Ocean deoxygenation and copepods: coping with oxygen minimum zone variability

Abstract

Abstract. Increasing deoxygenation (loss of oxygen) of the ocean, including expansion of oxygen minimum zones (OMZs), is a potentially important consequence of global warming. We examined present-day variability of vertical distributions of 23 calanoid copepod species in the Eastern Tropical North Pacific (ETNP) living in locations with different water column oxygen profiles and OMZ intensity (lowest oxygen concentration and its vertical extent in a profile). Copepods and hydrographic data were collected in vertically stratified day and night MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) tows (0–1000 m) during four cruises over a decade (2007–2017) that sampled four ETNP locations: Costa Rica Dome, Tehuantepec Bowl, and two oceanic sites further north (21–22∘ N) off Mexico. The sites had different vertical oxygen profiles: some with a shallow mixed layer, abrupt thermocline, and extensive very low oxygen OMZ core; and others with a more gradual vertical development of the OMZ (broad mixed layer and upper oxycline zone) and a less extensive OMZ core where oxygen was not as low. Calanoid copepod species (including examples from the genera Eucalanus, Pleuromamma, and Lucicutia) demonstrated different distributional strategies (implying different physiological characteristics) associated with this variability. We identified sets of species that (1) changed their vertical distributions and depth of maximum abundance associated with the depth and intensity of the OMZ and its oxycline inflection points; (2) shifted their depth of diapause; (3) adjusted their diel vertical migration, especially the nighttime upper depth; or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept). These distribution depths changed by tens to hundreds of meters depending on the species, oxygen profile, and phenomenon. For example, at the lower oxycline, the depth of maximum abundance for Lucicutia hulsemannae shifted from ∼600 to ∼800 m, and the depth of diapause for Eucalanus inermis shifted from ∼500 to ∼775 m, in an expanded OMZ compared to a thinner OMZ, but remained at similar low oxygen levels in both situations. These species or life stages are examples of “hypoxiphilic” taxa. For the migrating copepod Pleuromamma abdominalis, its nighttime depth was shallow (∼20 m) when the aerobic mixed layer was thin and the low-oxygen OMZ broad, but it was much deeper (∼100 m) when the mixed layer and higher oxygen extended deeper; daytime depth in both situations was ∼300 m. Because temperature decreased with depth, these distributional depth shifts had metabolic implications. The upper ocean to mesopelagic depth range encompasses a complex interwoven ecosystem characterized by intricate relationships among its inhabitants and their environment. It is a critically important zone for oceanic biogeochemical and export processes and hosts key food web components for commercial fisheries. Among the zooplankton, there will likely be winners and losers with increasing ocean deoxygenation as species cope with environmental change. Changes in individual copepod species abundances, vertical distributions, and life history strategies may create potential perturbations to these intricate food webs and processes. Present-day variability provides a window into future scenarios and potential effects of deoxygenation.

Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations

“…3-5). The low density of foraminifera in the oxycline is an interesting contrast to the vertical distributions of many metazoan species that often peak in abundance in the upper oxycline and decline in the core of the OMZ (Maas et al, 2014;Wishner et al, 1995Wishner et al, , 2013Wishner et al, , 2020b. Based on the mixed assemblage and low densities, we hypothesize that planktic foraminifera are largely absent from the upper oxycline, with populations restricted to either the oxygenated photic zone habitat above or the OMZ below.…”

“…Plankton tows sampled depths between 0 and 1000 m, across dissolved oxygen levels between 0.03 and 4.93 mL L −1 and temperatures ranging from 4.5 to 22.9 • C. Although smallscale oxygen features and their depth relative to the oxycline and OMZ varied somewhat (Wishner et al, 2018(Wishner et al, , 2020b, the overall structure of the water column was consistent across tows. A warm, oxygenated surface mixed layer overlaid an oxygen-depleted OMZ, with gradual cooling at increasing depth below the thermocline.…”

“…However, benthic foraminifera are widely understood to be among the ex-tremophiles that thrive in the OMZ through special adaptations (Levin, 2003;Bernhard and Bowser, 2008;Glock et al, 2012Glock et al, , 2018Glock et al, , 2019LeKieffre et al, 2017;Gooday et al, 2020). Benthic foraminiferal adaptations include nitrate respiration (Risgaard-Petersen et al, 2006;Hogsland et al, 2008;Pina-Ochoa et al, 2010;Bernhard et al, 2011Bernhard et al, , 2012aWoehle et al, 2018;Orsi et al, 2020), dormancy (Bernhard and Alve, 1996;Ross and Hallock, 2016;LeKieffre et al, 2017), and morphologies consistent with facilitating increased gas exchange (Bernhard, 1986;Perez-Cruz and Machain-Castillo, 1990;Glock et al, 2011Glock et al, , 2012Kuhnt et al, 2013Kuhnt et al, , 2014Rathburn et al, 2018). There they are important contributors to benthic food webs (e.g., Nomaki et al, 2008;Enge et al, 2014) and are used as indicators of low-oxygen environments (e.g., Kaiho, 1994;Bernhard et al, 1997;Cannariato et al, 1999;Jorissen et al, 2007;Ohkushi et al, 2013).…”

A Blueprint for an Inclusive, Global Deep-Sea Ocean Decade Field Program

“…At the seafloor, oxygen patterns resemble those for pH, with highest dissolved oxygen concentrations in the North Atlantic, and lowest concentrations in the North Pacific (Sweetman et al, 2017). Oxygen strongly influences benthic fauna density, biodiversity, species distributions, taxonomic composition, food web structure, biogeochemical cycling, body size and specieslevel population and physiological rates (Levin and Gooday, 2003;Muthumbi et al, 2004;Laffoley and Baxter, 2019;Wishner et al, 2020). In the deep sea, the strongest influence of oxygen occurs at bathyal depths (200-1200 m) within oxygen minimum zones, but particularly in the prevalent extreme OMZs in the North and Eastern Pacific Ocean, Northern Indian Ocean, and off west Africa (Helly and Levin, 2004) as well as the Western Indian Ocean (Muthumbi et al, 2004).…”

Characterizing deepwater oxygen variability and seafloor community responses using a novel autonomous lander

“…The lack of community-level response to diurnal and weekly oxygen variability seen in our data may not be surprising given that animals have several ways that they can respond to stressful conditions, which would not affect community-level abundance, diversity, or composition patterns. For example, fish can become less active and reduce metabolic demands (Richards, 2009(Richards, , 2010, or they can decrease feeding behavior (Wu, 2002;Nilsson, 2010) during periodic hypoxia. We observed that animals were less active in the deeper deployments, but it is unclear if this is due to the lower-oxygen conditions or other environmental covariates.…”