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Abstract and Figures

Using a combination of in situ observations and idealistic 2D non-hydrostatic numerical simulations, the relation between Cold-Water Coral (CWC) mound size and hydrodynamics is explored for the Rockall Bank area in the North-Atlantic Ocean. It is shown that currents generated by topographically-trapped tidal waves in this area cause large isopycnal depressions resulting from an internal hydraulic control above CWC mounds. The oxygen concentration distribution is used as a tracer to visualize the flow behavior and the turbulent mixing above the mounds. By comparing two CWC mounds of different size and located close to each other, it is shown that the resulting mixing is highly dependent on the size of the mound. The effects of the hydraulic control for mixing, nutrient availability and ecosystem functioning are also discussed.
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Geophysical Research Letters
On the influence of cold-water coral mound size
on flow hydrodynamics, and vice versa
Frédéric Cyr1, Hans van Haren1, Furu Mienis1, Gerard Duineveld1, and Daniel Bourgault2
1Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Netherlands, 2Institut des Sciences de la Mer de Rimouski,
Université du Québec à Rimouski, Rimouski, Québec, Canada
Abstract Using a combination of in situ observations and idealistic 2-D nonhydrostatic numerical
simulations, the relation between cold-water coral (CWC) mound size and hydrodynamics is explored for
the Rockall Bank area in the North Atlantic Ocean. It is shown that currents generated by topographically
trapped tidal waves in this area cause large isopycnal depressions resulting from an internal hydraulic
control above CWC mounds. The oxygen concentration distribution is used as a tracer to visualize the flow
behavior and the turbulent mixing above the mounds. By comparing two CWC mounds of different sizes
and located close to each other, it is shown that the resulting mixing is highly dependent on the size of the
mound. The effects of the hydraulic control for mixing, nutrient availability, and ecosystem functioning are
also discussed.
1. Introduction
Cold-water coral (CWC) reefs are known for hosting complex and rich ecosystems [Henry and Roberts, 2007;
Roberts et al., 2008]. Because CWC are essentially filter feeders, their distribution, shape, and growth are inti-
mately linked to particle and nutrient fluxes found in regions influenced by strong currents or turbulent
mixing. For example, the presence of CWC was associated to flow acceleration over topography [Genin et al.,
1986; Duineveld et al., 2007; Davies et al., 2009; Rüggeberg et al., 2011] or to internal wave mixing [Frederiksen
et al., 1992; Whiteetal., 2005; van Haren et al., 2014]. However, the exact causal relationship between CWC reef
development and the hydrodynamics is still an open question. Addressing this question is challenging, espe-
cially considering that by forming reefs or mounds, CWC can modify their own hydrodynamical environment.
Using a combination of in situ observations and idealized numerical simulations, the relationship between
the size of CWC mounds and the local hydrodynamics is explored.
The focus is on the northwest European shelf, known for hosting several CWC mound clusters (or provinces)
along the continental slopes [Roberts et al., 2008; White and Dorschel, 2010]. Geological evidence exists of suc-
cessive mound growth in the area over the past 2.6 Ma [Thierenset al., 2010]. From a physical point of view, this
region is also known for its “extraordinary” tidal currents [Moray, 1665; Cartwright, 1969] resulting from the
diurnal tidal waves being trapped and driven to resonance by the topographical features [Longuet-Higgins,
1968; Huthnance, 1974; Pingree and Griffiths, 1984]. This enhancement of the barotropic current velocities is
particularly important above the eastern slope of Rockall Bank (Figure 1). These trapped waves may be visual-
ized as alternating cyclonic/anticyclonic eddy-like cells moving southwest along the bank edge (see alternate
red and blue cells of Figure 1). The translation of these cells is such that a fixed station along the slope will
mainly experience a diurnal current periodicity (e.g., currents observed with 12 h delay in the red boxes of
Figure 1 have opposite signs).
A recent mooring deployment above the SE Rockall Bank slope confirmed the diurnal nature of near-bottom
currents in the area [van Haren et al., 2014; Cyr and van Haren, 2015]. These studies also revealed large verti-
cal excursions of the isopycnals (>100 m) and relatively strong diurnal currents (u<0.25 ms1) associated
with a diurnal succession of sharp temperature fronts generating high turbulence levels, with potential pos-
itive effects on exchange of particulate and dissolved compounds (e.g., oxygen and nutrients). It is therefore
conjectured that strong near-bottom currents caused by the trapped waves are intimately linked with CWC
growth and shapes in this area [Whiteetal., 2007; Mienis et al., 2007; van Haren et al., 2014].
In this area, numerous carbonate mounds of varying sizes were found with some of more than 300 m height
next to mounds of 100m[Mienis et al., 2006]. These observations thus raise questions concerning CWC
RESEARCH LETTER
10.1002/2015GL067038
Key Points:
• Explore the turbulent mixing caused
by trapped diurnal tides on deep coral
mounds
• Turbulent mixing efficiently
redistributes oxygen, and possibly
nutrients, in the water column
• Establish a link between mixing,
nutrient availability, and cold-water
coral mound growth
Correspondence to:
F. Cy r,
frederic.cyr@nioz.nl
Citation:
Cyr, F., H. van Haren, F. Mienis,
G. Duineveld, and D. Bourgault
(2016), On the influence of
cold-water coral mound size on
flow hydrodynamics, and vice versa,
Geophys. Res. Lett.,43, 775 783,
doi:10.1002/2015GL067038.
Received 15 NOV 2015
Accepted 2 JAN 2016
Accepted article online 8 JAN 2016
Published online 30 JAN 2016
©2016. American Geophysical Union.
All Rights Reserved.
