NP6.2

Bacteria and phytoplankton are abundant in aquatic environments, forming the base of the food web and mediating elemental cycling at a global scale. Understanding the interactions these microorganisms have with their turbulent fluid environments is an active area of research, largely conducted in laboratory-based flow experiments. In this work, we provide an open-source design and rigorous flow characterization for a 1L, dual oscillating grid turbulence facility, the smallest volume facility to date which produces near-isotropic, homogeneous turbulence. We optimized the tank geometry (grid-to-grid and grid-to-wall spacing), the grid geometry (for both classical and fractal grids: effective mesh size, blockage ratio, and fractal grid parameters), and the grid forcing regimes (for both coupled, antiphase and decoupled, randomized forcing: frequency range, stroke range, and randomized forcing parameters) to minimize mean flows and to produce acceptably homogeneous and isotropic turbulence within the unique constraints of a litre-scale volume. We acquired particle image velocimetry (PIV) measurements for both classical and fractal grids across a wide range of grid forcing regimes. We discuss the resulting length- and timescales relevant to microorganism-flow interactions, from the integral to the Kolmogorov scales. Finally, we discuss how the range of turbulent kinetic energy (TKE) dissipation rates achieved across the operational space of the facility mimics oceanographic turbulence in a range of in situ conditions, from the nearshore to the open ocean. This facility meets a long-standing need in the oceanography community in which feasible experimental working volumes are constrained by labor-intensive culturing requirements for large volumes of aquatic bacteria and phytoplankton.

How to cite: Wheeler, J. D., True, A. C., Michalec, F.-G., Holzner, M., Stocker, R., and Crimaldi, J. P.: A litre-scale turbulence facility for microorganism-flow interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13437, https://doi.org/10.5194/egusphere-egu22-13437, 2022.

The coast of Perú hosts the largest and most productive Eastern Boundary Upwelling System. Climate change is predicted to increase stratification, thereby increasing light availability and lowering nutrient concentrations at the surface. Moreover, the winds causing upwelling in this area are predicted to change their intensity and migrate polewards.

To better understand and predict the response of phytoplankton to changes in light and nutrient conditions, we recreated different light and nutrient scenarios in 9 off-shore mesocosms during the KOSMOS-Peru-2020 experiment in March-April 2020 off the coast of Callao (Perú). We recreated two light scenarios: high light (HL) and low light (LL); and four levels of upwelling by adding deep water (DW) in different proportions (0, 15, 30, 45 and 60 %). We monitored the phytoplankton composition every two days for 36 days. Photosynthetic pigments were measured using HPLC and the phytoplankton community composition was estimated using CHEMTAX and taxonomically determined by microscopic analyses, whereas chlorophyll-a (Chla) as a proxy for bulk phytoplankton biomass and particulate organic carbon, nitrogen and phosphorus (POC, PON and POP) provided information about biomass and stoichiometry of the total suspended matter.

The enclosed initial community was dominated by the red-tide forming raphidophyte Fibrocapsa japonica, detected for the first time off the coast of Perú during this experiment.

After an initial phase, during which F. japonica consumed the nutrients available, the DW was added and a second bloom, dominated by diatoms developed. As expected, more phytoplankton accumulated under HL and in higher DW treatments. The phytoplankton community under LL increased its Chla content per cell to maximize photosynthetic performance, whereas HL caused a significant increase in the POC:PON ratio.

Diatoms, coccolithophores and Phaeocystis were positively affected by HL, whereas the LL phytoplankton assemblage was dominated by smaller groups such as cryptophytes, prasinophytes, Synechococcus and especially the pelagophyte Octactis octonaria. F. japonica became more abundant under LL during the initial phase. Higher upwelling intensity favored diatoms as well as pelagophytes and chlorophytes under LL, whereas low nutrients conditions favored prasinophytes. Upwelling events were accompanied by high contributions of diatoms, whereas nutrient-depleted conditions were dominated by small phytoplankton groups and dinoflagellates.

From our results we conclude that although upwelling intensity did not affect stoichiometry significantly for the duration of the experiment, an intensification of stratification causing greater exposure to HL conditions might decrease the nutritional value of phytoplankton for upper trophic levels. Changes in light and nutrient availability caused by climate change will trigger a shift in the phytoplankton community composition. HL and intense upwelling areas might be dominated by diatoms and LL and low nutrient areas might be dominated by prasinophytes with distinct consequences for the trophic transfer and export efficiency of the Peruvian upwelling system.

