Earth & Environment: Dead zone shrank as winds declined: Warming may not enlarge North Pacific low-oxygen region

Science News ◽  
2014 ◽  
Vol 186 (5) ◽  
pp. 11-11
Author(s):  
Thomas Sumner
Keyword(s):  
North Pacific ◽  
Dead Zone ◽  
Low Oxygen ◽  
Earth Environment

scholarly journals Open ocean dead zones in the tropical North Atlantic Ocean

2015 ◽  
Vol 12 (8) ◽  
pp. 2597-2605 ◽  
Author(s):  
J. Karstensen ◽  
B. Fiedler ◽  
F. Schütte ◽  
P. Brandt ◽  
A. Körtzinger ◽  
...  
Keyword(s):  
North Atlantic ◽  
North Atlantic Ocean ◽  
Dead Zone ◽  
Marine Ecosystem ◽  
High Surface ◽  
Open Ocean ◽  
Depth Range ◽  
Low Oxygen ◽  
Dead Zones ◽  
Tropical North Atlantic

Abstract. Here we present first observations, from instrumentation installed on moorings and a float, of unexpectedly low (<2 μmol kg−1) oxygen environments in the open waters of the tropical North Atlantic, a region where oxygen concentration does normally not fall much below 40 μmol kg−1. The low-oxygen zones are created at shallow depth, just below the mixed layer, in the euphotic zone of cyclonic eddies and anticyclonic-modewater eddies. Both types of eddies are prone to high surface productivity. Net respiration rates for the eddies are found to be 3 to 5 times higher when compared with surrounding waters. Oxygen is lowest in the centre of the eddies, in a depth range where the swirl velocity, defining the transition between eddy and surroundings, has its maximum. It is assumed that the strong velocity at the outer rim of the eddies hampers the transport of properties across the eddies boundary and as such isolates their cores. This is supported by a remarkably stable hydrographic structure of the eddies core over periods of several months. The eddies propagate westward, at about 4 to 5 km day−1, from their generation region off the West African coast into the open ocean. High productivity and accompanying respiration, paired with sluggish exchange across the eddy boundary, create the "dead zone" inside the eddies, so far only reported for coastal areas or lakes. We observe a direct impact of the open ocean dead zones on the marine ecosystem as such that the diurnal vertical migration of zooplankton is suppressed inside the eddies.

scholarly journals Dead zone or oasis in the open ocean? Zooplankton distribution and migration in low-oxygen modewater eddies

2016 ◽  
Vol 13 (6) ◽  
pp. 1977-1989 ◽  
Author(s):  
Helena Hauss ◽  
Svenja Christiansen ◽  
Florian Schütte ◽  
Rainer Kiko ◽  
Miryam Edvam Lima ◽  
...  
Keyword(s):  
Surface Layer ◽  
Dead Zone ◽  
Trophic Levels ◽  
Zooplankton Abundance ◽  
Depth Range ◽  
Calanoid Copepods ◽  
Long Term Effects ◽  
Low Oxygen ◽  
Oxygen Concentrations ◽  
Ocean Deoxygenation

Abstract. The eastern tropical North Atlantic (ETNA) features a mesopelagic oxygen minimum zone (OMZ) at approximately 300–600 m depth. Here, oxygen concentrations rarely fall below 40 µmol O2 kg−1, but are expected to decline under future projections of global warming. The recent discovery of mesoscale eddies that harbour a shallow suboxic (< 5 µmol O2 kg−1) OMZ just below the mixed layer could serve to identify zooplankton groups that may be negatively or positively affected by ongoing ocean deoxygenation. In spring 2014, a detailed survey of a suboxic anticyclonic modewater eddy (ACME) was carried out near the Cape Verde Ocean Observatory (CVOO), combining acoustic and optical profiling methods with stratified multinet hauls and hydrography. The multinet data revealed that the eddy was characterized by an approximately 1.5-fold increase in total area-integrated zooplankton abundance. At nighttime, when a large proportion of acoustic scatterers is ascending into the upper 150 m, a drastic reduction in mean volume backscattering (Sv) at 75 kHz (shipboard acoustic Doppler current profiler, ADCP) within the shallow OMZ of the eddy was evident compared to the nighttime distribution outside the eddy. Acoustic scatterers avoided the depth range between approximately 85 to 120 m, where oxygen concentrations were lower than approximately 20 µmol O2 kg−1, indicating habitat compression to the oxygenated surface layer. This observation is confirmed by time series observations of a moored ADCP (upward looking, 300 kHz) during an ACME transit at the CVOO mooring in 2010. Nevertheless, part of the diurnal vertical migration (DVM) from the surface layer to the mesopelagic continued through the shallow OMZ. Based upon vertically stratified multinet hauls, Underwater Vision Profiler (UVP5) and ADCP data, four strategies followed by zooplankton in response to in response to the eddy OMZ have been identified: (i) shallow OMZ avoidance and compression at the surface (e.g. most calanoid copepods, euphausiids); (ii) migration to the shallow OMZ core during daytime, but paying O2 debt at the surface at nighttime (e.g. siphonophores, Oncaea spp., eucalanoid copepods); (iii) residing in the shallow OMZ day and night (e.g. ostracods, polychaetes); and (iv) DVM through the shallow OMZ from deeper oxygenated depths to the surface and back. For strategy (i), (ii) and (iv), compression of the habitable volume in the surface may increase prey–predator encounter rates, rendering zooplankton and micronekton more vulnerable to predation and potentially making the eddy surface a foraging hotspot for higher trophic levels. With respect to long-term effects of ocean deoxygenation, we expect avoidance of the mesopelagic OMZ to set in if oxygen levels decline below approximately 20 µmol O2 kg−1. This may result in a positive feedback on the OMZ oxygen consumption rates, since zooplankton and micronekton respiration within the OMZ as well as active flux of dissolved and particulate organic matter into the OMZ will decline.

