Description: Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean1, 2. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies3, 4, 5, 6. Regional observational data indicate a continuous decrease in oceanic dissolved oxygen concentrations in most regions of the global ocean1, 7, 8, 9, 10, with an increase reported in a few limited areas, varying by study1, 10. Prior work attempting to resolve variations in dissolved oxygen concentrations at the global scale reported a global oxygen loss of 550 ± 130 teramoles (1012 mol) per decade between 100 and 1,000 metres depth based on a comparison of data from the 1970s and 1990s10. Here we provide a quantitative assessment of the entire ocean oxygen inventory by analysing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years. We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (1015 mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths. We suggest that changes in the upper water column are mostly due to a warming-induced decrease in solubility and biological consumption. Changes in the deeper ocean may have their origin in basin-scale multi-decadal variability, oceanic overturning slow-down and a potential increase in biological consumption11, 12.

Description: The eastern tropical South Pacific (ETSP) upwelling region is one of the ocean’s largest sinks of fixed nitrogen, which is lost as N2 via the anaerobic processes of anammox and denitrification. One-third of nitrogen loss occurs in productive shelf waters stimulated by organic matter export as a result of eastern boundary upwelling. Offshore, nitrogen loss rates are lower, but due to its sheer size this area accounts for ~70% of ETSP nitrogen loss. How nitrogen loss and primary production are regulated in the offshore ETSP region where coastal upwelling is less influential remains unclear. Mesoscale eddies, ubiquitous in the ETSP region, have been suggested to enhance vertical nutrient transport and thereby regulate primary productivity and hence organic matter export. Here, we investigated the impact of mesoscale eddies on anammox and denitrification activity using 15N-labelled in situ incubation experiments. Anammox was shown to be the dominant nitrogen loss process, but varied across the eddy, whereas denitrification was below detection at all stations. Anammox rates at the eddy periphery were greater than at the center. Similarly, depth-integrated chlorophyll paralleled anammox activity, increasing at the periphery relative to the eddy center; suggestive of enhanced organic matter export along the periphery supporting nitrogen loss. This can be attributed to enhanced vertical nutrient transport caused by an eddy-driven submesoscale mechanism operating at the eddy periphery. In the ETSP region, the widespread distribution of eddies and the large heterogeneity observed in anammox rates from a compilation of stations suggests that eddy-driven vertical nutrient transport may regulate offshore primary production and thereby nitrogen loss.

Description: A strong El Niño developed in early 2015. Measurements from a research cruise on the R/V Sonne in October 2015 near the Equator east of the Galapagos Islands and off the shelf of Peru are used to investigate changes related to El Niño in the upper ocean in comparison with earlier cruises in this region. At the Equator at 85°30′ W, a clear temperature increase leading to lower densities in the upper 350 m had developed in October 2015, despite a concurrent salinity increase from 40 to 350 m. Lower nutrient concentrations were also present in the upper 200 m, and higher oxygen concentrations were observed between 40 and 130 m. In the equatorial current field, the Equatorial Undercurrent (EUC) east of the Galapagos Islands almost disappeared in October 2015, with a transport of only 0.02 Sv in the equatorial channel between 1° S and 1° N, and a weak current band of 0.78 Sv located between 1 and 2°30′ S. Such near-disappearances of the EUC in the eastern Pacific seem to occur only during strong El Niño events. Off the Peruvian shelf at  ∼  9° S, characteristics of upwelling were different as warm, saline, and oxygen-rich water was upwelled. At  ∼  12,  ∼  14, and  ∼  16° S, the upwelling of cold, low-salinity, and oxygen-poor water was still active at the easternmost stations of these three sections, while further west on these sections a transition to El Niño conditions appeared. Although from early 2015 the El Niño was strong, the October measurements in the eastern tropical Pacific only showed developing El Niño water mass distributions. In particular, the oxygen distribution indicated the ongoing transition from “typical” to El Niño conditions progressing southward along the Peruvian shelf.

Description: The El Niño Southern Oscillation (ENSO) with its warm (El Niño) and cold (La Niña) phase has strong impacts on marine ecosystems off Peru. This influence extends from changes in nutrient availability to productivity and oxygen levels. While several studies have demonstrated the influence of ENSO events on biological productivity, less is known about their impact on oxygen concentrations. In situ observations along the Peruvian and Chilean coast have shown a strong water column oxygenation during the 1997/1998 strong El Niño event. These observations suggest a deepening of the oxygen minimum zone (OMZ) along the continental shelf. However, due to reduced spatial coverage of the existing in situ observations, no studies have yet demonstrated the OMZ response to El Niño events in the whole Eastern Tropical South Pacific (ETSP). Furthermore, most studies have focused on El Niño events. Much less attention was given to the oxygen dynamics under La Niña influence. Here, we provide a comprehensive analysis of the ENSO influence on OMZ dynamics. Interannual variability of the OMZ during the period 1990–2010 is derived from a regional coupled physical-biogeochemical model forced with realistic atmospheric and lateral boundary conditions. Our results show a reduction of the vertical extent and a deepening of suboxic waters (SW) during the El Niño phase. During the La Niña phase, there is a vertical expansion of SW. These fluctuations in OMZ extent are due to changes in oxygen supply into its core depth mainly from lateral margins. During the El Niño phase, the enhanced lateral oxygen supply from the subtropics is the main reason for the reduction of SW in both coastal and offshore regions. During the La Niña phase, the oxygenated subtropical waters are blocked by the poleward transport along the southern margin of the OMZ. Consequently, oxygen concentrations within the OMZ are reduced and suboxic conditions expand during La Niña. The detailed analysis of transport pathways presented here provides new insights into how ENSO variability affects the oxygen-sensitive marine biogeochemistry of the ETSP.