CYR ET AL. COLD-WATER CORAL AND MIXING 775
Geophysical Research Letters 10.1002/2015GL067038
Figure 1. Bathymetry of the study area and surface tidal currents snapshots during the survey. (a) Rockall Bank (main figure), located about 400 km northwest of
Ireland in the North Atlantic (lower inset). The focus is made on the Logachev CWC mounds province (upper inset, multibeam mapping) on the southern slopes
of Rockall Bank. Approximate conductivity-temperature-depth (CTD) transects (white dashed lines) are drawn for reference. The model 2-D domain (magenta
and green dashed lines) follows the CTD transects and are oriented according to the Cartesian axes system on the lower right. The main figure highlights sea
surface anomalies 𝜂in color (difference between local sea surface elevation and the mean elevation of the 1×1region around it) for 15 October, 12:00.
Barotropic currents for the same period are also drawn as arrows showing current intensification at the slopes. These data (FES2012) were produced by Noveltis,
Legos, and Collecte Localisation Satellites Space Oceanography division and distributed by Aviso, with support from Centre National d’Etudes Spatiales
(http://www.aviso.altimetry.fr/). (b) Same data as in Figure 1a but 12 h later. The region of interest (red rectangle) is drawn in both panels for reference.
growth rates, in possible relation to different turbulence levels. This is because the strong variability of
physico-chemical conditions due to mixing on the Rockall mounds would, according to Findlay et al. [2014],
be beneficiary for the corals as it induces physiological flexibility. By comparing two coral mounds of differ-
ent sizes that are separated by less than 10 km, the primary goal of this study is to highlight the different flow
hydrodynamics in relation to CWC mounds sizes. Potential effects of the hydrodynamics for the redistribution
of oxygen and nutrients are also explored, together with the effects for CWC ecosystems functioning and,
indirectly, reef formation.
2. Methodology
2.1. In Situ Data
The data set used in this study was collected during the months of October 2012 and October 2013 during
two surveys aboard the R/V Pelagia that aimed to characterize environmental constraints on CWC reef and
mound development on the SE slopes of Rockall Bank (Figure 1). More specifically, two CWC mounds from
the Logachev province are under the scope of our study. The first structure is the Haas mound, a >300 m high
and 1.5 km wide (in the cross-isobath direction) structure that emerges from the bank (see cross-isobath
section in Figure 2a). This mound locally increases the slope of the bank from 3% (between 450 and 2400 m)
to 35% on the downslope side of the CWC mound (the depth increases from 560 to 900 m over 1 km). During
the 2012 survey, six conductivity-temperature-depth (CTD) transects were realized over the slope across the
Haas CWC mound during a period of 28 h.
The second structure is located less than 10 km northeast of the Haas mound, following approximately the
same isobaths (see Figure 2b). This mound is smaller than the Haas mound, and its slope is slightly gentler at
about 25% (the depth increases from about 790 to 975 m over 800 m). During the 2013 campaign, five CTD
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Figure 2. Numerical simulation configurations. (a) Idealized topography over the Haas CWC mound (magenta dashed
line in Figure 1a). (b) Idealized topography over the smaller CWC mound (green dashed line in Figure 1a). Both
topographies are built from the 30arcsec intervals General Bathymetric Chart of the Oceans (GEBCO_08 Grid, version
20100927, http://www.gebco.net) but with local refinement obtained from multibeam mapping (upper inset of Figure 1a).
The domains are clipped at their minimum and maximum depths and extended at these constant depths until the
boundaries located at ±5×105km (not shown). (c) Idealistic density (red dashed line) and oxygen concentration
(gray dashed line) profiles, calculated from mean profiles obtained during the 2012 CTD transect in the 0– 955 m depth
range (solid lines). In the 955–1250 m depth range, these profiles match one cast realized in the deepest part of the
slope during the 2013 campaign. Below 1250 m, profiles are extended to match the average of 15 Bio-Argo casts realized
between January and June 2013 within a box of latitude (54–56N) and longitude (12–18W ) encompassing our region
of interest (data downloaded on 13 May 2015 at https://www.nodc.noaa.gov/argo). These data were collected and made
freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.
edu, http://argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System. Since no climatology
is available below 2000 m, these profiles are extrapolated to a constant stratification and a constant oxygen
concentration in the 2000– 2500 m range, respectively.
transects were realized across this mound during a period of 21 h. During both surveys, a Seabird SBE-43
oxygen sensor was also mounted on the CTD. Oxygen concentration is used here as a conservative tracer to
visualize the flow interaction with the mounds. The R/V Pelagia was also equipped with a Kongsberg-Simrad
EM300 multibeam echosounder to map the sloping bottom in greater detail (see upper inset in Figure 1a).
Of the six CTD transects realized above the Haas mound, three are presented in Figures 3a, 3c, and 3e. This
figure shows the density (black lines) and oxygen concentration (color scale) distributions, together with the
topography (gray shade). These transects were composed of seven fixed CTD stations (vertical dashed lines),
and each transect was completed in about 5 h. The three transects displayed here are chosen to represent
the upslope flow (Figure 3a), the downslope flow (Figure 3e), and the flow reversal in between (Figure 3c).
Figures 4a, 4c, and 4e show similar CTD transect data but over the smaller CWC mound.
2.2. Idealized Simulations
In order to better interpret our observations, we performed idealized numerical simulations using a
two-dimensional, nonhydrostatic and fully nonlinear model initially developed by Bourgault and Kelley [2004]
that has been modified here to include the Coriolis force. The modified version is still two-dimensional by
resolving spatial gradients only in the x-zplane, but the model now incorporates an equation for an infinitely
uniform velocity in the ydirection (see axes system in Figure 1). We refer to related papers for specific details
about the numerical algorithm [Bourgault and Kelley, 2004; Bourgault et al., 2014], and only a few relevant
aspects of the simulations are provided here.
The numerical domains correspond to portions of the SE slope of the Rockall Bank (Figures 2a and 2b) along
two transects (respectively, magenta and green lines in Figure 1). While we use realistic topography for the
slope region, the upper and lower portions of the slope outside the region of interest are clipped at the maxi-
mum and minimum depth values, respectively, and extended to x5×105km to avoid contamination from
the model boundaries (model boundaries are not shown in Figure 2).
