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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.
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