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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L12603, doi:10.1029/2007GL029947, 2007

Monthly periodicity in acoustic reflections and vertical motions in the deep ocean

Hans van Haren

Royal Netherlands Institute for Sea Research, Den Burg, The Netherlands


Abstract

A recent, 1.5 years long record of acoustic Doppler current profiler (ADCP)-data from the Canary Basin (North-Atlantic ocean) likely reflects vertical zooplankton migration between 800 and 1400 m. This record clearly distinguishes 3 major periodicities of down- and upgoing motions to within a precision of ∼1/400: a daily, a seasonal and a monthly cycle. Largest daily excursions occur during full moon. The directly observed hourly mean vertical velocity amplitudes of ∣w∣ = 0.025 ± 0.01 m s−1 are too slow for particles from the observational depths to reach the zone of moon- (and only very weak sun-) light penetration in half a day. It is shown that no physical (internal wave), geochemical or sinking food mechanism can trigger the daily and monthly cycles, which are coupled. It is speculated that an entrained biorhythm running precise internal biochemical clocks controls the vertical migration.

Received 9 March 2007; accepted 16 May 2007; published 19 June 2007.

Keywords: monthly periodicity; open deep ocean ADCP-data; zooplankton migration.

Index Terms: 4227 Oceanography: General: Diurnal, seasonal, and annual cycles (0438); 4259 Oceanography: General: Ocean acoustics; 4890 Oceanography: Biological and Chemical: Zooplankton; 4894 Oceanography: Biological and Chemical: Instruments, sensors, and techniques; 4599 Oceanography: Physical: General or miscellaneous.

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Citation: van Haren, H. (2007), Monthly periodicity in acoustic reflections and vertical motions in the deep ocean, Geophys. Res. Lett., 34, L12603, doi:10.1029/2007GL029947.