scholarly journals Dead zone or oasis in the open ocean? Zooplankton distribution and migration in low-oxygen modewater eddies

2015 ◽  
Vol 12 (21) ◽  
pp. 18315-18344 ◽  
Author(s):  
H. Hauss ◽  
S. Christiansen ◽  
F. Schütte ◽  
R. Kiko ◽  
M. Edvam Lima ◽  
...  
Keyword(s):  
Surface Layer ◽  
Dead Zone ◽  
Trophic Levels ◽  
Target Strength ◽  
Zooplankton Abundance ◽  
Depth Range ◽  
Long Term Effects ◽  
Low Oxygen ◽  
Oxygen Concentrations ◽  
Ocean Deoxygenation

Abstract. The eastern tropical North Atlantic (ETNA) features a mesopelagic oxygen minimum zone (OMZ) at approximately 300–600 m depth. Here, oxygen concentrations rarely fall below 40 μmol O2 kg−1, but are thought to decline in the course of climate change. The recent discovery of mesoscale eddies that harbour a shallow suboxic (< 5 μmol O2 kg−1) OMZ just below the mixed layer could serve to identify zooplankton groups that may be negatively or positively affected by on-going ocean deoxygenation. In spring 2014, a detailed survey of a suboxic anticyclonic modewater eddy (ACME) was carried out near the Cape Verde Ocean Observatory (CVOO), combining acoustic and optical profiling methods with stratified multinet hauls and hydrography. The multinet data revealed that the eddy was characterized by an approximately 1.5-fold increase in total area-integrated zooplankton abundance. A marked reduction in acoustic target strength (derived from shipboard ADCP, 75kHz) within the shallow OMZ at nighttime was evident. Acoustic scatterers were avoiding the depth range between about 85 to 120 m, where oxygen concentrations were lower than approximately 20 μmol O2 kg−1, indicating habitat compression to the oxygenated surface layer. This observation is confirmed by time-series observations of a moored ADCP (upward looking, 300 kHz) during an ACME transit at the CVOO mooring in 2010. Nevertheless, part of the diurnal vertical migration (DVM) from the surface layer to the mesopelagic continued through the shallow OMZ. Based upon vertically stratified multinet hauls, Underwater Vision Profiler (UVP5) and ADCP data, four strategies have been identified followed by zooplankton in response to the eddy OMZ: (i) shallow OMZ avoidance and compression at the surface (e.g. most calanoid copepods, euphausiids), (ii) migration to the shallow OMZ core during daytime, but paying O2 debt at the surface at nighttime (e.g. siphonophores, Oncaea spp., eucalanoid copepods), (iii) residing in the shallow OMZ day and night (e.g. ostracods, polychaetes), and iv) DVM through the shallow OMZ from deeper oxygenated depths to the surface and back. For strategy (i), (ii) and (iv), compression of the habitable volume in the surface may increase prey-predator encounter rates, rendering zooplankton more vulnerable to predation and potentially making the eddy surface a foraging hotspot for higher trophic levels. With respect to long-term effects of ocean deoxygenation, we expect zooplankton avoidance of the mesopelagic OMZ to set in if oxygen levels decline below approximately 20 μmol O2 kg−1. This may result in a positive feedback on the OMZ oxygen consumption rates, since zooplankton respiration within the OMZ as well as active flux of dissolved and particulate organic matter into the OMZ will decline.

