Distribution and diel vertical movements of mesopelagic scattering layers in the Red Sea

Abstract

The mesopelagic zone of the Red Sea represents an extreme environment due to low food concentrations, high temperatures and low oxygen waters. Nevertheless, a 38 kHz echosounder identified at least four distinct scattering layers during the daytime, of which the 2 deepest layers resided entirely within the mesopelagic zone. Two of the acoustic layers were found above a mesopelagic oxygen minimum zone (OMZ), one layer overlapped with the OMZ, and one layer was found below the OMZ. Almost all organisms in the deep layers migrated to the near-surface waters during the night. Backscatter from a 300 kHz lowered Acoustic Doppler Current Profiler indicated a layer of zooplankton within the OMZ. They carried out DVM, yet a portion remained at mesopelagic depths during the night. Our acoustic measurements showed that the bulk of the acoustic backscatter was restricted to waters shallower than 800 m, suggesting that most of the biomass in the Red Sea resides above this depth.

Fig. 1

Map showing cruise track (blue line) and positions of the CTD casts (red points) in the Red Sea. The blue transparent rectangles indicate sections of the cruise track included in the EK60 data

Fig. 2

Echograms from April 27. a An entire 24 h period, with the different scattering layers indicated by numbers. Day and night periods are indicated with white and black bars above echogram. b The vertical profiles of salinity (black line), temperature (blue line), fluorescence (green line), and oxygen (red line) closest in time and space to the echogram in a

Fig. 3

Level plots of acoustic biomass profiles (10–1,000 m). Days are plotted along the x-axis, with one observation per day (the latitudes of the CTD stations are given for every second date in red). The depth is plotted along the y-axis, and the colours correspond to amount of acoustic biomass. Absence of data is shown as white fields. Fluorescence (red) and oxygen (blue) concentrations are overlaid. Upper panels daytime data; lower panels night-time data. Left column EK60 data. Right column LADCP (300 kHz) data. In the EK60 plots, each horizontal pixel-column represents a daily profile. In the LADCP plots, each pixel horizontally represents a single CTD cast. For the EK60, the overlay values are from the CTD cast closest in time to the centre of the profile; in the LADCP plots, the overlay values are concurrent. Blue lines oxygen concentrations in mL L⁻¹. The contour lines are 0.25 mL L⁻¹ apart. Red lines: contours of fluorescence

Fig. 4

Ratios of day to night acoustic biomass. The black line and points show the average ratio between the daytime and night-time 38 kHz EK60 backscatter, integrated in 10 m bins. The grey line and points also show the average day-to-night backscatter ratio, but based on the day and night 300 kHz LADCP casts. Note that the plots may be noisy in depth intervals with very low backscattering and that the ratio axis is logarithmic