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Oceanic fronts are often associated with enhanced biological activity. Depending on their generation mechanism, they are often linked to specific geographical areas. Here we use 25 years of high-resolution satellite sea surface temperature (SST) daily images to generate maps of SST fronts over Canadian coastal waters. Results show that fronts are ubiquitous features, but some fronts are more persistent than others. We confirmed the location of previously known major fronts, but some new persistent frontal areas were also detected as a result of the use of high-resolution (1.1 km) data and a methodology adapted to detect smaller-scale frontal features. Results also show that some of the frontal areas are associated with enhanced phytoplankton biomass or higher trophic level organisms (whales and birds) confirming the ecological importance of this physical process. RÉSUMÉ Les fronts océaniques sont souvent liés à une activité biologique accrue. Ils sont souvent associés à une aire géographique spécifique qui dépend du mécanisme qui les génèrent. Nous avons utilisé 25 années d'images journalières de températures de surface de la mer (TSM) à haute résolution spatiale afin de générer de cartes de fronts de TSM pour les eaux côtières canadiennes. Les résultats montrent que des fronts sont visibles partout mais que certains d'entre eux sont persistants dans le temps. Nous avons confirmé la position des fronts déjà connus mais de nouvelles zones ont aussi été détectées résultant de l'utilisation de données à haute résolution (1.1 km) et d'une méthodologie adaptée à la détection de fronts à plus petite échelle spatiale. Les résultats ont aussi montré que certains fronts sont associés à une biomasse phytoplanctonique plus élevée ou à la présence d'orga-nismes de niveaux trophiques supérieurs (baleines, oiseaux), confirmant l'importance écologique de ce processus physique.
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Thermal Fronts Atlas of Canadian Coastal Waters
Frédéric Cyr
1,2,
*
and Pierre Larouche
3
1
Institut des Sciences de la Mer, Université du Québec à Rimouski, Rimouski, Quebec, Canada
2
Physical Oceanography, Royal Netherlands Institute for Sea Research (NIOZ), Den Burg,
Netherlands
3
Ocean and Environmental Science Branch, Maurice Lamon tagne Institute, Department of Fisheries
and Oceans Canada, Mont-Joli, Quebec, Canada
[Original manuscript received 20 February 2014; accepted 15 September 2014]
ABSTRACT Oceanic fronts are often associated with enhanced biological activity. Depending on their generation
mechanism, they are often linked to specific geographical areas. Here we use 25 years of high-resolution satellite
sea surface temperature (SST) daily images to generate maps of SST fronts over Canadian coastal waters. Results
show that fronts are ubiquitous features, but some fronts are more persistent than others. We confirmed the location
of previously known major fronts, but some new persistent frontal areas were also detected as a result of the use of
high-resolution (1.1 km) data and a methodology adapted to detect smaller-scale frontal features. Results also
show that some of the frontal areas are associated with enhanced phytoplankton biomass or higher trophic
level organisms (whales and birds) confirming the ecological importance of this physical process.
RÉSUMÉ Les fronts océaniques sont souvent liés à une activité biologique accrue. Ils sont souvent associés à une
aire géographique spécifique qui dépend du mécanisme qui les génèrent. Nous avons utilisé 25 années dimages
journalières de températures de surface de la mer (TSM) à haute résolution spatiale afin de générer de cartes de
fronts de TSM pour les eaux côtières canadiennes. Les résultats montrent que des fronts sont visibles partout mais
que certains dentre eux sont persistants dans le temps. Nous avons confirmé la position des fronts déjà connus
mais de nouvelles zones ont aussi été détectées résultant de lutilisation de données à haute résolution (1.1 km)
et dune méthodologie adaptée à la détection de fronts à plus petite échelle spatiale. Les résultats ont aussi
montré que certains fronts sont associés à une biomasse phytoplanctonique plus élevée ou à la présence dorga-
nismes de niveaux trophiques supérieurs (baleines, oiseaux), confirmant limportance écologique de ce processus
physique.
KEYWORDS thermal fronts; frontal probability; Newfoundland Shelf; Scotian Shelf; Gulf of St. Lawrence;
Hudson Bay; Baffin Bay; Pacific Ocean
1 Introduction
An oceanic thermal front is a region of enhanced sea surface
temperature (SST) gradients. They are often linked to gradi-
ents of other physical, chemical, or biological properties
such as salinity, nutrients, plankton, suspended sediments.
Their spatial and temporal scales vary over many orders of
magnitude, ranging from small river plumes (a few metres
and a lifetime of a few days) to western boundary currents
such as the Gulf Stream (hundreds of kilometre s and coherent
in time for thousands of years). Because ocean fronts often
constitute exceptional biological habitats, they are considered
a key feature in physical oceanography (Belkin, Cornillon, &
Sherman, 2009; Sherman, 1990, 2005).
Past studies mostly focused on the description of large-scale
oceanic fronts associated with major currents (e.g., Belkin
et al., 2009; Bisagni, Kim, & Chaudhuri, 2009; Legeckis,
Brown, & Chang, 2002). Only a few studies have focused
on specific coastal ecosystems (e.g., Belkin, Cornillon, &
Ullman, 2001; Nieto, Demarcq, & McClatchie, 2012; Wall
et al., 2008). Many physical processes, such as geostrophic
currents, estuarine buoyancy currents, upwellings, water
mass convergence zones, or marginal ice zones, can generate
fronts in the coastal ocean. Depending on the generation
mechanisms, fronts are often closely related to bathymetric
features (shelves, capes, canyons, banks, shoals, etc.). The
generation mechanisms also determine whether the front is
associated with surface nutrient supply and enhanced
primary produc tivity. For example, fronts delimiting diver-
gence zones (such as tidal mixing or upwelling) from the sur-
rounding waters generally mark high primary productivity
areas. On the other hand, fronts associated with current con-
vergence or with temperature gradients caused by river
*Corresponding authors email: frederic.cyr@nioz.nl
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plumes or ice melting, may not be associated with high nutri-
ent supply.
