French Research Institute for Exploitation of the Sea
ArchiMer - Institutional Archive of IfremerNot a member yet
27944 research outputs found
Sort by
Turbulence Observations and Energetics of Diurnal Warm Layers
No full textDiurnal warm layers (DWLs) develop under relatively weak winds and strong solar radiation and have important consequences for ocean surface transport and air-sea interactions. In this paper we investigate DWLs during three consecutive days in the subtropical South Atlantic using observations from an underwater glider equipped with a turbulence microstructure package, a series of drifters at two different depths, and a 1-D turbulence model. The observations and modeling show that the DWLs create a near-surface stratification that partially decouples the surface current from the mixed layer below. However, we find that turbulent entrainment of momentum from below the DWL is important in the evolution of the surface current. We further derive buoyancy, potential and kinetic energy budgets, and identify the dominant terms. The upper ocean potential energy budget is dominated by the incoming solar radiation, with only a small contribution from turbulent mixing. Turbulent shear production, however, is found to be an important influence on the upper ocean mean kinetic energy, receiving a similar fraction of the wind work as the acceleration of the diurnal jet. The DWL evolution and energy budgets are corroborated with simulations from a freely-evolving 1-D turbulence model, which additionally shows that the exchange of mean kinetic energy with the surface wave field due to rotation is of minor importance to the development of the DWLs we observe
The Unaccounted Oceanic Sink of Anthropogenic Nitrous Oxide and Its Relationship With Anthropogenic Carbon Dioxide
No full textSince 1800, the concentration of greenhouse gases like nitrous oxide (N2O) and carbon dioxide (CO2) has significantly increased due to anthropogenic activities. Oceanic anthropogenic CO2 (Cant) uptake from the atmosphere has been quantified and periodically re-evaluated given the implications for climate change. However, the potential oceanic uptake of N2O has been largely overlooked. This study quantifies the uptake of N2O of anthropogenic origin (N2Oant) taken up by the global ocean and how it relates with the anthropogenic CO2. The oceanic inventory of N2Oant has been quantified using two approaches which consider the anthropogenic perturbation of N2O and CO2 as conservative tracers: first, a direct approach using the Transient Time Distribution (TTD) method; and second, indirectly through a novel method, founded on the direct proportionality between the excess of both N2O and CO2 in the atmosphere since 1800. Our results show that the North Atlantic Ocean is a key region of maximum accumulation of N2Oant due to the confluence of cold and ventilated waters. The global oceanic uptake of N2Oant from the pre-industrial times to 2010 was estimated to be 11.5 +/- 2.3 Tg-N, with an annual uptake rate of 0.23 +/- 0.05 Tg-N yr-1. The study shows that oceanic sequestration contributes to a small portion of global N2O inventories, but it is comparable to other N2O oceanic budget numbers derived from atmospheric nitrogen deposition. Furthermore, connecting the N2Oant and Cant oceanic distributions is a valuable tool for linking the perturbation of the Anthropocene on the N and C cycles in the ocean
The Role of Ballasting, Seawater Viscosity and Oxygen-Dependent Remineralization for Export and Transfer Efficiencies in the Global Ocean
No full textThe particulate organic carbon (POC) flux from the euphotic zone to the deep ocean is central to the biological carbon pump. It is typically evaluated using "export efficiency" and "transfer efficiency," which reflect POC formation and sinking and carbon sequestration efficiency in the ocean's interior, respectively. Since observations of these metrics are limited, biogeochemical models can elucidate the controls of large-scale patterns. This study uses the global ocean-biogeochemical model FESOM-REcoM, with a new sinking routine that accounts for ballast minerals, seawater viscosity, and oxygen-dependent remineralization in POC sinking and remineralization, to identify the drivers of global export and transfer efficiency. We find that export efficiency is highest at high latitudes, where diatoms, mesozooplankton, and macrozooplankton dominate the plankton community, but that high export efficiency does not always imply high transfer efficiency. Omitting ballast minerals decreases export efficiency by 20% in the Southern Ocean, yet the globally integrated POC flux out of the euphotic zone (5.4-5.6 Pg C yr-1 ) and the global average export efficiency (14.7%-15.4%) are relatively insensitive to seawater viscosity, mineral ballasting, or oxygen-dependent remineralization. In contrast, global transfer efficiency is more sensitive to these processes and varies between 21% and 25% in the simulations, with the largest reduction by 23% observed when omitting ballasting in subtropical, low-productivity regions. Our findings suggest that assumptions about ballasting and background sinking speed could explain previous discrepancies in the literature regarding the highest transfer efficiencies in low or high latitudes. Notably, while plankton community structure determines export efficiency regimes, zooplankton fecal pellets drive high transfer efficiencies in regions with high export efficiency, like the Southern Ocean
Can Biogeochemical Tracer Observations Constrain Southern Ocean Diapycnal Mixing Rates?
