Drastic changes in tidal hydrodynamics following seagrass decline and their seasonal variations in a shallow lagoon
Over the past decades, seagrasses have drastically declined worldwide, reducing their capacity to regulate flow conditions. Intertidal species have been particularly affected by this decline, yet there is limited understanding of how intertidal seagrass loss influences hydrodynamics in shallow coastal lagoons. In this study, we use a 3D flow-vegetation model that accounts for vegetation effect on mean and turbulent flow, as well as flow-induced leaf bending, to investigate how tidal hydrodynamics respond to seasonal and multi-decadal changes in intertidal seagrass characteristics. The model is applied to the Arcachon lagoon (France), colonized by extensive Zostera noltei and Zostera marina meadows. This study reveals that a short-leaf and flexible seagrass species such as Zostera noltei can regulate tidal hydrodynamics throughout the lagoon due to the presence of broad and dense meadows on the tidal flats. In summer, seagrass decline leads to a significant increase in the 75th percentile in bottom flow velocities (+100 %) on the tidal flats, but to a decrease in the channels (−20 %). However, in winter, the response of tidal hydrodynamics to the reduction in seagrass coverage is far less pronounced. Comparison of simulated scenarios reveals that the multi-decadal decline of Zostera meadows with summer characteristics and the seasonal loss between summer and winter lead to modifications in tidal-flow parameters (current velocities, tidal asymmetry, high-tide water level) of a comparable magnitude. These changes in hydrodynamics likely enhance suspended sediment concentration, reducing light availability, contributing to further seagrass loss, and modifying sediment management for stakeholders due to enhanced siltation in channels.
(Coastal Engineering. vol. 205, n° 0378-3839, pp. 104948 (34p.), 01/03/2026)
LERAR, COAST, IFREMER, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Seasonal dynamics of cable bacteria in an estuarine intertidal mudflat subject to green tides: Implications for the foraminiferal community and test preservation
As many coastal areas draining intensive agricultural activities, Ledano estuary mudflats (French Brittany) experience Ulva proliferation, causing green tides. We studied the seasonal dynamics of sulphoxidizing cable bacteria from April 2019 to July 2020 using microsensors (O 2 , pH, H 2 S). The activity of these filamentous bacteria, called electrogenic sulphur oxidation (eSOx), results in strong acidification and in pore-water CO 3 2depletion in the first few centimeters of sediment (within the suboxic zone). Living and dead benthic foraminiferal assemblages were studied in July 2020 to observe the effects of eSOx on the calcareous meiofauna and their shell preservation in the sediment. eSOx was patchy on the mudflat but persistent throughout the year. It contributed up to 45% of oxygen consumption during the algal mat decay, and exceeded 100% during the flooding period suggesting stimulation by nitrate inputs. The corrosive effect was maximal in July 2020 (ΔpH ~ 1.7, [CO$_3^{2-}$]calc < 10 μM). The living foraminiferal community was sparse and nearly monospecific, dominated by the calcareous species Haynesina germanica, probably due to green tides coupled with eSOx-driven acidification. However, living specimens of H. germanica showed no signs of advanced dissolution of their shell suggesting a biological capacity to survive in such environmental acidification, potentially linked to their photosynthetic capability. In contrast, the dead assemblages displayed greater diversity despite a loss of about 20% of the calcareous shell recording with depth due to the synergetic effect of low salinity and eSOx. Overall, this study shows that green tides strongly influence cable bacteria activity and then, sedimentary biogeochemical processes in eutrophic coastal environments.