Surrounded by three oceans, Canada has the longest
coastline in the world and oceans have a major role in the
Canadian economy generating more than $1 billion in
revenue. However, such a large body of water is difficult
to monitor in detail using conventional in situ observations.
Moreover, the remoteness and the harsh ice and meteorolo-
gical conditions that prevail in most of the northern basins
make observational datasets in these areas very scarce.
Although a few studies have looked at frontal structures in
Canadian waters from satellite data (e.g., Jardine,
Thomson, Foreman, & Leblond, 1993; Lavoie, Bonn,
Dubois, & El-Sabh, 1985; Mavor & Bisagni, 2001; Perry,
Dilke, & Parsons, 1983; Ullman & Cornillon, 1999), these
are generally limited to small areas. Using 25 years of remo-
tely sensed SST data, the aim of this study is to provide an
overview of frontal features in Canadian coastal waters.
Whenever possible, known associations between front
location, generation mechanism, and biological features are
also indicated.
This study is, therefore, an atlas for front location that scien-
tists can refer to in preparation for (or as a complement to) field
studies. This work may also help in mapping sensitive biological
habitats in a changing environment, such as the work recently
realized by the World Wildlife Fund (Christie & Sommerkorn,
2012). Seven regions come under the scope of this study:
Hudson Strait and the Labrador Shelf, the Newfoundland
Shelf, Scotian Shelf, and Gulf of Maine area, the Gulf of
St. Lawrence, Hudson Bay and Foxe Basin, Baffin Bay, and
the Northeast Pacific Ocean (Fig. 1). For ease of locating infor-
mation, each of these regions is presented and discussed separ-
ately in Sections 3a to 3g. These regions are mostly in Canadian
waters, but some regions naturally close to Canada will also be
described. The Beaufort Sea (hatched area in Fig. 1) is not dis-
cussed here because the region is under investigation in another
ongoing and independent study on thermal fronts.
2 Methodology and datasets
Mean daily maps of SST were obtained from the Maurice
Lamontagne Remote Sensing Laboratory for the period
Fig. 1 Global map of the areas studied: (A) Hudson Strait and the Labrador Shelf (52°65°N, 79°54°W), (B) Newfoundland Shelf (44°52°N, 60°44°W), (C) the
Scotian Shelf and the Gulf of Maine (41°45°N, 72°58°W), (D) the Gulf of St. Lawrence (45°52°N, 70°55°W), (E) Hudson Bay and Foxe Basin (51°
71°N, 96°72°W), (F) Baffin Bay (60°82°N, 85°50°W), (G) the Northeast Pacific Ocean (40°67°N, 180°110°W). (H)The Beaufort Sea (67°76°N,
145°115°W) is not within the scope of the present study because the region is under investigation in a separate study.
2 / Frédéric Cyr and Pierre Larouche
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19862010. These maps were generated using Advanc ed Very
High Resolution Radiometer (AVHRR) data. These daily SST
maps are composites of multiple satellite passes over Canadian
waters with a spati al resolution of 1.1 km. Before the compo-
site is created, each image undergoes a thorough series of tests
to eliminate clouds and ice. These tests are based on pixel
albedo in the two visible channels and temperature in the
three thermal channels. Tests are also made on the spatial uni-
formity of albedo and temperature over a 3 × 3 pixel box.
These flagged data are then compared with ice coverage
maps generated by the National Snow and Ice Data Center
(NSIDC) to detect abnormal pixels that may not have been
detected by the cloud screening. Any SST pixel corresponding
to a non-zero ice coverage region according to NSIDC is
flagged as ice covered. A final test is done to further eliminate
abnormal values that may not have been detected by cloud/ice
screening. This is done by first applying a temporal homogen-
eity filter that compares SST with all other measurements
made during a 15-day window centred on the image time.
Then, a daily climatological filter is used to eliminate outliers
above three standard deviations from the 25-year average. The
use of a climatological filter has been shown to greatly
improve the quality of SST retrievals for mid-latitude data
leading to a mean bias of 0.19°C (Pettigrew, Larouche, &
Gilbert,
2011).
Regions of interest for this study (Fig. 1) were extracted
from the daily national composites and processed for front
detection. Before processing images with the front detection
algorithm, land was masked using the National Oceanic and
Atmospheric Administrations (NOAA) Global Self-consist-
ent, Hierarchical, High-resolution Geography (GSHHG) data-
base (Wessel & Smith, 1996) so that only water pixels were
processed. When more than 85% of the water pixels of a
daily image are flagged as cloud, ice, or an invalid pixel, the
image is discarded and so not used in further analysis.
