The occurrence of mesoscale eddies that develop suboxic environments at
shallow depth (about 40–100 m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests that
the water mass inside the eddy is well isolated from ambient waters
supporting the development of severe near-surface oxygen deficits. So far,
hydrographic and biogeochemical characterization of these eddies was limited
to a few autonomous surveys, with the use of moorings, underwater gliders and
profiling floats. In this study we present results from the first dedicated
biogeochemical survey of one of these eddies conducted in March 2014 near the
Cape Verde Ocean Observatory (CVOO). During the survey the eddy core showed
oxygen concentrations as low as 5
New technological advances in ocean observation platforms, such as profiling floats, gliders and in sensors have greatly facilitated our knowledge about physical, chemical and biological processes in the oceans, particularly those occurring on small spatiotemporal scales (Johnson et al., 2009; Roemmich et al., 2009). Physical transport processes in frontal regions and mesoscale eddies have been found to generate biogeochemical responses that are very different from the general background conditions (Baird et al., 2011; Mahadevan, 2014; Stramma et al., 2013). A key process in driving the generation of anomalies is the vertical flux of nutrients into the euphotic zone, which enhances primary productivity, a process that is of particular importance in usually oligotrophic environments (Falkowski et al., 1991; McGillicuddy et al., 2007). Besides the locally generated response, the westward propagation of mesoscale eddies introduce a horizontal (mainly zonal) relocation of eddy properties. Satellite data and model studies show that eddies do play an important role in the offshore transport of organic matter and nutrients from the eastern boundary upwelling system (EBUS) into the open ocean. Considering their transport alone, eddies have been found to create a negative impact on productivity in the EBUS regions because of their net nutrient export (Gruber et al., 2011; Nagai et al., 2015; Rossi et al., 2009).
The eastern tropical North Atlantic (ETNA) hosts an eastern boundary oxygen minimum zone (OMZ), which is primarily created from sluggish ventilation
(Luyten et al., 1983) and high productivity in the EBUS along the western
African coast. In its western part, the ETNA is bounded by the Cabo Verde frontal zone (CVFZ, Fig. 1), separating the OMZ regime from the wind-driven and well-ventilated North Atlantic subtropical gyre. In the south, towards the
Equator, oxygen is supplied via zonal current bands (Stramma et al., 2005;
Brandt et al., 2015). The vertical oxygen distribution shows two distinct
oxygen minima, an upper one at about 75 m depth and a deep OMZ core at about
400 m (Brandt et al., 2015; Karstensen et al., 2008; Stramma et al., 2008b).
On the large scale, the minimum oxygen concentrations in the ETNA OMZ are
just below 40
Based on satellite data analysis, a statistical assessment of mesoscale eddies has been done for the North Atlantic in general (Chelton et al., 2011), in particular the ETNA (Chaigneau et al., 2009; Schütte et al., 2016b). However, Schütte et al. (2016a, b) were the first to further differentiate anticyclonically rotating eddies into “normal” anticyclones and ACMEs, by combining satellite data (sea level anomalies, sea surface temperature) with in situ data (CTD (conductivity, temperature and depth), profiling floats, glider). They found that about two to three ACMEs are generated each year at distinct regions in the EBUS and then propagate into the open ETNA waters.
An intense biogeochemical response in ACMEs has been reported for other ocean
regions as well. For instance, McGillicuddy et al. (2007) reported intense
phytoplankton blooms in ACMEs for the western North Atlantic, near Bermuda.
They explained the phenomenon as the result of a vertical nutrient flux
driven by the interaction of the eddy with the overlying wind field. Altabet
et al. (2012) observed enhanced production of biogenic nitrogen (N
Here, we present biogeochemical insights into low-oxygen ACMEs in the ETNA based on direct in situ sampling during two coordinated ship-based surveys. The main objective of this study is to reveal and quantify biogeochemical processes occurring inside a low-oxygen ACME in the ETNA. This publication is part of a series that describes biological, chemical and physical oceanographic processes and their interaction inside these eddies. In this publication we first present the vertical hydrographic structure of a surveyed ACME and discuss nutrient concentrations and the marine carbonate system. All the data are put into regional context by comparing ACME conditions with (1) ambient background conditions represented by CVOO and (2) the biogeochemical setting in the proximal EBUS off the western African coast, where the eddy originated. Derived estimates for transformation rates of various key parameters and for carbon export rates within the surveyed ACME highly exceed known values for the ETNA and also other open-ocean regions.
