Diel vertical migration (DVM) is a ubiquitous phenomenon in marine and freshwater plankton communities. Most commonly, plankton migrate to surface waters at dusk and return to deeper waters at dawn. Up until recently, it was thought that DVM was triggered by a relative change in visible light intensity. However, evidence has shown that DVM also occurs in the deep sea where no direct and background sunlight penetrates. To identify whether such DVM is associated with latitudinal and seasonal day light variation, one and a half years of recorded acoustic data, a measure of zooplankton abundance and movement, were examined. Acoustic Doppler current profilers, moored at eight different sub-tropical latitudes in the North-Atlantic Ocean, measured in the vertical range of 500–1600 m. DVM was observed to follow day length variation with a change in season and latitude at all depths. DVM followed the rhythm of local sunrise and sunset precisely between 500 and 650 m. It continued below 650 m, where the deepest penetrable irradiance level are <10 −7 times their near-surface values, but plankton shortened their time at depth by up to about 63% at 1600 m. This suggests light was no longer a cue for DVM. This trend stayed consistent both across latitudes and between the different seasons. It is hypothesized that another mechanism, rather than light, viz. a precise biochemical clock could maintain the solar diurnal and seasonal rhythms in deep sea plankton motions. In accordance with this hypothesis, the deepest plankton were consistently the first to migrate upwards.

In the Atlantic Ocean, the densest water mass Antarctic Bottom Water “AABW” can only cross the Mid‐Atlantic Ridge from its southwestern to northeastern basins in limited, because deep, conduits. At the southwestern entrance of one of these, the equatorial Romanche Fracture Zone, AABW crosses a sill at 4550 m depth in a 7 km narrow channel before plunging into the deep. At the sill‐slope, the rapidly flowing AABW causes shear‐induced turbulent mixing with the overlying water masses. We present an excerpt of 1 Hz sampled, half‐yearlong moored observations from 99 high‐resolution temperature sensors that demonstrate and quantify the turbulence details. On top of quasi‐steady shear flow, an internal tide modulates the mixing. Together, they constitute a means for an extremely long train of >250 consecutive Kelvin‐Helmholtz billows in a day that vary between 5 and 100 m in vertical scale.