Because the goal was to study coastal fronts, we used the vari-
able window single-image edge-detection (VW-SIED) algor-
ithm (Diehl, Budd, Ullman, & Cayula, 2002). This
algorithm is an improvement over SIED (Cayula & Cornillon,
1992) that uses a fixed overlapping window size for edge
detection. The advantage of VW-SIED is that the variable
window size offers better detection of smaller scale fronts
(Kahru, Di Lorenzo, Manzano-Sarabia, & Mitchell, 2012)
and is thus better suited to the study of coastal regions. The
algorithm exploits the idea that for a frontal region, SST
pixel values over a specific window should follow a bi-
modal distribution (many warm and cold pixels with a few
pixels in the temperature transition range). Fronts generally
correspond to this transition zone called the edge. To detect
fronts, each daily SST image is split into overlapping
windows of variable size for which SST distributions are cal-
culated and edges flagged. The VW-SIED algorithm optimizes
window size using semi-variance statistics, and th e overlap is
always half the size of neighbouring windows. When all of the
image is processed, individually detected edges that may be
disconnected are linked to each other with a contour-following
algorithm (see Cayula & Cornillon,
1992). After VW-SIED
algorithm processing, our dataset corresponds to daily
images indicating front/no front pixels with flagged pixels cor-
responding to ice or cloud. After front detection, statistics are
calculated using all available images (number of days × 25
years) to generate a frontal frequency index for each pixel
defined as:
f (%) =
#detected fronts
#cloud/ice free pixels
× 100%. (1)
Maps of phytoplankton biomass revealed by chlorophyll
(chl-a) concentration are also presented in this study. This
information was obtained through the Giovanni web portal
(http://disc.sci.gsfc.nasa.gov/giovanni) that provides an inter-
active tool to extract satellite-derived chl-a. We used that
tool to estimate a climatology of chl-a using the global time
series from the Sea-Viewing Wide Field-of-View Sensor
(SeaWIFS) (19982010). Unless otherwise stated, infor-
mation about bird colony locations was gathered from the
Important Bird Areas in Canada (www.ibacanada.ca) and
from the Canadian Wildlife Service (2012). Finally, bathy-
metric features were obtained from the 30 arc-second intervals
of the General Bathymetric Chart of the Oceans (GEBCO_08
Grid, version 20100927, http://www.gebco.net).
3 Observations and discussion
a Hudson Strait and Labrador Shelf
Hudson Strait, including Ungava Bay (Fig. 2), is a region
under the influence of freshwater runoff from Hudson Bay
and the high salinity waters of the Labrador Sea (Drinkwater,
1986). This region is known for strong tidal currents and high
elevation due to near-resonant M
2
frequency that enters the
strait from the Labrador Sea. Oceanic fronts are thus likely
to occur in this region as a result of water mass interaction
and tidal mixing. Since the pioneering work of Griffiths,
Pingree, and Sinclair (1981) who mapped tidal fronts from a
numerical study of the Simpson-Hunter parameter (Simpson
& Hunter, 1974), very little field work (e.g., Drinkwater &
Jones, 1987; Taggart et al., 1989) has been conducted to
confirm the existence of these fronts.
Some of the locations where high frontal frequencies are
observed corroborate predictions of Griffiths et al. (1981) in
western Hudson Strait (Fig. 2; frontal activity near Notting-
ham, Salisbury, and Charles islands). These areas are known
biological hotspots where higher phytoplankton biomass is
observed (Fig. 3) and where large bird colonies are found.
Besides these areas, our analysis also shows other regions
with high frontal frequencies. One of the most important
frontal region detected spans about 350 km on the southern
shore of Baffin Island, from west of Big Island to near Cape
Dorset. Its existence is likely not linked to tidal mixing
because the Simpson-Hunter parameter indicates a stratified
region (Drinkwater & Jones, 1987; Griffiths et al., 1981).
Because it follows the 200 m isobath, it may be the result of
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Fig. 2 Mean frontal frequency (19862010) for Hudson Strait, Ungava Bay, and the northern Labrador Shelf. Isobaths at 50, 200, and 1000 m are shown. The
locations of Cape Dorset (CD), Salisbury (SI), Nottingham (NI), Big (BI), Resolution (RI), and Akpatok (AI) islands, and Seven Islands Bay (SIB) are
shown. The red line near Akpatok Island corresponds to the cross-front transect described in Taggart, Drinkwater, Frank, McRuer, and Larouche (1989).
Fig. 3 Chl-a concentration climatology (19982010) for Hudson Strait, Ungava Bay, and the northern Labrador Shelf. The 50, 200, and 1000 m isobaths are
shown. See Fig. 2 for acronyms.
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the cold Baffin Island current that enters the strait from the east
and flows along the south edge of Baffin Island (Drinkwater,
1986; Straneo & Saucier, 2008). Another front is visible on the
opposite side of the strait, again following the 200 m isobath,
and may correspond to the northern edge of the Hudson Bay
outflow. Field observations of Hudson Strait inflow and
outflow along a transect offshore of Big Island revealed that
these are trapped within a distance of about 40 km from
each coast (Straneo & Saucier, 2008). This distance roughly
corresponds to the width of the sloping topography between
the coast and a little offshore of the 200 m isobath. Although
the frontal region al ong Baffin Island does not appear to be
linked to increased chl-a concentration, the region along the
Quebec coast does (Fig. 3). This may result from the different
nature of both currents. While the inflow current on the north
side of the channel is mostly barotropic and weakly stratified,
the outflow is strongly baroclinic, with maximum velocity in
approximately the upper 100 m (Straneo & Saucier, 2008).
The latter also carries nutrient-enriched water as the result of
strong tidal mixing that occurs in the strait (Drinkwater &
Harding, 2001).
At the eastern entrance to the strait, the most active tidal
mixing area is located off the southern tip of Resolution
Island. Our results show that fronts are more frequent a little
southeast of the island tip. Advection may be responsible for
the displacement of the surface signature of these fronts in
the direction of the outflow, since the study of Griffiths
et al. (1981) does not take into account the stabilizing effect
of runoff into their tidal mixing model (Drinkwater & Jones,
1987; Garrett, Keeley, & Greenberg, 1978; Sutcliffe,
Loucks, Drinkwater, & Coote, 1983). The entire region of
eastern Hudson Strait is characterized by higher chl-a concen-
tration with no obvious relation to the presence of that front.
