Internal wave–zooplankton interactions in the Alboran Sea (W-Mediterranean)

Abstract

An upward looking 75 kHz ADCP (Acoustic Doppler Current Profiler) was moored at 100 m above the bottom in 912 m water depth in the central-north Alboran Sea (W-Mediterranean Sea). The ADCP sampled current and acoustic echo-amplitude at a rate of once per minute for 9 days, thereby revealing particular internal wave–zooplankton interactions that varied with depth. The ADCP's echo-amplitude, a measure of zooplankton abundance, showed a dominant diurnal periodicity due to vertical migration, and occasionally intense reflections manifesting high-frequency interfacial internal waves close to the buoyancy frequency. These waves reached excursions of up to about 90 m crest-trough, around 250 m. They were only visible in acoustic data during daytime when the zooplankton gathered near these thin interfacial layers. Around 500 m where the vertical density stratification was 10 times smaller than at 250 m, larger amplitude internal waves were observed, measuring up to about 150 m crest-trough. These waves had the lowest internal wave frequency, close to the inertial frequency. Below 600 m, the stratification was so weak that large (~100 m) vertical convection in the direction of the earth rotational vector dominated over internal gravity waves. At these depths, diurnal vertical migrations were not observed, which suggests that zooplankton either avoid or become dispersed by waters with large vertical convection. Light limitation is not expected to be a key factor in this case, as vertical migration has been observed deeper than 1000 m in other areas where stratification is greater.