Interactive comment on “ Biogeochemical characteristics of a long-lived anticyclonic eddy in the eastern South Pacific Ocean ”

First, the premise that eddies constitute hotspots of denitrification is still not robustly confirmed in this work, although the evidence presented supports the idea. This is because the conclusion is based largely on one observation in a single vertical profile and some comparisons between an observation made in austral fall 2011 and measurements retrieved from observations in June 2010 (nearly 3 months after the estimated time of formation for the eddy under study). While there is substantiating

chemistry in the ESP not only by transporting physical and chemical properties from the coast to the ocean interior but also during advection, local biological consumption of oxygen within an eddy further generates conditions favorable to denitrification and loss of fixed nitrogen from the system.

Introduction
Mesoscale eddies play a major role in vertical and horizontal transport of heat, salts and other physical, chemical and biological constituents (Chelton et al., 2007;Chaigneau et al., 2008Chaigneau et al., , 2009)).In the eastern South Pacific (ESP) Ocean, mesoscale eddies frequently form in the coastal transition zone off central Chile due to the instability of the alongshore currents (Hormazábal et al., 2013;Morales et al., 2010Morales et al., , 2012)).These eddies transport water for long distances and over several months across biogeographic boundaries, from the productive Humboldt (Peru-Chile) Current to adjacent oligotrophic waters of the subtropical gyre (Pizarro et al., 2006).While cyclonic eddies transport surface waters and are seen as negative anomalies by altimetry data, anticyclonic eddies, including the Intrathermocline eddies (ITE) are deeper and transport low oxygen and salty subsurface waters from the Equatorial Subsurface Waters (ESSW; Chaigneau et al., 2011).
Although eddies have been considered a net loss of nutrients from the coastal zone (Gruber, 2011), they constitute a nutrient source in the open ocean that stimulates production in oligotrophic regions (McGillicuddy et al., 1998).In addition, eddies introduce spatial heterogeneity in productivity, community structure and particle flux, as has been observed in the Sargasso Sea (McGillicuddy et al., 1998;Sweeney et al., 2003).Impacts of eddies on biogeochemical processes are of particular interest for coastal transition zones of eastern boundary currents, where oxygen minimum zones (OMZs) and eddies interact (Altabet et al., 2012;Stramma et al., 2013).Open-ocean eddies associated with subsurface hypoxic or suboxic conditions have been observed in both the eastern tropical Atlantic and eastern tropical Pacific (e.g., Lukas and Santiago-Mandujano, 2001;Stramma et al., 2014;Karstensen et al., 2015).High surface productivity, downward particle flux and oxygen respiration, combined with sluggish exchange between the eddy interior and surrounding waters, have been proposed as mechanisms leading to the formation of "dead zones" observed within anticyclonic-mode water eddies (Karstensen et al., 2015).Oxygen consumption rates within an eddy can be 3 to 5 times higher than in the surrounding oligotrophic water (Karstensen et al., 2015).Eddies containing hypoxic or suboxic water can become hotspots for nitrogen cycling, including biogenic production of N 2 and loss of fixed nitrogen from the system (Altabet et al., 2012;Stramma et al., 2013).Recently, Stramma et al. (2013) suggested that in the eastern tropical Pacific Ocean, coastal mode water eddies are zones of active loss of fixed nitrogen while nitrogen loss associated with older, open-ocean mode water eddies is considerably lower .
In eastern boundary subtropical upwelling systems with a pronounced subsurface OMZ, such as in the ESP, eddies may play an important role in transporting OMZ waters and their microbial communities to the open ocean.The OMZ in the upwelling region of the ESP is associated with Equatorial Subsurface Water (ESSW) that has been transported poleward, from the Pacific Ocean equatorial belt, along the coast with the Peru-Chile undercurrent.The ESSW is characterized by high salinity and nutrient concentrations, low oxygen concentrations and an active, bacterially mediated nitrogen cycling, including production and consumption of the greenhouse gas nitrous oxide (N 2 O).Intense nitrification, denitrification and nitrous oxide (N 2 O) production associated with ESSW is generally confined to a narrow coastal band and contributes to net nitrogen loss in this region (Lam and Kuypers, 2011).
Intriguing questions about the role of coastally generated eddies abound: what biogeochemical transformations occur as this volume of water is advected offshore?Do concentrations of dissolved oxygen decrease, increase or remain the same?If changes in dissolved oxygen concentrations occur during transport, what are the other biogeochemical consequences in both surface and subsurface layers?Here, we present results from a study on the physical and chemical characteristics of a single anticyclonic eddy observed ∼ 900 km offshore, off central Chile.We tracked the eddy back to its region of formation and examined changes in concentration of oxygen and nitrogen species from the time it left the coast to the time it was sampled in the oligotrophic ocean.