scholarly journals Uncovering diversity and metabolic spectrum of animals in dead zone sediments

2020 ◽  
Vol 3 (1) ◽  
Author(s):  
Elias Broman ◽  
Stefano Bonaglia ◽  
Oleksandr Holovachov ◽  
Ugo Marzocchi ◽  
Per O. J. Hall ◽  
...  
Keyword(s):  
Oxygen Sensor ◽  
Dead Zone ◽  
Extreme Environments ◽  
Primary Concern ◽  
Rna Seq ◽  
Low Oxygen ◽  
Oxygen Gradient ◽  
Marine Life ◽  
Lack Of Information ◽  
Oxygen Conditions

AbstractOcean deoxygenation driven by global warming and eutrophication is a primary concern for marine life. Resistant animals may be present in dead zone sediments, however there is lack of information on their diversity and metabolism. Here we combined geochemistry, microscopy, and RNA-seq for estimating taxonomy and functionality of micrometazoans along an oxygen gradient in the largest dead zone in the world. Nematodes are metabolically active at oxygen concentrations below 1.8 µmol L−1, and their diversity and community structure are different between low oxygen areas. This is likely due to toxic hydrogen sulfide and its potential to be oxidized by oxygen or nitrate. Zooplankton resting stages dominate the metazoan community, and these populations possibly use cytochrome c oxidase as an oxygen sensor to exit dormancy. Our study sheds light on mechanisms of animal adaptation to extreme environments. These biological resources can be essential for recolonization of dead zones when oxygen conditions improve.

scholarly journals Open ocean dead-zone in the tropical North Atlantic Ocean

2014 ◽  
Vol 11 (12) ◽  
pp. 17391-17411 ◽  
Author(s):  
J. Karstensen ◽  
B. Fiedler ◽  
F. Schütte ◽  
P. Brandt ◽  
A. Körtzinger ◽  
...  
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Large Scale ◽  
North Atlantic Ocean ◽  
Dead Zone ◽  
Mixed Layer Depth ◽  
Euphotic Zone ◽  
Low Oxygen ◽  
Tropical North Atlantic ◽  
Swirl Velocity

Abstract. The intermittent appearances of low oxygen environments are a particular thread for marine ecosystems. Here we present first observations of unexpected low (<2 μmol kg-1) oxygen environments in the open waters of the eastern tropical North Atlantic, a region where typically oxygen concentration does not fall below 40 μmol kg-1. The low oxygen zones are created just below the mixed-layer, in the euphotic zone of high productive cyclonic and anticyclonic-modewater eddies. A dynamic boundary is created from the large swirl-velocity against the weak background flow. Hydrographic properties within the eddies are kept constant over periods of several months, while net respiration is elevated by a factor of 3 to 5 reducing the oxygen content. We repeatedly observed low oxygen eddies in the region. The direct impact on the ecosystem is evident from anomalous backscatter behaviour. Satellite derived global eddy statistics do not allow to estimate the large-scale impact of the eddies because their vertical structure (mixed-layer depth, euphotic depth) play a key role in creating the low oxygen environment.

Could Artificial Downwelling/Upwelling Mitigate Oceanic Deoxygenation in Western Subarctic North Pacific?

2021 ◽  
Vol 8 ◽  
Author(s):  
Canbo Xiao ◽  
Wei Fan ◽  
Ying Chen ◽  
Yao Zhang ◽  
Kai Tang ◽  
...  
Keyword(s):  
North Pacific ◽  
Subsurface Water ◽  
Low Oxygen ◽  
Physical Processes ◽  
Subarctic North Pacific ◽  
Rcp 8.5 ◽  
Ph Decrease ◽  
A Minor ◽  
Western Subarctic North Pacific ◽  
Precipitation Reduction

Subpolar gyre regions such as the Western Subarctic North Pacific (WSNP) contain sluggish, low-oxygen water, and are threatened by loss of oxygen (deoxygenation). Our simulations under RCP 8.5 emission scenario suggest that installing pipes to induce artificial downwelling and upwelling (AD and AU) provides short-term solutions to combat deoxygenation in the WSNP. With no engineering, the WSNP's subsurface oxygen decreases by 30–100 mmol/m3 by the year 2100. Continuous implementation of AD and AU instead counters this declining trend, and AD is more effective than AU. The oxygenation effect is primarily a consequence of how the two engineering schemes vertically redistribute oxygen via physical processes. AD directly improves oxygen at depth via advecting surface water toward the ocean interior and subsequent enhanced pycnocline mixing, and AU does so via generating compensatory downwelling outside of the pipes. Both schemes take near 40 years to complete the oxygenation. After that, oxygen reaches a new equilibrium state in the WSNP with no further improvement by the engineering. AD and AU both strongly increase primary production surrounding the deployment sites, but lead only to weak enhancement of aerobic respiration in subsurface water and thus a minor impact on the oxygenation. Other unwanted environmental side effects are negligible compared to those caused by rapid climate change within this century, including outgassing of carbon dioxide, pH decrease, and precipitation reduction.

Dead Zones Discovered in Coastal Waters

Dead Zones ◽  
2021 ◽  
pp. 21-35
Author(s):  
David L. Kirchman
Keyword(s):  
North America ◽  
Gulf Of Mexico ◽  
Chesapeake Bay ◽  
Dead Zone ◽  
The Black Sea ◽  
Low Oxygen ◽  
Coastal Regions ◽  
Dead Zones ◽  
The World ◽  
The Baltic

This chapter describes the discovery of coastal dead zones, such as the Gulf of Mexico and Chesapeake Bay in North America and the Baltic and Black Seas in Europe. Gene Turner sailed out of Pascagoula, Mississippi, in the spring of 1975, on the first of seven cruises that led to the discovery of the Gulf of Mexico dead zone. In the Chesapeake Bay, an unlikely environmentalist, Charles Officer, sounded the alarm in 1984. The biggest dead zone, however, is the Baltic Sea. Even as early as 1969, ecologists feared hypoxia was turning the Baltic into a “biological desert.” The northwest shelf of the Black Sea turned hypoxic in the 1970s, which killed bottom-dwelling fish like goby and flounder. Many coastal regions around the world have low oxygen waters that devastate marine life and habitats. The early studies emphasized one or two of three ingredients—sewage, fresh water, and plant nutrients—thought to be essential in creating a dead zone. This chapter and Chapter 3 discuss these ingredients before revealing which is most important.

scholarly journals Characterization of “dead-zone” eddies in the tropical Northeast Atlantic Ocean

2016 ◽  
Author(s):  
Florian Schütte ◽  
Johannes Karstensen ◽  
Gerd Krahmann ◽  
Helena Hauss ◽  
Björn Fiedler ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Mixed Layer ◽  
Dead Zone ◽  
Open Ocean ◽  
Low Oxygen ◽  
Northeast Atlantic ◽  
Eastern Boundary ◽  
Northeast Atlantic Ocean ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. Localized open-ocean low–oxygen dead-zones in the tropical Northeast Atlantic are recently discovered ocean features that can develop in dynamically isolated water masses within cyclonic eddies (CE) and anticyclonic modewater eddies (ACME). Analysis of a comprehensive oxygen dataset obtained from gliders, moorings, research vessels and Argo floats revealed that eddies with low oxygen concentrations at 50–150 m depths can be found in surprisingly high numbers and in a large area (from about 4°N to 22°N, from the shelf at the eastern boundary to 38°W). Minimum oxygen concentrations of about 9 µmol kg−1 in CEs and severely suboxic concentrations (< 1 µmol kg−1) in ACMEs were observed. In total, 173 profiles with oxygen concentrations below the minimum background concentration of 40 µmol kg−1 could be associated with 27 independent “dead-zone” eddies (10 CEs; 17 ACMEs) over a period of 10 years. The eddies’ oxygen minimum is located in the eddy core beneath the mixed layer at a mean depth of 80 m. Compared to the surrounding waters, the mean oxygen anomaly between 50 and 150 m depth for CEs (ACMEs) is −38 (−79) µmol kg−1. The low oxygen concentration right beneath the mixed layer has been attributed to the combination of high productivity in the eddies’ surface waters and the isolation of their cores with respect to lateral oxygen supply. Indeed, eddies of both types feature a cold sea surface temperature anomaly and enhanced chlorophyll concentrations in their center. The locally increased consumption within these eddies represents an essential part of the total consumption in the open tropical Northeast Atlantic Ocean and might be partly responsible for the formation of the shallow oxygen minimum zone. Eddies south of 12°N carry weak hydrographic anomalies in their cores and seem to be generated in the open ocean away from the boundary. North of 12°N, eddies of both types carry anomalously low salinity water of South Atlantic Central Water origin from the eastern boundary upwelling region into the open ocean. Water mass properties and satellite eddy tracking both point to an eddy generation near the eastern boundary.

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