How to cite: Behncke, J., Fernández-Méndez, M., and Riebesell, U.: Effect of Light and Upwelling Intensity on the Phytoplankton Community Composition in the Peruvian Upwelling System, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7020, https://doi.org/10.5194/egusphere-egu22-7020, 2022.

How to cite: Schmitt, F. G., Le Quiniou, C., Huang, Y., Calzavarini, E., Houliez, E., and Christaki, U.: The Agiturb laboratory turbulence generation system and its application to plankton studies: zooplankton and phytoplankton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8638, https://doi.org/10.5194/egusphere-egu22-8638, 2022.

Marine micro-organisms must cope with complex flow patterns and even turbulence as they navigate the ocean. To survive they must avoid predation and find efficient energy sources. A major difficulty in analysing possible survival strategies is that the time series of environmental cues in non-linear flow is complex, and that it depends on the decisions taken by the organism. One way of determining and evaluating optimal strategies is reinforcement learning. In a proof-of-principle study, Colabrese et al. [Phys. Rev. Lett. (2017)] used this method to find out how a micro-swimmer in a vortex flow can navigate towards the surface as quickly as possible, given a fixed swimming speed.  The swimmer measured its instantaneous swimming direction and the local flow vorticity in the laboratory frame, and reacted to these cues by swimming either left, right, up, or down. However, usually a motile micro-organism measures the local flow rather than global information, and it can only react in relation to the local flow, because in general it cannot access global information (such as up or down in the laboratory frame). Here we analyse optimal strategies with local signals and actions that do not refer to the laboratory frame. We demonstrate that symmetry-breaking is required to find such strategies. Using reinforcement learning we analyse the emerging  strategies for different sets of environmental cues that micro-organisms are known to measure. This talk is based on "Navigation of micro-swimmers in steady flow: the importance of symmetries" by Jingran Qiu, [Opens in a new win Navid Mousavi, Kristian Gustavsson[Opens in a new window], Chunxiao Xu, Bernhard Mehlig, and Lihao Zhao, Journal of Fluid Mechanics 932, A10. doi:10.1017/jfm.2021.978

How to cite: Mehlig, B.: Navigation of micro-swimmers in steady flow: the importance of symmetries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7811, https://doi.org/10.5194/egusphere-egu22-7811, 2022.

Finding mating partners can be challenging for copepods: the ocean is vast and turbulent, the average animal's concentration is sparse, and their swimming ability is limited. Therefore, the probability for locating a mate assuming a homogeneous distribution of animals and a random motion leads to low mating encounter rates. However, zooplankton distribution is not homogeneous; field observations since the 1950s have shown that plantkon have patchy distributions over multiple scales - from thousand of kilometers down to the millimeter scale¹. Of relevance to mating is patchiness at small scales (on the order of the animal's size), leading to increased probability for sexual encounters due to higher local concentrations. Indeed, such mating clusters have been identified in ship transect observations². However, how such clusters form in the diffusive turbulent environment is not fully understood.

In certain species, males actively search for females to achieve sexual encounters. When a male locates a female it pursuits her to achieve contact³, and this behavior is thought to drive small-scale clustering⁴. Nevertheless, the details of this process are not so straightforward. Specifically, the random swimming pattern males perform in their search, and the (super) diffusive nature of turbulence⁵, both increases the animals' dispersion, thus opposing patch formation. Therefore, the existence of mating clusters requires a detailed balance between diffusion and pair-interactions. However, this equilibrium in zooplankton patch formation was not examined in the past.

Our study examines the equilibrium between diffusion and pair-interactions in zooplankton. Specifically, we have formulated a numerical framework, the pair-interaction model, which allows to study patch formation. Remarkably, we observe that pair-interactions can lead to patches of numerous particles, similar to the field observations². We thus explore the model's parameter space, to reveal what is required for patchiness to be sustained. Furthermore, we compare our model's results with laboratory measurements of calanoid copepod trajectories³ and show good agreement between the model and the experiment. Our results support the hypothesis that small-scale patchiness is driven by the animal's behavior and thus explain the details of how zooplankton achieve high mating encounter rates in their complex environment.

¹ B. Pinel-Alloul and A. Ghadouani (2007). Spatial heterogeneity of planktonic microorganisms in aquatic systems, Springer Netherlands, Dordrecht.

² C. S. Davis, S. M. Gallager and A. R. Solow (1992). Science 257, 230-232.

³ F.-G. Michalec et al. (2017). Proc. Natl. Acad. Sci. U.S.A. 114 ; F.-G. Michalec et al. (2020). eLife 9, e62014.

⁴ C. L. Folt and C. W. Burns (1999). Trends in Ecology and Evolution, 14, 300–305.