The initial density and oxygen concentration distributions are horizontally uniform across the domain and
correspond to the vertical profile presented in Figure 2c. These profiles are synthetic, as they are a mixture
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Figure 3. Observations from CTD transects and numerical simulation results across the Haas CWC mound. (a, c, e) The
linearly interpolated oxygen (color) and density (solid lines) fields over seven CTD stations (vertical dashed lines).
Isopycnals are separated by 0.02 kg m3intervals. The timing (start to end in UTC time) of each CTD transect is as
follows: from 16 October 2012 at 20:43 to 17 October 2012 at 01:05 (Figure 3a); on 17 October 2012 from 01:05 to 06:22
(Figure 3c); and on 17 October 2012 from 06:22 to 10:52 (Figure 3e). The maximum upslope displacements is found in
Figure 3c. (b, d, f) Numerical results of the same quantities, where velocity vectors above the CWC mound are drawn in
purple. Note that the panels are vertically stretched, meaning that the arrow angles do not represent the true angle of
the currents. The phases of these snapshots relative to the phases of the nearest current reversal are identified in the
upper left corner (here 𝜙=𝜙0corresponds to 4 days and 15 h after the beginning of the simulation). These approximately
fit thetiming of the corresponding transects, with the maximum upslope velocities in Figure 3b, the maximum downslope
velocities in Figure 3f, and the reversal in Figure 3d. Sketches representing the phase of the measurements (thick lines)
and the phase of the model results (red dots) versus cross-isobath velocities uare given in Figures 3a, 3c, and 3e.
of several sets of observations (details in Figure 2 caption). Oxygen is treated as a passive and conservative
tracer in the model. It is therefore transported and modified by the same advective and diffusive processes as
those applied to the density field, without any sources or sinks. The fluid is initially at rest, and the only forcing
terms, Fxand Fy, are horizontally uniform barotropic pressure gradients that aim to mimic the diurnal tides
present in the region. The set of equations to be resolved is thus [see also Bourgault et al., 2014]
𝜕u
𝜕t+𝜕u2
𝜕x+𝜕(wu)
𝜕zfv =Fx1
𝜌0
𝜕p
𝜕x+𝜕
𝜕x𝜈h
𝜕u
𝜕x+𝜕
𝜕z𝜈v
𝜕u
𝜕z,(1)
𝜕v
𝜕t+𝜕(uv)
𝜕x+𝜕(wv)
𝜕z+fu =Fy+𝜕
𝜕x𝜈h
𝜕v
𝜕x+𝜕
𝜕z𝜈v
𝜕v
𝜕z,(2)
𝜕w
𝜕t+𝜕(uw)
𝜕x+𝜕w2
𝜕z=−1
𝜌0
𝜕p
𝜕z+𝜌
𝜌0
g+𝜕
𝜕x𝜈h
𝜕w
𝜕x+𝜕
𝜕z𝜈v
𝜕w
𝜕z,(3)
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Figure 4. Same as Figure 3 but for the smaller CWC mound northeast of Haas mound. The timing of each CTD transect
is as follows: (a) on 10 October 2013 from 17:47 to 21:10; (c) on 11 October 2013 from 02:27 to 05:54; and (e) on 11 October
2013 from 06:51 to 10:17.
𝜕u
𝜕x+𝜕w
𝜕z=0,(4)
𝜕𝜌
𝜕t+𝜕(u𝜌)
𝜕x+𝜕(w𝜌)
𝜕z=𝜕
𝜕x𝜅h
𝜕𝜌
𝜕x+𝜕
𝜕z𝜅v
𝜕𝜌
𝜕z,(5)
where tis time; xand zare, respectively, the across-channel and vertical axes (positive downward); u,v, and w
are, respectively, the velocities along the x,y, and zaxes; fis the Coriolis parameter; 𝜌is the water density; 𝜌0
is a reference density; pis the pressure; 𝜈hand 𝜈vare the horizontal and vertical eddy viscosity, respectively;
and 𝜅hand 𝜅vare the horizontal and vertical eddy diffusivity, respectively. Eddy viscosities are parametrized
using the Smagorinsky [1963] scheme described in Bourgault and Kelley [2004], e.g.,
𝜈h=(CsΔx)22S2N2for 2S2>N2
106m2s1otherwise,(6)
where Δxis the horizontal grid size, Cs=0.1the Smagorinsky coefficient, N2=(g𝜌0)𝜕𝜌𝜕zthe buoyancy
frequency squared, and S2is the square of the strain rate tensor as detailed in the appendix in Bourgault and
Kelley [2004] but with the addition of terms to take into account the strain rate caused by the gradients of
v(e.g., 𝜕v𝜕xand 𝜕v𝜕z). Similar expressions are used for the eddy diffusivities except that minimum values
are set to 107m2s1for 2S2<N2. Note that we used variable horizontal and vertical grid cell sizes, with
refinement around the CWC mounds (Δxmin =100 m, Δxmax =5×106m, Δzmin =2.5m, and Δzmax =50 m).
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Figure 5. Comparison between the numerical simulations after 4 days and 15 h. (a) Haas CWC mound (zoom from
Figure 3b). (b) Smaller CWC mound located 10 km north of Haas mound (zoom from Figure 4b). Oxygen (color),
density (solid lines), and velocities (arrows) are shown. The red star is the location of the mooring described in
van Haren et al. [2014].
In equations (1) and (2), the forcing terms are Fx=g𝜕𝜂
𝜕xsin(𝜔t2𝜋3)and Fy=g𝜕𝜂
𝜕ysin(𝜔t), with
𝜔=2𝜋86400 rad s1the diurnal frequency and 𝜕𝜂
𝜕x=5×107and 𝜕𝜂
𝜕y=7.5×107the approximated
slope of the surface deformation calculated from one month (October 2010) of sea surface elevation anomaly
(e.g., 𝜂in Figure 1). This can be visually understood from Figure 1 where cells having Δ𝜂0.05 m elevation
and length scales of about Δx100 km in xand Δy65 km in yare found near the study region. The phase
lag 2𝜋3in Fxwas set to adjust the phase differences between uand vto those observed by van Haren et al.
