Publication Date:
2015-06-19
Description:
Recent observations in the eastern tropical South Pacific (ETSP) demonstrated the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production/consumption processes on the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source-sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked and sampled during two surveys carried out in the ETSP in November-December 2012. A "two peak" structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification amoA and the gene marker for N2O production during denitrification, nirS. Conversely, we found strong N2O depletion in the core of the OMZ (O2 〈 5 μmol L−1) to be consistent with nitrite (NO2−) accumulation and low levels of nitrate (NO3−), thus suggesting active denitrification. N2O depletion within the OMZ's core was substantially higher in the center of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, "aging" of mesoscale eddies tends to decrease N2O concentrations through the water column in response to reduced supply of material to fuel N-loss, although hydrographic variability might also significantly impact the pace of the production/consumption pathways for N2O. Our results demonstrate the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.
Print ISSN:
1810-6277
Electronic ISSN:
1810-6285
Topics:
Biology
,
Geosciences
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