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 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 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 THE POTENTIAL EFFECT OF NITROGEN REMOVAL PROCESSES ON THE δ15N FROM DIFFERENT TAXA IN THE MEXICAN SUBTROPICAL NORTH EASTERN PACIFIC

2012 ◽  
Vol 27 (2) ◽  
pp. 25
Author(s):  
J. Camalich ◽  
A. Sánchez ◽  
S. Aguíñiga ◽  
E. F. Balart
Keyword(s):  
Potential Effect ◽  
Trophic Levels ◽  
Eastern Pacific ◽  
N Fixation ◽  
Low Oxygen ◽  
Top Predators ◽  
North Eastern ◽  
La Red ◽  
Oxygen Concentrations ◽  
Bahia Magdalena

The sub-tropical north eastern Pacific is one of the major zones in the ocean where nitrogen is removed by bacterial processes which are enhanced by low oxygen concentrations commonly found in the water column along the Pacific coast upwelling areas. It is well established that the nitrogen isotopic signal (δ15N) increases in relation to trophic levels but little is known about the transfer of this δ15N signal from the dissolved fraction to higher trophic levels in oceanic regions with low oxygen. The objectives of this study are: 1) to report δ15N values from different abiotic and biotic components collected in the low-oxygen oceanic region in front of Bahía Magdalena (Mexican subtropical north-eastern Pacific); 2) to compare the δ15N of different trophic levels with analogous organisms in regions where nitrogen fixation is the dominating process, which will allow us to evaluate the actual transfer of δ15N enriched in 15N through the trophic web up to top predators. The δ15N was higher in both abiotic and biological compared to those reported from zones where N fixation is the dominating process. Oxygen concentrations in the oceanic area in front of Bahía Magdalena are low (< 2ml/l) at shallow water depths (< 100m) but not anoxic. Despite this we found that the δ15N signal reflects denitrification and this signal is transferred up though the food web. Efecto potencial del proceso de remoción de nitrógeno sobre el δ15N de distintos taxa en el Pacífico noreste mexicano subtropical El Pacífico subtropical noroeste es una de las zonas más importantes del océano en las cuales el nitrógeno es utilizado por procesos bacterianos que se intensifican bajo condiciones bajas de oxígeno como las que se encuentran comúnmente en las zonas de surgencia a lo largo de las costas del Pacifico. El incremento en la señal isotópica de N con respecto al nivel trófico (δ15N) es bien conocido, sin embargo su transferencia desde la fracción disuelta hasta niveles tróficos altos no ha sido estudiada a profundidad en zonas del océano en las cuales las concentraciones de oxígeno son bajas. Los objetivos de este estudio son: 1) reportar valores de δ15N de diferentes compartimentos (abióticos y bióticos) recolectados en la zona oceánica de baja concentración de oxígeno frente a Bahía Magdalena (Pacifico subtropical noreste Mexicano); 2) comparar δ15N de diferentes niveles tróficos con organismos análogos de regiones en las cuales la fijación de nitrógeno es el procesos dominante; esto nos permitirá evaluar la transferencia real de δ15N enriquecido en 15N a través de la red trófica hasta depredadores tope. El δ15N de los componentes abióticos y abióticos fue más alto que los reportados en regiones con una alta tasa de fijación de N. Las concentraciones de oxígeno en la zona de estudio son bajas (< 2ml/l) a profundidades superficiales (< 100m) aunque no anóxicas. No obstante, la señal de δ15N refleja desnitrificación y esta señal es transferida a lo largo de la cadena trófica.

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.

scholarly journals Characterization of “dead-zone” eddies in the eastern tropical North Atlantic

2016 ◽  
Vol 13 (20) ◽  
pp. 5865-5881 ◽  
Author(s):  
Florian Schütte ◽  
Johannes Karstensen ◽  
Gerd Krahmann ◽  
Helena Hauss ◽  
Björn Fiedler ◽  
...  
Keyword(s):  
North Atlantic ◽  
Dead Zone ◽  
Open Ocean ◽  
Depth Range ◽  
Low Oxygen ◽  
Eastern Boundary ◽  
Tropical North Atlantic ◽  
Minimum Zone ◽  
Core Depth ◽  
Oxygen Minimum

