Abstract
BackgroundOxygen 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 FindingsUsing 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/SignificanceThis 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.
Highlights
Oceans include vast areas called oxygen minimum zones (OMZs) where subsurface layers are depleted in dissolved oxygen (DO) [1]
113 hydrographic stations were sampled (Fig. 2A) to acquire vertical profiles of physicalbiogeochemical parameters using a conductivity-temperaturedepth probe equipped with a dissolved oxygen sensor (CTDO)
Those 25 reference casts allowed highly precise measurement of the DO concentration at ZVEEC. Based on these reference DO profiles and the concomitant echograms, we determined that the mean DO concentration at ZVEEC was 0.80 mL L21, regardless of the diel period (Fig. 1B; ANOVA day-night effect: F[1,23] = 0.0005, p = 0.98) or the distance from the coast (ANOVA offshore-inshore effect: F[1,23] = 0.3518, p = 0.56)
Summary
Oceans include vast areas called oxygen minimum zones (OMZs) where subsurface layers are depleted in dissolved oxygen (DO) [1]. OMZs are separated from the well-oxygenated surface mixed-layer by strong vertical DO gradients forming the oxycline. These OMZs contribute to 25–75% of oceanic N2O production [2], a potent greenhouse gas, which influences the Earth’s heat budget and depletes stratospheric ozone [3]. A few species of zooplankton, mesopelagic fish, and squids have adapted their metabolism to temporarily (through diel vertical migration) or permanently inhabit OMZs, most marine species limit their distribution to the surface oxygenated layer [5,6]. The upper limit of OMZs is rising and the vertical extent of the well-oxygenated surface layer shrinks, constraining the vertical habitat of epipelagic organisms. Oxygen minimum zones (OMZs) are expanding in the World Ocean as a result of climate change and direct anthropogenic influence. Monitoring the variability of the upper limit of the OMZs relies on time intensive sampling protocols, causing poor spatial resolution
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