Ocean convergence and the dispersion of flotsam

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material

Technological Advances in Drifters for Oil Transport Studies

AbstractAdvances in drifter technology applied to oil spill studies from 1970 to the present are summarized here. Initially, drifters designed for oil spill response were intended to remotely track trajectories of accidental spills and help guide responders. Most recently, inexpensive biodegradable drifters were developed for massive deployments, making it possible to significantly improve numerical transport models and to investigate, via observations, the processes leading to dispersion and accumulation of surface pollutants across multiple scales. Over the past 50 years, drifters have benefited from constant improvements in electronics for accurate and frequent location and data transmission, as well as progress in material sciences to reduce fabrication costs and minimize the environmental impact of sacrificial instruments. The large amount of in-situ data provided by drifters, covering a broad area, is crucial to validate the numerical models and remote sensing products that are becoming more important in guiding response and policy decisions

On the transport and landfall of marine oil spills, laboratory and field observations

The dynamics of crude oil and different surface ocean drifters were compared to study the physical processes that govern the transport and landfall of marine oil spills. In a wave-tank experiment, drifters with drogue did not follow oil slicks. However, patches of undrogued drifters and thin bamboo plates did spread at the same rate and in the same direction as the crude oil slicks. Then, the trajectories of the Deepwater Horizon oil spill and 1300 drifters released near the spill source were investigated. Undrogued drifters were transported twice as fast as drogued drifters across the isobaths. 25% of the undrogued drifters landed, versus about 5% of the drogued ones, for the most part, on the same coastline locations where oil was found after Deepwater Horizon. Results highlight the importance of near surface gradients in controlling the cross-shelf transport and landing of surface material on the Gulf of Mexico's northern shores. •Undrogued drifters and crude oil slicks advect and disperse similarly in waves.•Undrogued drifters make landfall 5 times more than near-surface drogued drifters.•Near-surface vertical shear sets the cross-shelf transport of surface material

Fine-scale distribution of zooplankton over a mesoscale front explored through high frequency imaging

International audienc

Fine-scale distribution of larval fish and zooplankton over a mesoscale front explored through high frequency imaging

International audienc

Imperfect automatic image classification successfully describes plankton distribution patterns

International audienc

A Biodegradable Surface Drifter for Ocean Sampling on a Massive Scale

Abstract Targeted observations of submesoscale currents are necessary to improve science’s understanding of oceanic mixing, but these dynamics occur at spatiotemporal scales that are currently challenging to detect. Prior studies have recently shown that the submesoscale surface velocity field can be measured by tracking hundreds of surface drifters released in tight arrays. This strategy requires drifter positioning to be accurate, frequent, and to last for several weeks. However, because of the large numbers involved, drifters must be low-cost, compact, easy to handle, and also made of materials harmless to the environment. Therefore, the novel Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifter was designed following these criteria to facilitate massive sampling of near-surface currents during the Lagrangian Submesoscale Experiment (LASER). The drifting characteristics were determined under a wide range of currents, waves, and wind conditions in laboratory settings. Results showed that the drifter accurately follows the currents in the upper 0.60 m, that it presents minimal wave rectification issues, and that its wind-induced slip velocity is less than 0.5% of the neutral wind speed at 10 m. In experiments conducted in both coastal and deep ocean conditions under wind speeds up to 10 m s−1, the trajectories of the traditional Coastal Ocean Dynamics Experiment (CODE) and the CARTHE drifters were nearly identical. Following these tests, 1100 units were produced and deployed during the LASER campaign, successfully tracking submesoscale and mesoscale features in the Gulf of Mexico. It is hoped that this drifter will enable high-density sampling near metropolitan areas subject to stress by the overpopulation, such as lakes, rivers, estuaries, and environmentally sensitive areas, such as the Arctic

Near-Surface Current Mapping by Shipboard Marine X-Band Radar: A Validation

Abstract The Lagrangian Submesoscale Experiment (LASER) involved the deployment of ~1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F. G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR–drifter pairs, the root-mean-square error for the current speed is 4 cm and that for the current direction is 12°. The MR samples currents at a greater effective depth than the CARTHE drifters (1–5 m vs ~0.4 m). The mean MR–drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15°). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP

High throughput in situ imaging reveals complex ecological behaviour of giant mixotrophic protists

Although planktonic organisms have been the topic of scientific research for centuries, some organisms have fallen through the cracks. This is the case of Rhizaria, unicellular eukaryotes that are particularly delicate and often crushed by classical plankton nets. Yet, their substantial contribution to planktonic biomass was recently brought to light thanks to in situ imaging. Such an approach allows the study of these organisms in their undisturbed environment. Beyond their substantial biomass, their trophic ecology is poorly described (some taxa are mixotrophic and host photosynthetic symbionts, others do not) and knowledge regarding their reproductive cycle is even scarcer. Leveraging high frequency in situ imaging, we investigated the fine-scale distribution and orientation of ~230,000 organisms belonging to three groups of Rhizaria, including the mixotrophic taxa Acantharia and Collodaria, and the non-mixotrophic Phaeodaria. We brought to light differences in vertical distribution between subgroups, likely revealing different life strategies and contrasted abilities for buoyancy control. We also detected a previously undocumented preferential orientation of some organisms in each taxon. Finally, we infer from some of our observations presumptive steps of the obscure life cycle of Collodaria, which seems to involve fine buoyancy control to reach new environments and enable de novo symbiont acquisition. Altogether, these unprecedented results highlight that complex ecological behaviour can be achieved by “simple” unicellular organisms

High throughput in situ imaging reveals complex ecological behaviour of giant mixotrophic protists

International audienceAlthough planktonic organisms have been the topic of scientific research for centuries, some organisms have fallen through the cracks. This is the case of Rhizaria, unicellular eukaryotes that are particularly delicate and often crushed by classical plankton nets. Yet, their substantial contribution to planktonic biomass was recently brought to light thanks to in situ imaging. Such an approach allows the study of these organisms in their undisturbed environment. Beyond their substantial biomass, their trophic ecology is poorly described (some taxa are mixotrophic and host photosynthetic symbionts, others do not) and knowledge regarding their reproductive cycle is even scarcer. Leveraging high frequency in situ imaging, we investigated the fine-scale distribution and orientation of ~230,000 organisms belonging to three groups of Rhizaria, including the mixotrophic taxa Acantharia and Collodaria, and the non-mixotrophic Phaeodaria. We brought to light differences in vertical distribution between subgroups, likely revealing different life strategies and contrasted abilities for buoyancy control. We also detected a previously undocumented preferential orientation of some organisms in each taxon. Finally, we infer from some of our observations presumptive steps of the obscure life cycle of Collodaria, which seems to involve fine buoyancy control to reach new environments and enable de novo symbiont acquisition. Altogether, these unprecedented results highlight that complex ecological behaviour can be achieved by “simple” unicellular organisms