Ocean Deoxygenation and Copepods: Coping with Oxygen Minimum Zone Variability

Abstract

<p><strong>Abstract.</strong> 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 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 tows (0–1000 m) during 4 cruises over a decade (2007–2017) that sampled 4 ETNP locations: Costa Rica Dome, Tehuantepec Bowl, and 2 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. Copepod species (including examples from the genera <i>Eucalanus</i>, <i>Pleuromamma</i>, and <i>Lucicutia</i>) demonstrated different distributional strategies and physiologies 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).</p> <p>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.</p>

Eddies Affect Subsurface Phytoplankton and Oxygen Distributions in the North Pacific Subtropical Gyre

“…A recent study by Rasse and Dall'Olmo (2019) found a permanent layer of small particles linked to microbial communities located in the upper section of the oxygen minimum layer, and they further showed that attenuation of small‐particle fluxes was equivalent or significantly higher inside the oxygen minimum layer. The layer of small particles above the oxygen minimum layer could be caused by the zooplankton fragmentation of large particles into small particles (Briggs et al, 2020; Cavan et al, 2017), as many zooplankton migrate during the day into the low oxygen core or the upper boundary of the oxygen minimum layer (Steinberg et al, 2008; Wishner et al, 2018, 2020).…”

Mesoscale Variability in the Boundaries of the Oxygen Minimum Zone in the Eastern South Pacific: Influence of Intrathermocline Eddies

“…Biogeochemical processes in OMZs are largely driven by microbial communities associated with anaerobic metabolism (e.g., denitrification, anammox), which are highly relevant in terms of fixed nitrogen loss, as well as its impact on primary production rate, carbon sequestration, and greenhouse gas production. Vertical movements in the upper limit of the OMZ (oxycline) are expected to alter such rates (Arévalo‐Martínez et al., 2015; Jiao et al., 2014; Kalvelage et al., 2015), and also have a direct impact on most aerobic organisms through an expansion or contraction of their habitat within the upper mixed layer‐seasonal thermocline, changes in their depth distribution within the OMZ or by alterations in their patterns of diel vertical migration (Bianchi et al., 2013; Gilly et al., 2013; Wishner et al., 2020).…”

Characterizing deep-water oxygen variability and seafloor community responses using a novel autonomous lander

“…At this depth, we recorded severely hypoxic conditions (O 2 < 22.5 µmol kg -1 ) for ~1% of the deployment time (Table 1), suggesting that even though this community is above the depth frequently associated with the upper boundary of the OMZ (450 m), it is still periodically exposed to OMZ conditions. High spatial resolution-sampling of 485 the eastern tropical North Pacific OMZ documents considerable submesoscale oxygen variability with better oxygenated holes (Wishner et al 2019); such patchiness would not be unexpected off southern California. Oxygen decreases of 0.25-0.5 µmol kg -1 year -1 have been reported for depths of 400 m (Bograd et al 2008), and if these trends continue, in 13-26 years, this depth zone may become the upper boundary of the OMZ.…”