French Research Institute for Exploitation of the Sea
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Thermocline Dynamics and Nitrogen Cycling Drive Productivity Shifts in the South China Sea Across Glacial Cycles
No full textThe South China Sea (SCS) nitrogen cycle responds dynamically to glacial–interglacial climate changes, yet the drivers of nitrogen isotope variability remain debated. We present a synthesis of foraminifera‐bound δ15N (FB‐δ15N) records, together with complementary organic and amino‐acid δ15N data from across the SCS. In the eastern and northern SCS, FB‐δ15N peaks during glacials when intensified East Asian Winter Monsoons shoal the thermocline, enhance upwelling of 15N‐enriched deep nitrate, and stimulate export production. Conversely, western and southern SCS records exhibit δ15N minima during interglacials, reflecting enhanced nitrogen fixation and/or partial nitrate utilization. This east–west divergence highlights the important role of upper‐ocean structure in modulating nitrogen isotopes, alongside potential sea‐level–driven nutrient effects. Our results establish FB‐δ15N as a sensitive proxy for reconstructing nutrient dynamics and monsoon‐driven upwelling in marginal seas and underscore the influence of thermocline variability in shaping marine nitrogen cycling over glacial cycles
Distance Learning for Analog Methods (1st revision)
No full textAnalogs are similar states of a system, occurring at remote times within independent numerical simulations or previous observations. This concept has emerged in atmospheric sciences and was further used in ocean sciences for forecasting, downscaling, upscaling, and extreme event attribution. The distance used to find and rate analogs is a key feature of analog methods. Most studies are based on the Euclidean distance or other pre-defined metrics. In this investigation, we adapt distance learning algorithms originally designed for classification and regression to statistical forecasting objectives, using the continuous-ranked probability score as a loss function. Our algorithm allows to jointly optimize three key hyperparameters of analog methods: the feature space, distance, the number of analogs used. In particular, this algorithm allows to reduce the feature space dimension while preserving high performances, a key requirement for small-size datasets. We test our algorithm on an idealized chaotic system and a tropical cyclone dataset. These experiments suggest that the optimal distance depends on the forecast horizon and the number of available data, and that our algorithm allows for reasonable performances of analog ensemble methods even for small-size datasets. Our algorithm runs faster then existing grid-search-like analog distances optimization algorithms. This allows to test and optimize a wider class of distances, for instance weighting a large number of predictive variables. Our approach is not limited to forecasting and can assist the search for optimal hyperparameters of any analog method, enhancing exploration possibilities and improving overall performances
Working Group on Mixed Fisheries Advice Methodology (WGMIXFISH-METHODS)
No full textThe ICES Working Group on Mixed Fisheries Methodology (WGMIXFISH-METHODS) met to progress work on the improvement and development of the mixed fisheries considerations. The work addressed in 2025 included improving workflows for the advice process, presenting methodological advances and responding to issues encountered during WGMIXFISH-ADVICE 2024. The group reviewed the annual data call and processing procedures and discussed improved communication of mixed fishery advice products and collaborations with other Working Groups. The primary development affecting data workflows is the planned introduction of the Regional DataBase and Estimation System (RDBES) to support stock assessments carried out in ICES. In 2027, RDBES will replace InterCatch and the current ad hoc mixed fisheries data call, providing WGMIXFISH with data at a greater level of disaggregation than is currently possible. Whereas this presents an opportunity to improve the quality of mixed fisheries analyses carried out by the Working Group, the transition to RDBES is challenging because new processes are needed to assemble the inputs to mixed fisheries models. The Working Group identified the data requirements and steps needed to transfer workflows to using RDBES and outlined a timetable for comparing inputs derived from RDBES and current workflows in 2026. Several methodological advances were presented, many of which address outstanding limitations of current approaches or stakeholder feedback received during mixed fisheries scoping workshops. These included improved modelling of joint harvesting in FLBEIA, an age-structured mixed fisheries model framework for the Celtic Sea case study, and analyses of métier effort-share and fleet quota-share assumptions to reduce fleet ‘choking’ behaviour through artificial and ‘weak’ technical interactions. Additionally, applications of externally developed mixed fisheries models for the Bay of Biscay and Central Mediterranean, and an analysis mixed fisheries in the Ionian Sea were presented. Finally, each case study addressed outcomes and issues encountered during the previous year in preparation for WGMIXFISH-ADVICE 2025
