French Research Institute for Exploitation of the Sea
ArchiMer - Institutional Archive of IfremerNot a member yet
27944 research outputs found
Sort by
Variance-based shoreline extraction from nearshore video monitoring systems
No full textUnder the effect of various hydrodynamic forcings, the morphology of the coastal zone and more particularly the intertidal zone is subject to short-term variations. Capable of autonomous continuous recording, video camera systems offer a good opportunity to capture these short-term morphological variations. Extracting a topography of the intertidal zone from these videos involves detecting successive shorelines on the foreshore during a tidal cycle by assigning them an altitude. Detecting this shoreline is therefore a key stage. Here, considering the shoreline as the boundary between a moving zone (wave breaking) and a supposedly stable zone, we propose a method based on ‘variance images’ (in reality, calculated as the median for 5 min video of the standard deviations on radiometry of each pixel over 10 s intervals). The method was evaluated at four sites with different geo-morphological characteristics, different camera installations and with a wide range of weather conditions (including stormy ones). Of the 111 randomly selected images, 73% of the shorelines were correctly detected (reaching 95% for the Ruscumunoc site). These raw results could be further improved by adding a filtering step or by optimising the installation (avoiding mast vibrations, pruning vegetation, etc.). The Digital Elevation Models (DEMs) generated potentially on a daily basis over the intertidal zone will make it possible to study topographic variations under different conditions (reconstructive regime, impact of an extreme event, etc.)
Qualité du Milieu Marin Littoral. Bulletin de la surveillance 2023. Département du Morbihan
No full textL’Ifremer coordonne, sur l’ensemble du littoral métropolitain, la mise en œuvre des réseaux d’observation et de surveillance de la mer côtière. Ce bulletin contient une synthèse et une analyse des données collectées par l’ensemble des réseaux pour le département du Morbihan. Des représentations graphiques homogènes, assorties de commentaires, donnent des indications sur les niveaux et les tendances des paramètres mesurés
Aggregation Behaviour as an Adaptive Reproductive Strategy in a Marine Ecosystem Engineer
No full textAggregation behaviour is a key process shared by numerous organisms, providing benefits such as predator protection, resistance to abiotic stressors and enhancing reproductive success. In coastal waters, mussel aggregations shape ecosystems and increase biodiversity; however, many aspects of this ecologically key behaviour remain unexplored. Here, we investigated the potential link between aggregation rate and reproductive maturity stages in the blue mussel Mytilus edulis over a seasonal cycle. We showed that the seasonality of M. edulis aggregation rate was a significant sinusoidal function of the mean reproductive maturity stage. Specifically, aggregation increased during gamete maturation until the onset of spawning and subsequently declined, following a cyclical pattern, supporting a reproductive function of this aggregation behaviour in mussels. Given the ubiquity of plastics as marine contaminants and their known harmful effects on organisms, we subsequently assessed how the temporal patterns observed in aggregation behaviour may be impacted by an exposure to leachates from plastic pellets. Thus, an exposure to plastic leachates led to a loss of the cyclical pattern observed in control seawater, potentially compromising mussel reproduction and the stability of mussel bed habitat. Our study showed the relationship between aggregation behaviour, reproduction and contaminants in mussels. Understanding these complex interactions is crucial, given the pivotal role mussels and their aggregations play in shaping marine ecosystems, offering new insights into the resilience of this habitat facing contemporary challenges
Comparison of a global high-resolution ocean data assimilation system with SWOT observations
No full textSurface Water and Ocean Topography (SWOT) high-resolution sea surface height (SSH) data extend the capabilities of nadir altimetry, enabling the detection of small ocean features, up to submesoscales. Assimilating these new measurements has great potential to enhance the accuracy of high-resolution global models but requires a detailed understanding of the data physical content to adapt the assimilation system accordingly. The Cal/Val 1-day repeat phase of the SWOT mission offers a unique opportunity to evaluate model performance in the high-frequency and high-wavenumber domain. SWOT SSH data are compared with outputs from Mercator Ocean International 1/12° global analysis and forecasting system. This comparison aims to characterize differences between the dynamics captured by SWOT and those represented in the model. Maps of SSH variability and spectral analyses are presented. Frequency spectra reveal good agreement between SWOT and the model at large scales but significant differences at higher frequencies. These differences are attributed to submesoscale signals in SWOT observations that were not captured by nadir altimeters or that are too small for the model grid resolution. The analyses also reveal coherent and non-coherent internal tide residuals in SWOT data. These residuals are quantified to improve the characterization of the representation error for future assimilation experiments. Insights from this study will inform and pave the way for effectively integrating SWOT data into operational systems
Interspecies synchrony in departure decisions from stopover sites: evidence using a multi‐species capture–recapture model