Abstract. Localized open-ocean low-oxygen “dead zones” in the eastern tropical North Atlantic are recently discovered ocean features that can develop in dynamically isolated water masses within cyclonic eddies (CE) and anticyclonic mode-water eddies (ACME). Analysis of a comprehensive oxygen dataset obtained from gliders, moorings, research vessels and Argo floats reveals that “dead-zone” eddies are found in surprisingly high numbers and in a large area from about 4 to 22° N, from the shelf at the eastern boundary to 38° W. In total, 173 profiles with oxygen concentrations below the minimum background concentration of 40 µmol kg−1 could be associated with 27 independent eddies (10 CEs; 17 ACMEs) over a period of 10 years. Lowest oxygen concentrations in CEs are less than 10 µmol kg−1 while in ACMEs even suboxic (< 1 µmol kg−1) levels are observed. The oxygen minimum in the eddies is located at shallow depth from 50 to 150 m with a mean depth of 80 m. Compared to the surrounding waters, the mean oxygen anomaly in the core depth range (50 and 150 m) for CEs (ACMEs) is −38 (−79) µmol kg−1. North of 12° N, the oxygen-depleted eddies carry anomalously low-salinity water of South Atlantic origin from the eastern boundary upwelling region into the open ocean. Here water mass properties and satellite eddy tracking both point to an eddy generation near the eastern boundary. In contrast, the oxygen-depleted 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. In both regions a decrease in oxygen from east to west is identified supporting the en-route creation of the low-oxygen core through a combination of high productivity in the eddy surface waters and an isolation of the eddy 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 low-oxygen core depth in the eddies aligns with the depth of the shallow oxygen minimum zone of the eastern tropical North Atlantic. Averaged over the whole area an oxygen reduction of 7 µmol kg−1 in the depth range of 50 to 150 m (peak reduction is 16 µmol kg−1 at 100 m depth) can be associated with the dispersion of the eddies. Thus the locally increased oxygen consumption within the eddy cores enhances the total oxygen consumption in the open eastern tropical North Atlantic Ocean and seems to be an contributor to the formation of the shallow oxygen minimum zone.

scholarly journals Ocean deoxygenation and copepods: coping with oxygen minimum zone variability

2020 ◽  
Vol 17 (8) ◽  
pp. 2315-2339 ◽  
Author(s):  
Karen F. Wishner ◽  
Brad Seibel ◽  
Dawn Outram
Keyword(s):  
Mixed Layer ◽  
Calanoid Copepod ◽  
Copepod Species ◽  
Depth Range ◽  
Low Oxygen ◽  
Maximum Abundance ◽  
Vertical Distributions ◽  
Oxygen Profiles ◽  
Oxygen Minimum ◽  
Ocean Deoxygenation

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.

scholarly journals Ocean Deoxygenation and Copepods: Coping with Oxygen Minimum Zone Variability

2019 ◽  
Author(s):  
Karen F. Wishner ◽  
Brad Seibel ◽  
Dawn Outram
Keyword(s):  
Mixed Layer ◽  
Life History Strategies ◽  
Copepod Species ◽  
Depth Range ◽  
Low Oxygen ◽  
Vertical Distributions ◽  
Species Abundances ◽  
Oxygen Profiles ◽  
Oxygen Minimum ◽  
Ocean Deoxygenation

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 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 Eucalanus, Pleuromamma, and Lucicutia) 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). 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.

scholarly journals Ocean deoxygenation, the global phosphorus cycle and the possibility of human-caused large-scale ocean anoxia

2017 ◽  
Vol 375 (2102) ◽  
pp. 20160318 ◽  
Author(s):  
Andrew J. Watson ◽  
Timothy M. Lenton ◽  
Benjamin J. W. Mills
Keyword(s):  
Chemical Weathering ◽  
Large Scale ◽  
Oxygen Demand ◽  
Subsurface Water ◽  
Phosphorus Cycle ◽  
Low Oxygen ◽  
Positive Feedbacks ◽  
Future Possibility ◽  
Warming World ◽  
Ocean Deoxygenation

The major biogeochemical cycles that keep the present-day Earth habitable are linked by a network of feedbacks, which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes that control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 10 5 –10 6 years, these ocean anoxic events (OAEs) being particularly associated with warm ‘greenhouse’ climates. A powerful mechanism responsible for past OAEs was an increase in phosphorus supply to the oceans, leading to higher ocean productivity and oxygen demand in subsurface water. This can be amplified by positive feedbacks on the nutrient content of the ocean, with low oxygen promoting further release of phosphorus from ocean sediments, leading to a potentially self-sustaining condition of deoxygenation. We use a simple model for phosphorus in the ocean to explore this feedback, and to evaluate the potential for humans to bring on global-scale anoxia by enhancing P supply to the oceans. While this is not an immediate global change concern, it is a future possibility on millennial and longer time scales, when considering both phosphate rock mining and increased chemical weathering due to climate change. Ocean deoxygenation, once begun, may be self-sustaining and eventually could result in long-lasting and unpleasant consequences for the Earth's biosphere. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

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