Assessing Plastic Brittleness to Understand Secondary Microplastic Formation on Beaches: A Hotspot for Weathered Marine Plastics
No full textThe degradation and fragmentation of plastic debris into secondary microplastics pose significant environmental challenges, particularly on beaches where mechanical abrasion and chemical weathering accelerate plastic fragmentation. This study contributes to the understanding of secondary microplastic formation by addressing key questions related to plastic embrittlement, mechanical behavior, and degradation processes. We investigate the brittleness of polyethylene (PE) and polypropylene (PP) particles collected from Hawaiian beaches, focusing on their mechanical and chemical degradation states. Conventional tensile testing methods are impractical for irregularly shaped, small field-recovered particles. Therefore, we used a simple fragmentation test to evaluate brittleness under fixed applied pressure, enabling large-scale statistical analysis of PE and PP samples from beach field surveys. We show that the brittle samples have a very low molecular weight (Mw). Such low Mw, coupled with the appearance of oxidation products, suggests an advanced degradation state of the sampled plastics. Through our fragmentation test, we provide a large-scale, field-based quantification of plastic brittleness in beach samples, underscoring an increased propensity for further fragmentation and highlighting the severity of coastal and ocean plastic pollution
Unveiling the Influence of the Daily Oceanic (Sub)Mesoscale Thermal Feedback to the Atmosphere
No full textOceanic mesoscale (O(100 km)) Thermal Feedback (TFB) can modify the marine atmospheric boundary layer through two main mechanisms: Downward Momentum Mixing (DMM) and Pressure Adjustment (PA). In this study, we use ERA5 reanalysis, with the finest spatial resolution available (roughly 25 km globally), and a set of coupled ocean-atmosphere model experiments to investigate the extent to which the spatial resolution of oceanic fine scale structures (O(10)-100 km) influences the TFB mechanisms on a daily timescale in the northwestern tropical Atlantic. We show that the low-level wind magnitude response to sea surface temperature anomalies is controlled by large oceanic mesoscale structures (>100 km), whereas finer structures play a minor role. However, the TFB mechanisms vary considerably depending on whether the mesoscale structures are fully represented or not. The DMM and PA mechanisms are diminished by about 36% and 85%, respectively, when the full range of mesoscale structures in the ocean is resolved. This reduction is associated with both a stronger atmospheric frontogenesis mechanism induced by submesoscale (O(10 km)) oceanic thermal structures and the divergence of submesoscale ocean currents, which is not the case at the large mesoscale. These processes have the potential to exert a destructive influence on the daily oceanic mesoscale TFB by inducing opposite low-level atmospheric vertical velocities, which is particularly enhanced in the case of the PA mechanism. Finally, our findings suggest that climate models must accurately represent fine scale ocean thermal structures that directly influence TFB mechanisms and potentially affect cloud formation and precipitation patterns
Prokaryotic microbiota outperform eukaryotic microbiota in differentiating between infection states of iconic diseases of two commercial oyster species
No full textThe role of microbiota in health and disease is most often expressed by structural shifts of the taxonomic composition of prokaryote communities in infected and healthy individuals. In cultured aquatic animals with open circulatory systems, such as mollusks, microbiota also harbor a wide range of protists, which are unicellular eukaryotes that could also play an important role during infections. To evaluate the effectiveness of eukaryotic vs. prokaryotic microbiota in characterizing infection states, we examined both microbial compartments under natural conditions in two commercially important oyster species, the flat oyster Ostrea edulis and the Pacific oyster Magallana (Crassostrea) gigas. With O. edulis being infected by two protist parasites, Marteilia refringens and Bonamia ostreae, and M. gigas being infected by the ostreid herpes virus