[2014]. Comparison between modeled and observed near-bottom currents at the base of the Haas mound
(from van Haren et al. [2014]; see red star in Figure 5 for mooring location) shows that the model captures 56%
of the current variance in vand 24% in u, when computed in the 820– 900 m depth range, i.e., where velocity
measurements are available. The relatively low variance percentage resolved for this depth range, specially
for u, is due to the presence the strong nonlinear bores described in Cyr and van Haren [2015] that the model
is unable to reproduce (not shown).
3. Results
CTD transect observations and numerical results are analyzed for both CWC mounds in Figures 3 and 4.
A sketch representing the exact model phase and the approximate phase of the CTD transects accompany
each panel (sinusoidal plot overlay in the topography in Figures 3a, 3c, 3e, 4a, 4c, and 4e). Note that snapshots
from the fifth diurnal tidal cycle of the simulations are presented here (see Figure 3 caption), but each tidal
cycle exhibit similar behavior, although with some changes from one cycle to another due to the distortion
of the density and velocity fields caused by the interaction of the flow with the mound.
For the Haas mound (Figure 3), the numerical results exhibit qualitative resemblances with the observations.
Notable features include vertical isopycnal displacements of more than 100 m, both in the observations and
in the simulation, likely caused by the interaction of the flow with the coral mound. These occur during the
periods of maximum upslope and downslope velocities above the mound (respectively, Figures 3a and 3b
and Figures 3e and 3f), when a depression of the isopycnals occurs on the lee side of the flow (left of the
mound in Figure 3b and right in Figure 3f), suggesting hydraulic control of the flow. At the maximum ups-
lope displacements, isopycnals plunge down on the left side of the CWC, both in the observations and in
the model (Figures 3c and 3d). Vertical inversions in the oxygen field also suggest that turbulent mixing is
at work above the mound. The most important of these inversions occur during the reversal of the tidal cur-
rents (Figure 3c) and is seen as a 50 m thick layer of oxygenated waters (concentration increase from about
205 to 235 μmol kg1) injected between 500 and 600 m for x>1km. This high oxygen anomaly extends to at
least 23 km in the xdirection. Low oxygen anomalies for the same depth range are also found in Figure 3a,
likely resulting from mixing at the top of the mound. The shear present during the current reversal (suggested
by the numerical simulation, Figure 3d over the mound) may be responsible for the inversion of oxygen by
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overturning motions. While this oxygen anomaly remains in the observations after the cross-isobath flow
reversal (Figure 3e), it has disappeared in the simulation (Figure 3f). Numerical diffusion may be responsible
for the rapid mixing of this tracer in the model.
Another hydrodynamic aspect is that the isopycnals located deeper than 600 m on the downslope side of
the mound do not clearly rise above the summit of the mound during any tidal phase (both in observa-
tions and in the simulation). They rather plunge down when pushed against the mound (see the deepest
isopycnals in Figures 3a and 3b), suggesting a blocking of the flow. This is also visible in Figures 3c and 3d,
where the oxygen-poor layer (dark gray) is blocked on the right side of the mound (note that the dark gray
layer found on the left side of the mound in Figure 3e likely comes from the water flowing from the sides
of the mound later in the tidal cycle, not from above it). Since the oxygen minimum layer east of Rockall
Bank marks the bottom of the nutrient-depleted layer (this can be seen in McGrath et al. [2012, Figures 5b,
6a, and 6b]), it also means that the top of the Haas mound is not regularly washed by nutrient-rich waters
from below.
For the smaller CWC mound (Figure 4), the hydraulic control of the flow is different. Except for Figure 4a,
isopycnal depressions on the lee side are present but weaker than in Figure 3. The simulation also shows
less overturning than for the Haas mound, although density inversions in Figures 4b and 4d suggest some
mixing close to the top of the mound. This mound also does not clearly block the flow, and the incoming
oxygen-depleted waters from below the summit are able to flow over it, both in the observations and in
the simulation. This means that this smaller CWC mound is periodically washed by deep nutrient-rich waters
at a diurnal frequency. Below about 700 m, the observed sloshing motion of deep water flowing up and
down the mound shows qualitative similarities with the model results. However, the large isopycnals heav-
ing in Figure 4a is not reproduced by the model and therefore may not be due to the interaction of the
barotropic tidal flow with this CWC mound. One explanation for these observed vertical excursions is that
they may be due to the interaction of the flow with the 3-D topography which cannot be captured by the
2-D model. Indeed, for this phase, the barotropic flow first meets the Haas mound before meeting the smaller
mound northeast of it (see upper inset in Figure 1). Trapped internal tides generated by the interaction of the
barotropic flow with the topography may also explain these vertical isopycnal motions.
4. Discussion and Conclusion
4.1. Model Limitations
Unlike recent 3-D numerical work on CWC ecosystems [Henry et al., 2013; Moreno Navas et al., 2014], or more
specifically on Rockall Bank [Mohn et al., 2014], our approach here is to use an idealistic 2-D model in order
to focus on one specific mechanism: cross-isobath motions and internal wave generation by topographi-
cally trapped waves. This process-based approach allows us to perform relatively inexpensive nonhydrostatic
numerical simulations, preferable in this case due to the highly nonlinear internal bores and rapid density
perturbations seen in the observations [van Haren et al., 2014; Cyr and van Haren, 2015]. This approach may
be justified by the relative symmetry of the Rockall Bank slope and the small size of trapped waves compared
to the size of the bank (Figure 1). The present model, however, produces satisfying results for the purpose of
this study, especially in terms of reproducing the characteristics of the water column observed during two
transect surveys above two CWC mounds. It suggests that the modulation of currents and turbulence are pri-
marily caused by the trapped diurnal tidal wave, thereby discrediting the potential role of critical reflection of
freely propagating internal waves as main drivers explaining the presence of CWC in this area (for a discussion
on the role of freely propagating internal tides for CWC see, for example, White and Dorschel [2010] and Mohn
et al. [2014]). This model, however, falls short in representing the along-isobath residual circulation, hypothe-
sized by Mohn et al. [2014] to retain fresh organic material in close proximity of the Rockall slopes. Although
not under the scope if this study, it is worth noting that the model is also unable to reproduce the energetic
bore propagation observed at the base of the Haas mound [Cyr and van Haren, 2015].
