Sedimentation pathways in the Indian sector of the Southern Ocean during aproduction regime dominated by regeneration

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) a t 52 degrees S, and a second in the Seasonal Ice Zone (SIZ) at 63 degrees S , was dominated by regeneration (0.3 < f-ratio < 0.4). Two time series, eac h of about 4 d, were performed over pre-determined time intervals of 4 h us ing a free-floating sediment trap set at 200 m at the 2 sites. Hourly varia tions of C, N, chlorophyll a (chl a) and its degradation products, taxon-sp ecific pigments, lipid classes and dimethylsulfoniopropionate (DMSP) were m easured simultaneously. Measurements in the water column were done during t he sediment trap drifting. Fucoxanthin, a typical diatom pigment, was the m ajor accessory pigment found in the trap material at the 2 stations, wherea s, in the water column, the phytoplankton was dominated by flagellates in t he 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. Howeve r, 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 fr om vertical migration of euphausiids, since copepods crossing the pycnoclin e 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 tempera ture minimum (50 to 100 m). However, pigment signature in the trap material suggested the selective sedimentation of nanoflagellates (essentially pela gophytes). Correspondingly, there was high proportions of sterols up to 40% in the trap material. The diel variations somewhat resembling POOZs, the l ow chi a-to-phaeopigments ratio and the presence of phaeopigments in their most degraded forms were strong indications of the key role played by zoopl ankton in the export fluxes in the SIZ. Hence, the selective feeding of phy toplankton by zooplankton strongly influenced the sedimentation pathways in the Indian Sector of the Southern Ocean. Additionally, DMSP, which is prod uced by phytoplankton in superficial waters, is shown to be as labile as ch i a when processed by grazers.