(Journal of Sea Research (JSR). vol. 210, n° 1385-1101, pp. 102683, 01/03/2026)
LPG, UM, UA, INSU - CNRS, CNRS, Nantes univ - UFR ST, Nantes Univ, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
On the relations between morphotectonics, seismicity, perennial and sporadic hydrothermalism on the volcanic island complex of Milos (Greece)
The volcanic island complex of Milos, situated ∼200 km north of the Hellenic subduction zone, records the interplay of volcanic, tectonic, and gravitational processes within multidirectional, polyphased rift systems that developed throughout the Mio-Plio-Quaternary. Its active and seismogenic fault network hosts both perennial and transient hydrothermal venting zones, offering a natural laboratory to examine fluid circulation within a volcanic arc built on stretched continental crust. This study refines the relationships between magmatism, seismicity, and hydrothermal activity, with implications for local geological hazards. Two new morpho-tectonic sketches are presented. The first places the Milos volcanic center within the structural framework of the Aegean microplate; the second illustrates the spatial organization of the Milos tectono-hydrothermal-volcanic system. These reconstructions integrate detailed physiographic analyses (relief, slope), field observations, and multidisciplinary datasets on seismicity, hydrothermalism, and mineralization. At the scale of the Aegean microplate, (i) NE-SW faults are attributed to extensional deformation along the Mid-Cycladic Lineament; (ii) E-W faults relate to the opening of the Milos, Cretan, and Christiana basins, parallel to the mid-Miocene West Cycladic Detachments; (iii) NW-SE faults correspond to the Myrtoon Basin and Gulf of Milos openings, the latter being closed to the south by the Fyriplaka volcano; and (iv) N-S faults controlling the Zephyria graben and the eastern margin of Milos are continuous with Cretan fault systems and are parallel both to the Eocene trans-Cycladic thrust in the region and to magnetic anomalies in the subducted Tethyan oceanic crust south of the Hellenic subduction zone. At the archipelago scale, seismicity, hydrothermal venting, alteration zones, and phreatic explosions are concentrated in the hanging wall of the Achivadolimni fault, along or near the intersections of the N-S Zephyria and NW-SE Fyriplaka grabens, directly above the inferred magma chamber. Microearthquake hypocentres, likely linked to fluid-induced fault dilation, occur beneath the Gulf of Milos (∼7 km) and Fyriplaka volcano (∼5 km). Their distribution suggests a genetic link between the intersecting grabens, cooling of isotherms beneath the gulf by descending seawater, surface hydrothermal manifestations, and tectonic earthquakes (e.g., Mw 5.3 Milos, 1992). A shallow hydrothermal convection loop (<3 km) probably overlies a magmatic reservoir where fluids accumulate beneath a self-sealed, low-permeability cap, possibly corresponding to a thermal brittle-ductile transition (370-450 °C). Co-seismic ruptures may locally breach this barrier, inducing "arterial fault" behaviour along the Achivadolimni fault. Pulsating hydrothermal activity and historical phreatic eruptions are interpreted as surface expressions of transient injections of over-pressurized magmatic fluids, leading to decompression, phase transitions, and fluid
(Bulletin de la Société Géologique de France. vol. 197, n° 0037-9409, 22/02/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LGL-TPE, ENS de Lyon, UCBL, INSU - CNRS, UJM, UJM EPE, CNRS, LGENS, INSU - CNRS, CNRS, ENS-PSL, PSL, I2M-BX, UB, CNRS, INRAE, NKUA
A new open boundary condition for Boussinesq-type models, applied to irregular wave fields
We present a novel approach to handle open boundary conditions for a Boussinesq-type wave model coupled with the nonlinear shallow water equations. Traditional methods for managing open boundaries — such as sponge layers and source functions — are computationally intensive and require ad hoc calibration. To address this, we reformulate the Boussinesq equations as a system of conservation laws with nonlocal flux and a rapidly decaying source term. This reformulation is adapted to generate waves at the boundary of the numerical domain, from surface elevation data in situations where both incoming and outgoing waves are present. The proposed numerical scheme employs a MacCormack prediction-correction strategy combined with finite volume and finite difference methods, preserving key physical properties and ensuring stability. Comparison with laboratory experiments demonstrates that our approach avoids boundary reflection issues. In particular, it is able to accurately reproduce infragravity waves associated with a random wave field propagating over a sloping beach. This work opens important perspectives for improving phase-resolving coastal wave models, with the aim of forecasting complex random wave conditions in natural environments.
(Ocean Modelling. vol. 201, n° 1463-5003, pp. 102713, 21/02/2026)
INSA Toulouse, INSA, Comue de Toulouse, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, IMB, UB, Bordeaux INP, CNRS
Multi-centennial internal variability in the North Atlantic could drive additional warming over Europe
Europe has experienced abnormal warming over the recent decades. Model-based studies highlight that multi-centennial internal variability of the North Atlantic can strongly affect European temperatures. However, the limited availability of high-resolution proxy records has hindered observational assessment of the existence and amplitude of such variability in the real climate system. Here, we compile annual-to-decadal proxy-based Holocene reconstructions, instrumental observations, and climate model simulations to demonstrate the existence of this multi-centennial variability mode and quantify its amplitude. We show that this mode is closely tied to the internal variability of the Atlantic Meridional Overturning Circulation. Its temporal evolution explains part of the observed 20th century variability, and a shift towards a positive phase in the late 1990s can explain the recent amplified warming over Europe. When its amplitude is constrained by observations, this internal variability may enhance anthropogenic warming in Northern Europe by up to 30% over 2000-2035.