Hydrography and nitrogen data
Hydrographic data and water samples for analyses of nutrients, N 2 O and surface 15 N-POM were collected along a transect from Valparaíso, Chile (33 • S, 71.6 • W) to Easter Island (28.2 • S, 107.4 • W) during the Tara Oceans Expedition (11-31 March 2011, Fig. 1;Karsenti et al., 2011).Sampling consisted of 11 vertical profiles using a Sea-Bird 911 equipped with an oxygen sensor (SBE43, sampling rate 24 Hz; Picheral et al., 2014).Due to logistical constraints, the oxygen sensor could not be calibrated on board.It was calibrated at the start of the expedition (July 2009) and a year later (August 2010 during a stopover in Cape Town, South Africa).A third calibration was conducted in September 2011 (during a stopover in Papeete).The sensor showed mean drifts of 0.101 (August 2010) and 0.405 µmol kg −1 (September 2011), between successive calibrations.Because oxygen calibrations could not be done routinely, post-cruise validation 2 at Tara stations, 094-A (black triangles) and 094-B (red triangles).For a complete detail of the Tara CTD cast please refer to (Picheral et al., 2014).
Discrete water samples for N 2 O (in triplicate) and nutrient (in duplicate) analyses were obtained from a rosette equipped with 10 L Niskin bottles.Samples were collected at 0, 50,100,150,200,250,300,400,500, and 900 m.N 2 O samples (20 mL) were taken after the inorganic carbon samples, using a tygon tubing to avoid bubble formation.Samples were fixed with 50 µL of saturated mercuric chloride and stored in the dark.N 2 O concentrations were determined onshore using a gas chromatograph equipped with an electron capture detector (ECD), following a headspace technique (McAullife, 1971).A four-point calibration curve was determined with air (0.32 ppm) and N 2 O standards of 0.1, 0.5 and 1 ppm (Scotty gas mixture; Air Liquid Co.).

Eddy identification and tracking
Presence and position of mesoscale eddies in the region were determined by analyzing weekly maps of anomalies in sea level and geostrophic velocities from the multi-satellite AVISO product (Ssalto/Duacs, http://www.aviso.altimetry.fr/en/data.html),from April 2010 to September 2011.This gridded, multi-satellite altimeter product provides spatial resolution of 1/3 • and allows resolution of eddies with an efolding scale > 40 km (Chaigneau et al., 2011;Chelton et al., 2011).For eddy tracking, we used the Okubo-Weiss parameter method (W ), which evaluates the relative dominance of strain and vorticity (Chelton et al., 2007;Okubo, 1970;Sangrà et al., 2009;Weiss, 1991): (3) where S n and S s are the normal and shear components of strain, respectively, and ω is the relative vorticity.This approach confirmed that the water mass sampled in station 094-A (30 • S, 81 • W; TARA_20110316T1152Z_999_EVENT_CAST; http://doi.pangaea.de/10.1594/PANGAEA.836473;Fig. 1a) was associated with an eddy.The path of this eddy was reconstructed from its time of origin (28 April 2010) to the time of its decay (29 June 2011) by tracing the eddy center (i.e., the region with highest vorticity) in successive geostrophic fields.
Vertical hydrographic profiles obtained from station 094-A (within the identified eddy) were compared with vertical profiles of temperature, salinity and dissolved oxygen concentration collected during other studies in the area and with nearby Argo buoy profiles and climatological information from WOA13 data (Garcia et al., 2014).Data available for nearest grid (1 • × 1 • ) to station 094-A were used in this analysis.

