Introduction
The concentration of dissolved oxygen (DO) in seawater is of critical
importance to almost all marine life and oceanic biogeochemical cycling
.
Local DO concentrations are the result of a delicate balance between oxygen
supply and consumption, and eventually regions of extremely low DO content are
created: at the microscale at particle boundaries
, at the mesoscale as coastal dead zones
or at the large scale as eastern boundary oxygen
minimum zones (OMZs) .
Quantifying and fully understanding processes that control the DO supply and
consumption balance, and any possible alterations over time, remain
challenges in current research.
Critical DO concentration thresholds which lead to major reorganizations of
the marine ecosystems have been identified
. For higher
trophic levels, such as those of fish, the impact of a certain DO
level on metabolism, and as such fitness, is species-dependent
. Nevertheless, for DO below
20 µmolkg-1 (“severe hypoxia”), mass mortality of fish has
been reported . At severe hypoxia DO levels, microbes
begin to convert nitrite and ammonium to nitrogen gas, thus removing fixed
nitrogen from the water, which in turn limits primary productivity
. A next distinct DO threshold is
for concentrations below about 5 µmolkg-1, where microbes
begin to utilize nitrate (and other nitrogen species) as terminal electron
acceptors in anaerobic respiration (“denitrification”)
. Finally, when DO reach
concentrations around 1 µmolkg-1 (“anoxia”), only
specifically adapted microbes can exist .
The pelagic zones of the eastern tropical North Atlantic OMZ are considered
to be “hypoxic”, with minimal DO of marginally below
40 µmolkg-1 . As
such it is assumed that the DO levels pose some limitation in biodiversity on
the regional ecosystem, primarily through avoidance and possibly increased
mortality . The region is thus very much in contrast
to the major OMZs in the eastern North and South Pacific Ocean and the
northern Indian Ocean, where DO concentrations pass all DO thresholds
outlined above, and as such specifically adapted ecosystems must exist.
Here the discovery of extremely low DO in mesoscale eddies in the eastern
tropical North Atlantic is documented. The DO values fall in the range of
severe hypoxia and even anoxia. They extend over horizontal scales of about
100 km and vertical scales of about 100 m.
Overview map of the eastern tropical North Atlantic oxygen
minimum zone. The tracks of the two anticyclonic-modewater eddies
(CVOO2007, grey line; CVOO2010, black line) observed at CVOO
(plus sign; 17∘35.39′ N,
24∘15.12′ W) as well as the track of the
cyclonic eddy (Argo2008, red line), surveyed with an Argo float are
shown. The position (dots) and dates (label) of the first and last
identification of the three eddies are given. The RV METEOR
survey of the CVOO2007 (grey star) is labelled accordingly. For
reference, the average footprint (circle at CVOO) is
given. Positions of the Argo float profiles surveyed inside (white
circles) as well as outside the cyclonic eddy radius (crosses) are
shown.
Data and methods
Moored sensors
One set of DO time series we discuss below was acquired at the Cape Verde
Ocean Observatory (CVOO) mooring. CVOO is located in the eastern tropical
North Atlantic, about 100 km northeast of the São Vicente
(17∘35′ N, 24∘15′ W), Cabo Verde, and approximately
800 km from the Mauritanian coast (Fig. ). Since 2006 the
observatory has been equipped with oxygen sensors: first only one sensor was installed at about
140 m depth, but since the beginning of 2008 at least two
sensors have been installed, one of which measuring at depths shallower
than 60 m.
The oxygen measurements at the CVOO mooring were done with AADI Aanderaa
oxygen optode (type 3830) sensors. For the first two deployments (period from July
2006 to October 2009) we followed the recommendation of the manufacturer and
performed a calibration against zero oxygen concentration, by submerging the
optodes in a sodium sulfite solution, and against saturated waters. For
the following periods a more advanced technique was used, based on a number
of calibration points at different temperatures and oxygen concentrations
. In brief, one set of calibration values was obtained
from a comparison of oxygen data from an optode attached to a CTD rosette
and the accompanying CTD oxygen sensor (Sea-Bird Electronics 43 Clark
electrode) calibrated itself using the Winkler titration method. This
comparison was done by keeping the CTD over several minutes at a certain
depth where a weak vertical oxygen gradient was seen. This procedure was done
before and after the deployment of the respective optodes. In this way we
obtained >15 independent calibration points for each optode. In addition, a
lab calibration at zero oxygen was done. All calibration points were used to
derive a final calibration equation for one deployment of one certain optode.
