Title: Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations
Abstract. Oxygen-depleted regions of the global ocean are
rapidly expanding, with important implications for global
biogeochemical cycles. However, our ability to make projections
about the future of oxygen in the ocean is limited by
a lack of empirical data with which to test and constrain the
behavior of global climatic and oceanographic models. We
use depth-stratified plankton tows to demonstrate that some
species of planktic foraminifera are adapted to life in the
heart of the pelagic oxygen minimum zone (OMZ). In particular,
we identify two species, Globorotaloides hexagonus
and Hastigerina parapelagica, living within the eastern tropical
North Pacific OMZ. The tests of the former are preserved
in marine sediments and could be used to trace the extent
and intensity of low-oxygen pelagic habitats in the fossil
record. Additional morphometric analyses of G. hexagonus
show that tests found in the lowest oxygen environments are
larger, more porous, less dense, and have more chambers in
the final whorl. The association of this species with the OMZ
and the apparent plasticity of its test in response to ambient
oxygenation invites the use of G. hexagonus tests in sediment
cores as potential proxies for both the presence and intensity
of overlying OMZs. more »« less
Abstract. Oxygen-depleted regions of the global ocean are rapidly expanding, withimportant implications for global biogeochemical cycles. However, ourability to make projections about the future of oxygen in the ocean islimited by a lack of empirical data with which to test and constrain thebehavior of global climatic and oceanographic models. We usedepth-stratified plankton tows to demonstrate that some species of plankticforaminifera are adapted to life in the heart of the pelagic oxygen minimumzone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within theeastern tropical North Pacific OMZ. The tests of the former are preserved inmarine sediments and could be used to trace the extent and intensity oflow-oxygen pelagic habitats in the fossil record. Additional morphometricanalyses of G. hexagonus show that tests found in the lowest oxygen environments arelarger, more porous, less dense, and have more chambers in the final whorl.The association of this species with the OMZ and the apparent plasticity ofits test in response to ambient oxygenation invites the use of G. hexagonus tests insediment cores as potential proxies for both the presence and intensity ofoverlying OMZs.
Davis, Catherine V.; Sibert, Elizabeth C.; Jacobs, Peter H.; Burls, Natalie; Hull, Pincelli M.(
, Nature Communications)
Abstract
Oxygen minimum zones (OMZs) play a critical role in global biogeochemical cycling and act as barriers to dispersal for marine organisms. OMZs are currently expanding and intensifying with climate change, however past distributions of OMZs are relatively unknown. Here we present evidence for widespread pelagic OMZs during the Pliocene (5.3-2.6 Ma), the most recent epoch with atmospheric CO2analogous to modern (~400-450 ppm). The global distribution of OMZ-affiliated planktic foraminifer,Globorotaloides hexagonus, and Earth System and Species Distribution Models show that the Indian Ocean, Eastern Equatorial Pacific, eastern South Pacific, and eastern North Atlantic all supported OMZs in the Pliocene, as today. By contrast, low-oxygen waters were reduced in the North Pacific and expanded in the North Atlantic in the Pliocene. This spatially explicit perspective reveals that a warmer world can support both regionally expanded and contracted OMZs, with intermediate water circulation as a key driver.
Oxygen limited marine environments, such as oxygen minimum zones, are of profound importance for global nutrient cycling and vertical habitat availability. While it is understood that the extent and intensity of oxygen minimum zones are responsive to climate, the limited suite of viable proxies for low oxygen pelagic environments continues to pose a real barrier for paleoclimate interpretations. Here we investigate the proxy potential of an array of trace element (Mg, Mn, Zn, and Sr) to Ca ratios from the shells of Globorotaloides hexagonus , a planktic foraminifer endemic to tropical through temperate oxygen minimum zones. A species-specific relationship between Mg/Ca and temperature is proposed for quantitative reconstruction of oxygen minimum zone paleotemperatures. Both Mn/Ca and Zn/Ca ratios vary with oxygen concentration and could be useful for reconstructing G. hexagonus habitat where the primary signal can be d\istinguished from diagenetic overprinting. Finally, a robust correlation between Sr/Ca ratios and dissolved oxygen demonstrates a role for Sr as an indicator of oxygen minimum zone intensity, potentially via foraminiferal growth rate. The analysis of these relatively conventional trace element ratios in the shells of an oxygen minimum zone species has tremendous potential to facilitate multiproxy reconstructions from this enigmatic environment.
