Fluid motion and solute distribution around sinking aggregates. II. Implications for remote detection

Marine snow aggregates are colonized by copepods, and encounter rates inferred from observed abundances of colonizers are high. We examined the potential for hydromechanical and chemical remote detection. The fluid disturbance generated by a sinking aggregate was described by solving the Navier-Stokes’ equation for a sinking sphere at Reynolds numbers typical of marine snow (up to 20). Fluid deformation rate, the component of the flow that can be perceived by copepods, attenuates rapidly, and detection distances estimated from knowledge of the hydromechanical sensitivity in copepods are insufficient to account for the observed abundances of colonizers. We next solved the advection-diffusion equation to describe the chemical trail left by a leaking and sinking aggregate. The plume is long and slender and may be detected by a horizontally cruising copepod. From the model of the plume and literature-based estimates of size-dependent aggregate leakage rates of amino acids, we estimate that a threshold sensitivity to amino acids of 0.4 × 10–7 M is required to account for observed abundances of colonizers. This is consistent with knowledge of the amino acid concentrations needed to elicit behavioral responses in copepods.