(Nature Communications, n° 2041-1723, 11/02/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, VUB, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, MERMAID, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, RHUL
Microplastic transport and trapping in a highly turbid, tide-dominated estuary
Microplastics are an emerging pollutant in aquatic systems, with estuaries acting as key zones for their retention and transformation. However, limited field observations, complex estuarine hydrodynamics and diverse particle properties hinder a comprehensive understanding of microplastic transport and fate, limiting accurate risk assessment and evaluation of environmental impacts. The objective of this work is to better understand the physical processes governing the transport and trapping of microplastics in macrotidal turbid estuaries, using the Gironde estuary (SW France) as a case study. The methodology of this work is mainly based on a hydro-sedimentary numerical model coupled with a Lagrangian particle tracking model. This approach is complemented with in-situ observation data. A comprehensive review of process-based modelling approaches used to study microplastic dynamics in estuaries was first conducted to assess various parameterization strategies, identify key challenges, and offer recommendations and future directions to advance microplastic modelling strategies in estuaries. Building on insights from this review, the relative influence of estuarine physical processes on microplastic transport was examined through sensitivity scenarios using different release configurations. The results identify the shoreline interaction by beaching– refloating dynamics as a key process for buoyant particles, while resuspension and vertical mixing modulate the transport and vertical distribution of non-buoyant particles. Microplastic-sediment interactions, such as flocculation and temporary trapping in bottom sediments, play an important role in enhancing particle retention within the estuary. Model results also show that hydrodynamic processes alone can significantly trap microplastics, with seasonal variability modulating the intensity and location of trapping. Elevated river discharge during the spring season enhances seaward transport, particularly for buoyant particles, whereas in summer, microplastics are more likely to be retained, with denser particles accumulating near the estuarine turbidity maxima (ETM) in the tidal rivers. This accumulation forms a water-column estuarine microplastic maximum (EMPM), sustained by net upstream transport driven by tidal pumping. In-situ observations in the water column support these findings, confirming the presence of strong near-bottom microplastic concentrations in summer in the Garonne tidal river, particularly during strong flood and ebb current velocities, with a dominance of high-density fibrous particles. Model simulations also indicate that floating particles are consistently trapped along a frontal line near the main channel, generating a surface EMPM. In the upper estuary, this line of particle accumulation follows the primary convergence zone produced by the combined effects of tidal currents and estuarine bathymetry. However, in the middle estuary, the accumulation line shifts to a secondary convergence zone due to the combined effect of morphological features and the alternance between convergence and divergence patterns over the tidal cycle. Sensitivity tests confirm that baroclinic effects play a significant role in shaping frontal convergence, with sediment-induced water density modulating its strength. Overall, the results highlight that tide-dominated, highly turbid estuaries act as efficient microplastic retention zones due to the combined influence of tidal hydrodynamics, sediment-microplastic interactions, and morphological features.
(03/02/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Storm Wind Flow and Sediment Dynamics on Adjacent Coastal Dunes With Contrasting Morphologies During One Storm Event
This study presents the first high‐resolution spatial and temporal analysis of wind flow, sediment transport and topographic evolution under simultaneous storm conditions across two morphologically contrasting beach‐dune systems, characterized by a gently sloping dune face (11°) and a steep, scarped dune face (36°). Results demonstrate that the dune slope strongly controls near‐surface wind acceleration, the development of secondary airflow structures (amplitude, spatial positions), and the continuity of sediment transport pathways. Over the gentle slope, airflow accelerates progressively up the stoss face, promoting sustained, landward‐directed sediment fluxes across the entire beach–dune system and enabling efficient sediment recycling. In this configuration, beach‐derived contributions account for only 12%–15% of the total sediment flux. In contrast, the steep scarp induces flow deceleration and separation at the dune toe, limiting sediment transfer from the beach and favoring seaward‐directed transport associated with secondary vortices at the crest. These contrasting airflow organizations result in fundamentally different storm responses. The gently sloping dune undergoes landward translation with minimal net volume change, whereas the scarped dune experiences dominant marine erosion, leading to a 4 m retreat of the dune front and a sediment loss of ∼30 m 3 m −1 . A new conceptual model of storm‐driven airflow over contrasting dune morphologies is proposed, illustrating how inherited dune slope governs airflow structure and circulation patterns. Overall, these results identify inherited dune morphology as a primary control on airflow organization, sediment pathways, and dune resilience during extreme events.
