Hydroxylamine (NH2OH), a short‐lived intermediate in the nitrogen cycle, is a potential precursor of nitrous oxide (N2O) in the ocean. However, measurements of NH2OH in the ocean are sparse. Here we ...present a data set of depth profiles of NH2OH from the equatorial Atlantic Ocean and the eastern tropical South Pacific and compare it to N2O, nitrate, and nitrite profiles under varying oxygen conditions. The presence of NH2OH in surface waters points toward surface nitrification in the upper 100 m. Overall, we found a ratio of 1:3 between NH2OH and N2O in open ocean areas when oxygen concentrations were >50 μmol/L. In the equatorial Atlantic Ocean and the open ocean eastern tropical South Pacific, where nitrification is the dominant N2O production pathway, stepwise multiple regressions demonstrated that N2O, NH2OH, and nitrate concentrations were highly correlated, suggesting that NH2OH is a potential indicator for nitrification.
Plain Language Summary
Hydroxylamine (NH2OH) is a short‐lived intermediate in the nitrogen cycle. It could be a precursor of nitrous oxide (N2O) in the ocean. Nitrous oxide (N2O) is an important greenhouse gas and leads to the production of other nitrogen species that can deplete the ozone layer. In the ocean, N2O can be produced by two processes—nitrification and denitrification, while only during nitrification, the oxidation of ammonia to nitrate, is NH2OH involved. The key aim of this study is to decipher the role of NH2OH as a potential indicator for N2O production. We found out that NH2OH is strongly correlated with N2O in open ocean areas, where nitrification is the main N2O pathway and can therefore be used as an indicator for active nitrification and in situ N2O production.
Key Points
Hydroxylamine concentrations are significantly correlated with nitrous oxide concentrations in oxygenated waters
In waters that likely were influenced by denitrification the correlation between hydroxylamine and nitrous oxide breaks down
Hydroxylamine may be useful as an indicator for active in situ production of nitrous oxide by nitrification in the open ocean
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 NH4+
and the other from other substrates such as NO2-) was the dominant
species, comprising 70 %–85 % of total produced N2O from
NH4+, 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.
Methane (CH4) is a climate‐relevant trace gas that is emitted from the open and coastal oceans in considerable amounts. However, its distribution in remote oceanic areas is largely unknown. To fill ...this knowledge gap, dissolved CH4 was measured at nine stations at 75°S in the Ross Sea during austral summer in January 2020. CH4 undersaturation (mean: 82 ± 20%) was found throughout the water column. In subsurface waters, the distribution of CH4 mainly resulted from mixing of water masses and in situ consumption, whereas the CH4 concentrations in the surface mixed layer were mainly driven by air–sea exchange and diapycnal diffusion between the surface and subsurface layers, as well as consumption of CH4. With a mean air–sea CH4 flux density of −0.44 ± 0.34 μmol m−2 d−1, the Ross Sea was a substantial sink for atmospheric CH4 during austral summer, which is in contrast with most oceanic regions, which are known sources.
Carbon monoxide (CO) influences the radiative budget and oxidative capacity of the atmosphere over the Arctic Ocean, which is a source of atmospheric CO. Yet, oceanic CO cycling is understudied in ...this area, particularly in light of the ongoing rapid environmental changes. We present results from incubation experiments conducted in the Fram Strait in August–September 2019 under different environmental conditions: while lower pH did not affect CO production (GPCO) or consumption (kCO) rates, enhanced GPCO and kCO were positively correlated with coloured dissolved organic matter (CDOM) and dissolved nitrate concentrations, respectively, suggesting microbial CO uptake under oligotrophic conditions to be a driving factor for variability in CO surface concentrations. Both production and consumption of CO will likely increase in the future, but it is unknown which process will dominate. Our results will help to improve models predicting future CO concentrations and emissions and their effects on the radiative budget and the oxidative capacity of the Arctic atmosphere.
Ground-based atmospheric observations of CO2, δ(O2∕N2), N2O, and CH4
were used to make estimates of the air–sea fluxes of these species from the Lüderitz and Walvis Bay upwelling cells in the ...northern Benguela region, during upwelling events. Average flux densities (±1σ) were 0.65±0.4 µmol m−2 s−1 for CO2, -5.1±2.5 µmol m−2 s−1 for O2 (as APO), 0.61±0.5 nmol m−2 s−1 for N2O, and 4.8±6.3 nmol m−2 s−1 for CH4. A comparison of our top-down (i.e., inferred from atmospheric anomalies) flux estimates with shipboard-based measurements showed that the two approaches agreed within ±55 % on average, though the degree of agreement varied by species and was best for CO2. Since the top-down method overestimated the flux density relative to the shipboard-based approach for all species, we also present flux density estimates that have been tuned to best match the shipboard fluxes. During the study, upwelling events were sources of CO2, N2O, and CH4 to the atmosphere. N2O fluxes were fairly low, in accordance with previous work suggesting that the evasion of this gas from the Benguela is smaller than for other eastern boundary upwelling systems (EBUS). Conversely, CH4 release was quite high for the marine environment, a result that supports studies that indicated a large sedimentary source of CH4 in the Walvis Bay area. These results demonstrate the suitability of atmospheric time series for characterizing the temporal variability of upwelling events and their influence on the overall marine greenhouse gas (GHG) emissions from the northern Benguela region.
Large-scale climatic forcing is impacting
oceanic biogeochemical cycles and is expected to influence the water-column
distribution of trace gases, including methane and nitrous oxide. Our ability
as ...a scientific community to evaluate changes in the water-column inventories
of methane and nitrous oxide depends largely on our capacity to obtain robust
and accurate concentration measurements that can be validated across
different laboratory groups. This study represents the first formal
international intercomparison of oceanic methane and nitrous oxide
measurements whereby participating laboratories received batches of seawater
samples from the subtropical Pacific Ocean and the Baltic Sea. Additionally,
compressed gas standards from the same calibration scale were distributed to
the majority of participating laboratories to improve the analytical accuracy
of the gas measurements. The computations used by each laboratory to derive
the dissolved gas concentrations were also evaluated for inconsistencies
(e.g., pressure and temperature corrections, solubility constants). The
results from the intercomparison and intercalibration provided invaluable
insights into methane and nitrous oxide measurements. It was observed that
analyses of seawater samples with the lowest concentrations of methane and
nitrous oxide had the lowest precisions. In comparison, while the analytical
precision for samples with the highest concentrations of trace gases was
better, the variability between the different laboratories was higher:
36 % for methane and 27 % for nitrous oxide. In addition, the
comparison of different batches of seawater samples with methane and nitrous
oxide concentrations that ranged over an order of magnitude revealed the
ramifications of different calibration procedures for each trace gas.
Finally, this study builds upon the intercomparison results to develop
recommendations for improving oceanic methane and nitrous oxide measurements,
with the aim of precluding future analytical discrepancies between
laboratories.
The open ocean is a major source of nitrous oxide
(N2O), an atmospheric trace gas attributable to global warming and
ozone depletion. Intense sea-to-air N2O fluxes occur in major oceanic
upwelling ...regions such as the eastern tropical South Pacific (ETSP). The
ETSP is influenced by the El Niño–Southern Oscillation that leads to
inter-annual variations in physical, chemical, and biological properties in
the water column. In October 2015, a strong El Niño event was developing
in the ETSP; we conduct field observations to investigate (1) the N2O
production pathways and associated biogeochemical properties and (2) the
effects of El Niño on water column N2O distributions and fluxes
using data from previous non-El Niño years. Analysis of N2O natural
abundance isotopomers suggested that nitrification and partial
denitrification (nitrate and nitrite reduction to N2O) were occurring
in the near-surface waters; indicating that both pathways contributed to
N2O effluxes. Higher-than-normal sea surface temperatures were
associated with a deepening of the oxycline and the oxygen minimum layer.
Within the shelf region, surface N2O supersaturation was nearly an
order of magnitude lower than that of non-El Niño years. Therefore, a
significant reduction of N2O efflux (75 %–95 %) in the ETSP
occurred during the 2015 El Niño. At both offshore and coastal stations,
the N2O concentration profiles during El Niño showed moderate
N2O concentration gradients, and the peak N2O concentrations
occurred at deeper depths during El Niño years; this was likely the
result of suppressed upwelling retaining N2O in subsurface waters. At
multiple stations, water-column inventories of N2O within the top 1000 m were up to 160 % higher than those measured in non-El Niño years,
indicating that subsurface N2O during El Niño could be a reservoir
for intense N2O effluxes when normal upwelling is resumed after El
Niño.
The coastal upwelling regime off Peru in December 2012 showed considerable vertical concentration gradients of dissolved nitrous oxide (N2O) across the top few meters of the ocean. The gradients were ...predominantly downward, i.e., concentrations decreased toward the surface. Ignoring these gradients causes a systematic error in regionally integrated gas exchange estimates, when using observed concentrations at several meters below the surface as input for bulk flux parameterizations – as is routinely practiced. Here we propose that multi-day near-surface stratification events are responsible for the observed near-surface N2O gradients, and that the gradients induce the strongest bias in gas exchange estimates at winds of about 3 to 6 m s−1. Glider hydrographic time series reveal that events of multi-day near-surface stratification are a common feature in the study region. In the same way as shorter events of near-surface stratification (e.g., the diurnal warm layer cycle), they preferentially exist under calm to moderate wind conditions, suppress turbulent mixing, and thus lead to isolation of the top layer from the waters below (surface trapping). Our observational data in combination with a simple gas-transfer model of the surface trapping mechanism show that multi-day near-surface stratification can produce near-surface N2O gradients comparable to observations. They further indicate that N2O gradients created by diurnal or shorter stratification cycles are weaker and do not substantially impact bulk emission estimates. Quantitatively, we estimate that the integrated bias for the entire Peruvian upwelling region in December 2012 represents an overestimation of the total N2O emission by about a third, if concentrations at 5 or 10 m depth are used as surrogate for bulk water N2O concentration. Locally, gradients exist which would lead to emission rates overestimated by a factor of two or more. As the Peruvian upwelling region is an N2O source of global importance, and other strong N2O source regions could tend to develop multi-day near-surface stratification as well, the bias resulting from multi-day near-surface stratification may also impact global oceanic N2O emission estimates.
Dimethyl sulphide (DMS) and carbon monoxide (CO) are climate-relevant trace gases that play key roles in the radiative budget of the Arctic atmosphere. Under global warming, Arctic sea ice retreats ...at an unprecedented rate, altering light penetration and biological communities, and potentially affect DMS and CO cycling in the Arctic Ocean. This could have socio-economic implications in and beyond the Arctic region. However, little is known about CO production pathways and emissions in this region and the future development of DMS and CO cycling. Here we summarize the current understanding and assess potential future changes of DMS and CO cycling in relation to changes in sea ice coverage, light penetration, bacterial and microalgal communities, pH and physical properties. We suggest that production of DMS and CO might increase with ice melting, increasing light availability and shifting phytoplankton community. Among others, policy measures should facilitate large-scale process studies, coordinated long term observations and modelling efforts to improve our current understanding of the cycling and emissions of DMS and CO in the Arctic Ocean and of global consequences.
PDF
