The coccolithophores Emiliania huxleyi and Coccolithus pelagicus: Extant populations from the Norwegian–Iceland Seas and Fram Strait
The distributions of the coccolithophore species Emiliania huxleyi and Coccolithus pelagicus (heterococcolith-bearing phase) in the northern North Atlantic were investigated along two zonal transects crossing Fram Strait and the Norwegian–Iceland Sea, respectively, each conducted during both July 2011 and September–October 2007. Remote-sensing images as well as CTD and ARGO profiles were used to constrain the physico-chemical state of the surface water and surface mixed layer at the time of sampling. Strong seasonal differences in bulk coccolithophore standing stocks characterized the northern and southern transects, where the maximum values of 53×103 cells/l (fall) and 70×103 cells/l (summer), respectively, were essentially explained by E. huxleyi. This pattern confirms previous findings of a summer to fall northwestward shift in peak coccolithophore cell densities within the Nordic Seas. While depicting an overall zonal shift in high cell densities between the summer (Norwegian Sea) and fall (northern Iceland Sea) conditions, the southern transects were additionally characterized by local peak coccolithophore concentrations associated with a geographically and temporally restricted convective process (Lofoten Gyre, summer), as well as an island mass effect (in the vicinity of Jan Mayen Island, fall). Maximum coccolithophore abundances within Fram Strait were found during both seasons close to the western frontal zone (Polar and Arctic Fronts) an area of strong density gradients where physical and chemical properties of the surface mixed layer are prone to enhance phytoplankton biomass and productivity. Here, changes in species dominance from E. huxleyi in summer, to C. pelagicus in fall, were related to the strengthened influence during summer, of surface AW, as well as to high July solar irradiance, within an area usually characterized by C. pelagicus-dominated low density populations.
(Deep Sea Research Part I: Oceanographic Research Papers. vol. 98, n° 0967-0637, pp. 1-9, 24/04/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UiT
Radar altimetry backscattering signatures at Ka, Ku, C, and S bands over West Africa
This study presents a comprehensive comparison of radar altimetry signatures at Ka-, Ku-, C-, and S-bands using SARAL, ENVISAT and Jason-2 data over the major bioclimatic zones, soil and vegetation types encountered in West-Africa, with an emphasis on the new information at Ka-band provided by the recently launched SARAL–Altika mission. Spatio-temporal variations of the radar altimetry responses were related to changes in surface roughness, land cover and soil wetness. Analysis of time series of backscattering coefficients along the West African bioclimatic gradient shows that radar echoes at nadir incidence are well correlated to soil moisture in semi-arid savannah environments. Radar altimeters are able to detect the presence of water even under a dense canopy cover at all frequencies. But only measurements at Ka-band are able to penetrate underneath the canopy of non-inundated tropical evergreen forests.
(Physics and Chemistry of the Earth. Parts A/B/C. vol. 83-84, n° 1474-7065, pp. 96-110, 24/04/2026)
GET, IRD, UT3, Comue de Toulouse, INSU - CNRS, CNES, CNRS, LEGOS, IRD, UT3, Comue de Toulouse, INSU - CNRS, CNES, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, USTTB
Bidecadal North Atlantic ocean circulation variability controlled by timing of volcanic eruptions
While bidecadal climate variability has been evidenced in several North Atlantic paleoclimate records, its drivers remain poorly understood. Here we show that the subset of CMIP5 historical climate simulations that produce such bidecadal variability exhibits a robust synchronization, with a maximum in Atlantic Meridional Overturning Circulation (AMOC) 15 years after the 1963 Agung eruption. The mechanisms at play involve salinity advection from the Arctic and explain the timing of Great Salinity Anomalies observed in the 1970s and the 1990s. Simulations, as well as Greenland and Iceland paleoclimate records, indicate that coherent bidecadal cycles were excited following five Agung-like volcanic eruptions of the last millennium. Climate simulations and a conceptual model reveal that destructive interference caused by the Pinatubo 1991 eruption may have damped the observed decreasing trend of the AMOC in the 2000s. Our results imply a long-lasting climatic impact and predictability following the next Agung-like eruption.