The chemically forced (and thus more precise) zero oxygen calibration was
weighted 3 times higher than the CTD/oxygen cast reference values. The
difference between calibration point observations and calibrated optode
suggests an overall RMSE of 3 µmolkg-1. Comparison of
the chemically forced zero oxygen phase data and the phase readings at low-oxygen concentrations suggests a higher accuracy at low DO concentrations of
about 1 µmolkg-1. Pressure and salinity variability was
corrected according to the AADI manual.
Argo float data
A profiling float was launched in the tropical North Atlantic region to
document the seasonal variability in upper layer DO and particle load. By
accident it was entrained into the low-oxygen eddy. The float was a PROVOR
profiling Argo float (WMO 6900632; Martec Inc., France) equipped with
a standard CTD (SBE 41CP), an oxygen optode (AADI Aanderaa optode 3830) and
a transmissometer (CRV5, WETLabs). The float was programmed to conduct a vertical profile every
5th day between 400 dbar (nominal drift depth) and the
surface with a vertical resolution of 5 dbar throughout the profile. The DO
concentrations obtained from the float were corrected for salinity effects
(using the float CTD salinity) and a pressure correction was applied to the
data , increasing the oxygen linearly by 4 % per
100 bar.
Moreover, we had one in situ calibration CTD calibration cast available,
recorded a few hours after deployment of the float. The CTD oxygen sensor
(Sea-Bird Electronics 43 Clark electrode) was again calibrated using the
Winkler titration method. This procedure resulted in a post-offset correction
of float-based DO measurements. The transmissometer data were not further
calibrated and are reported here in units of m-1 (beam attenuation
coefficient) based on the factory calibration. The sensor data are presumably
impacted by bio-fouling within the optical path, and this was accounted for by
subtracting the minimum deep water beam attenuation value from each profile.
Satellite data and eddy tracking
The delayed time reference product of merged sea-level anomaly (SLA) data
(version 2010) provided by AVISO (Archiving, Validation, and Interpretation
of Satellite Oceanographic) was used for tracking of the three eddies under
discussion. The SSALTO/DUACS project constructs a merged satellite product
projected on a 1/3∘ horizontal resolution Mercator grid every 7
days e.g.and references therein.
Initially we tracked the three eddies under discussion visually by
inspecting individual SLA maps. This was possible as we knew the exact time and location of the appearance
of low-DO eddies from the in situ observations (mooring, float). By looking up subsequent SLA maps, the displacement of an
identified SLA that was associated with the three eddies was
charted and eddy tracks were constructed for the period before and after the
in situ observation (Fig. ).
However, in addition we used an automatic detecting and tracking algorithm,
based on the Okubo–Weiß method . The method is
robust and widely used to detect mesoscale eddies in satellite data as well
as numerical model output .
In brief, the method is based on quantifying the contribution of relative
vorticity on the strain tensor, and an eddy is defined as a region of
negative W (vorticity dominates over strain) surrounded by a region of
positive W (strain dominates over vorticity). A threshold
W0 has to be chosen, and we used W0=-2×10-12 s-2 for our eddy
detection limit. Tracking was done by following the centre of individual
W0 areas in SLA maps from 1 (maximum 10 km) to 3 weeks (maximum
distance 60 km). The automatic detection reproduced well the tracking that
was obtained using the visual inspection method.
Time series of DO from the CVOO site at 40 to 60 m
depth (black line) and at 140 m (grey
line) during the beginning of the time series. The passage of the two anticyclonic-modewater eddies in
February 2007 (CVOO2007) and February 2010 (CVOO2010) is labelled
accordingly. The theoretical oxygen surface saturation (red line) is shown,
as well as the 40 µmolkg-1 threshold reported in
the literature. The period from 15 January to 15 March for each year
is indicated by grey-shaded area.
(a) Time series of DO from the two sensors available
at nominal 42 m (black line) and 170 m depth (blue
line). For reference, the oxygen surface saturation (grey line) and
the 40 µmolkg-1 threshold (black broken line) are shown.
Corresponding time series of (b) meridional flow
(m s-1), (c)
salinity (in PSS-78), and (d) potential temperature
(∘C) in the upper 350 m as observed during the
CVOO2010 passage are shown. The black line in (b) indicates
zero meridional velocity, and the grey lines in (c) and (d)
indicate the
varying depth of the oxygen sensors shown in (a) during instalment. Selected
potential density anomaly surfaces are shown as white contours in
(b), (c) and (d) for reference, and the time series data
were converted into distance assuming an eddy translation speed of
5 km day-1.
Results and discussion
Open ocean low-oxygen events from moored observation
Anomalously low DO, in reference to the expected lower limit for the tropical
North Atlantic of about 40 µmolkg-1 ,
was first identified in the DO time series available from the CVOO mooring
(Fig. ). At the CVOO mooring the typical DO concentrations in the
upper 60 m are close to the oxygen saturation value >200 µmolkg-1;, with variability of about
50 µmolkg-1 over periods of a few days or so. However,
exceptionally low DO events were observed during boreal winters of 2007, 2010,
2011 and 2012. In the following we concentrate on the two most extreme low-DO events in 2007 and 2010.
The most intense low-DO event was recorded in February 2010 (CVOO2010,
Fig. a) at the mooring site and persisted over a period of about
1 month. During that period, DO concentrations at shallow depth (42 m)
were <2 µmolkg-1 and thus close to the DO threshold
for anoxia. A second sensor, installed deeper, at a nominal 170 m, also showed
a drastic DO decrease from the typical 100 µmolkg-1 to less
than 30 µmolkg-1 during the event. Inspecting the
hydrography and currents, recorded with multiple other moored instruments, we
observed that the low-DO event was accompanied by the appearance of a lens of
cold and less saline water (Fig. c, d) and a strong and reversing
meridional flow (Fig. b). The flow reversal (from a northward flow
to a southward flow) indicates the passage of an anticyclonic eddy across
the mooring.
Time series of (a) salinity and (b) oxygen
from profiling float data. The two grey boxes indicate the
period when the float was trapped in the cyclonic eddy; these represent the isolated period (dashed box, left) and the non-isolated period (solid
box, right). Potential density anomaly contours are shown as contour
lines. (c) Vertical profile of the aOUR derived from
successive dives during the period when the eddy was isolated. The thick broken line shows the background aOUR .
Further inspection of the temporal evolution of isopycnals (surfaces of
constant water density) during the eddy passage indicated that a special type
of anticyclonic eddy, a so-called anticyclonic-modewater or intrathermocline
eddy , crossed the mooring.
Anticyclonic-modewater eddies can be identified from downward/upward-bent
isopycnals towards the eddy centre below/above a subsurface swirl velocity
maximum. The transition between upward- and downward-bent isopycnals forms a lens
(or mode) of a specific water mass which can be at all water depths. Prominent
examples of intrathermocline eddies are so-called “meddies”, which
propagate at depths between 500 and 1500 m and have been formed from
instabilities of the Mediterranean outflow after entering the North Atlantic
through the Strait of Gibraltar . In our observations the
mode is at much shallower depth, centred at about 70 m, and had a height of
about 50 m or so. It contained the most extreme low DO concentrations. Below
this mode, the eddy had a structure of a typical anticyclone and reached
deeper than 1400 m (not shown). Along with the passage of the CVOO2010 eddy,
the surface mixed layer shoaled from a thickness of about 50–60 m before
(and after) the eddy passage to less than 20 m during the eddy passage.
Another extreme event in the DO record from the CVOO mooring time series
(Fig. ) is seen in February 2007 (CVOO2007), almost exactly
3 years before the 2010 event. Again the low-DO event was accompanied
by a flow reversal and hydrographic anomalies as seen during CVOO2010; as
such it was associated with the passage of an anticyclonic-modewater eddy.
At that time only a single oxygen sensor was installed at the CVOO mooring at
120 m depth (nominal), and the lowest DO concentrations were about
15 µmolkg-1, indicating severe hypoxia
(20 µmolkg-1) conditions at the given depth.
Open ocean low-oxygen events from Argo float data
One further severe hypoxia event was detected within a cyclonic eddy that
was surveyed with an Argo type float (Fig. a and b). The float was
operating from mid-February 2008 until the end of May 2009. Launched in the
Mauritanian upwelling region, the float remained in the coastal area until
the end of May 2008, when it began to move in a west-northwest direction into
open waters (Fig. ). Overlayed on the west-northwest movement, the
float trajectory revealed “loops”, which indicate rotational movement of a
drifter, and the direction of rotation indicated a movement in a cyclonic
eddy.
With the westward propagation, into the open ocean waters, a decrease in DO
at all depth levels below the mixed layer is observed. The decrease in DO
lasted until mid-December 2008, and lowest DO concentrations (about
14 µmolkg-1) were always found close to the mixed-layer
base, which, however, successively deepened. After December 2008 the DO rather
abruptly increased again (Fig. b), accompanied by drastic changes in
temperature (not shown here) and salinity (Fig. a). However, from the eddy trajectory analysis presented below, it
turned out that the float was still inside the eddy at the time of the abruptly changing interior structure.
Propagation of oxygen anomalies
The in situ data (CVOO2007, Argo2008, CVOO2010) provided us time periods and
positions of low-DO events. Therefore it was simple to identify associated
mesoscale eddies in SLA maps. The SLA maps revealed what was already seen
from the hydrography – the two eddies observed at CVOO were anticyclonic
eddies (the modewater character cannot be identified from the SLA data), and
the Argo float surveyed a cyclonic eddy. Considering the along-path
characteristics from concurrent SLA maps, all three eddies had roughly
similar diameters (about 130 km) and propagated westward, with a speed of
about 4.5 km day-1. As such they can be categorized as
“typical” for this latitude range .
All three eddies had a similar region of origin at about
18∘ N, 16.5∘ W. The cyclone was formed in May 2008 (the
float entered the eddy core about 1 month later), and the two
anticyclonic-modewater eddies in July 2006 and 2009, respectively.
The SLA across the eddy radius was rather weak, with an amplitude of only
1.5 (±1.5) cm (negative for the cyclone, positive for the
anticyclones). Such a SLA translates into maximum geostrophic surface
currents of about 0.05–0.10 m s-1, which is slow when compared
with global eddy characteristics .
However, this is not too much of a surprise, as we knew, at least for the
CVOO2007 and CVOO2010 eddies, from the in situ velocity data that the maximum
velocity was at the subsurface, at about 70 m depth, and velocity rapidly
decreased towards the surface (Fig. b). Thus the maximum in
SLA-derived surface geostrophic flow is only 10–20 % of the interior
maximal swirl velocity directly observed with an acoustic Doppler current
profiler (ADCP). We also used the density field derived from moored sensors
and calculated a geostrophic velocity under the assumption that there is a
layer with no motion at 1400 m. This approach resampled the velocity
structure fairly well and in particular the subsurface swirl velocity maximum
at about 70 m depth.
Surface chlorophyll concentration of the CVOO2010 anticyclone
at two life stages approximately 2 months (a) and 1
month (b) before the centre of the anticyclone crossed the
CVOO mooring. The SLA-derived track of the anticyclone centre
(see Fig. ) and the approximate diameter (130 km) are
shown for reference. The white plus sign marks the
CVOO position.
Respiration in isolated eddies
Various physical and biogeochemical processes have been identified as
possible drivers of the ecosystem responses in mesoscale eddies. In
particular, intense phytoplankton blooms have been reported for cyclonic and
anticyclonic-modewater eddies
. Phytoplankton blooms are likely important for creating a low-DO
zone, because subsequent sinking of detritus is accompanied by oxygen
consumption. Satellite-derived surface chlorophyll images would help to at
least identify strong near-surface bloom events in the three eddies, but
cloud-free periods are rare in the region. However, the few available images
with sufficient coverage (Fig. ) show high chlorophyll fluorescence signals related to the
eddies and suggest phytoplankton blooms to occur.
From the difference in DO concentrations between concurrent Argo float
profiles we were able to estimate a depth-dependent respiration rate
(Fig. c) or apparent oxygen utilization rate (aOUR) profile. Only
profiles where DO decreased (May to mid-December 2008) were considered. In
order to take the successive deepening of isopycnals over time into account
(e.g. Fig. a, contours), the aOUR was calculated in density classes
and subsequently projected back to depth, using the mean vertical density
profile.
The aOUR is highest, more than 0.15 µmolkg-1day-1, just
below the mixed layer and levels out to about
0.05 µmolkg-1day-1 between 120 m and the maximum depth
the float surveyed (400 m). The rates are 3 to 5 times higher than typical
rates for the thermocline .
Moreover, the rates must be seen as a lower bound of the real respiration
inside of the eddy, as we assume no supply of DO by vertical mixing or from
outside the eddy. Nevertheless, remarkable constant hydrography of the eddy
core over time (Fig. b for temperature) suggests that lateral
exchange across the eddy rim with surrounding waters is small. A connection
between sinking particles and oxygen respiration is also seen in the
transmissometer data (not shown). The transmissometer signal is at maximum
just at the base of the mixed layer, while minimal DO is observed about 5 m
below that particle maximum, indicating that the net oxygen respiration is
related to sinking particles.