Glider information at the origin of the eddy
To characterize the properties of water near the time and location of eddy formation we used temperature, salinity and oxygen data from a cross-shore transect (73.0-74.8• W, 36.5 • S) conducted with a Slocum glider (Teledyne Technologies) in June 2010 (Pizarro et al., 2015).The data covered an area that extended to within 160 km from the presumed starting point of the eddy based on satellite backtracking.The glider was equipped with an optical oxygen sensor (Aanderaa Data Instrument oxygen Optode model 3830).Oxygen sensor calibrations are routinely done at the Physical Oceanography Laboratory, Universidad de Concepción, with a two-point calibration (0 and 100 % of oxygen saturation).Details of the sensor and the physical principles involved in the measurements are described in Körtzinger et al. (2005) and Uchida et al. (2008).
surface oxygen minimum layer (with dissolved oxygen concentration as low as 1.17 µmol O 2 kg −1 ) was detected at station 094-A, 922 km offshore (30 • S, 81 • W, Fig. 1d).Dissolved oxygen concentrations observed at 094-A are anomalous given that at this latitude the boundary of the OMZ (defined as O 2 < 44.6 µmol kg −1 ) and its core (defined as O 2 < 22.3 µmol kg −1 ) do not typically extend beyond ∼ 600 and ∼ 500 km from the shoreline, respectively (Silva et al., 2009).Suboxic conditions (O 2 < 10 µmol kg −1 ) are thought to be confined to the shelf region and were reported only in some bottom waters over the continental shelf (Farías, 2003).Climatological data indicate that subsurface water in the vicinity of station 094-A is typically oxygenated (Fig. 1e).According to the above classification of dissolved oxygen concentrations, the OMZ observed at station 094-A has its upper and lower boundaries at depths of 164 and 536 m, respectively, while suboxic conditions were observed between 174 and 421 m (66 % of the OMZ).
The low-oxygen water mass was associated with higher salinities (34.60-34.66)compared to surrounding water (Fig. 1c) and climatology for this region (34.15-34.36, for the years 1965-2012).Historical data from previous cruises conducted along the P06-WOCE transect (32.5 • S) or north of the study region (e.g., 28 • S, Scorpio; Silva et al., 2009; 27 • S, CIMAR 5 cruise; Fuenzalida et al., 2006) do not show such anomalies in physical and chemical conditions at this longitude (Fig. 2a and b).The presence of the anomalous salinity signature observed in this study is supported by an independent Argo float that profiled in the vicinity of 094-A (33.86 • S, 79.84 • W) during the Tara Oceans' sampling period (Fig. 2).Vertical profiles of salinity and temperature anomalies at 094-A show a warm and salty core centered between 230 and 270 m, accompanied by the dome and bowl shapes of the upper and lower thermocline, respectively (Fig. 3).This feature is characteristic of an intrathermocline, anticyclonic eddy (ITE, Xiu and Chai 2011;Chaigneau et al., 2011;Hormazábal et al., 2013).Positive sea-level anomalies (SLA) and associated geostrophic currents further indicate that station 094-A was located close to the center of a mesoscale anticyclonic eddy (∼ 150 km in diameter), centered at 33.45 • S, 80.85 • W (Fig. 4).
Reconstruction of this mesoscale eddy suggests that it was formed in the coastal transition zone off Concepción (36.09 • S, 74.21 • W), approximately 315 d prior to our sampling at station 094-A, and advected northwest at a mean velocity of 2.4 cm s −1 , approx. 2 km d −1 (Fig. 4).Reconstruction of the eddy's velocity and direction agrees well with previously reported trajectories of eddies in the area (Chaigneau, 2005).

