Abstract
The oxygen deficient mesopelagic layer (ODL) off Peru has concentrations below 5 μmol O2 kg–1 and is delimited by a shallow upper oxycline with strong vertical gradient and a more gradual lower oxycline (lOx). Some regions show a narrow band of slightly increased oxygen concentrations within the ODL, an intermediate oxygen layer (iO2). CTD, oxygen and lowered Acoustic Doppler Current Profiler (LADCP, 300 kHz) profiles were taken on the shelf edge and outside down to mostly 2000 m. We evaluate here the acoustic volume backscatter strength of the LADCP signal representing organisms of about 5 mm size. Dominant features of the backscatter profiles were a minimum backscatter strength within the ODL, and just below the lOx a marked backscatter increase reaching a maximum at less than 3.0 μmol O2 kg–1. Below this maximum, the acoustic backscatter strength gradually decreased down to 1000 m below the lOx. The backscatter strength also increased at the iO2 in parallel to the oxygen concentration perturbations marking the iO2. These stable backscatter features were independent of the time of day and the organisms represented by the backscatter had to be adapted to live in this microaerobic environment. During daylight, these stable structures were overlapped by migrating backscatter peaks. Outstanding features of the stable backscatter were that at very low oxygen concentrations, the volume backscatter was linearly related to the oxygen concentration, reaching half peak maximum at less than 2.0 μmol O2 kg–1 below the lOx, and the depth-integrated backscatter of the peak below the lOx was higher than the integral above the Ox. Both features suggest that sufficient organic material produced at the surface reaches to below the ODL to sustain the major fraction of the volume backscatter-producing organisms in the water column. These organisms are adapted to the microaerobic environment so they can position themselves close to the lower oxycline to take advantage of the organic particles sinking out of the ODL.
Highlights
One of the most extreme mesopelagic Oxygen Deficient Layers is located in the Eastern Tropical South Pacific (ETSP) off Peru, with hypoxic oxygen concentrations extending from 500 m depth (Fuenzalida et al, 2009; Paulmier and Ruiz-Pino, 2009)
We used the 5th bin of the down-looking lowered ADCP (LADCP) Master beams (AEM5), and because this bin registers volume backscatter at 50 m distance and 20 degrees angle from the vertical, the shallowest data in a cast are at approximately 50 m water depth
The Oxygen Deficient Layer (ODL) is clearly distinguished in the oxygen and LADCP data in the depth layer between the top oxycline (Figure 2B, A) and the lower oxycline
Summary
One of the most extreme mesopelagic Oxygen Deficient Layers (hereafter ODLs) is located in the Eastern Tropical South Pacific (ETSP) off Peru, with hypoxic oxygen concentrations extending from 500 m depth (Fuenzalida et al, 2009; Paulmier and Ruiz-Pino, 2009). Off Peru, the ODL extends to the lower oxycline (lOx) at around 500 m and the low oxygen concentration within the ODL is expected to reduce the biogenic oxidation of organics outside the continental shelf, a pattern observed by various authors for mesopelagic oxygen deficient layers (Martin et al, 1987; Wishner et al, 1990; Dale et al, 2015; Cavan et al, 2017). We are reporting data from locations off the shelf, the flux of organic particles at the bottom of the ODL, corresponding to a strong gradient of biomass above/below the lower oxycline, is expected to be less attenuated than in mesopelagic waters with higher than suboxic oxygen concentrations
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