An eddy-stimulated hotspot for fixed nitrogen-loss from the Peru oxygen minimum zone

Altabet, Mark A.; Ryabenko, Evgeniya; Stramma, Lothar; Wallace, Douglas W.R.; Frank, Martin; Grasse, Patricia; Lavik, Gaute

doi:10.5194/bg-9-4897-2012

Cited by 62 publications

(96 citation statements)

References 42 publications

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“…The upper N* minimum, which is located above the OMZ, is most probably a residual of the uplifted PCU during its upwelling at the shelf. This is in contrast to the interpretations of Altabet et al (2012) who argued that the upper N* minimum observed in a comparable anticyclonic coastal eddy off Peru resulted from shelf waters that had been in contact with shelf sediments and thus were carrying signals of sedimentary dentrification and/or anammox (Altabet et al, 2012).…”

Section: The Cyclonic Feature (Eddy C)contrasting

confidence: 52%

“…A similar feature is seen in the O 2 and NO Barber and Huyer, 1979;CopinMontégut and Raimbault, 1994;Fernández et al, 2009;Altabet et al, 2012;Casciotti et al, 2013) Codispoti et al (1986).…”

Section: Discussionmentioning

confidence: 53%

“…Moreover, the surveyed open ocean cyclonic eddy did not show any significant active nitrogen loss. Thus, we conclude that coastal mode water eddies are indeed a "hotspot" of nitrogen loss off Peru as proposed by Altabet et al (2012). However, the open ocean mode water eddy and the cyclonic eddy seem to be of reduced importance or even negligible, respectively, as hotspots of active nitrogen loss.…”

Section: Discussionmentioning

confidence: 77%

“…Tracking the eddy forward in time showed that its core reached 15 • 30 S, 85 • W on 13 February 2013 which results in a northwestward velocity of 7.1 cm s −1 . Altabet et al (2012), the second N* minimum in eddy A and the negative N* in eddy B are attributed to active nitrogen loss processes within eddies of the OMZ. The upper N* minimum, which is located above the OMZ, is most probably a residual of the uplifted PCU during its upwelling at the shelf.…”

Section: The Cyclonic Feature (Eddy C)mentioning

confidence: 99%

“…The high eddy activity in austral spring (October to November) near 15 • S coincides with areas of reduced upwelling and an important inshore advection of warmer and saltier subtropical water from 9 • S to 16 • S (Chaigneau et al, 2008). Correa-Ramirez et al (2012) suggested that the observed seasonal discrepancy between the occurrence of the maximum upwelling (in austral winter) and satellite observation of the maximum chlorophyll concentrations (in austral summer) in the coastal area of Peru may be caused by an intensification and seasonal displacement of eddy kinetic energy associated with eddies north of 20 • S. Moreover, coastal eddies off Peru have been shown to be sites of significant loss of fixed nitrogen (Altabet et al, 2012). Therefore, eddies off Peru seem to play an important role for biological productivity and the nitrogen cycle.…”

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confidence: 99%

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On the role of mesoscale eddies for the biological productivity and biogeochemistry in the eastern tropical Pacific Ocean off Peru

et al. 2013

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Abstract. Mesoscale eddies seem to play an important role for both the hydrography and biogeochemistry of the eastern tropical Pacific Ocean (ETSP) off Peru. However, detailed surveys of these eddies are not available, which has so far hampered an in depth understanding of their implications for nutrient distribution and biological productivity. In this study, three eddies along a section at 16 • 45 S have been surveyed intensively during R/V Meteor cruise M90 in November 2012. A coastal mode water eddy, an open ocean mode water eddy and an open ocean cyclonic eddy have been identified and sampled in order to determine both their hydrographic properties and their influence on the biogeochemical setting of the ETSP. In the thermocline the temperature of the coastal anticyclonic eddy was up to 2 • C warmer, 0.2 more saline and the swirl velocity was up to 35 cm s −1 . The observed temperature and salinity anomalies, as well as swirl velocities of both types of eddies were about twice as large as had been described for the mean eddies in the ETSP. The observed heat and salt anomalies (AHA, ASA) of the anticyclonic eddy near the shelf-break of 17.7 × 10 18 J and 36.6 × 10 10 kg are more than twice as large as the mean AHA and ASA for the ETSP. We found that the eddies contributed to the productivity by maintaining pronounced subsurface maxima of chlorophyll of up to 6 µg L −1 . Based on a comparison of the coastal (young) mode water eddy and the open ocean (old) mode water eddy we suggest that the ageing of eddies when they detach from the shelf-break and move westward to the open ocean influences the eddies' properties: chlorophyll maxima are reduced to about half (2.5-3 µg L −1 ) and nutrients are subducted. However, different settings at the time of formation may also contribute to the observed differences between the young and old mode water eddies. The coastal mode water eddy was found to be a site of nitrogen (N) loss in the OMZ with a maximum NO − 3 anomaly (i.e. N loss) of about −25 µmol L −1 in 250 m water depth, whereas, the open ocean mode water and cyclonic eddies were of minor and negligible importance for the N loss, respectively. Our results show that the important role of eddies for the distribution of nutrients, as well as biogeochemical processes in the ETSP (and other OMZ/upwelling regions) can only be fully deciphered and understood through dedicated high spatial and temporal resolution oceanographic/biogeochemical surveys.

