Ross T et al. (2020), Rapid deep ocean deoxygenation and acidificatio...

ECB-ART-49687

Glob Chang Biol 2020 Nov 01;2611:6424-6444. doi: 10.1111/gcb.15307.

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Rapid deep ocean deoxygenation and acidification threaten life on Northeast Pacific seamounts.

Ross T , Du Preez C , Ianson D .

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Anthropogenic climate change is causing our oceans to lose oxygen and become more acidic at an unprecedented rate, threatening marine ecosystems and their associated animals. In deep-sea environments, where conditions have typically changed over geological timescales, the associated animals, adapted to these stable conditions, are expected to be highly vulnerable to any change or direct human impact. Our study coalesces one of the longest deep-sea observational oceanographic time series, reaching back to the 1960s, with a modern visual survey that characterizes almost two vertical kilometers of benthic seamount ecosystems. Based on our new and rigorous analysis of the Line P oceanographic monitoring data, the upper 3,000 m of the Northeast Pacific (NEP) has lost 15% of its oxygen in the last 60 years. Over that time, the oxygen minimum zone (OMZ), ranging between approximately 480 and 1,700 m, has expanded at a rate of 3.0 ± 0.7 m/year (due to deepening at the bottom). Additionally, carbonate saturation horizons above the OMZ have been shoaling at a rate of 1-2 m/year since the 1980s. Based on our visual surveys of four NEP seamounts, these deep-sea features support ecologically important taxa typified by long life spans, slow growth rates, and limited mobility, including habitat-forming cold water corals and sponges, echinoderms, and fish. By examining the changing conditions within the narrow realized bathymetric niches for a subset of vulnerable populations, we resolve chemical trends that are rapid in comparison to the life span of the taxa and detrimental to their survival. If these trends continue as they have over the last three to six decades, they threaten to diminish regional seamount ecosystem diversity and cause local extinctions. This study highlights the importance of mitigating direct human impacts as species continue to suffer environmental changes beyond our immediate control.

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	FIGURE 1. Map of our Northeast Pacific seamounts study region. Line P oceanographic monitoring stations are indicated by red squares; major stations (P12, P16, and P26) are highlighted in yellow (main map) and black (inset). The triangle symbols indicate seamount summits in our study region (DFO, 2019), the four in black were surveyed during the 2017 benthic imaging survey cruise (Table 1). The proposed Offshore Pacific marine protected area is outlined in blue. Bathymetry is from GEBCO 2014 Grid, version 20141103, http://www.gebco.net
	FIGURE 2. False color plots of dissolved oxygen (a) and aragonite saturation state (ΩAr, b) over the Line P time series, averaged over our NEP seamounts study region (Figure 1). Overlaid on (a) are the calcite saturation depth (yellow), and the upper (red) and lower (blue) boundaries of the oxygen minimum zone. The legend indicates the trends associated with each of these boundaries. In (b), the ΩAr = 0.7 (red) horizon is shown. Also overlaid are the isopycnal depths for the 26.83 (thick gray dotted line), 27.04 (thick gray dash‐dotted line), 27.4 (thick gray dashed line), and 27.62 kg/m3 (thick gray line) sigma levels. Depths of the bottle samples that were interpolated to create the contour plot of ΩAr are shown (black dots)
	FIGURE 3. Time series of dissolved oxygen at fixed depth (475 m, a; 1,000 m, b; 1,700 m, c; thick black lines), at fixed potential density (1,027.04 kg/m3, a; 1,027.4 kg/m3, b; 1,027.62 kg/m3, c; blue lines/dots), and the estimated oxygen change due to movement of these constant potential density surfaces (mean oxygen has been added to put it on the same axis; red lines/squares). The gray patch behind the black lines represents the intra‐annual natural variability in the data at that depth (it is the inter‐month standard deviation from station P12 in 2013 [N = 4])
	FIGURE 4. Images of the indicator taxa: (a) rougheye rockfish (Sebastes aleutianus) were observed as solitary individuals and in schools; (a–c) brittle stars (Ophiacantha diplasia and Ophiopholis spp.) could form dense living mats covering the seafloor often collocated with (also b, indicated with arrow) the bubblegum coral (Paragorgia cf. jamesi), which was small but dense while (c) the bamboo coral (Isidella tentaculum) was slightly less dense but could grow over 2 m high; (d, indicated with arrows) the black coral (Chrysopathes speciosa) formed extensive gardens; (e) the tall vase‐shape bugle sponge (Pinulasma n. sp.); (f, indicated with arrows) the cup coral (Flabellidae) is solitary coral; (g, indicated with arrows) the undulated glass sponges (cf Tretodictyum n. sp.) increased seafloor structural complexity with its body morphology; and (h) the sea lily was observed as solitary individuals and as dense fields (Florometra serratissima). The white scale bars in the corner of each panel are 10 cm long
	FIGURE 5. Plot of the observed depth distributions of the nine indicator taxa identified based on the benthic visual surveys along with simultaneously collected depth profiles of Union, Dellwood, Unnamed (UN) 16 and 18 seamounts (Figure 1). The red background shows the mean oxygen concentration over the full time series, highlighting the OMZ (i.e., white is outside the OMZ). The dashed white lines delineate the region between 800 and 1,200 m which corresponds to oxygen <0.5 ml/L and dotted blue line indicates the mean depth of the calcite saturation horizon. A range of metrics on the species’ depth ranges are plotted (mean, horizontal black lines; ±1 standard deviation, dark gray boxes; maximum and minimum depth observed, thin dark gray lines). *The sea lily has a bimodal depth distribution (gap between 1,150 and 700 m); the black coral also has a bimodal distribution (dip between 900 and 850 m); the rest have roughly bell‐shaped unimodal distributions (except for the rockfish, which is truncated at the top) (S5). All taxa were found on all seamounts with habitat (seafloor) in their depth range (mean ± 1 standard deviation) with the exception of the bugle sponge which was not observed on UN 16 (S5)

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