In Un gava Bay, our analysis showed that the more frequent
fronts are located along the southern and eastern coasts. The
presence of rivers emptying large amounts of freshwater into
this area (approximately 4500 m
3
s
1
, seen as high chl-a con-
centrations on Fig. 3) is probably responsible for the gener-
ation of the observed fronts. One of the fronts predicted by
Griffiths et al. (1981) in Ungava Bay has already been con-
firmed by direct sampling (Taggart et al., 1989; red line in
Fig. 2). Our study shows that this front is likely small and
less frequent compared with the other fronts present in
Ungava Bay. Despite this, it is of high biological significance
as indicated by the presence of a large thick-billed mure
colony on Akpatok Island.
Along the Labrador Shelf, a straight and sharp front appears
close to the coast southeast of the mouth of Hudson Strait. This
front is likely the transition between coastal waters and Labra-
dor Shelf waters that flow on the shelf after their formation in
eastern Hudson Strait. The latter is a nutrient-rich mixture
formed from Hudson Bay freshwater outflow, Baffin Island
Current, and West Greenland Current waters (Drinkwater &
Harding, 2001; Dunbar, 1951; Sutcliffe et al., 1983). Despite
its persistence, this front does not appear to be directly
linked to high chl-a. The region of the northern Labrador
shelf is, however, reputed to sustain high primary production
throughout the summer (Drinkwater & Harding,
2001), and
a bird colony can be found at Seven Island Bay, close to
that area.
Further south, the most frequen t frontal activity on the inner
shelf occurs near the mouth of Byron, Groswater, and Sand-
wich bays (Fig. 4). The presence of these fronts might be
related to the inshore branch of the Labrador Current that
flows shoreward of Hamilton and Nain banks and recirculates
around them (Chapman & Beardsley, 1989; Colbourne,
DeYoung, Narayanan, & Helbig, 1997; Wu, Tang, &
Hannah, 2012). This likely causes a transition in temperature
properties between the current and the coastal waters
coming from the many fjords and bays in this area. Many
important bird areas are located in this vicinity.
At the shelf break, a persistent front is present offshore of
the 1000 m isobath (Figs 2 and 4). This front was briefly ident-
ified by Belkin et al. (2009 ) using 9 km pixel resolution and is
associated with the main branch of the Labrador Current. The
high frequency of this front is a signature of the stability of the
Labrador Current. The separation between this front and the
inner branch of the Labrador Current described above consists
of more complex flow patterns over the several banks in this
area (Drinkwater & Harding, 2001; Fissel & Lemon, 1991).
b Newfoundland Shelf
Offshore circulation on the northern Newfoundland Shelf is a
continuation of that on the Labrador Shelf resulting in a large
occurrence of fronts along the 200 m isobath. Inshore, high
frontal probability is found on the north shore of Newfound-
land ( Fig. 5), off the Northern Peninsula near the Strait of
Belle Isle. This front likely marks the limit between inner Lab-
rador Current (LC) inflow into the Strait of Belle Isle and the
outflow from the Gulf of St. Lawrence. Other locations with
high frontal probabilities are located off different bays
(Notre Dame, Bonavista, Trinity, and Conception). Those
are probably the result of onshore excursio ns of the inner
branch of the cold LC flowing close to shore, contrasting
with warmer coastal waters in these bays during ice-free
months. This area is characterize d by relatively high chl-a con-
centration near the shore (Fig. 6) and the presence of large
colonies of eiders, common mures, storm petrels, and spawn-
ing sites of capelin (Davoren, 2007).
On the southern shore of Ne wfoundland there is a very high
frequency of fronts (8%) on the eastern side of the mouth of
Laurentian Channel (Fig.
5). A recent numerical study (Wu
et al., 2012) reports that the LC extension that flows around
the Avalon Peninsula also splits in two branches, one
flowing west following the southern coast of Newfoundland
and the other, not previously reported in literature, flowing
south into Halibut Channel to the mouth of the Laurentian
Channel. The large number of fronts detected in this area
may be a result of the confluence of LC water flowing
through Halibut Channel with the LC flowing along the
Grand Banks towards the west and the outflow from the
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Gulf of St. Lawrence. Despite the large number of fronts, this
area does not appear to be characterized by a large concen-
tration of chl-a (Fig. 6).
Other high frontal probabilities are found south and west of
Green Bank and on the southern side of the Avalon Channel.
These can also be the result of local small-scale circulation
not well established in the literature. Fronts with lower prob-
abilities are also found in shallower areas close to the shore,
south of the Avalon Peninsula and southwest of Saint-Pierre
et Miquelon islands. The very low frontal probability over
St. Pierre Bank and the Grand Banks may indicate high
tidal mixing over these shallower areas. This important
mixing can contribute to the enhancement of the surface
temperature contrast between the banks and the surrounding
areas.
Another area where a large occurrence of fronts has been
detected is along the continental shelf from east of the
Grand Banks to Flemish Cap. This is an area of intense
current convergence between the North Atlantic Current
(NAC) and the LC generating the observed fronts (a complete
description of the frontal zone near Flemish Cap can be found
in Stein (2007)). Strong frontal activity is also present in the
southern part of Flemish Pass, possibly because part of the
Gulf Stream/NAC sometimes enters the pass from the south
and meets the LC (e.g., Gil, Sánchez, Cerviño, & Garabana,
2004). These areas of frontal activity are all characterized by
higher chl-a contents than the oceanic region (Fig. 6).
c Scotian Shelf and Gulf of Maine area
Many areas with high frontal probabilities are located along
the southwest tip of Nova Scotia, Grand Manan Island, and
the upper reaches of the Bay of Fundy (Fig. 7), confirming
previous observations that also showed these fronts to have
a strong seasonal variability (Ullman & Cornillon, 1999). In
this area, Garrett et al. (1978 ) and Loder and Greenberg
Fig. 4 Mean frontal frequency (19862010) for the southern Labrador Shelf. The 50, 200, and 1000 m isobaths are shown. Locations of Nain (NB) and Hamilton
(HB) banks and Byron (BB), Groswater (GB) and Sandwich (SB) bays are shown.