(Nature Communications. vol. 6, n° 2041-1723, pp. 6545, 24/04/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LOCEAN, IPSL, ENS-PSL, PSL, UVSQ, UPMC, CEA, INSU - CNRS, X, IP Paris, CNES, CNRS, MNHN, IRD, UPMC, INSU - CNRS, CNRS, PARVATI, LOCEAN, IPSL, ENS-PSL, UVSQ, UPMC, CEA, INSU - CNRS, X, CNES, CNRS, MNHN, IRD, UPMC, INSU - CNRS, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, GLACCIOS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CNRM, INSU - CNRS, CNRS, Comue de Toulouse
Carbon fixation and emission within dense aquatic weed mats of a temperate shallow lake
Dense aquatic weed mats are likely to enhance oxygen depletion and build-up of organic matter. As a result of a thick biomass layer, water flow is reduced and hypoxic conditions prevail; the imbalance between aerobic and anaerobic processes can influence the net ecosystem metabolism and its carbon budget. We here present results from a multi-year investigation (2013-2015) carried out on a mesotrophic shallow lake of south-west of France (Lacanau Lake), where 10% of surface is occupied by dense beds of Egeria densa and Lagarosiphon major. Investigations were carried out through multiple approaches, such as 24h-cycle water collection, GIS mapping, growth rates measurements and sedimentary organic matter assessment. Results from this study revealed that, thanks to advantageous hydrodynamics local conditions (strong wind), those plants do not necessarily trigger water stagnation and hypoxia. Nevertheless, their fast growth generates the accumulation of a huge quantity of labile litter, which feeds respiration processes and methanogenesis on the bottom of the macrophytes bed. Our study underlines that dense aquatic weed mats may constitute important hotspots for greenhouse gases emissions at our latitude, notably enhanced by mild temperature and strong irradiation all over the year.
(pp. 17, 24/04/2026)
UR EABX, IRSTEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Atlas des diatomées des eaux douces de la Réunion
L’ile de la Réunion, de par son caractère insulaire et son isolement, présente de grandes particularités des assemblages biologiques, doublée d’un fort taux d’endémisme. Les diatomées des cours d’eau ne font pas exception à ce constat, et étaient fortement méconnues jusqu’à un passé récent. Or la connaissance des espèces présentes et de leur écologie est le premier pas pour leur protection face aux pollutions et aux pressions qui s’exercent sur leurs habitats. Un programme de recherche-développement financé par l’Office de l’Eau de la Réunion, la DEAL Réunion, le FEDER et l’ONEMA a été lancé à partir de 2008 et a duré jusqu’à fin 2013. Il a été conduit par le consortium Asconit-Cemagref (devenu depuis Irstea). Le but était d’identifier les espèces diatomiques locales, de les nommer, de leur affecter une écologie et une écologie de l’altération, de fonder un nouvel indice diatomique adapté à l’évaluation des cours d’eau de la Réunion (l’IDR) et de transférer cette nouvelle méthode avec la production des documents et outils d’accompagnement (guide méthodologique, guide taxonomique, outil de calcul sous R et sa notice d’utilisation etc. Au cours de ce programme, 343 taxons ont pu être identifiés ou reconnus à l’espèce, certains non trouvés dans la littérature taxonomique mondiale se voyant affecter un code de genre et un numéro d’espèce. Le présent Atlas taxonomique édité par l’Office de l’Eau de la Réunion contient un grand nombre de fiches-taxons décrivant la plupart de ces espèces locales, avec des indications morphologiques et des supports iconographiques au microscope optique et électronique permettant de les reconnaître et de les déterminer : des cartes chorologiques permettant de résumer à quels sites ou dans quels cours d’eau on les trouve, de façon différenciée aux deux saisons ; et enfin, des figurations permettant de situer et de résumer leurs caractéristiques autoécologiques par rapport à des variables abiotiques naturelles et à d’autres sous forte influence des altérations anthropiques.