4.2. Influence of CWC Mound Size on the Hydrodynamics
The different flow behavior between the two mounds is summarized in Figure 5. It can be seen that the Haas
mound induces a stronger hydraulic control on the flow compared to the smaller CWC mound. The parameter
space that describes this periodic flow is difficult to establish for two reasons: (1) the ridge is not isolated but
rather consists of a bump on a slope; and (2) the frequency of the flow is diurnal, such that the slope cannot
be characterized as being subcritical/supercritical because the internal waves are trapped (no tidal rays can
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escape from the slope). In this regard, the flow is better described by an analogy with steady flows because
the tidal excursion distance is larger than the width of the obstacle 2𝜋U0
𝜔L>1, where U0101ms1,an
order of magnitude for the modeled velocity far from the topography, and L103m an order of magnitude
for the width of the mounds. In steady flows, or in this case when the flow is affected by the full topographic
height of the mound, the topographic Froude number better describes the obstruction of the flow by the
topography, Fr =U0
Nh0
, where h0is the topography height and Nthe buoyancy frequency [Legg and Klymak,
2008]. For Fr 1, strongly nonlinear lee waves aregenerally expec ted tooccur [Legg and Klymak , 2008; Klymak
et al., 2010].
For the two numerical simulations presented here, only the parameter h0is varied, with h0=305 m for the
Haas mound and h0=75 m for the smaller CWC mound (Figures 2a and 2b). Using N2×103s1, the
mean buoyancy frequency calculated from the density profile of Figure 2c between 250 and 1000 m, we find
Fr 1011and Fr 1for the Haas mound and the smaller CWC mound, respectively. These calculations
are in line with those depicted in Figure 5, i.e., a strong blocking with nonlinear lee wave/hydraulic jump
generation for the Haas mound and a weaker blocking for the smaller mound. Given the acceleration of the
flow above the higher mound to about U=0.35 ms1(Figure 5b), the vertical excursion of the isopycnals
associated with the hydraulic jump may be predicted to be Δz=U
N175 m[Legg and Klymak, 2008], i.e., in
line with in situ observations and model results (Figure 3).
This also indicates that when the tidal flow relaxes, the lee wave should evolve in an upstream-propagating
(toward the summit) bore-like structure with considerable implication for mixing [Legg and Klymak, 2008;
Klymak et al., 2010]. The latter is also suggested by the numerical results, where an overturning structure
(just left of the summit in Figure 3d) remnant of the lee wave of Figure 3b is swept on the top of the mound.
Steep isopycnals just left of the summit in Figure 3c may also suggest such behavior in the observations. It is
likely that the mixing energy released during this flow reversal is responsible for the oxygen input in depth
(and possible nutrient redistribution) observed in observations (Figures 3c and 3e, 500– 600 m, right of the
mound summit). Such bore-like structures were observed by van Haren et al. [2014] and Cyr and van Haren
[2015] at the base of the Haas mound between about 800 and 900 m (red star in Figure 5b). However, these
structures are likely not generated by the same mechanism since the mooring was deployed in the region
where the flow is blocked and in a depth range deeper than what the hydraulic jump can reach (maximum
depth is Δz175 m below the summit; thus, z725 m). Further investigation is needed regarding the
different origin of these structures.
4.3. Influence of the Hydrodynamics on CWC Ecosystem Functioning
While the size of the CWC mounds has been shown to play a role in the hydraulic control of the flow over them,
questions are raised regarding the influence of hydrodynamics on CWC ecosystem functioning and reef and
mound growth. Recent video footage by our team (unpublished results) suggests that while healthy corals
are found at the summit of smaller mounds, they are virtually absent from the summit of bigger mounds
(although found on their flanks; see van Bleijswijk et al. [2015]). This may suggest that the Haas mound has
reached its maximum height while the smaller mound still grows vertically. This observation may be coun-
terintuitive since it is generally accepted that dynamical environments (strong currents and mixing zones)
such as over the top of Haas mound are favorable for CWC [e.g., Genin et al., 1986; Frederiksen et al., 1992;
Roberts et al., 2006; Davies et al., 2009; White and Dorschel, 2010]. In other words, a challenging question
for CWC ecosystem functioning and reef and mound development on the slopes of Rockall Bank is raised:
How does mound size influence the hydrodynamics, and vice versa? This work partly answers this question by
suggesting that by modifying the hydrodynamics, CWC mounds maintain, up to a certain extent, a turbu-
lent environment favorable for their growth. However, by outgrowing themselves they significantly block
the flow (Fr <1) and prevent the deep nutrient-rich water to periodically wash their summit. Then, these
mounds may reach a steady state where they can no longer grow vertically in their actual physical and bio-
geochemical environment. Environmental parameters driving this maximum height may be different from
one region to another, since they depend on the barotropic tidal flow and on the vertical distribution of
nutrients. More field work is, however, needed to verify this hypothesis, which should include physiologi-
cal and geochemical tracers measurements above these mounds and geological analyses of CWC mound
growth rates.
CYR ET AL. COLD-WATER CORAL AND MIXING 782
Geophysical Research Letters 10.1002/2015GL067038
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Acknowled gments
The authors would like to thank
the captain and crew of the
R/V Pelagia . F. Cyr would like to thank
R. Chassagne, A. Cimatoribus, L. Maas,
and H. Malschaert for their help.
F. Mienis was funded by the Innova-
tional Research Incentives Scheme
of the Netherlands Organization for
Scientific Research (NWO VENI) and
F. Cyr by the Fonds de recherche
du Québec - Nature et technologies
(FRQNT) postdoctoral fellowship
program. D. Bourgault is a member of
the strategic network Québec-Océan.
