9 research outputs found
Concentration and isotopic composition of marine nitrous oxide, in particular the oxygen-17 isotope excess
Get PDFThe oxygen isotope excess Δ(17O) is a potential tracer of biological nitrous oxide (N2O) cycling.
This study presents the first measurements of Δ(17O) in marine N2O together with details about
the design and development of a custom-built Gas Chromatography-Isotope Ratio Mass
Spectrometry (GC-IRMS) analytical system for δ(15N), δ(18O) and Δ(17O) of N2O.
Δ(17O) values between 0.1 and 4.6 ‰ (relative to VSMOW) were observed during three field
campaigns in the temperate, subtropical and tropical Atlantic Ocean, the Scotia Sea and the
Weddell Sea. This indicates a biological source for oxygen isotope excess in N2O, as oceanic
N2O was not in equilibrium with the atmosphere (0.9 ‰).
δ(15N) values ranged from 2.3 to 25.1 ‰ (relative to Air-N2). δ(18O) measured in the Weddell
Sea ranged from 44.9 to 48.8 ‰ (relative to VSMOW). The dataset from the Atlantic Ocean
and the Atlantic sector of the Southern Ocean suggest nitrification and nitrifier-denitrification as
the main N2O production pathway in the oxic, deep ocean. High δ(15N) values in the south
Atlantic Gyre are presumably associated with a denitrification source.
A novel off-the-shelf N2O analyser was tested in combination with an equilibrator for semiautonomous
concentration measurements in the surface ocean. The subtropical gyres in the
Atlantic Ocean were confirmed to be weak sinks ((-0.14±0.31) μmol m-2 d-1 N2O flux to the
ocean in the northern, and (-0.16±0.33) μmol m-2 d-1 in the southern gyre) and the equatorial
region was a source of N2O to the atmosphere (flux of 0.53 μmol m-2 d-1). New data from the
Scotia Sea identified a strong source region ((2.9±2.7) μmol m-2 d-1), while the Weddell Sea was
closer to equilibrium with the atmosphere ((0.9±1.0) μmol m-2 d-1)
The impact of lake discontinuities on nitrogen biogeochemistry in river networks
Get PDFRiver networks connect terrestrial and marine ecosystems through transport of pollutants and nutrients. Lakes represent discontinuities within these river networks, which can be important biogeochemical hotspots, introducing substantial changes to the aquatic environment. Nitrogen is a key macronutrient that can potentially limit or co-limit primary production, but the processes that determine the fate of nitrogen during transport through river-lake networks are poorly understood. We studied three river systems and their lake discontinuities, spanning a range of trophic states and average water residence times, to understand the changes introduced to riverine nitrogen biogeochemistry by lake discontinuities. In-lake processes noticeably altered the concentration and speciation of nitrogen. Annually, lakes reduced up to 44% of nitrate compared to main inflow concentrations, while there was large variability in nitrate dynamics seasonally. The drawdown in surface nitrate concentrations resulted at times in phytoplankton co-limitation by nitrogen in-lake, as well as in the downstream river, where altered nitrogen patterns could persist for several kilometers. However, lakes occasionally subsidized N to downstream rivers as ammonium or dissolved organic nitrogen. Assimilation of nitrate in lake surface waters was one of the dominant processes impacting nitrogen availability; however, stable isotope data revealed an unexpected contribution of nitrification on nitrogen cycling in the epilimnion throughout the year and across trophic gradients. These changes in nitrogen concentration, as well as speciation introduced by lake discontinuities have potentially important consequences for the composition and metabolism of communities in downstream rivers and contribute to our fundamental understanding of freshwater processes
Hydrogen concentrations, underway measurements, equilibrator, reducing compound photometer
No full textEnvironmental factors influencing nitrogen fixation and diazotroph distribution in the Atlantic Ocean
No full textNitrogen fixation is an important process that supplies new fixed nitrogen to N-depleted regions of the ocean. The distribution and relative abundance of diazotrophs were evaluated along two meridional transects in the Atlantic ocean in April 2008 and October 2009, using phylotype-specific qPCR assays. Broad distribution patterns could be identified for most phylotypes, who tracked specific temperature ranges. Trichodesmium and the unculturable unicellular cyanobacterial diazotroph (UCYN-A) dominated the surface waters reaching over 1 million nifH copies L-1. However, the Trichodesmium and UCYN-A phylotypes were mutually exclusive, with Trichodesmium dominating at temperatures greater than 26°C, while UCYN-A was most abundant at 22°C. During the October 2009 transect, nitrogen fixation rates, measured with the 15N2 tracer technique, showed high peaks in activity in both the North and South Atlantic. Nitrogen fixation rates were however highest in the South Atlantic and were strongly correlated with the nifH copy numbers of the UCYN-A phylotype. The distribution patterns of both Trichodesmium and UCYN-A are explained in terms of the Saharan dust input and physiological temperature range
The impact of lake discontinuities on nitrogen biogeochemistry in river networks
No full textRiver networks connect terrestrial and marine ecosystems through transport of pollutants and nutrients. Lakes represent discontinuities within these river networks, which can be important biogeochemical hotspots, introducing substantial changes to the aquatic environment. Nitrogen is a key macronutrient that can potentially limit or co-limit primary production, but the processes that determine the fate of nitrogen during transport through river-lake networks are poorly understood. We studied three river systems and their lake discontinuities, spanning a range of trophic states and average water residence times, to understand the changes introduced to riverine nitrogen biogeochemistry by lake discontinuities. In-lake processes noticeably altered the concentration and speciation of nitrogen. Annually, lakes reduced up to 44% of nitrate compared to main inflow concentrations, while there was large variability in nitrate dynamics seasonally. The drawdown in surface nitrate concentrations resulted at times in phytoplankton co-limitation by nitrogen in-lake, as well as in the downstream river, where altered nitrogen patterns could persist for several kilometers. However, lakes occasionally subsidized N to downstream rivers as ammonium or dissolved organic nitrogen. Assimilation of nitrate in lake surface waters was one of the dominant processes impacting nitrogen availability; however, stable isotope data revealed an unexpected contribution of nitrification on nitrogen cycling in the epilimnion throughout the year and across trophic gradients. These changes in nitrogen concentration, as well as speciation introduced by lake discontinuities have potentially important consequences for the composition and metabolism of communities in downstream rivers and contribute to our fundamental understanding of freshwater processes
A tropical hotspot:Marine biogeochemistry
No full textThe ocean is an important source of the potent greenhouse gas N2O. Measurements in the tropical South Pacific have revealed a massive efflux of N2O from the coastal upwelling zone
