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On the biogenic hydrodynamic transport of upward and downward cruising copepods

Yunxing Su, Rui Zhu, Eckart Meiburg, Monica M. Wilhelmus

Abstract

Mesozooplankton aggregations undergoing vertical migrations in the upper ocean have been hypothesized to have an important role in the redistribution of carbon, nutrients, and oxygen via biogenic hydrodynamic transport (BHT). While laboratory studies have demonstrated how swarm-induced hydrodynamic instabilities can drive large-scale transport in strongly stratified environments, measurements are usually performed with model organisms that differ in morphology and swimming mode from ecologically relevant marine species. To bridge this gap, we conducted experiments with copepods and analyzed upward and downward trajectories to identify differences in flow fields, force distribution, and BHT for these two scenarios. Using two-dimensional bright-field Particle Image Velocimetry (PIV), we quantified the near-body velocity field and found that the average downward swimming speed significantly exceeds the average upward swimming speed, with the flow fields exhibiting direction-dependent characteristics. We incorporated these findings into a continuum squirmer model to estimate the swimmer-induced drift volume and mixing efficiency, focusing on the effects of the reduced gravity of the swimmers and the density stratification of the surrounding fluid. Our simulations reveal that both the excess weight of the organisms and the fluid stratification strongly constrain the net BHT. This study provides a critical step toward integrating lab-based models of marine mesozooplankton with remote sensing data to incorporate vertical migrations into global ocean models with realistic biogeochemistry and assess their ecological significance in actively sustaining local ecosystems.

On the biogenic hydrodynamic transport of upward and downward cruising copepods

Abstract

Mesozooplankton aggregations undergoing vertical migrations in the upper ocean have been hypothesized to have an important role in the redistribution of carbon, nutrients, and oxygen via biogenic hydrodynamic transport (BHT). While laboratory studies have demonstrated how swarm-induced hydrodynamic instabilities can drive large-scale transport in strongly stratified environments, measurements are usually performed with model organisms that differ in morphology and swimming mode from ecologically relevant marine species. To bridge this gap, we conducted experiments with copepods and analyzed upward and downward trajectories to identify differences in flow fields, force distribution, and BHT for these two scenarios. Using two-dimensional bright-field Particle Image Velocimetry (PIV), we quantified the near-body velocity field and found that the average downward swimming speed significantly exceeds the average upward swimming speed, with the flow fields exhibiting direction-dependent characteristics. We incorporated these findings into a continuum squirmer model to estimate the swimmer-induced drift volume and mixing efficiency, focusing on the effects of the reduced gravity of the swimmers and the density stratification of the surrounding fluid. Our simulations reveal that both the excess weight of the organisms and the fluid stratification strongly constrain the net BHT. This study provides a critical step toward integrating lab-based models of marine mesozooplankton with remote sensing data to incorporate vertical migrations into global ocean models with realistic biogeochemistry and assess their ecological significance in actively sustaining local ecosystems.
Paper Structure (19 sections, 16 equations, 11 figures, 3 tables)

This paper contains 19 sections, 16 equations, 11 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. Model organism
  4. Experimental setup
  5. Simulation approach
  6. Governing equations
  7. Non-dimensional formulation
  8. Numerical set-up
  9. Results
  10. Experimental observations
  11. Copepod swimming and free-sinking speeds
  12. Near-field body flows
  13. Discrete swimmer simulations
  14. Validation of numerical setup
  15. Drift volume analysis
  16. ...and 4 more sections

Figures (11)

  • Figure 1: Model organism and experimental setup. (a) Adult copepods ( Parvocalanus crassirostris). (b) Experimental setup (in-house micro-bright-field PIV setup) for individual copepod swimming speed and flow field measurements. The focal plane (red region) is vertical, allowing for measurements in the vertical direction. More details about the experimental setup can be obtained in gemmell2014new.
  • Figure 2: Swimming speed measurements of individual copepods. Panel (a) shows the copepod swimming angle, $\theta$, with respect to the horizontal direction, with negative values for downward swimming and positive values for upward swimming. Note that the focal plane is vertical. Panel (b) shows the swimming speed at different swimming angles as defined in panel (a). The dashed lines indicate the averaged upward (green) and downward (red) swimming speed, respectively. The black circle denotes the freely sinking speed of a copepod. Panel (c) shows the free-body diagram of a swimming copepod at steady state, where $T$ is the thrust force, $\Delta G$ is the excess weight, $D$ is the drag force, $R$ is the resultant force of gravity and thrust. The angle between the resultant force, $R$, and the horizontal direction is denoted as $\theta_1$. Panel (d) shows the measured individual copepod swimming speeds as a function of organism body length.
  • Figure 3: Cycle-averaged vorticity fields ($s^{-1}$) and streamlines of the flow fields induced by individual swimming copepods in the lab frame of reference (gravity direction is in the negative y-direction). Panel (a) shows a front-view (dorso), upward swimming copepod; panel (b) shows a front-view (dorso), downward swimming copepod.
  • Figure 4: Cycle-averaged vorticity fields (contours, unit $s^{-1}$) and streamlines of the flow fields induced by individual swimming copepods in the lab frame (gravity direction is in negative y direction). Panel (a) shows a side view, upward swimming copepod; panel (b) shows a side view, downward swimming copepod.
  • Figure 5: Near-field flow. The time-averaged image sequence in panels (a-c) shows the particle streaks induced by a swimming copepod (side view, with gravity pointing downward). In this sequence, from (a) to (c), the copepod is changing its swimming direction from an approximately horizontal direction to an approximately vertical direction. The red and green arrows denote the general direction of the seed particles and the near-body drift flow direction, respectively. The black dashed lines denote the body axis of the copepod, and the red dashed lines show the range of the flow induced by the organism.
  • ...and 6 more figures