Mesoscale eddies can be detected and tracked from space (Chelton et al., 2011; Schütte et al., 2016b). However, only a few of such eddies develop an oxygen-depleted core; therefore, surveying an oxygen-depleted mesoscale eddy in the ETNA (and elsewhere) is somewhat challenging. Schütte et al. (2016a) analyzed satellite and corresponding in situ data in the ETNA and found that on average about 20 % of all anticyclones (10 % of all eddies) are ACMEs, exhibiting a pronounced low-oxygen core. CEs also develop a low-oxygen core but not as low as ACMEs do.
In order to enable a targeted survey of the one particular ACME, the following strategy was designed (“Eddy Hunt” project; Körtzinger et al., 2016): we combined satellite data (sea level anomaly, SLA, and sea surface temperature, SST) with Argo float data in a near-real time mode. Although we did not have access to oxygen data in near-real time, we knew from earlier observations (Karstensen et al., 2015) that low-oxygen ACMEs have a low salinity core. As such, detecting an eddy with high SLA and low SST (Schütte et al., 2016b) and confirming low salinity at shallow depth from appropriate Argo float data, potential low-oxygen ACMEs were detected. An ACME with a low-oxygen core was discovered during a pre-survey using an autonomous underwater glider, initiating ship surveys.
Map of the study area between the Mauritanian coast and the Cabo
Verde archipelago. The ACME trajectory (dotted line) is based on satellite
sea level anomaly data and starts off the Mauritanian shelf edge in September 2013.
In March 2014, the ACME was surveyed twice north of Cabo Verde with two
different research vessels: RV
Here, we use ship data as well as data from a profiling float of a variety of biogeochemical parameters in order to investigate the marine carbonate system functioning in low-oxygen eddies. The following sections will provide a brief overview of samples collected during two ship cruises and the applied analytical methods. Moreover, the general setting of the CVOO ship time series, as well as data from hydrographic cruises and the profiling float will be introduced.
Dedicated eddy surveys were done during the RV
Along with CTD casts, an underwater vision profiler 5 (UVP; Picheral et al., 2010) was deployed during both cruises in order to quantify particle distribution in the water column (see results in Hauss et al., 2016). During both cruises, CTD casts down to 600 m were deployed, attempting to survey as close as possible to the eddy core (guided by the near-real time satellite SLA maps). CTD casts were also performed outside of the eddy to be able to investigate the horizontal contrast of the eddy to the surrounding waters. Based on the SLA data the “outside stations” during ISL and M105 were located 43 and 54 km away from the supposed eddy center, respectively. However, shipborne acoustic Doppler current profiler data (ADCP; see Hauss et al., 2016) as well as SLA data (Löscher et al., 2015) suggest a radius of this eddy of approx. 50–55 km. This points out that these stations were more at the rim of the eddy rather than in the surrounding water, therefore they may not represent typical background conditions. In order to compare the eddy observations to the typical background conditions, we used data collected during M105 at the CVOO time series station (see Sect. 2.2).
For comparison, we also used data from an Argo profiling float (WMO
no. 6900632) that got trapped in a low-oxygen cyclonic eddy (Karstensen et
al., 2015; Ohde et al., 2015). This float was equipped with an oxygen sensor
(AADI Aanderaa optode 3830) and a transmissometer (CRV5, WETLabs). The given
uncertainties of the float measurements were
Overview of detected concentration anomalies (
Based on satellite SLA data, the formation location of the target eddy is
reconstructed to be close to the shelf edge off Mauritania at approx.
