, each of about 4 d, were performed over pre-determined time intervals of 4 h using a free-floating sediment trap set at 200 m at the 2 sites. Hourly variations of C, N, chlorophyll a (chl a) and its degradation products, taxon-specific pigments, lipid classes and dimethylsulfoniopropionate (DMSP) were measured simultaneously. Measurements in the water column were done during the sediment trap drifting. Fucoxanthin, a typical diatom pigment, was the major accessory pigment found in the trap material at the 2 stations, whereas, in the water column, the phytoplankton was dominated by flagellates in the POOZ and diatoms in the SIZ. This suggests selective grazing of diatoms by zooplankton and/or mass sinking of diatoms, at least in the POOZ. However, since the set of compounds exhibited strong diel cycles in the POOZ, the export flux appears to mainly result from the zooplankton. The results are ambiguous as to whether the intensified sedimentation at night resulted from vertical migration of euphausiids, since copepods crossing the pycnocline were rare, and/or the nocturnal increase of feeding activity of copepods and microzooplankton. At the SIZ, diatoms dominated in the mixed layer and at the deep phytoplankton maximum (DPM) located at the depth of the temperature minimum (50 to 100 m). However, pigment signature in the trap material suggested the selective sedimentation of nanoflagellates (essentially pelagophytes). Correspondingly, there was high proportions of sterols up to 40% in the trap material. The diel variations somewhat resembling POOZs, the low chl a-to-phaeopigments ratio and the presence of phaeopigments in their most degraded forms were strong indications of the key role played by zooplankton in the export fluxes in the SIZ. Hence, the selective feeding of phytoplankton by zooplankton strongly influenced the sedimentation pathways in the Indian Sector of the Southern Ocean. Additionally, DMSP, which is produced by phytoplankton in superficial waters, is shown to be as labile as chl a when processed by grazers.

Description: In the framework of the projects DYFAMED and PICASSO, diel variations of particulate dimethylsulfoniopropionate (DMSPp) and of its size fraction higher than 10 µm (DMSPp〉10 µm) were studied in surface waters of the central 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 µm was undergoing diurnal variations. DMSPp-containing particles in the size range higher than 10 µm also markedly affected the DMSPp-to-chlorophyll (chl) a ratio of surface waters on a daily basis. The ratios were 35% to 72% lower at dawn than at 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 µm. 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 µm. The results of May 1995 are ambiguous as to the role of dinoflagellates because, in the absence of specific cell counts, DMSPp〉10 µm and the pigment peridinin, which is usually present in dinoflagellates (but peridinin-free dinoflagellates exist) showed very different vertical and temporal patterns.

Description: 234Th was used to quantify sinking fluxes and residence times of particles in surface waters of the north-western Mediterranean Sea. Measurements of dissolved and particulate 234Th were made at the DYFAMED station (43°25'N-7°51'E, JGOFS-France program). Sampling covered 1 year on four cruises in 1994 (February 9, April 29, June 3, October 1) and focused on a transition period in mid-spring with six repeated profiles collected during May 1995. 234Th was nearly in equilibrium with its parent 238U most of the year, except in spring. The intensive sampling in May shows a rapid evolution throughout the month from a moderate 234Th deficit to near-equilibrium values. The time-series of 234Th were treated with steady-state and non-steady-state models. 234Th particulate fluxes clearly indicate large variability in export, with the highest values observed in spring. Particle residence times in the upper 40 m range from 〈10 to 〉250 days, and could increase by a factor of 10 within 2 weeks. POC fluxes from the upper 40 m and export ratios (ThE: ratio of 234Th-derived POC export to primary production), derived from the 234Th/238U disequilibrium in the water column and POC/234Th ratio on trapped material, decrease from about 9.5 mmol C m**-2 d**-1 and 〉22% in early May to less than 5 mmol C m**-2 d**-1 and 15% after mid-May. The 234Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity.