Distribution of phytoplanktonic parameters in the Indian sector of the Southern Ocean during Indigo III Cruise

The distribution of chlorophyll-a obtained by HPLC along vertical and horizontal profiles during Indigo I11 cruise in the Indian sector of the Southern Ocean was used to correlate the phytoplankton and watermasses patterns. They confirm that the Southern Ocean is not ecologically uniform as is implied by the general term "Antarctic and Subantarctic ecosystem" (Hempel,1985). Highest chlorophyll-a standing crops are associated with the main frontal systems (the Subtropical Convergence, the Antarctic Polar Front and the Antarctic Divergence), In these particular areas, significant amounts of living phytoplankton are found as far as 70 meters deep, suggesting a downwards transport of chlorophyll-a along isopycnals, in relation with the frontal hydrodynamics. In interfrontal areas, chlorophyll-a concentrations are generally very low and restricted to the upper layers of the water column. Therefore, the vertical distribution of phytoplankton in the Southern Ocean is not as uniform as expected, referring to El Sayed (1978) or to Jacques et Minas (1981) . These preliminar results show that a more intensive and detailed study of the Antarctic frontal systems might lead to a better understanding of the phytoplankton distribution and productivity

Comparison of marine copepod outfluxes: Nature, rate, fate and role in the carbon and nitrogen cycles

We compare the nature of copepod outfluxes of nonliving matter, the factors controlling their rate (aid their Jaw, and finally their role, particularly their relative importance in the carbon and nitrogen cycle. Copepods release dissolved matter through excretion and respiration and particulate matter through production of faecal pellets, carcasses, moults, and (lead eggs. Excretion liberates several organic C, N, and P compounds and inorganic N and P compounds, with inorganic compounds constituting the larger part. The faecal pellets of copepods are covered by a peritrophic membrane and have a highly variable size and content. There is less information on the nature of other copepod particulate products. The weight-specific rates of posthatch mortality, respiration, excretion, and faecal pellet production have similar C or N levels and are higher than those of moulting and egg mortality. In general, most important factors controlling these rates are temperature, body mass, food concentration, food quality, and faunistic composition. Physical and biological factors govern the vertical fate of copepod products by affecting their sedimentation speed and concentration gradient. The physical factors are sinking speed, advection, stratification, turbulent diffiusion, and molecular diffusion. They influence the sedimentation speed and degradation of the copepod products. The biological factors are production, biodegradation (by zooplankton, nekton, and microorganisms) and vertical migration of copepods (diel or seasonal). Physical degradation and biodegradation by zooplankton and nekton are faster than biodegradation by microorganisms. The most important copepod outfluxes are excretion and fecal pellet production. Excretion offers inorganic nutrients that can be directly used by . Excretion of primary producers. Faecal pellets have a more important role in the vertical transport of elements than the other particulate products. Most investigation has focused on carbon burial in the form of copepod faecal pellets, measured by sediment traps, and on the role of ammonia excretion in nutrient recycling. Full evaluation of the role of copepod products in the transport and recycling of elements and compounds requires a quantification of all copepod products and their different fates, particularly detritiphagy, remineralization, and integration as marine snow

Water fluxes at an ocean margin in the presence of a submarine canyon

A 3-D, unsteady, nonlinear, high-resolution model is used to estimate shelf/slope exchanges through Calvi Canyon (NW Corsica, Mediterranean Sea) in various regimes of stratification and wind patterns. To evaluate the alongshore and cross-shore fluxes within the canyon area as well as the water exchanges between the canyon and Calvi Bay, volume transports are computed at the sides of two closed, interconnected boxes encompassing the canyon on the shelf and slope domains. Model results show that water transports between Calvi Bay and the open sea are determined by flow modifications in the canyon area. The mean horizontal flow deviates southwestward upstream of the canyon, generating an onshore transport in the western part of Calvi Bay. Within the canyon, the circulation is cyclonic and is responsible for an offshore transport downstream of the canyon and in the eastern part of the bay. The effect of stratification is shown to limit the vertical extent of the influence of canyon topography so that the alongshore flow above the canyon is quasi-undisturbed in strong stratified conditions, resulting in weak cross-shore exchange. Wind events are shown to be responsible for a strong increase of cross-shore transports between the bay and the canyon area

Factors controlling the distribution of diatoms and <i>Phaeocystis</i> in the Ross Sea

he spatial and temporal distributions of phytoplankton pigments were investigated in the western and south central Ross Sea during austral spring 1994 and summer 1990. Large gradients in biomass and phytoplankton community composition were observed both in the east-west and south-north directions, in relation to differences in water column structure and stability, which themselves depend on the processes of ice retreat within the different areas. Important are melting in the western Ross Sea, which induces strong stratification, and ice breakup and wind stress in the south central Ross Sea, which result in deep mixing and weakly stratified waters. In the western Ross Sea, the highest chlorophyll a (chl. a) concentrations observed in this study (129-358 mg m-2 in the upper 100 m) were tightly coupled to the stratified region of meltwater influence and were dominated by diatoms, as indicated by elevated fucoxanthin concentrations (89-239 mg m-2 in the upper 100 m). In the diatom bloom area, high levels of phaeophorbides a (maximum value of 192 mg m-2 in the upper 100 m) indicated that the dominant grazers, identified as Limacina helecina and copepods [Hecq, J.H., Magazzù, G., Goffart, A., Catalano, G., Vanucci, S., Guglielmo, L., 1992. Distribution of planktonic components related to vertical structure of water masses in the Ross Sea and the Pacific sector of the Southern Ocean. In: Anonymous, Atti del 9° congresso A.I.O.L., Santa Margherita Ligure, 20-23 Novembre 1990, 665-678], transferred a sustained part of the diatom production to the herbivore trophic level. Synthesis of our data with published information suggests that the diatom bloom we observed in the western Ross Sea was dominated by the species Fragilariopsis curta, and occurs annually off the coast of Victoria Land from 72°30'S to 77°S within 100-250 km from the coastal ice edge. It is assumed to persist on the order of 2.5-3 months during summer. In the south central Ross Sea, which was characterized by a poorly or unstratified water column, moderate chl. a concentrations (55-186 mg m-2 in the upper 100 m) were found in the polynya and in the ice-edge area in early spring. The 19'-hexanoyloxyfucoxanthin (19'-HF), a biomarker for Phaeocystis, was the major contributor to the total carotenoid abundance (29-132 mg m-2 in the upper 100 m), corroborating the hypothesis that Phaeocystis are well adapted to develop in a relatively mixed water column. At the northern limit of the polynya, a quite narrow (ca. 30 km) diatom bloom (31-67 mg fucoxanthin m-2 in the upper 100 m) overlapped with the Phaeocystis bloom in the slightly stratified marginal ice zone. This was an area of increased grazing pressure by various types of herbivorous zooplankton, among which very large amounts of krill were observed