⁵ J. P. Salazar and L. R. Collins (2009). Ann. Rev. Fluid Mech. 41, 405-432.

How to cite: Shnapp, R. and Holzner, M.: Copepods counter dispersion to maintain high mating-encounter rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7710, https://doi.org/10.5194/egusphere-egu22-7710, 2022.

A detailed understanding of the physical mechanisms driving gyrotactic species to migrate vertically towards the surface allows better quantification of biogeochemical fluxes across the ocean. We focus on marine phytoplankton cells that are motile under gyrotactic forcing. Some species spontaneously swim in the direction opposite to gravity [1]. Gyrotaxis is originating either from morphological aspects (elongated shape, density heterogeneity) or the coupled effect of swimming and settling which results in an inertial torque. Indeed, fluid inertial torque may have a potential impact on the gyrotaxis for elongated planktonic swimmers, especially for those forming long chains and thus having large swimming and settling speeds
Based on numerical simulations of hundreds of thousands of micro-organisms swimming in homogeneous isotropic turbulence, we will comment on the different sources of gyrotactic induced spatial clustering [2, 3] and vertical migration [4]. Some specific configurations lead to the accumulation of elongated plankton cells in upwelling flow regions enhancing their ability to move across turbulence through the water column.  

[1] Kessler J.O. (1985), Nature - 313, 218–220.
[2] Durham W. M., et al. (2013) Nat. Commun. - 4, 2148.
[3] De Lillo F., et al. (2014) Phys. Rev. Lett. – 112, 044502
[4] Lovecchio S., et al. (2019) Sci. Adv. - 5: eaaw7879

How to cite: Climent, E., Qiu, J., Cui, Z., and Zhao, L.: Gyrotactic plankton cells in turbulence: the effects of motility, shape, fluid acceleration and inertia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-895, https://doi.org/10.5194/egusphere-egu22-895, 2022.

Using DNS-based Eulerian-Lagrangian simulations, we investigate the dynamics of small gyrotactic swimmers in free-surface turbulence. We consider open channel flow turbulence in which bottom-heavy swimmers are dispersed. Swimmers are characterized by different vertical stability, so that some realign to swim upward with a characteristic time smaller than the Kolmogorov time scale, while others possess a re-orientation time longer than the Kolmogorov time scale. We cover one order of magnitude in the flow Reynolds number, and two orders of magnitude in the stability number, which is a measure of bottom heaviness. We observe that large-scale advection dominates vertical motion when the stability number, scaled on the local Kolmogorov time scale of the flow, is larger than unity: This condition is associated to enhanced migration towards the surface, particularly at low Reynolds number, when swimmers can rise through surface renewal motions that originate directly from the bottom-boundary turbulent bursts. Conversely, small-scale effects become more important when the Kolmogorov-based stability number is below unity: Under this condition, migration towards the surface is hindered, particularly at high Reynolds, when bottom-boundary bursts are less effective in bringing bulk fluid to the surface. In an effort to provide scaling arguments to improve predictions of models for motile micro-organisms in turbulent water bodies, we demonstrate that a Kolmogorov-based stability number around unity represents a threshold beyond which swimmer capability to reach the free surface and form clusters saturates.

How to cite: Marchioli, C., Bhatia, H., Sardina, G., Brandt, L., and Soldati, A.: Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3718, https://doi.org/10.5194/egusphere-egu22-3718, 2022.

Many phytoplankters are able to swim, and are thus not passively transported by the flow. Although usually weak, ocean turbulence can affect the motion of one-celled organisms in nontrivial ways. It is known that an ellipsoidal body can be rotated by the fluid gradients, depending on its aspect ratio.  On the other hand, directed swimming (e.g. following chemical or physiscal cues, in any form of taxis) can play an important role in determining the fitness of an individual, whether for finding food, light or escaping from predators.

By means of theoretical and numerical investigation [1,2], we show how a microswimmer's orientation can be influenced by different scales of the flow and in what condition relevant correlation with the orientation of the flow can be expected.   

[1] Alignment of nonspherical active particles in chaotic flows M Borgnino, K Gustavsson, F De Lillo, G Boffetta, M Cencini, B Mehlig, Physical review letters 123 (13), 138003

[2] M Borgnino, K Gustavsson, F De Lillo, G Boffetta, M Cencini (2021) in preparation.

How to cite: De Lillo, F., Borgnino, M., Boffetta, G., Gustafsson, K., Mehlig, B., and Cencini, M.: Orientation of swimmers in turbulent flows, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13293, https://doi.org/10.5194/egusphere-egu22-13293, 2022.