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- Title
Nitrogen cycling in shallow low oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes.
- Authors
Hu, H.; Bourbonnais, A.; Larkum, J.; Bange, H. W.; Altabet, M. A.
- Abstract
O2 minimum zones (OMZ) of the world's oceans are important locations for microbial dissimilatory NO3- reduction and subsequent loss of combined nitrogen (N) to biogenic N2 gas. This is particularly so when the OMZ is coupled to a region of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N isotope ratios (and O in the case of NO3- and NO2-) can be used as natural tracers of OMZ N-cycling because of distinct kinetic isotope effects associated with microbially-mediated N-cycle transformations. Here we present NO2- and NO3- stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface O2 was generally depleted below about 30 m depth with O2 less than 10 µM, while NO2- concentrations were high, ranging from 6 to 10 µM and NO3- was in places strongly depleted to near 0 µM. We observed for the first time, a positive linear relationship between 2- δ15N and δ18O at our coastal stations, analogous to that of NO3- N and O isotopes during assimilatory and dissimilatory reduction. This relationship is likely the result of rapid 2- turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of 2- facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ15N of [NO3-], DIN, or biogenic N2, values for ε are generally much lower than previously reported, reaching as low as 6.5. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.
- Subjects
NITROGEN cycle; TERRITORIAL waters; NITRITES; NITROGEN isotopes; OXYGEN isotopes
- Publication
Biogeosciences Discussions, 2015, Vol 12, Issue 19, p7257
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bgd-12-7257-2015