CYR ET AL. COLD-WATER CORAL AND MIXING 783
... Coral mounds can be tens of meters to over 300 m high (Mienis et al., 2006) and their presence directly influences the hydrodynamics in coral mound regions: The large structures partially block or deflect the (tidal) flow around them (Cyr et al., 2016;Juva et al., 2020), so that internal waves can be generated, amplified, and locally converted to turbulent motions, increasing the rate at which corals encounter suspended food particles Mohn et al., 2014;van Haren et al., 2014). With isopycnal-displacements of 100-200 m Mohn and White, 2007), downward velocities are increased (Mohn et al., 2014) and hydraulic jumps and internal bores emerge over mound slopes during the turn of the tide (Jackson et al., 2012;Legg and Klymak, 2008). ...
... Also, since most coral mound tops at Logachev are situated around the depth of the permanent pycnocline, it has been hypothesized that the mounds stop growing taller around this depth there (Mienis et al., 2006;Wheeler et al., 2007;White and Dorschel, 2010). In a 2D modelling study Cyr et al. (2016) compared flow over a small and a large protrusion in the area and suggested that this stabilization of the mound top is controlled by the way the mounds grow: vertically when small until they "outgrow" themselves, after which they mainly grow sideways. This finding was supported by observations of large mounds in the area that seemed to be lacking any live corals on the top. ...
... This finding was supported by observations of large mounds in the area that seemed to be lacking any live corals on the top. However, due to the inherent symmetry implied in a 2D model, the results of Cyr et al. (2016) represent hydrodynamics around an infinitely long ridge rather than a conical mound. ...
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Cold-water corals rely on currents to transport food towards them and when external conditions are favourable, they can form coral mounds. These structures, which can be over 300 m high, influence the hydrodynamics around the reefs that grow on the mounds, which feeds back to affect coral- and therefore mound-growth. We investigated these feedbacks at the Logachev coral mound province, by running simulations with a 3D hydrodynamic model (Roms-Agrif), using different seafloor bathymetries that represent consecutive stages of mound development. Simulations ranged from a fully smoothened bathymetry without mounds, to a coral mound (Haas mound) at 1.5 times its current size. The effect of mound height on coral growth was investigated by looking at the baroclinic (internal) tide, turbulent energy dissipation, vertical velocities, and horizontal bottom currents. The simulations suggest that with increasing mound height horizontal velocities increase, while turbulent energy dissipation and vertical velocities around the mound foot decrease. This supposedly limits coral growth at the mound foot and hence lateral mound extension in later stages of development. An increase in turbulent energy dissipation and vertical velocities on the mound top and upper flanks, indicates vertical mound growth at all subsequent stages. Our findings of continued vertical mound growth provide an explanation for recently published data on benthic cover from a transect over Haas mound, that show a dominance of live corals on the mound top. We find areas of increased energy conversion rates from the barotropic (surface) to the baroclinic tide on the bathymetry where we artificially eliminated the mounds from (i.e. smoothened bathymetry). Interestingly, these areas overlap with the region where coral mounds are located at present. So, the baroclinic tide is likely an important mechanism in the process of coral mound establishment. Given the relative ease with which the energy conversion rate from the barotropic to the baroclinic tide can be deduced from hydrodynamic model simulations, our results provide opportunities to investigate where coral mounds may be initiated worldwide.
... The CWC reefs can form large carbonate mounds ten to hundreds of metres high and several kilometres wide (Kenyon et al., 2003;van Weering et al., 2003). Carbonate mounds alter the surrounding hydrodynamic environment in various ways by enhancing current velocities Mohn et al., 2014), inducing episodic downward transport of waters from shallower depths Soetaert et al., 2016), and breaking of internal waves (van Haren et al., 2014;Cyr et al., 2016). The interaction of the tide (barotropic) with the topography generates internal (baroclinic) tidal processes, which enhance mixing and transport of fresh organic matter, produced at the sunlit surface ocean, to the coral mounds (Duineveld et al., 2004. ...
... The bottom currents are also mainly in a south-west direction and can peak at 75 cm s − 1 (Mienis et al., 2009a). A dominant diurnal baroclinic (internal) tide has been measured (van Haren et al., 2014) and can be seen as a modified Kelvin wave (Cyr et al., 2016), which causes cross-slope transport of water over a diurnal tidal cycle (Gerkema, 2019). Internal waves have been observed in the vicinity of these coral mounds (Huthnance, 1973;Mienis et al., 2007;Mohn et al., 2014), along with breaking of internal waves. ...
... Internal waves have been observed in the vicinity of these coral mounds (Huthnance, 1973;Mienis et al., 2007;Mohn et al., 2014), along with breaking of internal waves. This breaking of the wave can be caused by the interaction between the tide and the topography, and are especially predicted after the turning of the tide, which is the period of highest turbulence (van Haren et al., 2014;Cyr et al., 2016). ...
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Cold-water corals (CWCs) are important ecosystem engineers in the deep sea that provide habitat for numerous species and can form large coral mounds. These mounds influence surrounding currents and induce distinct hydrodynamic features, such as internal waves and episodic downwelling events that accelerate transport of organic matter towards the mounds, supplying the corals with food. To date, research on organic matter distribution at coral mounds has focussed either on seasonal timescales or has provided single point snapshots. Data on food distribution at the timescale of a diurnal tidal cycle is currently limited. Here, we integrate physical, biogeochemical, and biological data throughout the water column and along a transect on the south-eastern slope of Rockall Bank, Northeast Atlantic Ocean. This transect consisted of 24-hour sampling stations at four locations: Bank, Upper slope, Lower slope, and the Oreo coral mound. We investigated how the organic matter distribution in the water column along the transect is affected by tidal activity. Repeated CTD casts indicated that the water column above Oreo mound was more dynamic than above other stations in multiple ways. First, the bottom water showed high variability in physical parameters and nutrient concentrations, possibly due to the interaction of the tide with the mound topography. Second, in the surface water a diurnal tidal wave replenished nutrients in the photic zone, supporting new primary production. Third, above the coral mound an internal wave (200 m amplitude) was recorded at 400 m depth after the turning of the barotropic tide. After this wave passed, high quality organic matter was recorded in bottom waters on the mound coinciding with shallow water physical characteristics such as high oxygen concentration and high temperature. Trophic markers in the benthic community suggest feeding on a variety of food sources, including phytodetritus and zooplankton. We suggest that there are three transport mechanisms that supply food to the CWC ecosystem. First, small phytodetritus particles are transported downwards to the seafloor by advection from internal waves, supplying high quality organic matter to the CWC reef community. Second, the shoaling of deeper nutrient-rich water into the surface water layer above the coral mound could stimulate diatom growth, which form fast-sinking aggregates. Third, evidence from lipid analysis indicates that zooplankton faecal pellets also enhance supply of organic matter to the reef communities. This study is the first to report organic matter quality and composition over a tidal cycle at a coral mound and provides evidence that fresh high-quality organic matter is transported towards a coral reef during a tidal cycle.
... The results reported in the present paper are based on a similar approach of a combination of the observational data set and highresolution modeling applied to a different area, Rosemary Bank Seamount. Our findings are in line with the conclusions of the paper by Cyr et al. (2016) who acknowledged a strong influence of the baroclinic tidal hydrodynamics on the sustainability of the coral communities in the Rockall Bank area. ...
... Their peaks are comparable to the semidiurnal signal or even higher in the energy level (station 31), which testifies the strong nonlinearity of the internal wave process. Such waves will increase the mixing process that can contribute to better larvae dispersion (van Harren et al., 2014;Cyr et al., 2016). A general tendency of all graphs is that the energy level at the bottom and in the surface layer (red and blue lines in Figure 7, respectively) substantially exceeds a similar level calculated for the middepth segments (green-yellow colors). ...
... The bottom sub-inertial trapped internal waves are concentrated in the area of the main pycnocline below 600 m depth. Both types of waves can produce water mixing that supplies nutrients and food for deep-water corals and facilitate coral larvae dispersion (Mohn et al., 2014;Cyr et al., 2016). ...
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The internal wave dynamics over Rosemary Bank Seamount (RBS), North Atlantic, were investigated using the Massachusetts Institute of Technology general circulation model. The model was forced by M2-tidal body force. The model results are validated against the in-situ data collected during the 136th cruise of the RRS “James Cook” in June 2016. The observations and the modeling experiments have shown two-wave processes developed independently in the subsurface and bottom layers. Being super-critical topography for the semi-diurnal internal tides, RBS does not reveal any evidence of tidal beams. It was found that below 800-m depth, the tidal flow generates bottom trapped sub-inertial internal waves propagated around RBS. The tidal flow interacting with a cluster of volcanic origin tall bottom cones generates short-scale internal waves located in 100 m thick seasonal pycnocline. A weakly stratified layer separates the internal waves generated in two waveguides. Parameters of short-scale sub-surface internal waves are sensitive to the season stratification. It is unlikely they can be observed in the winter season from November to March when seasonal pycnocline is not formed. The deep-water coral larvae dispersion is mainly controlled by bottom trapped tidally generated internal waves in the winter season. A Lagrangian-type passive particle tracking model is used to reproduce the transport of generic deep-sea water invertebrate species.
... The entrapped sediments stabilize the reef framework, prevent its collapse, and thus, support vertical mound growth (e.g., Titschack et al., 2016;Wang et al., 2021). Moreover, model simulations showed that also the mounds themselves influence the hydrodynamics around them (Cyr et al., 2016;Mohn et al., 2014;van der Kaaden et al., 2021). With increasing mound height, tidetopography interactions may lead to an increase in vertical velocities. ...
... In turn, the development of coral mounds not just requires vivid CWC reefs, but also the concurrent supply of terrigenous sediments, which derive from fluvial or aeolian input but also from iceberg discharge, and are delivered or resuspended by moderate to strong bottom currents (e.g., Pirlet et al., 2011). Since an energetic hydrodynamic regime commonly prevails on top of a coral mound (Dorschel et al., 2007b;Mohn et al., 2014;Cyr et al., 2016;Juva et al., 2020;van der Kaaden et al., 2021), bypassing suspended sediments only become deposited when large and densely distributed coral (reef) frameworks are present, as those have the capability to reduce the velocity of near-bottom currents allowing sediment deposition between their branches (baffling; Huvenne et al., 2009;Titschack et al., 2009;Wang et al., 2021). Moreover, a flourishing reef needs the stabilising effect of a sediment infill for preventing successive fragmentation (by bioerosion, currents, gravity) and for developing into a mound several tens or even hundreds of metres high. ...
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... Of all paleoenvironmental parameters considered here, only the grain-size data used as a proxy for the hydrodynamic setting can be directly affected by the distance between the off-mound site and the coral mound site. However, even though coral mounds (being able to accelerate local currents due to their elevated topography, e.g., [62]) and thriving coral reefs on their top (being able to decelerate local currents due to the baffling effect of the coral framework, e.g., [63]) may induce local effects, their ability to modulate the regional current regime is limited. Hence, the regional hydrodynamic setting is best recorded in the off-mound records that are generally unaffected by local coral mound-related effects. ...
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Cold-water corals (CWCs) are the engineers of complex ecosystems forming unique biodiversity hotspots in the deep sea. They are expected to suffer dramatically from future environmental changes in the oceans such as ocean warming, food depletion, deoxygenation, and acidification. However, over the last decades of intense deep-sea research, no extinction event of a CWC ecosystem is documented, leaving quite some uncertainty on their sensitivity to these environmental parameters. Paleoceanographic reconstructions offer the opportunity to align the on- and offsets of CWC proliferation to environmental parameters. Here, we present the synthesis of 6 case studies from the North Atlantic Ocean and the Mediterranean Sea, revealing that food supply controlled by export production and turbulent hydrodynamics at the seabed exerted the strongest impact on coral vitality during the past 20,000 years, whereas locally low oxygen concentrations in the bottom water can act as an additional relevant stressor. The fate of CWCs in a changing ocean will largely depend on how these oceanographic processes will be modulated. Future ocean deoxygenation may be compensated regionally where the food delivery and food quality are optimal.