(pp. 230, 24/04/2026)
UR EABX, IRSTEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LEFE, INEE-CNRS, CNRS, UT3, Comue de Toulouse, CNRS, Toulouse INP, Comue de Toulouse
Divergent biophysical controls of aquatic CO2 and CH4 in the World's two largest rivers
Carbon emissions to the atmosphere from inland waters are globally significant and mainly occur at tropical latitudes. However, processes controlling the intensity of CO2 and CH4 emissions from tropical inland waters remain poorly understood. Here, we report a data-set of concurrent measurements of the partial pressure of CO2 (pCO2) and dissolved CH4 concentrations in the Amazon (n = 136) and the Congo (n = 280) Rivers. The pCO2 values in the Amazon mainstem were significantly higher than in the Congo, contrasting with CH4 concentrations that were higher in the Congo than in the Amazon. Large-scale patterns in pCO2 across different lowland tropical basins can be apprehended with a relatively simple statistical model related to the extent of wetlands within the basin, showing that, in addition to non-flooded vegetation, wetlands also contribute to CO2 in river channels. On the other hand, dynamics of dissolved CH4 in river channels are less straightforward to predict and are related to the way hydrology modulates the connectivity between wetlands and river channels.
(Scientific Reports. vol. 5, n° 2045-2322, pp. 15614, 24/04/2026)
ULiège, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, KU Leuven, UENF
Mercury mobilization and speciation linked to bacterial iron oxide and sulfate reduction: A column study to mimic reactive transfer in an anoxic aquifer
Mercury (Hg) mobility and speciation in subsurface aquifers is directly linked to its surrounding geochemical and microbial environment. The role of bacteria on Hg speciation (i.e., methylation, demethylation and reduction) is well documented, however little data is available on their impact on Hg mobility. The aim of this study was to test if (i) Hg mobility is due to either direct iron oxide reduction by iron reducing bacteria (IRB) or indirect iron reduction by sulfide produced by sulfate reducing bacteria (SRB), and (ii) to investigate its subsequent fate and speciation. Experiments were carried out in an original column setup combining geochemical and microbiological approaches that mimic an aquifer including an interface of iron-rich and iron depleted zones. Two identical glass columns containing iron oxides spiked with Hg(II) were submitted to (i) direct iron reduction by IRB and (ii) to indirect iron reduction by sulfides produced by SRB. Results show that in both columns Hg was leached and methylated during the height of bacterial activity. In the column where IRB are dominant, Hg methylation and leaching from the column was directly correlated to bacterial iron reduction (i.e., FeII release). In opposition, when SRB are dominant, produced sulfide induced indirect iron oxide reduction and rapid adsorption of leached Hg (or produced methylmercury) on neoformed iron sulfides (e.g., Mackinawite) or its precipitation as HgS. At the end of the SRB column experiment, when iron-oxide reduction was complete, filtered Hg and Fe concentrations increased at the outlet suggesting a leaching of Hg bound to FeS colloids that may be a dominant mechanism of Hg transport in aquifer environments. These experimental results highlight different biogeochemical mechanisms that can occur in stratified sub-surface aquifers where bacterial activities play a major role on Hg mobility and changes in speciation.
(Journal of Contaminant Hydrology. vol. 180, n° 0169-7722, pp. 56 - 68, 24/04/2026)
BRGM, UNIGE, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
The region of the Strandja Sill (North Turkey) and the Messinian events
The two sides of the Strandja Sill show a highly discontinuous stratigraphic succession since the Late Oligocene. This area, together with the Sea of Marmara Basin, is usually proposed as the gateway for the Paratethyan freshwaters and organisms that constituted the Lago Mare facies in the Mediterranean Sea during the Messinian Salinity Crisis (MSC). Our investigations involving new field observations and datings, together with previous studies, suggest that the sill has possibly experienced such a connection at around 8 Ma, i.e. significantly before the crisis. The proposal of a sea-level drop of the Black Sea before 7 Ma is not supported by our data on dinoflagellate cysts. Consistency of calcareous nannofossil succession at DSDP Site 380 is reinforced, allowing to reassert that subaerial erosion impacted both the southwestern Black Sea and the central Marmara – Dardanelles area during the peak of the MSC. At that time, this region was crossed by two oppositely directed fluvial networks, further supporting the absence of a marine gateway through the Strandja Sill. It is concluded that none of the Lago Mare events recorded in the Mediterranean during the MSC were the consequence of the passage of Paratethyan waters and organisms through this area. In the Black Sea, the well-dated Messinian fluvial erosion can be followed offshore. The overlying prograding deltaic deposits attest to a fast marine reflooding after the crisis. This constitutes a comprehensive erosion-sedimentation model in an area intensively explored for hydrocarbons.
(Marine and Petroleum Geology. vol. 66, n° 0264-8172, pp. 149-164, 24/04/2026)
iSTeP, UPMC, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, ITÜ, IFREMER, IPGP, INSU - CNRS, UPD7, UR, IPG Paris, CNRS, GeoEcoMar, CR2P, MNHN, UPMC, CNRS, CEREGE, IRD, INRA, AMU, CdF (institution), INSU - CNRS, CNRS, AYBE, ITÜ, Cerema, NIHWM
Pervasive multidecadal variations in productivity within the Peruvian Upwelling System over the last millennium
There is no agreement on the pluri-decadal expression of El Ni~ noeSouthern Oscillation (ENSO) in the Pacific over the last millennium. Marine records from the Peruvian margin indicate humid conditions (El Ni~ no-like mean conditions) over the Little Ice Age, while precipitation records from the eastern equatorial Pacific infer arid conditions (La Ni~ na-like mean conditions) for the same period. We here studied diatom assemblages, nitrogen isotopes, and major and minor elements at the lamination level in three laminated trigger cores located between 11 S and 15 S on the Peruvian shelf within the oxygen minimum zone (OMZ) to reconstruct precipitation and ocean productivity at the multiannual to multidecadal timescales over the last millennium. We respected the sediment structure, thus providing the first records of the mean climatic conditions at the origin of the lamination deposition, which ones represent several years. Light laminations were deposited under productive and dry conditions, indicative of La Ni~ na-like mean conditions in the system, while dark laminations were deposited under non-productive and humid conditions, representative of El Ni~ no-like mean conditions. La Ni~ na-like mean conditions were predominant during the Medieval Warm Period (MWP; 1000e600 years BP) and Current Warm Period (CWP; 150 years BP to present), while El Ni~ no-like mean conditions prevailed over the Little Ice Age (LIA; 600e150 years BP). We provide evidence for persistent multidecadal variations in productivity over the last millennium, which were disconnected from the mean climate state. Multidecadal variability has been stronger over the last 450 years concomitantly to increased variability in the NAO index. Two intervals of strong multidecadal variability were also observed over the MWP, congruent to decreased solar irradiance and increased volcanic activity.
(Quaternary Science Reviews. vol. 125, n° 0277-3791, pp. 78-90, 24/04/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CAU
Is the toxicity of pesticide mixtures on river biofilm accounted for solely by the major compounds identified?
Pesticides are present as a cocktail of compounds at low concentrations in water bodies. Nevertheless, studies assessing the toxic effects of pesticide mixtures at realistic environmental concentrations are still scarce. Polar Organic Chemical Integrative Sampler (POCIS) allows the concentration of several organic chemicals from large volumes of natural water [1]. This high pre-concentration of compounds makes POCIS a powerful tool to assess extract toxicity via biological testing and therefore to study the impact of realistic mixtures . PE can be used as a black box with the advantages of dealing with mixtures “as they came”; this approach gives an estimation of an integrative measure of the toxic potential of a group of compounds including unknown toxicants without using an a priori approach. With this protocol, the global toxicity of the extract is assessed although the compounds responsible for the observed toxicity are not identified. The present study aimed to characterize the effects of long-term and low-dose exposure to POCIS extracts in a context of contamination by cereal crops on a natural biofilm community and to evaluate whether the effects observed due to exposure to PE were explained by the major compounds identified in the extracts [2].
(pp. 2, 24/04/2026)
UR EABX, IRSTEA, IRSTEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS