ALBERT

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 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.

Description: In the current era of rapid climate change, accurate characterization of climate-relevant gas dynamics – namely production, consumption, and net emissions – is required for all biomes, especially those ecosystems most susceptible to the impact of change. Marine environments include regions that act as net sources or sinks for numerous climate-active trace gases including methane (CH4) and nitrous oxide (N2O). The temporal and spatial distributions of CH4 and N2O are controlled by the interaction of complex biogeochemical and physical processes. To evaluate and quantify how these mechanisms affect marine CH4 and N2O cycling requires a combination of traditional scientific disciplines including oceanography, microbiology, and numerical modeling. Fundamental to these efforts is ensuring that the datasets produced by independent scientists are comparable and interoperable. Equally critical is transparent communication within the research community about the technical improvements required to increase our collective understanding of marine CH4 and N2O. A workshop sponsored by Ocean Carbon and Biogeochemistry (OCB) was organized to enhance dialogue and collaborations pertaining to marine CH4 and N2O. Here, we summarize the outcomes from the workshop to describe the challenges and opportunities for near-future CH4 and N2O research in the marine environment.

Description: 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.

Description: Nitrous oxide (N2O) is one of the most important long-lived greenhouse gases (GHG) responsible for the overall warming of the Earth system, and the strongest ozone-depleting compound in the stratosphere. While the ocean is thought to be a net source of N2O to the atmosphere, at regional and basin-scales there is a large range of variability in terms of its mid-depth water production and consumption, as well as a wide range of physical processes which impact the pool of this GHG in surface waters, posing a significant challenge to both observational and model-based estimates of the annual emissions. Furthermore, although environmental changes such as warming, eutrophication and decrease in sea ice coverage are expected to affect the cycling of N2O, the particular direction of the projected trends is highly uncertain, mostly due to the paucity of measurements in strong source regions and areas of difficult access due to their remote location or extreme weather conditions. In this talk I will present an overview of the current state of knowledge with respect to the exchanges of N2O across the sea-air-ice interfaces, and discuss their relevance in the context of ongoing climate change. To this end, I will use examples from ship-based observations in Eastern Boundary Upwelling Systems (EBUS), the subpolar North Atlantic and the Arctic Ocean. By looking at the contrasting features of these ecosystems and how they affect the marine budget of N2O, I aim to highlight the need of new approaches and priority areas for research on GHGs such as N2O.

Description: The Arctic Ocean is particularly sensitive to climate change. Its ecosystem structure and function are prone to be disturbed by fast warming and massive retreat of sea-ice, which in turn, might result in feedbacks on climate. Moreover, such drastic changes are expected to influence the meridional fluxes of heat, freshwater and biogeochemical tracers between subpolar areas and the Arctic. As the third most important greenhouse gas and major ozone-depleting substance in the stratosphere, nitrous oxide (N2O) is a crucial gas to study in order to assess the ocean’s role in the production and exchange of climate-relevant compounds to the atmosphere. Between 2018 and 2019 we conducted ship-based surveys to elucidate the source-sink dynamics of N2O in the subpolar-polar North Atlantic. Based on results from those campaigns, we show the distribution and spatial variability of surface N2O, which ranged from moderate supersaturation (positive sea-air fluxes) in ice-free subpolar areas to unusually strong undersaturation (negative sea-air fluxes) in partially or fully ice-covered areas. We also present a comprehensive overview of the water column distribution of N2O in the region, and by combining this data with hydrographic and chemical (O2 and inorganic nutrients) information, we trace back the origin of the dominant water masses so as to illustrate the connectivity between the Fram Strait and the Nordic Seas off southeast Greenland. This analysis is used to discuss how the meridional water mass exchange in the region influences the balance of local vs. remote N2O production and its spatial variability. Furthermore, we use the results from collocated molecular analyses (functional gene markers) to infer the occurrence and abundances of the main microbial communities responsible for the cycling of N2O. This contribution is relevant for assessments of expected changes in trace gas emissions with further climate-driven changes in the Arctic Ocean.

Description: Given the climatic relevance of marine-derived trace gases, the investigation of their distribution and emissions from key oceanic regions is a crucial need in our efforts to better understand potential responses of the ocean and the overlying atmosphere to environmental changes such as warming and deoxygenation. Low-oxygen waters connected to coastal upwelling systems and the associated oxygen minimum zones(OMZ) are well-recognized strong sources of several trace gases. Our main goal during the M135-M138 cruises was to assess the distribution of different gases which are relevant for the biogeochemical cycling of carbon and nitrogen in the OMZ off Peru, as well as the spatial and temporal variability of their sea-air fluxes.

Description: Human activities are changing the Arctic environment at an unprecedented rate resulting in rapid warming, freshening, sea ice retreat and ocean acidification of the Arctic Ocean. Trace gases such as nitrous oxide (N2O) and methane (CH4) play important roles in both the atmospheric reactivity and radiative budget of the Arctic and thus have a high potential to influence the region’s climate. However, little is known about how these rapid physical and chemical changes will impact the emissions of major climate-relevant trace gases from the Arctic Ocean. The combined consequences of these stressors present a complex combination of environmental changes which might impact on trace gas production and their subsequent release to the Arctic atmosphere. Here we present our current understanding of nitrous oxide and methane cycling in the Arctic Ocean and its relevance for regional and global atmosphere and climate and offer our thoughts on how this might change over coming decades.