No full textDirect observations of background diapycnal mixing rates in the Southern Ocean (SO) are limited spatially and temporally, making the choice of an appropriate value to parameterize this mixing in Earth system models a challenge. However, the deployment of Argo floats throughout the SO has provided an extensive range of observations of both physical and biogeochemical parameters. We use an ocean state estimate run with various background diapycnal mixing coefficients to assess if biogeochemical tracer observations can be used to better constrain SO diapycnal mixing rates. We find that vertical tracer distributions in the SO are highly sensitive to the rate of background diapycnal mixing and can provide an upper limit on background mixing rates. This demonstrates the importance of biogeochemical tracer observations throughout the full depth of the water column to validate ocean models
Sedimentary record of submarine gravity‐flow events in the southern Ryukyu forearc during the last 200 000 years: archive of mega‐earthquakes and tsunamis
No full textDespite high convergence velocity, the southern Ryukyu subduction has relatively low and sparse instrumental seismicity, in contrast with the Yaeyama Islands, hit by huge tsunamis over the last few thousand years. This study explores the potential of deep marine sediments to record past large earthquakes and tsunamis. During the MD214/EAGER cruise (2018), four sediment cores (12 to 23 m long) were collected in the forearc area (south of Yaeyama Islands), which is an efficient trap for gravity‐flow deposits. The chrono‐stratigraphic framework is established with radiocarbon dating, δ18O stratigraphy, U‐series data and tephra analysis including major/trace elements and Hf‐Pb‐Sr‐Nd isotopes. In the basin floor, bulk sedimentation rates locally exceed 60 cm/ka, while hemipelagic sedimentation rates range from 3 to 8 cm/ka, similar to values derived from U‐series data in recent sediments of the boxcores. Twenty‐four gravity‐flow deposits were identified over the last 20 000 years, a frequency consistent with the past tsunamis recorded in the coastal sediments over the last thousand years, emphasising their interest as archives of earthquakes and major tsunamis. At a regional scale, bulk sedimentation rates fall under 10 cm/ka for 48 large‐scale gravity‐flow deposits over the last 200 000 years. With average recurrence periods about 3500–4000 years, these regional events could be triggered by subduction large earthquakes (M > 7). Finally, four exceptionally large submarine gravity‐flow events (mega‐deposits) of unclear origin have been documented over the last 200 000 years. This work provides an unprecedented time series of major submarine gravity instabilities, probably initiated by great earthquakes on this active subduction. It implies that millennial recurrence periods of great earthquakes must be considered on subduction segments where instrumental seismicity is relatively low and underlines the importance of deep marine sediments in tracing these events
A Passive Environmental DNA Sampler for Aquatic Biodiversity Detection Tested in Marine Mesocosms
No full textIn aquatic ecosystems, environmental DNA (eDNA) can be collected from water samples to produce species inventories. One method for this is passive eDNA sampling, whose development for aquatic biodiversity monitoring is in its early stages. While several materials have been successfully tested for passive eDNA samplers (PEDS), methodological advances are still needed to explore their versatility as a complement to the more common method of active filtration. This study tested for the first time a PEDS using human‐crafted material in controlled marine mesocosms (1 m3) containing one species, the European seabass (Dicentrarchus labrax) in different conditions of fish density (1, 5, 10, 100 fish per m3) and exposure times (30 min, 2 h, 8 h, 24 h). We then tested the influence of another source of eDNA on the sampler's performance by introducing another species, the Pacific oyster (Magallana gigas). In addition, we compared the efficacy of the method with active filtration. The PEDS we produced consisted of a small electrospun polyacrylonitrile (PAN) membrane encapsulated in a custom‐made 3D‐printed frame. Each sampler is low‐cost, easy to manipulate, highly replicable, and customizable. A specific quantitative polymerase chain reaction‐based assay for the seabass was developed. Results were analyzed with multiscale occupancy modeling and continuous response variable modeling. We found that the PAN‐PEDS efficiently collected eDNA in a large volume (1 m3) of renewed water (1 m3/h), with a clear positive effect of high fish density on fish detection. The introduction of oysters did not significantly influence detection. Regarding exposure times, a range of results were obtained that could be attributed to the unreached equilibrium between eDNA shedding and degradation. While active eDNA collection (30 L) outperformed PAN‐PEDS, the ongoing development of passive methods can provide new insights in aquatic species monitoring when spatio‐temporal eDNA collection is required
Vulnerability and resilience of rainforests to plant invasions in a tropical island of the south pacific: a sixteen-year survey