18
Likewise, representative background conditions for the actual survey area
northwest of the Cabo Verde islands were estimated from data collected during
M105 at the near-by CVOO (17.58
All discrete seawater samples collected for this study were analyzed for
dissolved oxygen after Hansen (2007) with manual end-point determination.
Samples were stored dark after sampling and fixation and were analyzed within
12 h on board. Regular duplicate measurements were used to ensure high
precision of measurements (ISL: 0.27
Samples for nutrients were analyzed with autoanalyzer systems following the
general method by Hansen and Koroleff (2007). Nutrient samples during ISL and
M105 surveys were always taken as triplicates, stored at
Samples for dissolved inorganic carbon (DIC) and total alkalinity (TA) were
preserved and stored for later onshore analysis, following procedures
recommended by Dickson et al. (2007). Briefly, 500 mL borosilicate glass
bottles were filled air-bubble-free with seawater and then poisoned with
100
The transient tracers CFC-12 and SF6 were measured on-board M68/3 and M105 from 200 mL water samples using purge-and-trap, followed by a gas-chromatographic separation and detection technique slightly modified from Bullister and Wisegarver (2008).
Samples for dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) were collected into combusted (8 h, 500
Filtration of seawater (1 L of seawater
Karstensen et al. (2015) suggested that the low-oxygen cores of the eddies were created by an enhanced subsurface respiration due to high surface productivity. Subsequent sinking of particulate matter produced in the surface layer fuels this process. At the same time, an efficient isolation of the core from surrounding waters hinders oxygen ventilation. The high productivity is proposed to be driven by vertical nutrient flux at the rim of the eddy into the euphotic zone, a situation that resembles coastal upwelling regions. Therefore, we compare our results of the analysis of the eddy in spring 2014 (e.g., production and respiration of organic matter and related export fluxes) with observations from the Mauritanian shelf (refer to Sect. 2.2).
This reference data from the shelf was then used to determine the changes in biogeochemical parameters that occurred on the way from the formation to the survey area northwest of Cabo Verde. Again, the anomalies were determined along isopycnals and mapped back to depth. We assumed that the core of the eddy was not significantly affected by either horizontal or vertical mixing, due to such ACMEs being known to host highly isolated water bodies due to their physical structure (Karstensen et al., 2015). This assumption allows us to derive estimates for biogeochemical rates being independent of mixing processes.
Changes of oxygen and carbon due to remineralization of organic matter are
expressed as the apparent oxygen utilization rate (aOUR) and the carbon remineralization rate (CRR). In order to determine these rates, not only the
anomaly but also the age of the eddy, the time between formation on the
shelf and the time the eddy surveys took place needs to be known. The age
was determined from the SLA tracking algorithm, that was also used to
determine the area of origin (Schütte et al., 2016b;
Fig. 1). Biogeochemical rates were then estimated
along multiple isopycnal surfaces between the shelf and the eddy interior as
shown here for determination of CRRs:
Data from the Argo float trapped inside a CE in 2008 were processed as
described in Karstensen et al. (2015). Corresponding CRRs were derived from
aOURs by applying a Redfield stoichiometric ratio of O
In order to estimate the amount of carbon exported from the euphotic zone as
sinking particulate organic matter (POM), we used CRRs to derive the shape of the vertical export flux
curve for particulate organic carbon (POC). This approach assumes the absence
of major physical transport processes between the mixed layer and the ACME
core beneath, except for sinking particles of POM, which is generally being
described by the established Martin curve (Martin et al., 1987a):
The
The rates we derive from CRRs assume that the changes can exclusively be ascribed to the biogeochemical processes and no major transport processes (ventilation) play a role, as such reported rates in this study are to be seen as lower order estimates. However, from the comparison of the hydrographic properties in the eddy formation area and the survey area, this assumption is plausible for the core of the eddy (see detailed discussion in Sect. 3.1).
Based on SLA data analysis, the surveyed eddy was clearly identified for the
first time in November 2013 near the Mauritanian shelf edge at
17.65
Temperature–salinity (TS) diagram containing data from both eddy surveys (colored triangles and gray dots), the nearby CVOO station (large black dots) and accumulated CTD hydrocast data from multiple surveys on the shelf (small black dots). Branches of NACW and SACW water masses were labeled according to Schütte et al. (2016b).