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.
Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growthof higher organisms. They also alter the marine nitrogen cycle, which isstrongly bound to the carbon cycle and climate. While higher organisms ingeneral start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygenconcentration below a threshold of about 20 µM (microbial hypoxia),whereas anoxic processes dominate the nitrogen cycle at oxygenconcentrations of < ∼ 0.05 µM (functionalanoxia). The Arabian Sea and the Bay of Bengal are home to approximately21 % of the total volume of ocean waters revealing microbial hypoxia.While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionallyanoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Eventhough there are a few isolated reports on the occurrence of anoxia prior to1960, anoxic events have so far not been reported from the open northernIndian Ocean (i.e., other than on shelves) during the last 60 years.Maintenance of functional anoxia in the Arabian Sea OMZ with oxygenconcentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses thesurface ocean circulation twice a year and turns vast areas of the ArabianSea from an oligotrophic oceanic desert into one of the most productiveregions of the oceans within a few weeks. Thus, the comparably lowvariability of oxygen concentration in the OMZ implies stable balancesbetween the physical oxygen supply and the biological oxygen consumption,which includes negative feedback mechanisms such as reducing oxygenconsumption at decreasing oxygen concentrations (e.g., reduced respiration).Lower biological oxygen consumption is also assumed to be responsible for aless intense OMZ in the Bay of Bengal. According to numerical model results,a decreasing physical oxygen supply via the inflow of water masses from thesouth intensified the Arabian Sea OMZ during the last 6000 years, whereas areduced oxygen supply via the inflow of Persian Gulf Water from the northintensifies the OMZ today in response to global warming. The first issupported by data derived from the sedimentary records, and the latterconcurs with observations of decreasing oxygen concentrations and aspreading of functional anoxia during the last decades in the Arabian Sea.In the Arabian Sea decreasing oxygen concentrations seem to have initiated aregime shift within the pelagic ecosystem structure, and this trend is alsoseen in benthic ecosystems. Consequences for biogeochemical cycles are asyet unknown, which, in addition to the poor representation of mesoscalefeatures in global Earth system models, reduces the reliability of estimatesof the future OMZ development in the northern Indian Ocean.
Davis, C.V., Wishner, K., Renema, W., and Hull, P.M. Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations. Retrieved from https://par.nsf.gov/biblio/10220573. Biogeosciences 18. Web. doi:10.5194/bg-2020-280.
Davis, C.V., Wishner, K., Renema, W., & Hull, P.M. Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations. Biogeosciences, 18 (). Retrieved from https://par.nsf.gov/biblio/10220573. https://doi.org/10.5194/bg-2020-280
Davis, C.V., Wishner, K., Renema, W., and Hull, P.M.
"Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations". Biogeosciences 18 (). Country unknown/Code not available. https://doi.org/10.5194/bg-2020-280.https://par.nsf.gov/biblio/10220573.
@article{osti_10220573,
place = {Country unknown/Code not available},
title = {Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations},
url = {https://par.nsf.gov/biblio/10220573},
DOI = {10.5194/bg-2020-280},
abstractNote = {Abstract. Oxygen-depleted regions of the global ocean are rapidly expanding, with important implications for global biogeochemical cycles. However, our ability to make projections about the future of oxygen in the ocean is limited by a lack of empirical data with which to test and constrain the behavior of global climatic and oceanographic models. We use depth-stratified plankton tows to demonstrate that some species of planktic foraminifera are adapted to life in the heart of the pelagic oxygen minimum zone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within the eastern tropical North Pacific OMZ. The tests of the former are preserved in marine sediments and could be used to trace the extent and intensity of low-oxygen pelagic habitats in the fossil record. Additional morphometric analyses of G. hexagonus show that tests found in the lowest oxygen environments are larger, more porous, less dense, and have more chambers in the final whorl. The association of this species with the OMZ and the apparent plasticity of its test in response to ambient oxygenation invites the use of G. hexagonus tests in sediment cores as potential proxies for both the presence and intensity of overlying OMZs.},
journal = {Biogeosciences},
volume = {18},
author = {Davis, C.V. and Wishner, K. and Renema, W. and Hull, P.M.},
editor = {null}
}
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