Description: Oxygen minimum zones (OMZs) that impinge on continental margins favor the release of phosphorus (P) from the sediments to the water column, enhancing primary productivity and the maintenance or expansion of low-oxygen waters. A comprehensive field program in the Peruvian OMZ was undertaken to identify the sources of benthic P at six stations, including the analysis of particles from the water column, surface sediments, and pore fluids, as well as in situ benthic flux measurements. A major fraction of solid-phase P was bound as particulate inorganic P (PIP) both in the water column and in sediments. Sedimentary PIP increased with depth in the sediment at the expense of particulate organic P (POP). The ratio of particulate organic carbon (POC) to POP exceeded the Redfield ratio both in the water column (202 ± 29) and in surface sediments (303 ± 77). However, the POC to total particulate P (TPP = POP + PIP) ratio was close to Redfield in the water column (103 ± 9) and in sediment samples (102 ± 15). This suggests that the relative burial efficiencies of POC and TPP are similar under low-oxygen conditions and that the sediments underlying the anoxic waters on the Peru margin are not depleted in P compared to Redfield. Benthic fluxes of dissolved P were extremely high (up to 1.04 ± 0.31 mmol m−2 d−1), however, showing that a lack of oxygen promotes the intensified release of dissolved P from sediments, whilst preserving the POC / TPP burial ratio. Benthic dissolved P fluxes were always higher than the TPP rain rate to the seabed, which is proposed to be caused by transient P release by bacterial mats that had stored P during previous periods when bottom waters were less reducing. At one station located at the lower rim of the OMZ, dissolved P was taken up by the sediments, indicating ongoing phosphorite formation. This is further supported by decreasing porewater phosphate concentrations with sediment depth, whereas solid-phase P concentrations were comparatively high.

Description: 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.

Description: The temporal evolution of the physical and biogeochemical structure of an oxygen-depleted anticyclonic modewater eddy is investigated over a 2-month period using high-resolution glider and ship data. A weakly stratified eddy core (squared buoyancy frequency N2  ∼  0.1  ×  10−4 s−2) at shallow depth is identified with a horizontal extent of about 70 km and bounded by maxima in N2. The upper N2 maximum (3–5  ×  10−4 s−2) coincides with the mixed layer base and the lower N2 maximum (0.4  ×  10−4 s−2) is found at about 200 m depth in the eddy centre. The eddy core shows a constant slope in temperature/salinity (T∕S) characteristic over the 2 months, but an erosion of the core progressively narrows down the T∕S range. The eddy minimal oxygen concentrations decreased by about 5 µmol kg−1 in 2 months, confirming earlier estimates of oxygen consumption rates in these eddies. Separating the mesoscale and perturbation flow components reveals oscillating velocity finestructure ( ∼  0.1 m s−1) underneath the eddy and at its flanks. The velocity finestructure is organized in layers that align with layers in properties (salinity, temperature) but mostly cross through surfaces of constant density. The largest magnitude in velocity finestructure is seen between the surface and 140 m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450 m. For both regions a cyclonic rotation of the velocity finestructure with depth suggests the vertical propagation of near-inertial wave (NIW) energy. Modification of the planetary vorticity by anticyclonic (eddy core) and cyclonic (eddy periphery) relative vorticity is most likely impacting the NIW energy propagation. Below the low oxygen core salt-finger type double diffusive layers are found that align with the velocity finestructure. Apparent oxygen utilization (AOU) versus dissolved inorganic nitrate (NO3−) ratios are about twice as high (16) in the eddy core compared to surrounding waters (8.1). A large NO3− deficit of 4 to 6 µmol kg−1 is determined, rendering denitrification an unlikely explanation. Here it is hypothesized that the differences in local recycling of nitrogen and oxygen, as a result of the eddy dynamics, cause the shift in the AOU : NO3− ratio. High NO3− and low oxygen waters are eroded by mixing from the eddy core and entrain into the mixed layer. The nitrogen is reintroduced into the core by gravitational settling of particulate matter out of the euphotic zone. The low oxygen water equilibrates in the mixed layer by air–sea gas exchange and does not participate in the gravitational sinking. Finally we propose a mesoscale–submesoscale interaction concept where wind energy, mediated via NIWs, drives nutrient supply to the euphotic zone and drives extraordinary blooms in anticyclonic mode-water eddies.

Description: Benthic nitrogen (N2) fixation and sulfate reduction (SR) were investigated in the Peruvian oxygen minimum zone (OMZ). Sediment samples, retrieved by a multiple corer were taken at six stations (70–1025 m) along a depth transect at 12° S, covering anoxic and hypoxic bottom water conditions. Benthic N2 fixation was detected at all sites, with high rates measured in OMZ mid-waters between the 70 and 253 m and lowest N2 fixation rates below 253 m down to 1025 m water depth. SR rates were decreasing with increasing water depth, with highest rates at the shallow site. Benthic N2 fixation depth profiles largely overlapped with SR depth profiles, suggesting that both processes are coupled. The potential of N2 fixation by SR bacteria was verified by the molecular analysis of nifH genes. Detected nifH sequences clustered with SR bacteria that have been demonstrated to fix N2 in other benthic environments. Depth-integrated rates of N2 fixation and SR showed no direct correlation along the 12° S transect, suggesting that the benthic diazotrophs in the Peruvian OMZ are being controlled by additional various environmental factors. The organic matter availability and the presence of sulfide appear to be major drivers for benthic diazotrophy. It was further found that N2 fixation was not inhibited by high ammonium concentrations. N2 fixation rates in OMZ sediments were similar to rates measured in other organic-rich sediments. Overall, this work improves our knowledge on N sources in marine sediments and contributes to a better understanding of N cycling in OMZ sediments.