6 / Frédéric Cyr and Pierre Larouche
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Fig. 5 Mean frontal frequency (19862010) for the Newfoundland Shelf. The 100 and 1000 m isobaths are shown. Locations of the Strait of Belle Isle, the Northern
and Avalon peninsulas, the Gulf of St. Lawrence, Laurentian, Avalon, and Halibut channels, St. Pierre Bank, Grand and Green (GB) banks, Saint-Pierre et
Miquelon islands (SPM) along with Notre Dame (NDB), Bonavista (BB), Trinity (TB), and Conception (CB) bays are shown.
Fig. 6 Chl-a concentration climatology (19982010) for the inner Newfoundland Shelf. The 100 and 1000 m isobaths are shown. See Fig. 5 for acronyms.
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(1986) highlighted regions where tidal mixing is sufficiently
strong to sustain well-mixed water even in summer when
the heat flux from the atmosphere is high. These regions all
have nearby high frontal probabilities in the transition zone
between the well-mixed (usually low probabilities) and strati-
fied areas. In addition to tidal mixing, Tee, Smith, and Lefaivre
(1993) suggested that topographic upwelling can also generate
fronts at the southwest tip of Nova Scotia, off Cape Sable. All
regions mentioned above are reputed to be fish feeding and
spawning areas (Sinclair & Iles, 1985). High frontal probabil-
ities are also observed around Browns and Georges banks,
known for their enhanced tidal mixing and upwelling (e.g.,
Hannah, Shore, Loder, & Naimie, 2001; Mavor & Bisagni,
2001; Tee et al., 1993). This is consistent with the higher
chl-a concentrations observed in these regions where tidal
mixing occurs (Fig. 8). Another high frontal probability area
is located at the mouth of the Northeast Channel. These
fronts are not associated with higher chl-a concentrations
and are likely due to convergent water masses, where the
southwestward flow along the shelf edge enters the channel
and meets the northwestward moving slope water (Hannah
et al., 2001).
On the Scotian Shelf, the highest frontal probability occurs
around Sable Island bank, probab ly as a result of tidal mixing
combined with the anticyclon ic gyre that exists around the
bank (Hannah et al., 2001). Although less frequent, another
frontal area corresponds to the location of the inner shelf
current, shoreward of Lahave and Emerald basins. This
feature appears to be connected to the Sable Island Bank
frontal region. This observation supports the presence of a
cross-shelf current off Cape Canso that was recently detected
in a numerical study by Wu et al. (
2012). Finally, some frontal
activity is also visible in the Gully, possibly becaus e of the
northward excursion of the slope current in this gap on the
shelf break (Hannah et al., 2001). Of all these areas,
only the fronts near the southwest tip of Nova Scotia, at the
Sable Island Bank and along the cross-shelf front appear
associated with higher chl-a content (Fig. 8). This suggests
that these fronts are related to tidal mixing while the others
(e.g., the Gully and off Cape Canso) are related to current
convergence.
d Gulf of St. Lawrence
For convenience and in line with many regional studies (e.g.,
Bugden, 1981; Cyr, Bourgault, & Galbraith, 2011; Galbraith,
2006; Gilbert & Pettigrew, 1997), the Lower St. Lawrence
Estuary (LSLE) is considered here to be part of the Gulf of
St. Lawrence. Fronts and mesoscale dynamics are reputed to
be frequent features in the gulf as a result of high buoyancy
input from several rivers, combined with the importance of
Coriolis effects in this large system (Ingram & El-Sabh,
1990; Koutitonsky & Bugden, 1991). Such processes are
thought to greatly influence biological production and devel-
opment and the behaviour of species at different life stages
(e.g., Demers, Legendre, & Therriault, 1986; Le Fouest,
Fig. 7 Mean frontal frequency (19862010) for the Scotian Shelf, Gulf of Maine, and Bay of Fundy. The 50, 100, 200, and 1000 m isobaths are shown. Location of
the Northeast Channel, Cape Canso (CC), Cape Sable (CS), the Gully, Sable (SI), and Grand Manan (GMI) islands, Lehave (LB) and Emerald (EB) basins,
and Browns (BB) and Georges (GB) banks are shown.
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Zakardjian, & Saucier, 2005; Legendre & Demers, 1984;
Legendre, Demers, Therriault, & Boudreau, 1985). Meso-
scale th ermal fronts are also thought to be good proxies for
food availability for whales and thus closely related to their
dynamical habitat (Doniol-Va lcroze, Berteaux, Larouche, &
Sears, 2007).
The highest probabilities of frontal structures in th e Gulf of
St. Lawrence (Fig. 9) appear in regions with a low Simpson-
Hunter parameter (i.e., where tidal mixing is likely at work;
Lu, Thompson, & Wright, 2001, see their Fig. 10a). These
regions include the Strait of Belle Isle, western area of
Détroit de Jacques Cartier, the northeastern tip of Nova
Scotia, the mouth of Chaleur Bay, several spots in Northum-
berland Strait and, to a lesser extent, both tips of Îles de la
Madeleine. High frontal activity is also present along the
north shore of the Gulf of St. Lawrence and on the south
shore of Anticosti Island. For these locations, fronts are
likely the result of frequent upwelling due to dominant north-
western winds (Lacroix, 1987). The region of high frontal
probability at the southern entrance to the Strait of Belle Isle
is probably the result of the convergence between LC water
entering the Gulf of St. Lawrence from this strait and the
West Newfoundland Current (recently identified in Bourgault,
Cyr, Dumont, & Carter, 2014) that flows along the western
coast of Newfoundland.