OMZ evolution in the eddy
In the ESP, eddies could frequently transport suboxic water from the coastal OMZ to oceanic regions (Hormazábal et al., 2013).Low-oxygen water masses (O 2 < 44.6 µM) have been detected in the middle of the South Pacific Ocean (2000 km offshore at 28 • S; Silva et al., 2009), but at the time the authors did not associate these water masses with mesoscale eddies.In the North Pacific Ocean, low concentrations of dissolved oxygen in the subsurface open ocean have been previously described in association with anticyclonic mesoscale eddies (Altabet et al., 2012;Chaigneau et al., 2011;Johnson and McTaggart, 2010;Lukas and Santiago-Mandujano, 2001), and have been attributed to low-oxygen source waters (Lukas and Santiago-Mandujano, 2001) or to local consumption during the transport of an eddy (Karstensen et al., 2015).
To estimate temporal changes in concentrations of dissolved oxygen in the subsurface layer of our sampled eddy, we used underwater glider measurements, collected in June 2010, at 36.5 • S/74 • W (120 km from shore).This is the estimated location of the eddy 1 month after its formation and oxygen concentrations of source water are in agreement with those reported for other ITEs in the same region (Hormazábal et al., 2013).Glider measurements of dissolved oxygen showed a well-developed OMZ (O 2 < 44.6 µM; Fig. 5) between 104 and 352 m, with a suboxic layer located between 135 and 226 m.We refer to these values as the eddy's "initial" subsurface oxygen concentrations.Dissolved oxygen concentrations were then compared along isopycnals between the estimated location of the eddy's origin and its offshore location at station 094-A (Fig. 6).By the time the eddy reached its offshore location, the OMZ in the eddy was deeper and thicker (located between 164 and 537 m) than the OMZ measured in the coastal transition zone, and the lowest dissolved oxygen concentration decreased from 7.34 µmol kg −1 at 173 m to 1.17 µmol kg −1 at a depth of 338 m.The suboxic layer of the eddy in the open-ocean was located between 174 and 422 m, almost three times thicker than the suboxic layer of the "source water" (Fig. 6).This suggests that the observed OMZ offshore is a result of both advection of low-oxygen coastal water by an eddy and continuing www.biogeosciences.net/13/2971/2016/Biogeosciences, 13, 2971-2979, 2016 biological consumption during its transport.Estimated oxygen consumption rate in the OMZ of the eddy ranged from 0.29 to 44 nmol O 2 L −1 d −1 (∼ 0.1-15 µmol O 2 L −1 yr −1 ) in the core of the eddy (Fig. 6), which is quite high compared to previously reported oxygen consumption rates in other OMZs in this depth range (Karstensen et al., 2008).Considering the eddy's area (20 × 10 3 km 2 ; transport of ESSW of ∼ 1.38 Sv) and the thickness of the layer with dissolved oxygen concentrations < 44.6 µM (373 m), an oxygen deficit of 12.2 Tg is expected in the subsurface eddy's layer.This calculation is likely to underestimate the oxygen deficit since mixing with surrounding oxygen-rich water has not been taken into account here.If the whole sub-saturated oxygen layer of the eddy is considered (from ∼ 80 to 1000 m), the estimated oxygen deficit transported to the oceanic region is even larger.While this crude, back-of-the-envelope calculation of oxygen deficit should be taken with a grain of salt, it highlights the significant influence of eddies on OMZs in the open ocean.When the eddy dissipates, the oxygen deficit in the subsurface layer will be redistributed and will contribute to the overall oxygen budget of the ESP OMZ region.
The area of the "birth" of the studied eddy (∼ 36 • S) has been identified as a hotspot of eddy generation (Hormazábal et al., 2013), with ∼ 5-7 ITEs that can reach a diameter > 100 km being formed every year.If the eddy studied here is representative of eddies generated in the OMZ of the eastern boundary of the Pacific Ocean, between 31 and 36 • S, advection of and continued respiration within oxygen-deficient waters could produce a deficit as high as 60-85 Tg O 2 yr −1 within the oceanic region.