(Journal of Geophysical Research: Earth Surface. vol. 131, n° 2169-9003, 02/02/2026)
CEFREM, UPVD, INSU - CNRS, CNRS, BRGM, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, ONF
Exploring the environmental distribution of the oyster parasite Haplosporidium costale
The protozoan parasite Haplosporidium costale is known to occur in the USA where it has been associated with sharp seasonal mortality of the Eastern oyster Crassostrea virginica since the 1960’s. In 2019, the parasite was detected for the first time in the Pacific oyster Magallana gigas in France in the context of light mortality and was subsequently detected in archived material collected since 2008. This detection raised several questions regarding the ability of the parasite to maintain in the ecosystem and the potential involvement of other species in its life cycle. To answer these questions, an integrated sampling approach was deployed seasonally in three oyster farming areas where the parasite was already known to occur. Parasite presence was evaluated after checking the presence of PCR inhibitors and using a previously developed and validated Real Time PCR assay, optimized in this study to detect parasite DNA in various environmental compartments. Parasite DNA was almost only detected in cupped oysters. Considering the high number of oysters found positive with low infection intensity, a complementary experiment was undertaken to better characterize sub-clinical infections in oysters. The presence of the parasite was tested twice a week in water and sediment from aquaria hosting cupped oysters from a known infected site. After one month, oysters were sacrificed and tested regarding the presence of the parasite at the tissular level. Altogether, field and experimental results indicate that the parasite is stably established in oyster, particularly in gills, which may act as a reservoir all along the year. The detection of parasite DNA in nanoplankton and sediment suggests that H. costale is released from the oysters outside mortality event. Our results do not support the involvement of other species than cupped oyster in the parasite life cycle except periwinkles, whose role would deserve to be further investigated
(Journal of Invertebrate Pathology. vol. 214, n° 0022-2011, pp. 108462 (9p.), 01/02/2026)
ASIM, IFREMER, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
On the performance of radiocarbon and quartz OSL dating in macrotidal estuarine environments: Four case studies from Western France
The study of estuarine sedimentary archives provides valuable insights into their geomorphological evolution over the past two centuries, enhancing our understanding of estuarine responses to climate change. Establishing a reliable and precise geochronological framework is therefore essential for monitoring these changes. This study evaluates the performance of quartz Single-Aliquot Regenerative (SAR) OSL and AMS 14C dating in four estuaries along the western coast of France. The results are compared with cartographic data, serving as an independent age control. Of the 14 OSL dated samples, 10 yield depositional ages consistent with cartographic data, whereas the remaining 4 appear to overestimate ages by 20–100 years. In contrast, AMS 14C dating reveals numerous stratigraphic inversions, with at least 12 out of the 16 measured samples overestimating the depositional age in some cases by up to 5000 years, in total disagreement with cartographic data. The discrepancy between the OSL and radiocarbon ages reflects the constant reworking of allochthonous material, to which is added the further uncertainty associated with the local reservoir age. These factors fundamentally limit the reliability of 14C dating regardless of the material analyzed. By contrast, the OSL signal displays remarkable resilience, with any age overestimation linked to partial bleaching remaining minor (on the order of decades) compared with the errors affecting 14C ages. This underscores the capacity of OSL dating to resolve short-term environmental changes and positions it as the most reliable tool for constructing high-resolution chronologies of the last centuries in macrotidal estuarine settings.
(Quaternary Geochronology. vol. 92, n° 1871-1014, pp. 101723, 01/02/2026)
M2C, UNICAEN, NU, INSU - CNRS, UNIROUEN, NU, CNRS, DTU, UCPH, LPG, UM, UA, INSU - CNRS, CNRS, Nantes univ - UFR ST, Nantes Univ, Cerema, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Early Pleistocene (1.94–1.46 Ma) records for the upper Mediterranean Outflow Water branch reveal low and high latitude climate influences
The Mediterranean Outflow Water (MOW), modified by paleoceanographic conditions and tectonic processes, played a significant role in the formation of sediments drifts along the Iberian Margin. Using sediment samples from IODP Hole U1387C, we explore the Early Pleistocene history of the upper MOW core above the central Faro Drift in the Gulf of Cádiz. The time series of benthic foraminifer stable isotope and grain size related data have a rigorous stratigraphic framework consisting of nannofossil biostratigraphy and paleomagnetic and δ 18 O stratigraphy. The paleoenvironmental records are supplemented by natural gamma ray downhole logging data. Above the hiatus associated with the youngest dolostone, sandy to muddy contourite sedimentation started at 1.946 Ma, i.e., within Marine Isotope Stage (MIS) 74, at IODP Site U1387, slightly younger than at IODP Site U1389. Formation of contourite layers, reflected in the sortable silt and sand percentage records, strongly reacted to precession forcing, including semi-and quarter-precession cycles. The majority of the contourite beds developed during stadial (colder) climate periods, like previous observations from the Early to Late Pleistocene. Formation of contourite layers within MIS 53, MIS 55 and MIS 65, however, appear to be linked to the prevailing atmospheric conditions over North Africa. Periods of poor ventilation in the upper MOW were linked to insolation maxima and reduced ventilation in the Mediterranean Sea. Here, MIS 51 presents a peculiar case as poor ventilation reached from the surface to the lower North Atlantic Deep Water range, reflecting unique interglacial conditions that merit future exploration.
(Marine Geology. vol. 492, n° 0025-3227, 01/02/2026)
IPMA, CCMAR, UAlg, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UC Santa Cruz, UC, UC Davis, UC, NOC, INSU - CNRS, CNRS, UA, UM, LNEG, MARUM, TAMU