New guidelines for the application of Stokes’ models to the sinking velocity of marine aggregates

, ,

Numerical simulations of ocean biogeochemical cycles need to adequately represent particle sinking velocities (SV). For decades, Stokes’ Law estimating particle SV from density and size has been widely used. But while Stokes’ Law holds for small, smooth, and rigid spheres settling at low Reynolds number, it fails when applied to marine aggregates complex in shape, structure, and composition. Minerals and zooplankton can alter phytoplankton aggregates in ways that change their SV, potentially improving the applicability of Stokes’ models. Using rolling cylinders, we experimentally produced diatom aggregates in the presence and absence of minerals and/or microzooplankton. Minerals and to a lesser extent microzooplankton decreased aggregate size and roughness and increased their sphericity and compactness. Stokes’ Law parameterized with a fractal porosity modeled adequately size‐SV relationships for mineral‐loaded aggregates. Phytoplankton‐only aggregates and those exposed to microzooplankton followed the general Navier‐Stokes drag equation suggesting an indiscernible effect of microzooplankton and a drag coefficient too complex to be calculated with a Stokes’ assumption. We compared our results with a larger data set of ballasted and nonballasted marine aggregates. This confirmed that the size‐SV relationships for ballasted aggregates can be simulated by Stokes’ models with an adequate fractal porosity parameterization. Given the importance of mineral ballasting in the ocean, our findings could ease biogeochemical model parameterization for a significant pool of particles in the ocean and especially in the mesopelagic zone where the particulate organic matter : mineral ratio decreases. Our results also reinforce the importance of accounting for porosity as a decisive predictor of marine aggregate SV.

Sinking velocities vs. ESD (equivalent spherical diameter) for aggregates formed in each tank of the four treatments and comparison with theoretical expectations from different parameterizations of Stokes’ Law and the general Navier‐Stokes’ drag equation. P: phytoplankton; PZ: phytoplankton + microzooplankton (rotifers); PM: phytoplankton + mineral (calcite); PMZ: phytoplankton + mineral + microzooplankton. (a) Model 1, Stokes’ Law with constant porosities of 0% (dashed lines) and 99% (solid lines). (b) Model 2, general Navier‐Stokes’ drag law with constant drags of 1 (dashed lines) and 5 (solid lines), and a constant porosity of 99%. (c) Model 3, Stokes’ Law with a porosity scaled on a fractal geometry with coefficient a = 0.03 and D3 = 1.4 (dashed lines) and D3 = 1.8 (solid lines). (d) Model 4, general Navier‐Stokes’ drag law with a porosity scaled on a fractal geometry with coefficient a = 0.03 and D3 = 1.4 (solid lines) and 1.8 (dashed lines). See the text for details on drag calculation

Reference

Laurenceau-Cornec, E.C., Le Moigne, F.A.C., Gallinari, M., Moriceau, B., Toullec, J., Iversen, M.H., Engel, A., and De La Rocha, C.L. 2020. New guidelines for the application of Stokes’ models to the sinking velocity of marine aggregates. Limnol. Oceanogr. doi:10.1002/lno.11388.
Link to the journal:

Ika Paul-Pont, guest at the CNRS 80th anniversary panel discussion

,

Ika Paul-Pont, récemment distinguée par le CNRS d’une médaille de Bronze pour les travaux qu’elle mène avec ses collègues sur la contamination par les microplastiques, a été invitée à participer à une table ronde sur l’état de la recherche scientifique en France. Cet échange, en présence d’Emmanuel Macron et de Frédéric Vidal et animée par Mathieu Vidard, s’est déroulé dans le cadre d’une cérémonie qui a marqué la clôture d’une année de festivités autour des 80 ans du CNRS. Ce fut l’occasion pour les scientifiques invités de mettre en lumière des problématiques au cœur du fonctionnement de la recherche française.

(début de la table ronde à 12’40)

 

 

The TONGA mission

,

David Gonzalez Santana and Géraldine Sarthou from the Chibido team are on board the R/V Atalante in the South Pacific for the TONGA cruise (PIs Sophie Bonnet (MIO) and Cécile Guieu (LOV), + link on the CHIBIDO team website).

The objective of TONGA is to study the inputs of trace elements from shallow hydrothermal sources and their potential impact on biological productivity and the biological carbon pump.

Halfway through the cruise, they are on their way to the ultra-oligotrophic waters (their reference station) after having studied a shallow area (~ 200 m) particularly intense in terms of hydrothermal activity.

Then, the search of a second hydrothermal site is scheduled.

You can follow them on their Twitter account.

(C) Hubert Bataille/ IRD

The final conference of the VIVALDI project

,

26-27 November at the PNBI in Plouzané will consist in a Scientific congress dedicated to VIVALDI‘s research results.

28 November will be open to all stakeholders and will present VIVALDI’s main applied outcomes and potential innovations. On that day, interpretation will be available in French, English, Spanish and Italian. Session will take place at Océanopolis in Brest.

More information and programme can be found in the project’s website.

RESSAC Festival from 16 to 22 November

,

The first edition of the Ressac festival (Research in the arts and creation sciences), proposed and organised by the cultural service of the UBO (University of Western Brittany), begins on Saturday 16 November with the project “Auris maris, une oreille de mer à l’écoute du changement climatique” at Océanopolis.

LEMAR participates in this project, in particular through the strong involvement of Christine Paillard (DR at CNRS), and the work of the last year of high school students at Lycée Vauban “Myctophidae : Voyage en eau profonde” supervised by Gildas Roudaut (AI at IRD).

This festival is also part of the 80th anniversary of the CNRS.

The festival is free and open to all.

Read here an article of “Le Télégramme”:

and the complete program on the Festival website:
https://festival-ressac.univ-brest.fr/programmation-spectacles-vivants-expositions-creations-sonores/