Subsurface biogeochemical implications of the eddy
OMZ regions of eastern boundary current systems are considered important with respect to the nitrogen cycle.These are areas where nitrogen is lost due to denitrification and anammox, and are often associated with the buildup of nitrite (NO − 2 ; Lam and Kuypers, 2011).Water masses with these features can be advected and the processes enhanced offshore as indicated by the observed accumulation of high subsurface NO − 2 in anticyclonic coastal eddies (Stramma et al., 2013).
Off the Chilean coast, elevated NO − 2 concentrations are generally found within the core of the OMZ (Cornejo and Farías, 2012a;Silva et al., 2009).Previous studies conducted in the region of the eddy's formation show that nitrite concentrations in hypoxic waters (< 7 µmol O 2 kg −1 ) are < 0.1 µM (Cornejo and Farías, 2012b).In the present study, initial oxygen concentrations in the OMZ of the eddy were 2 maximum (up to 0.56 µM) at a depth of 250 m (Fig. 1f).The apparent buildup of NO − 2 concentration within the eddy corresponds to a NO − 3 deficit (N*) of −13.90 µM (Fig. 6) relative to surrounding water (determined from the deviation of the NO − 3 : PO −3 4 molar ratio from the Redfield ratio according to Deutsch et al., 2001), suggesting that conditions in the eddy's anoxic zone were favorable for denitrification.A subsurface water mass with low oxygen concentrations and marked nitrate deficit has been previously observed in the oligotrophic, oceanic water of the EPS, during the Scorpio cruise (Fig. 7, Silva et al., 2009), but other nitrogen species indicative of denitrification were not measured, and tools to link this water mass to eddy activity were not well developed at that time.
Another indication of the existence of anaerobic conditions within the OMZ of the eddy is the presence of a N 2 O consumption layer.This greenhouse gas is produced by nitrification and denitrification under hypoxic conditions and consumed by denitrification under suboxic conditions (< 8 µM of O 2 ; Bonin et al., 1989).The vertical distribution of N 2 O in the eddy (station 094-A) shows a double peak with a supersaturation at the upper and lower boundaries of the subsurface OMZ (up to 224 %) and subsaturation (44 %) in the upper region of the OMZ core (Fig. 1e), suggesting that both production and consumption of N 2 O occurred in the eddy.Layers depleted in N 2 O have been previously observed in different OMZs in coastal environments where denitrification takes place (Cornejo and Farías, 2012a).To the best of our knowledge, this is the first reported N 2 O consumption layer in oceanic subsurface waters of the South Pacific Ocean.
Oxygen concentrations in the area where the eddy was formed are often too high to support significant N 2 O consumption (Cornejo and Farías, 2012b;Cornejo et al., 2015).Thus N 2 O is being accumulated alongshore in the coastal OMZ (up to 25 nM) and consumed only in bottom waters associated with suboxic conditions.In the absence of direct measurements of N 2 O concentrations at the time of eddy formation, we use the mean coastal N 2 O concentrations (22.68 ± 2.99 nM) from three previous cruises (FIP ;Cornejo et al., 2015) observed at the isopycnal σ t = 26.56kg m −3 (depth with N 2 O undersaturation at station 094-A) as the "initial" concentration of N 2 O in the eddy.We estimated a consumption rate of 3.92 nmol N 2 O L −1 d −1 during the time the eddy was advected offshore.This N 2 O consumption rate is half of those reported from incubation experiments conducted at the upper (shallower) boundary of the OMZ off Perú (8.16 nmol L −1 d −1 ; Dalsgaard et al., 2012).Higher rates might have occurred in the center of the eddy, where more active denitrification is expected.Furthermore, it is likely that we have underestimated N 2 O production since mixing with upper and lower layers that are supersaturated with respect to N 2 O, was not taken into account in our calculation.
Although N 2 O was sampled from discrete depths, the thickness of the N 2 O consumption layer can be estimated considering that N 2 O consumption does not occur at dissolved oxygen concentrations > 8 µM (Bonin et al., 1989).Within the eddy, this layer was 60 m thick (210-270 m) meaning that net consumption was at least ∼ 0.15 Gg N 2 O during the offshore transfer.This N 2 O consumption represents only 14 % of the N 2 O accumulation estimated above and below the consumption layer, suggesting that the eddy provided a net supply of ∼ 0.85 Gg of N 2 O to the oceanic region.Observing both production and consumption of N 2 O in the eddy signifies net nitrogen loss, with possible impacts on even global nitrogen and N 2 O balances.
This study provides new evidence that anticyclonic mesoscale eddies play important roles in the biogeochemistry of the ESP (Altabet et al., 2012;Stramma et al., 2013).Previous studies have shown that mesoscale eddies act as hotspots for microbially mediated nitrogen loss via denitrification in coastal water; here we show that open-ocean anticyclonic eddies can play a similar role in parts of the ocean that are far removed from productive coastal waters and their associated OMZs.