No full textIsland biodiversity is considered to be particularly vulnerable to biological invasions. However, the direct impacts of invasive alien plants on native and endemic flora are often difficult to assess. Indeed, invasion by woody species is a relatively slow process, and natural or anthropogenic disturbances that favor some plant invaders, also contribute to biodiversity loss, and might act as confounding factors. Hence, we conducted a long-term monitoring of rainforest composition and structure in the small tropical high volcanic island of Moorea (French Polynesia, South Pacific) over a 16-year (2006–2022) period without any major natural or human disturbances. All endemic, indigenous and alien woody species with a stem ≥ 1 cm in diameter were identified and measured every four years in ten 20 m × 20 m plots. The variation of species richness, Shannon diversity, Pielou evenness and abundance over time was analyzed using generalized linear mixed-effects models. Results revealed an increase in the number of stems of alien plants (+ 50% in 16 years), especially the two most common invasive alien trees, Miconia calvescens (Melastomataceae) and Spathodea campanulata (Bignoniaceae). In contrast, we found a decrease in the richness (from 28 species to 19) and abundance (−20% of stems) of endemic and indigenous species, especially in the higher-elevation plots. Three plots remained relatively stable in terms of invasive species diversity according to Shannon and Pielou indices, and some common indigenous and endemic woody species persisted over time. Although a continuous decline of plant diversity, including species extirpation, was observed, our results also suggest a relative resilience of certain forest types and associated communities to plant invasions. Efforts to control invasive alien plants should therefore target the most vulnerable native habitats
Symbiodiniaceae and Bacterial Microbiome Dynamics Differentially Impact the Survival of Dominant Reef-Flat Porites Corals
No full textCoral reefs face significant threats across the globe, prompting a surge in restoration efforts aimed at mitigating their global decline. The health, resilience, and adaptability of corals are greatly influenced by their microbial communities, and while the response of coral microbiomes to many environmental stressors has been extensively studied, less is known about their natural dynamics following transplantation, which is an essential process for restoring degraded reef habitats. In this study, we integrated DNA metabarcoding (16S & ITS2) with ecological monitoring to investigate the dynamics of Symbiodiniaceae and bacterial communities in two dominant coral spp., Porites lobata and Porites cylindrica, and their different colour morphs, as they underwent transplantation and an 18-week acclimatisation period. We saw significant differences in microbial communities between the two Porites spp., outplanting sites, and individual coral colonies, as well as a colour morph-related difference in P. lobata bacterial communities. We saw reduced relative abundances of Endozoicomonadaceae, specifically from the genus Parendozoicomonas, following transplantation. P. lobata colonies with later Symbiodiniaceae shifts (18 weeks) had lower long-term survival. Changes in Symbiodiniaceae and bacterial communities have implications for holobiont function and colony survival, which should be considered when designing and implementing coral reef rehabilitation projects
9th EMB Forum Proceedings. Addressing coastal and water resilience on the land-sea interface
No full textThe 9th EMB Forum was held as a hybrid event on 2 April 2025 at the Institute of Natural Sciences in Brussels and online. The event was also co-hosted by the Institute of Natural Sciences. This event was endorsed as supporting the UN Decade of Ocean Science for Sustainable Development. It was also a recognised action in support of the EU Mission: Restore our Ocean and Waters. Over 40% of the European population lives at the coast and expects to continue to do so. However, this critical interface faces multiple threats including the impacts of climate change such as sea-level rise, water scarcity and increases in extreme weather, pollution, and habitat- and biodiversity loss. The land and sea are also addressed by different governance systems, making it difficult to manage the coastal region. This EMB Forum considered these challenges and how they can be overcome to ensure we retain coastal- and water resilience on the landsea interface. These discussions were of particular importance at this time given the new European Water Resilience Strategy and Ocean Pact, the objectives of the EU Mission: Restore our Ocean and Waters (Mission Ocean), and the challenges of the UN Decade of Ocean Science for Sustainable Development (Ocean Decade), such as increased community resilience to Ocean- and coastal risks. During the event, two new EMB Policy Briefs2 were presented: Policy Brief N°. 12 on Requirements for Coastal Resilience in Europe, and Policy Brief N°. 13 on Navigating the Future VI: The Ocean’s role in addressing global crises. This document is a summary of the discussions that took place during the 9th EMB Forum. The full recordings of the sessions are available on the EMB YouTube Channel
Dried fish provide widespread access to critical nutrients across Africa
No full textAquatic foods are essential in supporting food security and nutrition across the tropics, with “dried” fish particularly affordable, available, and nutritious. However, dried fish food systems are often hidden and overlooked due to data scarcity, limiting understanding of how dried fish contribute to nutrient intakes. Here, we combine nutrient analysis of fish samples with national household surveys from across East and West Africa to understand the importance of dried fish in diets. We find that small portions of dried fish contribute over 15% of recommended intakes for multiple essential dietary nutrients (calcium, iodine, iron, selenium, zinc, and vitamins B12 and D), with low heavy metal concentrations, and are consumed weekly by ~one-third of households in six countries (Côte d’Ivoire, Ghana, Nigeria, Malawi, Tanzania, and Uganda) (~144 million people). Dried fish consumption was more prevalent than fresh fish, reaching 54% more people, particularly those in poor households and near to marine coastlines or urban centers. The widespread prevalence of nutritious dried fish suggests that these foods and their distribution networks play critical roles in food security and nutrition, even in households distant from fisheries or urban centers. Dried fish can fill nutrient gaps across the tropics but will require policies that mitigate negative effects of overfishing, environmental changes, and competition with international markets, while providing postharvest support to fish processors