OsHV-1, we chose iconic diseases responsible for substantial mortalities and economic damage within the two species. We analyzed and compared the structural and compositional differences between healthy and infected oysters and used random forest machine learning to classify infection states and identify indicator taxa that distinguish healthy from infected individuals. Both at the structural and compositional levels, bacterial microbiota proved to be better predictors of infection states. By eliminating noisy taxa through variable selection in the random forest models, we enhanced the compositional differences between infection states. In all host-pathogen combinations, only a few taxa (<31) were required to achieve optimal separation. While the identity of indicator taxa will partly reflect the specific environmental conditions at the time of sampling, we recovered several previously described indicator taxa, such as Mycoplasma, Vibrio, Photobacterium, and Arcobacter. Next to these we also discovered new taxa like Motiliproteus that exhibited the potential to differentiate between infection states of the investigated O. edulis specimen. The simultaneous characterization of prokaryotic and eukaryotic microbiota suggests that only few prokaryotic indicator species might be needed to reliably differentiate between infected from healthy individuals and monitor infection risks
Impact Of Carbon Black Content In Relationships Between Mechanical Properties And Crosslinking Of Filled Polychloroprene During Oxidative Aging
No full textCarbon black (CB) fillers are widely used in elastomer compounds to improve their mechanical performance. Extensive studies conducted over more than 80 yr now have provided a satisfactory understanding of the influence of CB content on the mechanical properties of elastomers. However, in the majority of the studies devoted to aging, similar elastomer formulations with different CB contents are often considered as independent materials, which hinders the applicability of structure–property relationships. Herein, we reconcile the effects of aging and CB content by investigating the oxidative aging at 80 °C in air of a polychloroprene rubber with four different CB contents (from 10 to 45 parts per hundred of rubber). Both macromolecular network and mechanical properties are determined via swelling, uniaxial tensile, and fatigue tests. It is shown that the response of the materials can be decomposed into the contributions of elastomer matrix and fillers. The behavior of the matrix during aging is the same regardless of the CB content. Relationships between the microstructural characteristics of the matrix and the local mechanical properties are then derived. Their relevance for aged materials and for a wide range of CB content are demonstrated. Oxidative ageing is performed on polychloroprene at different carbon black contents.The apparent crosslink density is related to the one of the unfilled rubber matrix.Crosslinking of the matrix behaves the same independently of fillers content.Mechanical properties are linked to the matrix network through ageing.At moderate contents, fillers effect follows established reinforcement theories
Tracing suspended sediment fluxes using a glider: observations in a tidal shelf environment
No full textUnderwater gliders equipped with current profilers and optical turbidity sensors offer a low-energy solution for high-resolution measurements of currents, suspended particle properties, and sediment transport in coastal waters. Because the spatial structure of hydrosedimentary processes often changes on short time scales (hours to weeks), especially in coastal areas, validating the distribution of glider observations is required to assess our capacity to represent hydrosedimentary processes. Here we propose to validate in a shelf tide-dominated environment, both i) glider-based currents, and ii) glider-based acoustic backscatters and optical turbidities in full resolution delayed mode, using in situ colocated and synchronous ancillary observations. The deployed glider system correctly measures the periodic pattern of the tidal current, with a satisfying RMSD of O(3 cm s -1). Glider optical turbidities highly correlate with the ancillary observations (R2 up to 0.83). They also correlate well with their glider acoustic counterpart for most of the campaign period (R2 = 0.76), allowing an estimation of suspended particulate matter concentrations from the acoustic. In this study, we showed the presence of bottom nepheloid layers of several mg·l-1 on the shelf probably due to advection of coastal turbid waters as evidenced by estimated glider sediment fluxes. These results highlight the potential of gliders for quantifying sediment fluxes and advancing our understanding of coastal hydrosedimentary processes