Figure 1 .
Figure 1.(a) Climatology of dissolved oxygen distributions (µmol kg −1 ) at a depth of 200 m in the eastern South Pacific Ocean.Yellow triangles indicate locations of stations during the Tara Oceans cruise.Vertical distributions of (b) temperature ( • C), (c) salinity and (d) dissolved oxygen along the transect of the Tara Ocean cruise, from coastal water off Chile to the open ocean.Black lines in (b) and (c) indicate the 26.0 and 27.0 kg m −3 σ t isopycnals.Black line in (d) indicates the isoclines of 40 µmol kg −1 of dissolved oxygen.(e) Vertical distribution of N 2 O at Tara stations 093 (34.0 • S/73.0 • W, blue triangles; TARA_20110312T1637Z_093_EVENT_CAST), 094-A (33.2 • S/81.1 • W, black triangles; TARA_20110316T1152Z_999_EVENT_CAST) and 094-B (32.8 • S/84.8 • W, red triangles; TARA_20110317T1815Z_999_EVENT_CAST).(f) Vertical distribution of NO −2 at Tara stations, 094-A (black triangles) and 094-B (red triangles).For a complete detail of the Tara CTD cast please refer to(Picheral et al., 2014).

Figure 2 .
Figure 2. (a) T − S diagram from various sampling programs in the study area; (b) dissolved oxygen profiles at Tara station 094-A (red line) and stations in its vicinity from cruises conducted on 1992 (black dashed line), 2003 (gray dashed line), 2010 (thick gray line) and 2004 (thin gray line); from Argo buoys (blue line, without oxygen), and the WOA 2013 climatology (green line).More details on data sources are provided in Table1.

Figure 4 .
Figure 4. Sea surface hight from satellite data at the time of sampling in Tara station 094-A (March 2011).The position of the sampling stations (black crosses) and estimated eddy trajectory (black line) with start and end locations (black circles) of the trajectory are also indicated.The red circle off Concepción (36 • S) shows the probable eddy generation zone.

Figure 5 .
Figure 5. Vertical distributions of dissolved oxygen (µmol kg −1 ) in a cross-shore section at 36.5 • S near the time of eddy formation (June 2010).Black line indicates the isoline of 25 µmol kg −1 .

Figure 6 .
Figure 6.(a) Vertical distribution of Oxygen as a function of σ t at Tara station 094-A (within the eddy) in March 2011 (red line) and at the coast in June 2010 (black line).Estimated O 2 depletion rates are shown as gray bars, where the vertical dotted line marks zero O 2 consumption rate.(b) Vertical distributions of Nitrate deficit (N*) and Nitrous oxide saturation (black triangles and line) as a function of σ t within the eddy at station 094-A.

Figure 7 .
Figure 7. Vertical distributions of σ t , dissolved oxygen and nitrate deficit (N*; according to Deutsch et al., 2001) in a cross-shore transect along 28 • S in the Eastern Southern Pacific Ocean (72 to 100 • S) during Scorpio cruise (June 1967).Black lines refer to σ t of 26 and 27 kg m −3 .

Table 1 .
Sources of hydrographic data used in this study.The name, location, and distance between the nearest location of the measurements and station 094-A are reported.