Another prominent frontal structure visible in Fig. 9 corre-
sponds to the separation between the Gaspé Current, that flows
on the northern tip of Gaspésie peninsula, and the Anticosti
Gyre, a cyclonic cell located north of the peninsula and west
of Anticosti Island. Frontal activity is also visible in the tran-
sition zone between the LSLE and the Gulf of St. Lawrence
near Pointe-des-Monts, where a southward flow separates
both sub-regions and participates in channel-wide anticyclonic
eddies that evolve on a synoptic time scale (Koutitonsky &
Bugden, 1991; Larouche, Koutitonsky, Chanut, & El-Sabh,
1987). Relatively high frontal probabilities are seen through-
out the St. Lawrence estuary region. These result from la rge-
scale internal circulation features including cross-estuary
currents and gyres (El-Sabh, 1979; Ingram & El-Sabh, 1990;
Larouche et al., 1987) and freshwater plumes extending
from the estuarys north shore rivers.
Finally, although not shown in Lu et al. (2001), the front
present at the head of the Laurentian Channel near Tadoussac
is due to active tidal mixing that occurs on a sill that marks the
upstream limit of the LSLE. The relatively cold water that
results from this enhanced vertical mixing is a well-known
feature of the St. Lawrence (e.g., Gratton, Mertz, & Gagné,
1988; Ingram, 1976, 1979, 1983). A cold anomaly, as it
was referred to in Gratton et al. (1988), can also emanate
from this sill and travel up to 100 km on the south shore of
the LSLE, but the sporadic nature of such a phenomenon
makes it hard to detect from our 25-year time-series. In
winter (not shown here) the head of the Laurentian Channel
is sometimes referred to as the Tadoussac polynya because
its water rarely freezes during the cold season as a result of
upwelling water above the freezing point.
Fig. 8 Chl-a concentration climatology (19982010) for the Scotian Shelf and the Gulf of Maine. The 50, 100, 200, and 1000 m isobaths are shown. See Fig. 7 for
acronyms.
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Fig. 9 Mean frontal frequency (19862010) for the Gulf of St. Lawrence. The 50 and 300 m isobaths are shown.
Fig. 10 Chl-a concentration climatology (19982010) for the Gulf of St. Lawrence. The 50 and 300 m isobaths are shown.
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Some of the frontal features presented above are associated
with higher chl-a concentrations (Fig. 10). This is the case for
Îles de la Madeleine, the northeastern tip of Nova Scotia, and
to a lesser extent Jacques-Cartier Strait and the southern Antic-
osti coast. Jacques-Cartier Strait is known to support a com-
munity of marine mammals (Doniol-Valcroze et al., 2007;
Kingsley & Reeves, 1998) and is located close to the
Mingan Islands archipelago known to host a wide variety of
marine birds. Data for chl-a in the estuary, including the
Gaspé current, extension are unreliable because of the high
content of dissolved matter in this estuary (Nieke et al.,
1997). In Northumberland Strait, chl-a data are contaminated
by high concentrations of suspended matter (Ollerhead, 2005).
e Hudson Bay and Foxe Basin
Early thoughts from pioneering work on the Hudson Bay
system (Hachey, 1931; Huntsman, 1932; Vladykov, 1933)
suggested that the biological production of the region was gen-
erally low, driven in part by the short ice-free season and the
highly stratified surface water (see also Bajkov, 1975). More
recent work, however, suggests that the production is hetero-
geneous rather than weak, with higher phytoplankton and
chl-a concentrations near the coasts and in Hudson Strait
(e.g., Ferland, Gosselin, & Starr, 2011; Harvey, Therriault,
& Simard, 1997, 2001; Lapoussière et al., 2013; Roff &
Legendre, 1986). Small-scale mixing processes, such as tidal
mixing, upwelling, and river plumes, may thus contribute sig-
nificantly to the primary production of Hudson Bay. These
mixing processes are often accompanied by thermal front
signatures.
The frequency of fronts in the Hudson Bay system is pre-
sented in Fig. 11. The highest activity is nearly always in
the proximity of topographic features, with very low frontal
probability in the central part of the bay where the water
column is deeper than 100 m. Belkin et al. (2009) only
detected four major fronts in Hudson Bay: in northern
Hudson Bay, east of the Belcher Islands, in southern James
Bay, and in southwest Hudson Bay (see Fig. 63 in their sup-
plementary material). Our results confirm the presence of
these fronts, but the use of higher resolution SST imagery
better highlights the complexity of the frontal patterns.
Tidal amplitudes are small in the centre of the bay, increas-
ing to 1.25 m on the western shore. The southeastern portion
of the bay is also characterized by small tidal amplitudes
(<0.25 m; Prinsenberg & Freeman, 1986). Tidal mixing is
thus expected mostly along the western coasts. The position
of tidal mixing fronts predicted by Griffiths et al. (1981) and
verified by Drinkwater and Jones (1987) generally agree
with our observations for the southern tips of Southampton
and Coats islands and for the southern part of Hudson Bay
and James Bay.
