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, 4 (12), eaau5180
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Ocean Deoxygenation and Zooplankton: Very Small Oxygen Differences Matter

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Ocean Deoxygenation and Zooplankton: Very Small Oxygen Differences Matter

K F Wishner et al. Sci Adv.

Abstract

Oxygen minimum zones (OMZs), large midwater regions of very low oxygen, are expected to expand as a result of climate change. While oxygen is known to be important in structuring midwater ecosystems, a precise and mechanistic understanding of the effects of oxygen on zooplankton is lacking. Zooplankton are important components of midwater food webs and biogeochemical cycles. Here, we show that, in the eastern tropical North Pacific OMZ, previously undescribed submesoscale oxygen variability has a direct effect on the distribution of many major zooplankton groups. Despite extraordinary hypoxia tolerance, many zooplankton live near their physiological limits and respond to slight (≤1%) changes in oxygen. Ocean oxygen loss (deoxygenation) may, thus, elicit major unanticipated changes to midwater ecosystem structure and function.

Figures

Fig. 1
Fig. 1. Oxygen sections from Wire Flyer transects, showing oxygen concentration (color) with distance.
Diagonal black lines are the Wire Flyer path as it oscillated through the indicated depth zone. The light horizontal line is the horizontal MOCNESS tow path that targeted the edges of specific oxygen features seen in the earlier Wire Flyer tow (table S1). (A) Wire Flyer tow #9 and MOCNESS tow #724 (upper oxycline). (B) Wire Flyer tow #10 and MOCNESS tow #726 (upper oxycline). (C) Wire Flyer tow #12 and MOCNESS tow #728 (lower oxycline).
Fig. 2
Fig. 2. Hydrographic parameters from the three horizontal MOCNESS tows during zooplankton sample collection at depth.
Charts (top to bottom within each set) show MOCNESS depth, density (σt), oxygen, temperature, and salinity. Vertical lines and numbers (depth panel) indicate when each of the eight nets was open. Distance is ~8 km, and sampling time at depth is ~2.5 to 3 hours for each tow. (A) Tow #724 (425-m depth). (B) Tow #726 (430-m depth). (C) Tow #728 (800-m depth).
Fig. 3
Fig. 3. Zooplankton abundances, biomass, and oxygen from MOCNESS horizontal tows.
Each column shows data from one tow. For oxygen (top row), purple shading indicates low oxygen (≤5 μM, 0.11 ml/liter), and gray shading indicates high oxygen (≥8 μM, 0.18 ml/liter). For copepods (second row), Pleuromamma abdominalis is shown for shallower tows and L. hulsemannae is shown for the deep tow. The next rows show total euphausiids, the fish Cyclothone spp., and zooplankton biomass. All graphed taxa show significant abundance differences between samples in high versus low oxygen categories (see table S2 for data in each category), except 800-m euphausiids and 800-m total biomass (which are sparse at depth) (see fig. S2 for photographs of these taxa).
Fig. 4
Fig. 4. Day and night vertical distributions of zooplankton abundance for the same taxa shown in Fig. 3 and for total zooplankton biomass.
Data come from vertically stratified MOCNESS tow #716 and #725 (table S1). Oxygen is shown in the left graph (tow #725). Abundances are plotted at the mid-depth of each net (table S2). Open circles, day; closed circles, night. For copepods, blue lines represent P. abdominalis and red lines represent L. hulsemannae.
Fig. 5
Fig. 5. Hypoxia tolerance influences the abundance of dominant mesopelagic crustaceans in the OMZ.
(A) Critical PO2 (Pcrit, kPa) in diverse marine crustaceans (gray; table S4), including species from the current study (L. hulsemannae, Lh; N. flexipes, Nf; Gennadas spp., G; and Megacalanus spp., M; red, means ± SE; table S3) and from the eastern Pacific OMZ (blue). Lines connect intraspecific measurements at different temperatures. Note that the y axis is on a log scale. (B to E) MI calculated for N. flexipes (B and C) at 400 m and L. hulsemannae (D and E) at 800 m from metabolic and environmental data across the horizontal MOCNESS [red lines in (B) and (D)] and Wire Flyer transects (C and E). The yellow line on MI plots illustrates the corresponding MOCNESS tows. The corresponding abundance of each species is also plotted [blue lines in (B) and (D)]. Gray shading (B and D) indicates metabolically unavailable habitat (MI < 1).

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