Description of the biogeochemical features of the subtropical southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer of the International Polar Year

International audienceMeridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (Chl a), phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively), four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC), and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of Chl a and phaeopigments never exceeded 0.5 µg L−1 and 0.25 µg L−1, respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isoclines. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area, Chl a level of 0.4-0.5 µg L−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 µM and 1.5 µM, respectively, indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a post-bloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 µM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 µM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid, suggesting a late stage of a relatively minor diatom bloom possibly driven by iceberg drifting releases of iron. In this domain the estimated range of silica daily net production rate (e.g. 21.1 ± 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region

Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics

Phytoplankton size classes competitions at sub-mesoscale in a frontal oceanic region

International audienceThe effects of mesoscale and sub-mesoscale dynamics on the competition between two different phytoplankton size classes are investigated with a 3D primitive equations model. The model reproduces realistic simulations of mesoscale turbulence generated by a westward current in the southern hemisphere at statistical equilibrium in a summer situation. Effects of two different grazing pressures on phytoplankton competitions are compared and the role of eddy variability is quantified comparing high and low resolution simulations. High resolution simulations reveal a filamentary distribution of biomass and nutrients induced by the combination of vertical advection and horizontal stirring. This fine scale variability is observed not only on the horizontal but also on the vertical into the subsurface chlorophyll maximum. One of the key results is that such a dynamics induces a spatial segregation of the phytoplankton in the southern part of the frontal region that is mainly filamentary. This spatial segregation consists in biomass maxima for large phytoplankton in rich nutrients filaments and maxima for small phytoplankton outside these filaments. This anti-correlation is particularly strong when grazing pressure is low and is confirmed by statistical analysis. In the central frontal region, dominated by mesoscale dynamics, the two phytoplankton classes are strongly correlated together and biomass maxima are located close to downwelling regions that are poor in nutrients. It is shown that the effect of grazing is significantly amplified by the fine scale dynamics and that the combination of these two mechanisms is responsible of a switch of the ecosystem dominance in the surface layers. In addition, the effect of frontal dynamics. on the detritus export is very sensitive to gazing pressure: increasing grazing induces a significant decrease of the export in the presence of frontal dynamics whereas it induces an increase of the export without small-scale variability

Marine producers, consumers and climate change: From an ecological stoichiometry perspective

participantEcological stoichiometry is a rapidly expanding research field investigating how the elemental composition of organisms affects ecological processes. Owing to the fact that the ratios of carbon (C) :nitrogen (N) : phosphorus (P) are different among aquatic organisms, their recycling nutrients may thus affect the supply and the stoichiometry of C, N and P in the ocean. In contrast, the changing ocean (e.g. by climate change and human perturbation) may also affect the C:N:P stoichiometry of the ocean, and eventually the C:N:P of those organisms living in the ocean. Here, we investigated the impact of consumer-driven nutrient recycling (CNR) on oceanic primary production and the distribution of nitrogen (N) and phosphorus (P) in the deep ocean. For this purpose, we used and extended two existing models: a 2-box model of N and P cycling in the global ocean (Tyrrell, 1999), and the model of Sterner (Sterner, 1990) which formalised the principles of CNR theory. The resulting model showed that marine herbivores may affect the supply and the stoichiometry of N and P in the ocean, thereby exerting a control on global primary production. The predicted global primary production was higher when herbivores were included in the model, particularly when these herbivores had higher N:P ratios than phytoplankton. The potential impacts of climate change on the C and nutrient availability in aquatic ecosystems, its consequences for the C:N:P stoichiometry of plankton communities, and its implications for the structure of aquatic food webs are briefly discussed

Influence of consumer-driven nutrient recycling on primary production and the distribution of N and P in the ocean

