Figure 3.

(a) Schematic of the hydrodynamic model of a negatively buoyant copepod performing partial sinking (i.e. by definition, the sinking speed is less than the copepod's terminal sinking speed). The model copepod consists of a spherical body and a point force, f, located outside the spherical body. The frame of reference is fixed on the body, so that the steady partial sinking motion of the copepod is transformed into a constant flow coming from below the copepod. The curved arrow indicates the torque acting on the model copepod due to the reaction force of f. Note that for simplicity the balance of the torques on the model copepod is not considered here. We assume that choosing a suitable centre of mass relative to the centre of volume enforces the torque balance. (b) The FBD for the model copepod performing partial sinking. W stands for the copepod's weight acting at the centre of mass; B stands for buoyancy acting at the centre of volume of the copepod; F stands for the drag force of the surrounding water flow over the surface of the body; f′ is the reaction force of the water acting on the copepod's cephalic appendages. (c) FBD of the surrounding water. f is the point force exerted on the water by the copepod, representing the effect of the beating movement of the cephalic appendages; F′+B′ stands for the force of the copepod on the surrounding water through the body–fluid interface. Note that f′=−f, F′=−F, and B′=−B (Newton's third law).