No full textDuring migration, migratory bird species often aggregate at the same stopover sites due to geographical features that channel migratory routes (coasts, valleys) or locally abundant food resources (e.g. reedbeds, fruiting bushes). In migration ecology, however, stopover behavior is often studied on a single species, limiting the generality of inferences and their application to habitat management. If stopover decisions are similar across co‐occurring species, characterizing stopover ecology at a community level could provide more comprehensive insights. Using recent advances in modelling, we adapted a multi‐species capture‐recapture model to 1) quantify synchrony in stopover departure probabilities across species and 2) identify covariates driving this synchrony. We applied this model to three migratory songbirds commonly captured at French stopover sites: the sedge warbler Acrocephalus schoenobaenus, reed warbler Acrocephalus scirpaeus and bluethroat Luscinia svecica. Departure decisions were largely synchronous across species, with time since arrival (TSA) as the primary synchronizing factor and weather conditions as a secondary influence. High synchrony in departure timing produced waves of migrant departures and consistent patterns in departure decisions across species, suggesting shared fuelling and timing strategies. Although this phenomenon has been documented via visual counts and radar, it has not been formally quantified at the species‐level using capture‐recapture methods. Our model is flexible and can test hypotheses regarding spatial synchrony in departure decisions. With decades of capture–recapture data available across Europe and North America, our approach offers new potential for studying stopover ecology at the community level, over large geographic and temporal scales
Reconstruction of the volcanic history of Grande Comore and volcano-tectonic implications for the Comoros Archipelago
No full textSince the onset of Fani Maore submarine eruption, the volcanism of the Comoros Archipelago has gained renewed interest. Geochronological constraints have been recently obtained for Mohéli, Anjouan and Mayotte islands and nearby seamounts. Here we provide new groundmass K–Ar ages for Grande Comore. Along with major and trace element geochemistry and geomorphological analyses, these ages enable us to propose a three stages reconstruction of the island. The first, or M’Badjini stage, is much older than previously thought, with ages as old as 2.2 Ma. Following a period of quiescence and dismantling, the second phase occurred between 280 and 230 ka, probably on both La Grille and Karthala volcanoes. Then, volcanic activity resumed for a third phase from ca. 130 ka until present time. The Comoros geochronological database, updated with our new results, confirms that the construction history of the archipelago occurred as pulses and follows a northward propagation of volcanism (i.e. a widening of the volcanically active region)
Scenario set-up and the new CMIP6-based climate-related forcings provided within the third round of the Inter-Sectoral Model Intercomparison Project (ISIMIP3b, group I and II)
No full textThis paper describes the climate-related forcings (CRFs) provided within the 'b' part of the third simulation round of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3b). While ISIMIP3a comprises historical impact models simulations forced by observational CRF and direct human forcings (DHF), the ISIMIP3b CRFs are based on climate model simulations generated within the sixth phase of the Coupled Model Intercomparison Project (CMIP6). In a first set of experiments (ISIMIP3b, group I) the CMIP6-based CRFs for the historical period are combined with historical observation-based DHF also considered in ISIMIP3a (e.g. land use patterns, water and agricultural management, and fishing efforts). These group I simulations allow for the quantification of impacts of historical climate change by comparison to simulations where the observational DHF are combined with simulated pre-industrial CRFs. In addition, the impacts of observed changes in CRFs can be compared to the impacts of simulated changes in CRFs by comparing the ISIMIP3a simulations to the ISIMIP3b, group I simulations. The second group of experiments (ISIMIP3b, group II) comprises future projections assuming constant observational direct human forcings at 2015 levels to estimate the impact of climate change given today’s direct human influences for the low emission scenario SSP1-2.6, the high and the very high emission scenarios SSP3-7.0, SSP5-8.5, respectively. The very high emissions scenarios and the assumption of fixed present day direct human forcings particularly allow for testing the scalability of impacts in terms of global temperature change. The provided CRFs comprise atmospheric CO2 and CH4 concentrations, atmospheric and oceanic climate data, coastal water levels, tropical cyclone tracks and their associated wind speed and precipitation fields. In addition to the CRFs data, this paper describes the experiments belonging to group I and II and the rationale behind them. Another set of future projections accounting for changing DHFs (ISIMIP3b, group III) is in preparation and will be described in another paper
PROSE+ project: offshore seismic measurements on the seabed to test the ability to assess the spatial variation of the small-strain shear modulus in the subsurface environment