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Section: The Cyclonic Feature (Eddy C)contrasting

confidence: 52%

Section: Discussionmentioning

confidence: 53%

Section: Discussionmentioning

confidence: 77%

Section: The Cyclonic Feature (Eddy C)mentioning

confidence: 99%

mentioning

confidence: 99%

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On the role of mesoscale eddies for the biological productivity and biogeochemistry in the eastern tropical Pacific Ocean off Peru

et al. 2013

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A model‐based insight into the coupling of nitrogen and sulfur cycles in a coastal upwelling system

et al. 2014

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The biogeochemical cycling in oxygen-minimum zones (OMZs) is dominated by the interactions of microbial nitrogen transformations and, as recently observed in the Chilean upwelling system, also through the energetically less favorable remineralization of sulfate reduction. The latter process is masked, however, by rapid sulfide oxidation, most likely through nitrate reduction. Thus, the cryptic sulfur cycle links with the nitrogen cycle in OMZ settings. Here, we model the physical-chemical water column structure and the observed process rates as driven by formation and sinking of organic detritus, to quantify the nitrogen and sulfur cycles in the Chilean OMZ. A new biogeochemical submodule was developed and coupled to the Regional Ocean Model System (ROMS). The model results generally agree with the observed distribution of reactive species and the measured process rates. Modeled heterotrophic nitrate reduction and sulfate reduction are responsible for 47% and 36%, respectively, of organic remineralization in a 150 m deep zone below mixed layer. Anammox contributes to 61% of the fixed nitrogen lost to N2 gas, while the rest of the loss is through canonical denitrification as a combination of organic matter oxidation by nitrite reduction and sulfide-driven denitrification. Mineralization coupled to heterotrophic nitrate reduction supplies ∼48% of the ammonium required by anammox. Due to active sulfate reduction, model results suggest that sulfide-driven denitrification contributes to 36% of the nitrogen loss as N2 gas. Our model results highlight the importance of considering the coupled nitrogen and sulfur cycle in examining open-ocean anoxic processes under present, past, and future conditions.

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N‐loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment

Bourbonnais

Altabet

Charoenpong

et al. 2015

Global Biogeochemical Cycles

105

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Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent "natural tracer experiments" with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO 3 À ), nitrite (NO 2 À ), and biogenic N 2 associated with an anticyclonic mode-water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO 3 À was nearly exhausted, we measured the highest δ 15 N values for both NO 3 À and NO 2 À (up to~70‰ and 50‰) ever reported for an OMZ.Correspondingly, N deficit and biogenic N 2 -N concentrations were also the highest near the eddy's center (up tõ 40 μmol L À1 ). δ 15 N-N 2 also varied with biogenic N 2 production, following kinetic isotopic fractionation during NO 2 À reduction to N 2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO 3 À reduction (up to~30‰ in the presence of NO 2 À ).However, the overall ε for N-loss was calculated to be only~13-14‰ (as compared to canonical values of 20-30‰) assuming a closed system and only slightly higher assuming an open system (16-19‰). Our results were similar whether calculated from the disappearance of DIN (NO 3 À + NO 2 À ) or from the appearance of N 2 and changes in isotopic composition. Further, we calculated the separate ε values for NO 3 À reduction to NO 2 À and NO 2 À reduction to N 2 of~16-21‰ and~12‰, respectively, when the effect of NO 2 À oxidation could be removed. These results, together with the relationship between N and O of NO 3 À isotopes and the difference in δ 15 N between NO 3 À and NO 2 À , confirm a role for NO 2 À oxidation in increasing the apparent ε associated with NO 3 À reduction. The lower ε for N-loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N-loss.

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An eddy-stimulated hotspot for fixed nitrogen-loss from the Peru oxygen minimum zone

Cited by 62 publications

References 42 publications

On the role of mesoscale eddies for the biological productivity and biogeochemistry in the eastern tropical Pacific Ocean off Peru

On the role of mesoscale eddies for the biological productivity and biogeochemistry in the eastern tropical Pacific Ocean off Peru

A model‐based insight into the coupling of nitrogen and sulfur cycles in a coastal upwelling system

N‐loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment

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