Vertical profiles for all parameters measured from sensors mounted on CTD rosette systems. Data from the nearby CVOO station (blue) represent local background conditions, the gray area emphasizes the local anomaly against the background introduced by the ACME (yellow and red) and the green curve represents mean initial conditions of the ACME at the shelf (light green indicates standard deviation of the mean profile). Note that not all surveys were carried out with the same sensor package.
The temperature–salinity (TS) characteristics of the subsurface core of ACMEs
in the open ETNA (Karstensen et al., 2015; Schütte et al., 2016b) were
found to be nearly unchanged, compared to coastal regions. They resemble
South Atlantic Central Water (SACW), the dominating upper layer water mass in
the Mauritanian upwelling region, whereas the region around CVOO is actually
dominated by high salinity North Atlantic Central Waters (NACW; Pastor et
al., 2008). As expected for a low-oxygen eddy, the TS characteristic in the
2014 eddy core for the two surveys matched very well with the characteristic
found from the Mauritanian shelf reference stations (Fig. 2). This underlines
the isolation of the eddy against mixing processes with surrounding waters
during its westward propagation from the shelf into the open ocean. This
hypothesis is further corroborated by the calculation of mean water ages
(using the transit time distribution (TTD) method) derived from transient
tracer analysis (Sect. 2.3). Mean water age in the core of the eddy
(
Despite quasi-constant physical water mass properties over the course of the
eddy's lifetime, changes in biogeochemical variables are observed. In
comparison to the reference profile from the Mauritanian shelf, we find a
maximum oxygen decrease in the eddy core at a depth of 100 m (
Discrete bottle data for nutrients from the different ACME surveys. The gray shading illustrates the anomaly of the ACME (ISL) with respect to the regional background situation (CVOO).
We observe elevated nutrient concentrations (nitrate, phosphate, silicate)
inside the ACME core, which indicate the remineralization of organic matter
(Fig. 4). Nutrient data obtained during the ISL survey showed also elevated
concentrations for nitrate (2.92
In accordance with the oxygen decrease already discussed, a clear respiration
signal was also found in carbon parameters (Fig. 5). Values for DIC (max.
2258.8
Discrete bottle data for DIC and TA and calculated parameters of the
carbonate system (pH,
Vertical distribution of particulate and dissolved organic matter
(first four panels) based on discrete samples and particle density (60–530
The horizontal gradient of pH between inside and outside eddy conditions is
up to 0.3 pH units at a water depth of approx. 100 m. It is interesting to
note that a pH of 7.63 is close to values expected for future surface ocean
conditions in the year 2100 (approx. pH of 7.8) as predicted by models
assuming a global high CO
Above the core, DIC concentrations in the surface mixed layer vary between the two eddy surveys and CVOO. Slightly higher values were found during the ISL survey when compared to the M105 survey. The same was found for nutrient concentrations (Sect. 3.2), which consistently points towards a very recent or even ongoing upwelling event encountered during the ISL sampling. Episodic upwelling within ACMEs have been reported for other regions in the past (McGillicuddy et al., 2007). Below the eddy core at a depth of approx. 250 m, the DIC anomaly disappears and parameters fall back close to shelf background conditions (Fig. 5).
A slightly different picture is found in profile data for TA. Here, only a
small change of up to 17
Estimated biogeochemical rates within the ACME as derived along isopycnals between the shelf (green) and the ACME at the time of the two surveys (red, yellow). This approach is illustrated for oxygen and DIC profile data (large panels). Corresponding aOUR and CRR are peaking in the core of the ACME (small panels). Note that the matching between shelf and ACME data was made in density space, whereas the resulting rates are plotted in depth space.