The frontal pattern around the Belcher Islands (Fig. 12 )
allows us to hypothesize that vertical mixing is also import ant
in this area. This is supported by the recent study of Galbraith
and Larouche (2011) showing that this area has the second
coldest surface water in the Hudson Bay system. Since no
fronts were predicted by Griffiths et al. (
1981) near these
islands because the M
2
tidal amplitude is lower than elsewhere
on the coast (an amphidromic point is present north of the
islands), it is not clear whether this colder water is the result
of tidal or wind mixing.
The Belcher Islands region and its surroundings have been
reported to have some of the highest biological activity in the
Hudson Bay system (Anderson & Roff, 1980; Anderson, Roff,
& Gerrath, 1981; Harvey et al., 1997; Roff & Legendre, 1986).
Indeed, higher chl-a levels around the Belcher Islands are con-
sistent with the location of high frontal frequencies (Fig. 13).
Many of these frontal areas are also associated with the pres-
ence of marine birds.
James Bay also has high frontal probabilities. In contrast to
the Belcher Islands, James Bay, is the warmest part of Hudson
Bay, on average (Galbraith & Larouche, 2011). Together with
tidal mixing, river runoff can also explain the high frontal fre-
quency observed in James Bay. Contamination by dissolved
matter prevents accurate measurement of chl-a in James Bay.
The long front present onshore of the 100 m isobath in
western Hudson Bay (Fig. 11, south of Chesterfield Inlet)
may have a different origin than tidal mixing because the
front is detached from the coast. Galbraith and Larouche
(2011) reported this onshore region to be colder than the off-
shore region, creating a persistent thermal front. They hypoth-
esized that this is at least partly due to the advection of cold
water from northeastern Hudson Bay. What resembles a
plume from Chesterfield Inlet also creates a very persistent
front that is entrained by the general cyclonic circulation of
Hudson Bay. Besides water advection from the north, wind-
driven upwelling may also be responsible for the increased
presence of fronts along the western coast because winds
often blow from the southwest. The fact that there is almost
no increase in chl- a along that coast indicates that either the
upwelling process is not sufficiently intense or that the water
brought to the surface is low in nutrient content (Fig. 13).
Fronts observe d along the southern coast of Hudson Bay are
associated with freshwater plumes from the Churchill and
Nelson rivers with fresh water lying along the coast as it
moves cyclonically towards the east.
West of Hudson Strait, a long front stretches along the 100 m
isobath and marks the division between the shallow Foxe
Basin and the deeper Foxe Channel. In Foxe Basin
(Fig. 14), persistent fronts coexist with regions of very low
frontal probabilities (deep blue patch northeast of Foxe Penin-
sula). Front positions north of Prince Charles Island generally
agree with the study of Griffiths et al. (1981). Although no
fronts were predicted by these authors between this island
and Foxe Peninsula, our observations show that the strongest
fronts in Foxe Basin are located in this area, over a submarine
canyon of depth >50 m. This large front corresponds to a
region of westward current intensification that is part of a
cyclonic cell spaning most of southern Foxe Basin (Defossez,
Saucier, Myers, Caya, & Dumais, 2012, their Figs 5e and 5f).
The discrepancy between observed and predicted front
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locations found by Griffiths et al. (1981) may also come from
the poor bathymetry used by these authors.
Foxe Basin, except in the northwest, is generally thought to
have a well-mixed water column and very low biological
activity because no nutrient-rich deep water is available to
replenish the surface layer (Sibert et al., 2011; Smith et al.,
1985). In the northwest, higher biological production is sus-
tained by nutrient-rich water that flows through Fury and
Hecla Strait. The numerous and persistent fronts in the
eastern part of the basin probably originate from tidal
mixing as indicated by the near-freezing surface water
observed in that area during the warmest week of the year
Fig. 11 Mean frontal frequency (19862010) for Hudson Bay. The 100 m isobath is shown. Belcher and Coat islands are identified with the acronyms BI and CI,
respectively.
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(Galbraith & Larouche, 2011). Satellite chl-a data shows a
large patch of higher phytoplankton biomass located in south-
eastern Foxe Basin (Fig. 13) in the area where there is an
increased frontal presence. This indicates that nutrient replen-
ishment is possible, probably through the presence of the
deeper canyons. The discrepancy with numerical model
results (Sibert et al., 2011), which show almost no phytoplank-
ton biomass in that area, probably results from insufficient
spatial resolution of the model (10 km). Considering the rela-
tively low phytoplankton biomass generally observed in Foxe
Basin, this area may well be a local biological hot spot as
indicated by the large colonies of marine birds on Prince
Charles Island and the adjacent coast of Baffin Island. This
area is also known to be important for various marine
mammals.
f Baffin Bay
Baffin Bay is a large abyssal plain connected to the Arctic
Ocean through Smith, Jones, and Lancaster sounds and to
the Atlantic Ocean through Davis Strait. Very few studies
discuss the presence of fronts in Baffin Bay. Its remoteness
Fig. 12 Details of Belcher Islands and James Bay from Fig. 11. The 25, 50, and 100 m isobaths are shown.
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and the presence of ice for most of the year are factors explain-
ing this lack of information. Muench (1990) discussed the
presence of fronts east and west of Greenland, but the study
was limited to the Labrador Sea, without entering Baffin
Bay. He noted, however, the importance of fronts for vertical
and horizontal fluxes of dissolved material such as nutrients.
He also noted that frontal systems in polar seas often coincide
with the marginal ice zone.
Fig. 15 shows the frequency of fronts for Baffin Bay. The
area with the highest frontal frequency is located in Melville
Bay. This front was identified by Belkin et al. (2009) but
without indication of its much higher frontal probability
Fig. 13 Chl-a concentration climatology (19982010) for Hudson Bay. The 100 m isobath is shown. The Belcher and Coat islands are identified with the acronyms
BI and CI, respectively.