No full textThe increase in the number of offshore wind farm siting projects, combined with the multiplicity of developments in anchoring techniques, means that foundations and anchors need to be optimized for the conditions of the offshore subsurface, requiring precise knowledge of the mechanical characteristics of the medium. According to the recommendations of the CFMS (French committee of Soils Mechanics), detailed geophysical reconnaissance is then necessary during the project phase (design and execution) to obtain the most accurate information possible at the locations of the structures. In this context, the PROSE+ project aims to increase knowledge and provide new methodological and technical elements, based on surface seismic and geoelectrical techniques. This will make possible to approach heterogeneous environments in a quantitative and non-destructive way, thereby reducing the number of costly and invasive geotechnical surveys. To this end, we developed numerically a 2D seismic inversion technique using Surface Seismic Waves based on Particle Swarm optimization methods. In order to validate it on experimental data, we carried out measurements off Concarneau using 70 4-components sensors (GPR - Sercel nodes) placed on the seabed, in an unprecedented manner. 241 seismic shots were fired over this sensor’s network using an air gun at variable water depth. Finally, the sensors were left recording on the seafloor for 28 days. The recorded seismic data allow to test the capacity of both active and passive seismic imaging process to assess the shear modulus in a 2D medium under seabed
A Comparison of New and Established Foraminiferal Dissolution Proxies in Tropical Indian Ocean Deep‐Sea Sediments
No full textAssessing the preservation state of calcium carbonate microfossils preserved in deep‐sea sediments allows us to understand the impact of ocean chemistry on deep‐sea carbonate distribution and provides key information on potential biases affecting microfossil‐based proxies applied in the geological past. Established dissolution indices potentially fail to capture some finer details of how dissolution affects foraminiferal assemblages, especially in intermediate depths above the lysocline. Here, we present a new dissolution index, the Fragment Variance Index (FVIndex), which estimates test preservation using the ratio between fragment size variance and whole planktonic foraminiferal test variance. We hypothesize that increasing fragment size variance will reflect better preservation, whereas increasing whole test variance will indicate dissolution linked to water depth or chemistry. We test the FVIndex and other dissolution indices on a set of 62 coretop sediment samples from the tropical Indian Ocean (TIO) using an automated imaging system (MiSo) and the ParticleTrieur software, analyzing over 220,000 foraminifera and fragments. Generally, a higher FVIndex is associated with better preservation (at shallower water depths) and vice versa. The FVIndex, supported by other indices, indicates strong dissolution impact on planktonic foraminifera assemblages in the Mozambique Channel (MC), but very weak impact in the TIO due to the significant influence of local processes. We have tested FVIndex as a dissolution index on data sets from the MC and TIO; the next step would be to examine its performance in other more oceanographically homogenous regions
Evolutionary history and association with seaweeds shape the genomes and metabolisms of marine bacteria
No full textSeaweeds harbor a rich diversity of bacteria, providing them with metabolic resources and a surface for attachment and biofilm development. The host’s unique environment potentially shapes the bacterial genomes and promotes adaptations for a symbiotic lifestyle. To investigate whether the genomes of seaweed-associated bacteria are genetically and metabolically distinct from their close free-living relatives in seawater, we compared both the seaweed-associated and free-living counterparts of 72 bacterial genera across 16 seaweed hosts using whole-genome sequences or high-quality metagenome-assembled genomes. While taxonomic affiliation strongly influenced genome characteristics such as GC content, gene number, and size, host association had a lower effect overall. A reduced genome size was suggested only in Nereocystis luetkeana -associated microbes, while only Ascophyllum nodosum -associated bacteria had an increased GC content. Metabolic adaptations were indicated from the genomes of seaweed-associated bacteria, including enriched pathways for B vitamin production, complex carbohydrate utilization, and amino acid biosynthesis. In particular, Flavobacteriia showed the most pronounced differences between host-associated and free-living strains. We further hypothesized that bacteria associated with seaweed might have evolved to complement their host’s metabolism and tested this inference by analyzing the genomes of both the seaweed Ectocarpus subulatus and its 28 bacterial associates but found no evidence for such complementarity. Our analyses of 72 paired bacterial genomes highlighted significant metabolic differences in seaweed-associated strains with implications for carbon, nitrogen, and sulfur cycling in the coastal ocean. IMPORTANCE We hypothesized that the unique environment of seaweeds in coastal oceans shapes bacterial genomes and promotes a symbiotic lifestyle. We compared the genomes of bacteria isolated from seaweed with bacteria from the same genus found free-living in seawater. For genome features that included the number of genes, the size of the genome, and the GC content, taxonomy was of greater importance than bacterial lifestyle. When we compared metabolic abilities, we again found a strong effect of taxonomy in determining metabolism. Although several metabolic pathways differed between free-living and host-associated bacteria, this was especially prominent for Flavobacteriia in the phylum Bacteroidota . Notably, bacteria living on seaweeds had an increased occurrence of genes for B vitamin synthesis, complex carbohydrate use, and nitrogen uptake, indicating that bacterial genomes reflect both their evolutionary history and the current environment they inhabit