International audienceIn this study we investigated the impact of consumer-driven nutrient recycling (CNR) on oceanic primary production and the distribution of nitrogen (N) and phosphorus (P) in the deep ocean. For this purpose, we used and extended two existing models: a 2-box model of N and P cycling in the global ocean (Tyrrell, 1999), and the model of Sterner (1990) which formalised the principles of CNR theory. The resulting model showed that marine herbivores may affect the supply and the stoichiometry of N and P in the ocean, thereby exerting a control on global primary production. The predicted global primary production was higher when herbivores were included in the model, particularly when these herbivores had higher N:P ratios than phytoplankton. This higher primary production was triggered by a low N:P resupply ratio, which, in turn, favoured the P-limited N-2-fixation and eventually the N-limited non-fixers. Conversely, phytoplankton with higher N:P ratios increased herbivore yield until phosphorus became the limiting nutrient, thereby favouring herbivores with a low P-requirement. Finally, producer-consumer interactions fed back on the N and P inventories in the deep ocean through differential nutrient recycling. In this model, N deficit or N excess in the deep ocean resulted not only from the balance between N-2-fixation and denitrification, but also from CNR, especially when the elemental composition of producers and consumers differed substantially. Although the model is fairly simple, these results emphasize our need for a better understanding of how consumers influence nutrient recycling in the ocean

Le Parlement et la politique extérieure sous la IVe République / Philippe Pondaven,.. ; préface de Georges Berlia

Collection : Travaux et recherches de l'Université de droit, d'économie et de sciences sociales de Paris. Série Science politique ; 2Collection : Travaux et recherches de l'Université de droit, d'économie et de sciences sociales de Paris ; 2Contient une table des matièresAvec mode text

Effects of an iron-light co-limitation on the elemental composition (Si, C, N) of the marine diatoms Thalassiosira oceanica and Ditylum brightwellii

International audienceWe examined the effect of iron (Fe) and Fe-light (Fe-L) co-limitation on cellular silica (BSi), carbon (C) and nitrogen (N) in two marine diatoms, the small oceanic diatom Thalassiosira oceanica and the large coastal species Ditylum brightwellii. We showed that C and N per cell tend to decrease with increasing Fe limitation (i.e. decreasing growth rate), both under high light (HL) and low light (LL). We observed an increase (T. oceanica, LL), no change (T. oceanica, HL) and a decrease (D. brightwellii, HL and LL) in BSi per cell with increasing degree of limitation. The comparison with literature data showed that the trend in C and N per cell for other Fe limited diatoms was similar to ours. Interspecific differences in C and N quotas of Fe limited diatoms observed in the literature seem thus to be mostly due to variations in cell volume. On the contrary, there was no global trend in BSi per cell or per cell volume, which suggests that other interspecific differences than Fe-induced variations in cell volume influence the degree of silicification. The relative variations in C:N, Si:C and Si:N versus the relative variation in specific growth rate (i.e. mu:mu(max)) followed the same patterns for T. oceanica and D. brightwellii, whatever the irradiance level. However, the variations of C:N under Fe limitation reported in the literature for other diatoms are contrasted, which may thus be more related to growth conditions than to interspecific differences. As observed in other studies, Si:C and Si:N ratios increased by more than 2-fold between 100% and 40% of mu(max). Under more severe limitation (HL and LL), we observed for the first time a decrease in these ratios. These results may have important biogeochemical implications on the understanding and the modelling of the oceanic biogeochemical cycles, e.g. carbon and silica export. [Bucciarelli, E.] Univ Europeenne Bretagne, Rennes, France; Univ Brest, CNRS, IUEM, IRD,LEMAR,UMR 6539, F-29280 Plouzane, France Bucciarelli, E, Univ Europeenne Bretagne, Rennes, France [email protected] European Commission [EVK2-1999-00227]; BOA [ANR-05-BLAN-0153] The authors would like to thank two anonymous reviewers and D. Hutchins for their insightful comments which improved this manuscript. This work was funded by the support from the European Commission's Marine Science and Technology Programme under Contract EVK2-1999-00227 (IRONAGES, "Iron Resources and Oceanic Nutrients - Advancements of Global Environment Simulations") and the BOA project (ANR-05-BLAN-0153). The iron concentration in the background medium was measured by M. Gallinari. A. Masson performed carbon and nitrogen measurements. J. Helias is thanked for his help with the Fe limited cultures of T. oceanica