For the anticyclonic-modewater eddies a net DO respiration can only be
derived for the CVOO2007 eddy and for one depth only. This is because the
eddy was surveyed only twice during its lifetime: once by RV METEOR off Mauritania (Fig. ), and 7 months later
from the moored sensors at the CVOO mooring. Between these two surveys the DO
concentrations at the 120 m depth (only depth with DO instrument at
CVOO2007) changed by more than 50 µmolkg-1, which translate
into an aOUR of 0.25 µmolkg-1day-1. This is an even
higher aOUR when compared with the aOUR profile derived from the cyclonic
eddy at a corresponding depth, which might be related to a higher
productivity (and subsequently oxygen drawdown through sinking particles)
reported for anticyclonic-modewater eddies in the past
. Moreover, comparison of the ship data from July
2006 and the mooring data from February 2007 (CVOO2007) reveals that the core
of the eddy remained rather unchanged in temperature and salinity over
a period of 7 months (Fig. ), as well as after propagating more than
650 km westward.
A key process in the context of productivity is the vertical transport of
nutrients into the euphotic zone. Different processes, operating on the
sub-mesoscale, have been identified as being responsible for intense vertical
velocities within eddies. However, the exact details are a topic that has
been under debate for more than a decade seefor further references. Also, the trapping of
surface waters by eddies should play a role . The data
at hand do not allow for conclusions to be made on nutrient pathways within eddies, nor can
we estimate productivity. However, a bulk estimate for the vertical velocity
across the eddies can be done, making use of an approach based on wind stress
variations generated by wind/surface current shear
. In brief, on one
side of the eddy, where the wind blows against the eddy rotation, the wind
stress is elevated while the contrary happens on the opposite side. The
resulting wind stress curl drives an Ekman flux divergence, which in turn is
compensated for by an upwelling in the case of anticyclonic surface eddy rotation
. Using typical wind (10 m s-1)
and current speed (0.5 m s-1) across an eddy with a diameter of
130 km (as observed for the CVOO2010 and CVOO2007 eddy), we estimate an
upwelling of about 9 m month-1, corresponding to 65 m over the 7
months – the time it takes the eddies to propagate from the formation region,
off West Africa, to the CVOO site. However, controversy exists regarding the
validity of this concept .
Vertical distribution of (a) oxygen
(µmolkg-1) and (b) meridional velocity
(m s-1) surveyed with RV METEOR (cruise M68/3)
on the 18 July 2006, at 18∘ N and from 17 to
19∘ W (see Fig. 1 for position). (c) Hydrographic
characteristics of the eddy core as observed with RV METEOR
(red lines) and as observed 7 months later during the CVOO2007
passage (blue dots). For reference typical background conditions at
CVOO are shown (magenta dots and stars).
Besides productivity, and the related sinking of detritus, the “isolation”
of the eddy core from surrounding waters will also contribute to an increased
net respiration. A clear indication of minimal exchange was seen in the
constancy of the hydrographic structure of the eddy core, comparing
properties of the eddy during the RV METEOR survey and from CVOO2007
(Fig. ), as well as during the Argo2008 eddy survey. However,
further support comes from dynamical considerations. A proxy for the
coherence of an eddy, which also indicates the isolation of the eddy core, is
the ratio (α) of swirl velocity to translation speed
e.g.. For the anticyclonic-modewater
eddies (CVOO2007, CVOO2010) we have direct velocity observations, and just below the mixed-layer base they
show a maximum α>9 and a clear
indication of the coherence of this part of these anticyclonic-modewater
eddies.
For the Argo2008 survey of the cyclonic eddy, no direct swirl velocity
observation exists, and as such α cannot be calculated. However, we
used float profiles recorded before and after the float entered
(May/June 2008) and left (March/April 2009) the eddy, and observed
a fundamental change in the velocity shear profile – from a rotation with
nearly constant velocity from just below the mixed layer (30 m) to 400 m
depth (maximum observation depth) at the beginning of the survey to a profile
with a distinct peak in swirl velocity at about 110 m depth at the end of
the float survey. Such a change in the flow structure indicates that the
maximum α moved to deeper levels. We can only speculate that this
vertical movement of maximum α and associated local decrease in
α allowed surrounding waters to enter the eddy core and ended the
isolation (Fig. ).
