Plankton patchiness in high-frequency internal waves

Internal waves are ubiquitous elements of the physical dynamics on continental shelves, yet their effect on the spatial distribution of planktonic organisms throughout the water column is not well understood. We explore the effects of high-frequency internal waves on the patch dynamics of swimming plankters using a 2-layered model with 2 idealized wave forms (linear and weakly nonlinear interfacial waves). Our analysis suggests that even weakly swimming planktonic organisms may experience internal wave-induced changes in concentration. We find that maximum increases in concentration occur over the trough of the wave, and that changes in concentration above the interface are opposite to those below the interface. To a first approximation, the magnitude of the changes in concentration increases linearly with the wave amplitude and the swimming proficiency of the plankters. Maximum increases in concentration occur within the upper layer and, for a given stratification, increase linearly with the wave amplitude. These maximum increases in concentration are predicted to be less than twice the local background concentration. Such localized changes in the concentration of organisms should be ephemeral, lasting no longer than the wave period. Based on these results, we propose that coincident measurements of density (or temperature) and concentration of organisms through the water column can be used to determine the existence of 3 relationships that are consistent with internal wave-induced changes in the concentration of swimming plankters: (1) a relationship between along-isopycnal concentration anomalies and isopycnal depth that is out of phase above the pycnocline and in phase below the pycnocline (depth negative downwards), (2) a linear increase in the maximum along-isopycnal concentration anomalies with scaled isopycnal displacement amplitudes, and (3) maximum along-isopycnal concentration anomalies that are less than twice the background along-isopycnal concentration.