Description: The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which – for some, yet unconstrained, reason – are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key in understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N2) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one-quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB's nitrogen input, suggesting a potential role of N2 fixation in those waters. Here, we present a survey of N2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N2 fixers comparable to other oxygen minimum zone (OMZ) regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N2 fixers present throughout the water column (samples collected between 10 and 560 m water depth). While similar communities of N2 fixers were shown to actively fix N2 in other OMZs, N2 fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ. Consistent with this, no N2 fixation signal was visible in δ15N signatures. We suggest that the absence of N2 fixation may be a consequence of a micronutrient limitation or of an O2 sensitivity of the OMZ diazotrophs in the BoB. Exploring how the onset of N2 fixation by cyanobacteria compared to non-phototrophic N2 fixers would impact on OMZ O2 concentrations, a simple model exercise was carried out. We observed that both photic-zone-based and OMZ-based N2 fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which can exhaust remaining O2 traces in the BoB.

Description: Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O production from ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH+4 and the other from other substrates such as NO−2) was the dominant species, comprising 70 %–85 % of total produced N2O from NH+4, regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.

Description: Ocean alkalinity enhancement (OAE) is an emerging strategy that aims to mitigate climate change by increasing the alkalinity of seawater. This approach involves increasing the alkalinity of the ocean to enhance its capacity to absorb and store carbon dioxide (CO2) from the atmosphere. This chapter presents an overview of the technical aspects associated with the full range of OAE methods being pursued and discusses implications for undertaking research on these approaches. Various methods have been developed to implement OAE, including the direct injection of alkaline liquid into the surface ocean; dispersal of alkaline particles from ships, platforms, or pipes; the addition of minerals to coastal environments; and the electrochemical removal of acid from seawater. Each method has its advantages and challenges, such as scalability, cost effectiveness, and potential environmental impacts. The choice of technique may depend on factors such as regional oceanographic conditions, alkalinity source availability, and engineering feasibility. This chapter considers electrochemical methods, the accelerated weathering of limestone, ocean liming, the creation of hydrated carbonates, and the addition of minerals to coastal environments. In each case, the technical aspects of the technologies are considered, and implications for best-practice research are drawn. The environmental and social impacts of OAE will likely depend on the specific technology and the local context in which it is deployed. Therefore, it is essential that the technical feasibility of OAE is undertaken in parallel with, and informed by, wider impact assessments. While OAE shows promise as a potential climate change mitigation strategy, it is essential to acknowledge its limitations and uncertainties. Further research and development are needed to understand the long-term effects, optimize techniques, and address potential unintended consequences. OAE should be viewed as complementary to extensive emission reductions, and its feasibility may be improved if it is operated using energy and supply chains with minimal CO2 emissions.

Description: Marine diazotrophs convert dinitrogen (N-2) gas into bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the first version of the global oceanic diazotroph database (version 1) was published. Here, we present an updated version of the database (version 2), significantly increasing the number of in situ diazotrophic measurements from 13 565 to 55 286. Data points for N-2 fixation rates, diazotrophic cell abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and 809 %, respectively. Version 2 includes two new data sheets for the nifH gene copy abundance of non-cyanobacterial diazotrophs and cell-specific N2 fixation rates. The measurements of N-2 fixation rates approximately follow a log-normal distribution in both version 1 and version 2. However, version 2 considerably extends both the left and right tails of the distribution. Consequently, when estimating global oceanic N-2 fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43-57 versus 45-63 TgNyr (-1); ranges based on one geometric standard error). In contrast, when using arithmetic means, version 2 suggests a significantly higher rate of 223 +/- 30 TgNyr (-1) (mean +/- standard error; same hereafter) compared to version 1 (74 +/- 7 TgNyr (-1)). Specifically, substantial rate increases are estimated for the South Pacific Ocean (88 +/- 23 versus 20 +/- 2 TgNyr 1), primarily driven by measurements in the southwestern subtropics, and for the North Atlantic Ocean (40 +/- 9 versus 10 +/- 2 TgNyr (-1)). Moreover, version 2 estimates the N-2 fixation rate in the Indian Ocean to be 35 +/- 14 TgNyr (-1), which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N-2 fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional N-15(2) bubble method yields lower rates in 69% cases compared to the new N-15(2) dissolution method. This updated version of the database can facilitate future studies in marine ecology and biogeochemistry. The database is stored at the Figshare repository (https://doi.org/10.6084/m9.figshare.21677687; Shao et al., 2022).