Conclusions
Dead zones are observed in the open tropical North Atlantic at shallow depth,
just below the mixed layer. The dead zones are generated in either cyclonic or anticyclonic-modewater
eddies. Tracking of the eddies reveals them to be generated off the
northwestern African coast. They propagate westward,
with a speed of about 4.5 kmday-1 into the open ocean. We find,
from direct observations of respiration within one cyclonic eddy, a 3- to
5-fold elevated aOUR profile when compared with the typical rates reported
for the thermocline waters. High respiration rates were also found in
anticyclonic-modewater eddies, but from measurements at one single depth
only. From the few observations available, it seems that
anticyclonic-modewater eddies may create more intense dead zones (DO close to
zero) when compared with those in cyclonic eddies. This is possibly related
to higher productivity in connection with the eddy–wind interaction or other
mechanisms
.
Moreover, the mixed-layer depth in anticyclonic-modewater eddies is very
shallow, only a few tens of metres; as such nutrients from below will be
lifted far up into the euphotic zone.
There is clearly a local impact of dead zone eddies on the ecosystem. During
the passage of the anticyclonic-modewater eddies at the CVOO, we observed, in
the target strength data from moored ADCPs, that acoustic backscatterers, such
as zooplankters, stopped their diurnal migration cycle (Fig. 7). Such
absence of vertical migration is indicative of zooplankters in the major
OMZ regions . While in the open ocean mobile organisms
may escape from the dead zone, other organisms, such as the wide range of
prokaryotes, may need to adapt to the environment in order to survive. In
that sense the dead zone eddies can be seen as gigantic natural laboratories
where an extreme environment is created in a relatively short period of time (a
few months). These features may open new ways in investigating the adaptation
techniques of organisms to survive low-DO environments.
Time series of (a) oxygen at nominal 42 m depth and
(b) relative target strength between 65 and 70 m depth against hours of the day (in dB).
Target strength was calculated from the 300 kHz acoustic Doppler current
profiler data at CVOO. Minimal target strength
during all hours of the day is seen during the passage of the
low-DO anticyclonic-modewater eddy between 8 and 25 February 2010.
In principle, open ocean dead zones in cyclonic and anticyclonic-modewater
eddies could be created in all oceanic regions. Sufficient productivity, and
particle sinking and remineralization, as well as non-linearity (and thus
isolation) of the eddies, must be ensured for long enough periods of time.
One other important control parameter is presumably the initial DO
concentration. At the West African coast, where we report here the eddies are created, DO concentrations are around 40 to 70 µmolkg-1
in the depth level that will later be occupied by low-DO waters. However, in
the Pacific or the Indian Ocean, coastal DO concentrations are lower and
extremes in other biogeochemical parameters may be generated. Here, anomalous
nitrogen isotope compositions or anomalous
phytoplankton distributions have been reported to
exist in anticyclonic-modewater eddies in the past.
In order to detect dead zone eddies from space, via SLA data, concurrent
in situ observations of the vertical structure of the water column are
required. A combination of Argo float data and SLA data is a promising
technique that has been already applied regionally southeastern
Pacific; and globally but without a
focus on detecting anticyclonic-modewater eddies or water mass anomalies in
general. We did a preliminary analysis for the North Atlantic OMZ region,
using SLA data and Argo float data, that revealed about 10% of the
anticyclones are anticyclonic-modewater eddies (Florian Schütte, personal
communication). However, information about the oxygen distribution
would still be required to quantify the impact of the dead zone eddies on the
large-scale oxygen budget.
Eddies were observed less than 100 km north of the Cabo Verde islands; thus a possible interaction of a dead zone eddy with an island must be
considered. Given the shallow depth of a few tens of metres where lowest DO
concentrations are found, a sudden flooding of a coastal areas with low-DO
waters may occur. A dramatic impact on the local ecosystems and sudden fish
or crustacean death may be the consequence. In retrospect, such
eddy–island interactions may explain events that have been reported in the
past (O. Melicio, personal communication, National Fisheries Institute INDP,
Mindelo, São Vicente, Cabo Verde).
One may wonder why dead zone eddies have not been discovered before. Besides
possible undersampling issues for the tropical eastern North Atlantic in the
past, it is likely that such low DO concentrations were
disregarded as “outliers” in the data sets. In fact, we first interpreted the low
DO at CVOO2007 as an outlier related to an instrumental error, and only
the more recent events recorded with the double-sensor package at CVOO2010, combined with sophisticated optode calibration procedures, gave us
confidence that our observations were real.