We used data from the UVP to illustrate vertical distribution of small
particles (60–530
Discrete bottle samples for organic carbon (POC, DOC), particulate organic nitrogen (PON) and DON
were collected during the M105 survey only (Fig. 6). Both POC and DOC
concentrations are elevated inside the eddy compared to concentrations found
at CVOO. In particular, POC shows a major peak in the surface mixed layer
that exceeds not only concentrations at CVOO, but also all other POC
concentrations measured during the M105 cruise (including data between Cabo
Verde and 7
Based on the differences between the observed concentrations in the eddy and the reference profiles in the Mauritanian upwelling region, the oxygen and DIC changes with respective rates (Sect. 2.4) were estimated (Fig. 7). As outlined before, the data were compared in density space in order to consider the large-scale differences in the depth–density relation that primarily reflects the difference in ocean dynamics (Fig. 7, larger panels). As outlined in Sect. 2.4, the corresponding rates, presented here against depth (Fig. 7, smaller panel), were then calculated based on the estimated lifetime of the eddy (derived from satellite data). Thus, examined rates represent mean rates over the lifetime of the eddy and do not contain any information about their temporal evolution.
The data show clear anomalies for all parameters within the eddy core, which were most pronounced at a depth of 98 m (M105) and 105 m (ISL). Rates for all parameters are presented in Table 1. Below the eddy core, however, rates are vanishing and become indistinguishable from the uncertainty introduced by the applied isopycnal approach. For instance, the assumption of a well-isolated water body holds true for the core of the eddy only, but not necessarily for deeper parts of the eddy. Here, admixture of ambient waters becomes more likely in agreement with the TS characteristic approaching the background signature (Fig. 2), which significantly alters water mass properties of this part of the eddy. As a consequence of the non-isolation of the water underneath the core (below approx. 250 m), rates cannot be derived using this approach and are not further discussed. Similarly, rates can also not be derived for the surface mixed layer where multiple processes modify the parameter field (gas, heat and freshwater exchange).
Comparison of
The aOUR within the eddy peaks at
0.26
Rate estimates for other biogeochemical parameters within the investigated ACME are also exceptionally high (Table 1). We compared estimated rates with each other by looking at stoichiometric ratios such as C : N, N : P and O : C (data not shown). In fact, all ratios were found to be close to, or not distinguishable from, the stoichiometry proposed by Redfield et al. (1963). This finding provides indication for a reliable assessment of biogeochemical rates, based on the assumptions that were made and on independent samples of multiple parameters taken during two independent cruises.
The observed DIC increase rate within the eddy core can be referred to as the CRR resulting from continued respiration of organic matter. As illustrated in Fig. 5, the peak in DIC coincides with the depth of the sharpest decrease of POM and DOM. This is to be expected, as the CRR should equal the derivative of the vertical POC flux curve with respect to the depth. Following the approach of Jenkins (1982), one can derive the vertical flux of POC from aOUR or CRR values, respectively. Downward fluxes for POC can be seen as the major export process of carbon out of the euphotic zone.
Derived downward POC fluxes based on a model after (Martin et al.,
1987b) for the two ACME surveys (blue and red), a cyclonic eddy sampled by an
Argo float (CE, dashed line; Karstensen et al., 2015) and the general ETNA
(Karstensen et al., 2008). Flux estimates for the two ACME surveys are based
on CRRs estimated from DIC sample data. For the CE, aOURs derived from oxygen
measurements on an Argo float were converted to CRRs by applying a
stoichiometric O
We used these CRRs within the eddy core for determination of the vertical POC
flux at different depths by means of a power-law function (Martin et al.,
1987b). Vertical integration of the data between 100 and 1000 m yielded
estimates of the vertical POC flux at 100 m during the ISL and M105 cruises
of 0.19 (
POC fluxes derived here generally show higher values than those found in other open-ocean studies but are comparable to values associated with a North Atlantic spring bloom event (Berelson, 2001). Moreover, POC fluxes for this ACME were also in line with estimates made for other eddies, such as enhanced POC fluxes determined at the rim of a CE in the Western Pacific (Shih et al., 2015) or inside a CE in the ETNA (Fig. 8, derived from aOUR data in Karstensen et al., 2015). In general, estimated POC fluxes for the surveyed ACME based on the method described in Sect. 2.5 may represent a rather conservative estimate as the aOUR was derived based on the assumption of complete absence of vertical and horizontal ventilation processes. Thus, any minor ventilation process affecting the eddy core would cause our aOURs and POC flux estimates to be biased low.