... Periods of growth and/or decline of CWCs have been linked to changes in the physicochemical properties and productivity of the water masses [35,38,[40][41][42][43][44][45][46]. Bottom currents can also create small erosional and depositional sedimentary structures and modulate the shape of the mound [47]), resulting in flattened top mounds (within the storm wave base), cone-shaped mounds (quiet and deeper waters) [4], elongated mound shapes (due to the preferred growth of the reefforming organisms towards the main current) [48]), and mounds with current scours and moats around them (in strong bottom deep water along slope bottom currents) [35,41,[49][50][51]. Active CWC mounds are covered by thriving CWC reefs and background sedimentation rate is generally lower than CWCs growth rate, however, contouritic and/or hemipelagic sediments have generally been found as major structural components contributing to the accretion of the CWC mounds (sometimes representing up to two-thirds of the mound deposits) [18,52,53]. ...
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Carbonate mounds clustering in three fields were characterized on the upper continental slope of the northern Alboran Sea by means of a detailed analysis of the morphosedimentary and structural features using high-resolution bathymetry and parametric profiles. The contemporary and past benthic and demersal species were studied using ROV underwater imagery and some samples. A total of 325 mounds, with heights between 1 and 18 m, and 204 buried mounds were detected between 155 to 401 m water depth. Transparent facies characterize the mounds, which root on at least six erosive surfaces, indicating different growth stages. At present, these mounds are covered with soft sediments and typical bathyal sedimentary habitat-forming species, such as sea-pens, cerianthids and sabellid polychaetes. Nevertheless, remains of colonial scleractinians, rhodoliths and bivalves were detected and their role as potential mound-forming species is discussed. We hypothesized that the formation of these mounds could be related to favorable climatic conditions for cold-water corals, possibly during the late Pleistocene. The occurrence on top of some mounds of abundant rhodoliths suggests that some mounds were in the photic zone during minimum sea level and boreal guest fauna (e.g., Modiolus modiolus), which declined in the western Mediterranean after the Termination 1a of the Last Glacial (Late Pleistocene).
... This points to rather calm hydrodynamic conditions prevailing on top of the coral mound which was capped by dense CWC frameworks that allowed even the fine fraction of the current-transported sediment load to become deposited. At first glance, this conflicts with the reconstructed enhanced hydrodynamics at the nearby seafloor and also contradicts with the common sense that a seafloor obstacle, such as the positive topography of a coral mound, is expected to accelerate bottom-current velocities (e.g., Cyr et al., 2016). ...
Article
Full-text available
The formation of cold-water coral (CWC) mounds is commonly seen as being the result of the sustained growth of framework-forming CWCs and the concurrent supply and deposition of terrigenous sediments under energetic hydrodynamic conditions. Yet only a limited number of studies investigated the complex interplay of the various hydrodynamic, sedimentological and biological processes involved in mound formation, which, however, focused on the environmental conditions promoting coral growth. Therefore, we are still lacking an in-depth understanding of the processes allowing the on-mound deposition of hemipelagic sediments, which contribute to two thirds of coral mound deposits. To investigate these processes over geological time and to evaluate their contribution to coral mound formation, we reconstructed changes in sediment transport and deposition by comparing sedimentological parameters (grain-size distribution, sediment composition, accumulation rates) of two sediment cores collected from a Mediterranean coral mound and the adjacent seafloor (off-mound). Our results showed that under a turbulent hydrodynamic regime promoting coral growth during the Early Holocene, the deposition of fine siliciclastic sediments shifted from the open seafloor to the coral mounds. This led to a high average mound aggradation rate of >130 cm kyr −1 , while sedimentation rates in the adjacent off-mound area at the same time did not exceed 10 cm kyr −1. Thereby, the baffling of suspended sediments by the coral framework and their deposition within the ecological accommodation space provided by the corals seem to be key processes for mound formation. Although, it is commonly accepted that these processes play important roles in various sedimentary environments, our study provided for the first time, core-based empirical data proving the efficiency of these processes in coral mound environment. In addition, our approach to compare the grain-size distribution of the siliciclastic sediments deposited concurrently on a coral mound and on the adjacent seafloor Frontiers in Marine Science | www.frontiersin.org 1 December 2021 | Volume 8 | Article 760909 Wang et al. Cold-Water Coral Mound Formation allowed us to investigate the integrated influence of coral mound morphology and coral framework on the mound formation process. Based on these results, this study provides the first conceptual model for coral mound formation by applying sequence stratigraphic concepts, which highlights the interplay of the coral-framework baffling capacity, coral-derived ecological accommodation space and sediment supply.
... It occupies a wide range of habitats, including seamounts, submarine canyons, and fjords (Freiwald et al. 2004), in which it provides itself habitat for species-rich communities by forming complex three-dimensional structures of variable size and age, such as colonies, patches, reefs, and mounds (Wilson 1979, Mortensen et al. 1995, Rogers 1999, Jonsson et al. 2004, Costello et al. 2005, Wienberg and Titschack 2015, Henry and Roberts 2017. These structures persist after death of the organism and influence water flow and sedimentation within and around them (Cyr et al. 2016, Mienis et al. 2019. L. pertusa colonies originate with larval settlement on current-swept hard substrates (Wilson 1979, Freiwald et al. 2004) and enlarge through asexual reproduction of individual polyps by intratentacular budding (Cairns and Kitahara 2012). ...
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... To answer this question, the interplay of the prevailing hydrodynamics and sediment transport with corals should be investigated on a high spatial and temporal scale. Many successful in situ surveys have been done over the last decades using various devices, such as landers or moorings (e.g., Guihen et al., 2013;Mohn et al., 2014;Cyr et al., 2016). However, the data acquired with such investigations provides information about the general flow patterns near the reefs, but does not resolve the flow characteristics in the direct vicinity of the cold-water coral colonies. ...
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