Description: Cruise M135 was a contribution to the DFG Collaborative Research Project (SFB) 754: “Climate-Biogeochemistry Interactions in the Tropical Ocean” with the main goal to better understand the the role of diffusive and advective pathways connecting water within the bottom boundary layer (i.e. the water directly affected by sediment processes) to the pelagic and surface ocean. To achieve this, we have injected a conservative tracer (CF3SF5) within the bottom boundary layer at three different sites along the Peruvian coast at a depth of about 300 m in October 2015 that was mapped during M135. Tracer sampling was carried out by measuring water samples from the CTD-rosette water bottles. In total 144 CTD casts were carried out. From 132 CTD profiles 2828 samples for CF3CF5 investigations were gained and on most stations the tracer could be found. In addition 48 trace metal CTD’s were recorded and trace metal and chemical samples taken from the rosette bottles. On 166 of the CTD profiles oxygen samples were taken and on 94 CTD profiles nutrient samples were collected. Microstructure measurements were made on 24 stations and 2 gliders were deployed. For geological investigations at 5 locations multicorer and long gravity cores were taken. Continuous underway measurements of CO2,N2O and CO as well as continuous ADCP and thermosalinograph recording was made on 37 days. The cruise M135 was very successful; most systems on METEOR worked well and all planned objectives were reached.

Description: Summary • The global oxygen inventory has decreased by ~2% over the period 1960 to 2010, this finding is supported by regional time series data that indicate a continuous decrease in oceanic dissolved oxygen. • Ocean model simulations predict a decline in the dissolved oxygen inventory of the global ocean of 1 to 7% by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean. • Open-ocean deoxygenation is resulting mainly from a warming ocean, increased stratification and changing circulation which interact with eutrophication-induced hypoxia (oxygen concentration below ~60 to 120 μmol O2 kg-1) and biological activity in shelf regions. • Climate change related longer-term oxygen trends are masked by oxygen variability on a range of different spatial and temporal scales. • The decline in the oceanic oxygen content can affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries, ecosystems and coastal economies. • Oxygen loss is closely related to ocean warming and acidification caused by CO2 increase driven by CO2 emissions as well as biogeochemical consequences related to anthropogenic fertilization of the ocean; hence a combined effort investigating the different stressors will be most beneficial to understand future ocean changes.

Description: Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N-loss was calculated to be only ~13-14‰ (as compared to canonical values of ~20-30‰) assuming a closed system and only slightly higher assuming an open system (16-19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε for NO3− reduction to NO2− and NO2− reduction to N2 of ~16-21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2-, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for NO3− and NO2− reduction as well as N-loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N-loss.

Description: A subsurface low oxygen zone is located in the eastern tropical North Atlantic Ocean (ETNA) in the upper ocean with the core of the hypoxic (O2 ≤ 60 μmol kg−1) oxygen minimum zone (OMZ) at 400 to 500 m depth. The poorly known subsurface circulation in the OMZ region is derived from observations and data assimilation results. Measurements in the eastern tropical North Atlantic in November/December 2008, in November/December 2009 and October/November 2010 of velocity, oxygen and of a tracer (CF3SF5) that was released in April 2008 at ∼ 8° N, 23° W (at ∼ 330 m depth) show circulation in the upper part of the OMZ with spreading to the east in the North Equatorial Countercurrent (NECC) region and northwestward around the Guinea Dome. Three floats equipped with oxygen sensors deployed at ∼ 8° N, 23° W with parking depths at 330, 350 and 400 m depths were used to estimate velocity along the float trajectory at the surface and at the park depth. South of 9° N, the zonal surface velocity estimate from float data alternate seasonally. At the 350 m park depth north of 9° N a cyclonic northwestward flow across the OMZ was observed. The northward shift into the upper OMZ and the cyclonic flow around the Guinea Dome seem to be connected to a strong Atlantic Meridional Mode (AMM) event in 2009. A near-surface cyclonic circulation cell east of the Cape Verde Islands expands into the OMZ layer. The circulation of the upper OMZ mirrors the near surface circulation. Oxygen measurements from the cruises used here, as well as other recent cruises up to the year 2014 confirm the continuous deoxygenation trend in the upper OMZ since the 1960's near the Guinea Dome. The three floats deployed with the tracer show spreading paths consistent with the overall observed tracer spreading. Mesoscale eddies may modify the oxygen distribution in the OMZs. Oxygen sensors on the floats remained well calibrated for more than 20 months and so the oxygen profiles can be used to investigate mesoscale eddy signatures. However, in general eddies are less energetic in the ETNA south of the Cape Verde Islands compared to similar latitudes in the Eastern Tropical South Pacific.