The corresponding
We performed two biogeochemical surveys within an ACME in the open ETNA off
western Africa near the CVOO time-series site. The core of this mesoscale eddy
was found to host an extreme biogeochemical environment just beneath the
surface mixed layer. The concentration of oxygen had dropped to suboxic
levels (
We also investigated magnitudes of biogeochemical processes occurring within the eddy during its westward propagation, such as apparent oxygen utilization and carbon remineralization, by comparing our survey data with conditions prevailing during the ACME's initial state (Mauritanian shelf). Results showed mean aOURs over the lifetime of the ACME that exceed typical rates in the open-ocean ETNA by an order of magnitude (Karstensen et al., 2008). Resulting POC fluxes inside the ACME were also found to exceed background fluxes in the oligotrophic ETNA by a factor of 2–3, therefore comparable to meso- and eutrophic regions such as the Mauritanian upwelling region or the subpolar North Atlantic spring bloom. This finding is also in line with a 3-fold enhanced primary productivity in the same ACME's surface layer derived from Löscher et al. (2015) based on seawater incubations. Our results confirm that ACMEs in the ETNA can be seen as open-ocean outposts that clearly exhibit their origin in the EBUS but through their continued biogeochemical activity at the same time represent alien biogeochemical environments in a tropical ocean setting. As revealed by Schütte et al. (2016a), these ACMEs appear to play a small but significant role in maintaining the shallow OMZ in the ETNA.
The results of this study, however, are based on two independent surveys carried out at a certain point of time in the lifetime of the ACME. We are not able to address questions about the evolution and (non-) linearity of processes within the ACME throughout its lifetime. Therefore, future surveys should resolve not only spatial structure but also temporal evolution of biogeochemical processes at different life stages of these eddies.
In addition to this biogeochemical investigation, two other studies have
documented the impacts of this low-oxygen ACME on zooplankton and microbial
communities (Hauss et al., 2016; Löscher et al., 2015). There is
empirical indication that future scenarios such as deoxygenation and ocean
acidification can also affect higher trophic species (Munday et al., 2010;
Stramma et al., 2012). Any possible influence of this ACME on higher trophic
levels, however, remains unknown and would require a different observational
approach. The discovered anomalies within this eddy can be seen as a large
(50–100 km diameter) and relatively long-lived (
All data used for this publication are publicly available and can be accessed
at the Pangaea data repository (
The authors would like to thank Meteor M105 chief scientists M. Visbeck and
T. Tanhua for their spontaneous support of the “Eddy Hunt” project, as well
as H. Bange and S. Sommer for providing hydrographic data for the Mauritanian
shelf area. Conducting field work at Cabo Verde would not have been possible
without the tremendous support and engagement of the CVOO team at INDP
(Ivanice Monteiro, Nuno Vieira and Carlos Santos) as well as S. Christiansen
and T. Hahn. For DIC, TA, nutrient and DOC/TDN sample analysis we thank
S. Fessler, M. Lohmann and J. Roa. Processing of CTD data were performed by G.
Krahmann and S. Milinski and proofreading of the manuscript was kindly
provided by A. Canning. We also appreciate professional support from captains
and crews of RV
This project was funded by the Cluster of Excellence 80 “The Future Ocean” (grant no. CP1341, “Eddy Hunt”). The “Future Ocean” is funded within the framework of the Excellence Initiative by the Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. Further funding was provided by the BMBF project SOPRAN (grant no. 03F0662A), the DFG Collaborative Research Centre 754 and the European Commission for FP6 and FP7 projects CARBOOCEAN (264879) and CARBOCHANGE (264879). Edited by: L. Cotrim da Cunha Reviewed by: two anonymous referees