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. 2010 Apr 30;5(4):e10330.
doi: 10.1371/journal.pone.0010330.

Acoustic observation of living organisms reveals the upper limit of the oxygen minimum zone

Arnaud Bertrand  1 , Michael Ballón, Alexis Chaigneau
Affiliations

Acoustic observation of living organisms reveals the upper limit of the oxygen minimum zone

Arnaud Bertrand et al. PLoS One. .

Abstract

Background: Oxygen minimum zones (OMZs) are expanding in the World Ocean as a result of climate change and direct anthropogenic influence. OMZ expansion greatly affects biogeochemical processes and marine life, especially by constraining the vertical habitat of most marine organisms. Currently, monitoring the variability of the upper limit of the OMZs relies on time intensive sampling protocols, causing poor spatial resolution.

Methodology/principal findings: Using routine underwater acoustic observations of the vertical distribution of marine organisms, we propose a new method that allows determination of the upper limit of the OMZ with a high precision. Applied in the eastern South-Pacific, this original sampling technique provides high-resolution information on the depth of the upper OMZ allowing documentation of mesoscale and submesoscale features (e.g., eddies and filaments) that structure the upper ocean and the marine ecosystems. We also use this information to estimate the habitable volume for the world's most exploited fish, the Peruvian anchovy (Engraulis ringens).

Conclusions/significance: This opportunistic method could be implemented on any vessel geared with multi-frequency echosounders to perform comprehensive high-resolution monitoring of the upper limit of the OMZ. Our approach is a novel way of studying the impact of physical processes on marine life and extracting valid information about the pelagic habitat and its spatial structure, a crucial aspect of Ecosystem-based Fisheries Management in the current context of climate change.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Acoustic detection of the VEEC during the ‘Filamentos 2008’ survey.
A. Example of acoustic echogram showing a CTDO track and the VEEC. The superimposed black solid line is the corresponding DO vertical profile (mL L−1, lower axis). B. Box plot of DO concentration at ZVEEC according to the diel period. C. Relationship between ZVEEC and Z0.8 for the 25 stations with detectable CTDO tracks on echograms (full red circles; the full red square in the upper right corner corresponds to the station presented in A) and the other 71 CTDO stations (full blue circles); Red and black solid lines correspond to the linear regression for the 25 stations with detectable CTDO tracks and for all the 96 stations, respectively. D. Vertical range of the lower oxycline (shaded area) for all 96 stations ranked toward increasing Z0.8 (black solid line); full dots represent ZVEEC for the 25 stations with “visible” CTDO tracks (red) and the other 71 CTDO stations (blue).
Figure 2
Figure 2. Spatial distribution of the upper OMZ depth.
Upper OMZ depth estimated from Z0.8 determined from CTDO measurements (A) and Niskin bottles profiles (D) and ZVEEC estimated from acoustic measurements (B, E). Black crosses indicate the position of hydrographic stations (A, D) whereas black lines indicate acoustic tracks (B, E). C and F differences between Z0.8 and ZVEEC; black contours correspond to a null difference; boxplots of the differences are displayed on the upper right part of (C) and (F). Upper panel (A, B, C) corresponds to the ‘Filamentos 2008’ survey; dotted lines indicate the depth of the 200 m bottom depth. Lower panel (D, E, F) corresponds to the ‘Pelagic 2005’ survey. Left colour-bars correspond to figures (A, B, D, E) while right colour-bars correspond to figures (C, F). G. Meridional variation of ZVEEC averaged between the coast and 200 km offshore during the ‘Pelagic 2005’ survey (black solid line) and corresponding ± one standard deviation (grey shaded area).
Figure 3
Figure 3. Volume of anchovy habitat along the Peruvian coast.
A. Volume (red volume) estimated by integrating ZVEEC over the area occupied by the cold coastal water and its mixing with adjacent water masses during ‘Pelagic 2005’ survey. The upper part of the volume shows anchovy distribution estimated during the same survey. B. Zoom of the study area between 9°S and 15°S (black dotted rectangle) showing a region of shallower ZVEEC. This region corresponds to a mesoscale filament associated with strong westward geostrophic currents and high chlorophyll concentration as observed from geostrophic currents (black quivers) from satellite altimetry AVISO product and chlorophyll-a concentration (colours, in mg m−3) from satellite SeaWiFS data for the same time period (C). The black solid line south of Pisco in (A) corresponds to the transect presented in (D) showing the echogram and the ZVEEC (black solid line) along this transect. E. Wavelet power spectrum (in m2) of ZVEEC in this transect showing the presence of mesoscale (≥10 km) and submesoscale (F) Features.
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