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compared with the rest of the western Greenland large marine
ecosystem (see Fig. 18 in their supplementary material).
According to Belkin et al. (2009), the Melville Bay front is
part of the Mid-Shelf Front (MSF) running along the west
Greenland coast. Our results show that the MSF is not well
detected by the variable-window approach, probably as a con-
sequence of its small surface temperature gradients. Only
some areas closer to the coast near topographic features,
such as islands and fjords, show higher frontal probabilities.
A succession of calving glaciers along more than 400 km of
the coastline in this area (e.g., Van As, 2011) may modify
water properties and partly explain the frontal characteristics
observed near the coast. This area along the west Greenland
coast is characterized by relatively high chl-a concentrations
up to 74°N (Fig. 16).
Quite surprisingly, even though Melville Bay has been a
well-known area for whales to concentrate since the mid-nine-
teenth century, it does not have high chl-a concentrations. This
indicates that the Melville Bay fronts do not generate nutrient
replenishment of the surface layer. It is probable that the nature
of these fronts is more rel ated to surface density gradients gen-
erated by the many glaciers in the area than to upwelling pro-
cesses. Very few oceanographic measurements were made in
this area, but there are some indications that surface coastal
waters of Melville Bay are fresher than further offshore, gen-
erating an important baroclinic density front that also has a
surface temperature signature (Lobb, Weaver, Carmack, &
Ingram, 2003; Valeur, Hansen, Hansen, Rasmussen, & Thing-
vad, 1996).
Further offshore, there is a higher frontal frequency located
above the 1000 m isobath, corresponding to the West Green-
land Current Front (WGCF; Belkin et al., 2009). Some
onshore excursions that coincide with the presence of cross-
shelf channels are also observed. On the western side of
Baffin Bay, the highest frontal frequencies are located at the
mouth of Lancaster Sound. High frontal probabilities are
found on both sides of the sound above the 500 m isobath.
Direct observations (current meters) and tracked icebergs
and drifters (de Lange Boom, Macneill, & Buckley, 1982;
Fissel, Lemon, & Birch, 1982; Marko, Birch, & Wilson,
Fig. 14 Details of Foxe Basin from Fig. 11. The 25, 50, and 100 m isobaths are shown.
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1982) have shown that the southward Arctic flow from Smith
and Jones sounds makes significant excursions into eastern
Lancaster Sound. The frontal structure of this current has
already been observed in conductivity-temperature-depth
transects (Fissel et al., 1982). Numerical simulations suggest
that the inflow (mostly a barotropic advection of salty
waters from the Baffin Current) enters the sound from its
northern side and recirculates cyclonically in the eastern part
before exiting on the southern side, where the outflow
(strongly baroclinic with maximum current velocity over the
upper 100 m) also carries fresher runoff from the sound
(Wang, Myers, Hu, & Bush, 2012).
Fig. 15 Mean frontal frequency (19862010), for Baffin Bay. The 500 and 1000 m isobaths are shown. The locations of the Carey Islands (CI), Nares and Davis
straits, and Smith (SS), Jones (JS), and Lancaster (LS) sounds are shown.
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The high frontal probabilities observed off the mouth of
Lancaster Sound do not have a clear generation mechanism.
Very few measurements are available from that area, but the
study of Fissel et al. ( 1982 ) indicates that the area is character-
ized by small and variable currents. This is confirmed by
results from numerical experiments (Wang et al., 2012).
This area is, however, associated with higher levels of chl-a
(Fig. 16) suggesting the presence of a sustained phytoplankton
biomass possibly through a nutrient-regeneration mechanism.
The entire area near the entrance to Lancaster Sound is also
known as a hot spot for marine birds (Wong, Gjerdrum,
Morgan, & Mallory, 2014) and marine mammals
Fig. 16 Chl-a concentration climatology (19982010) for Baffin Bay. The 500 and 1000 m isobaths are shown. See Fig. 15 for acronyms.
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(Davis & Koski, 1980; Davis, Richardson, Johnson, &
Renaud, 1978).
Along the 1000 m isobath on the west side of the Baffin Bay,
the front that could be expected because of the presence of the
southward flowing Baffin Island Current (Tang et al., 2004; Wu
et al., 2012) is not clearly visible until it passes Davis Strait. At
this point, the current intensifies and meets the northernmost
extension of the West Greenland Current, generating a long
front, extending from Davis Strait to the mouth of Hudson
Strait, above the 500 m isobath (Wu et al., 2012). The area
where the two currents meet has been identified as a high
primary production region (Tremblay et al., 2012).
Northern Baffin Bay is the host of the North Water (NOW)
polynya, one of the largest and most biologically productive
polynyas in the world (see Barber, Marsden, Minnett,
Ingram, & Fortier, 2001; Deming, Fortier, & Fukuchi, 2002,
and articles therein for an exhaustive study of the polynya).
Ingram, Bâcle, Barber, Gratton, and Melling (2002) defined
the spatial extent of the polynya from Smith Sound, where
constriction of ice in Nares Strait is responsible for the
opening of the polynya (e.g., Dumont, Gratton, & Arbetter,
2009; Melling, Gratton, & Ingram, 2001) to 76°N, an arbitrary
limit. The entire polynya area is characterized by high satel-
lite-derived chl-a concentrations confirming the in situ
measurements carried out during the NOW polynya project
(Mei et al., 2002). Within the NOW polynya, some areas of
high frontal probability were found near the Carey Islands,
seaward of Jones Sound and southeast of Smith Sound near
Qaanaaq. A very large colony of marine birds is located on
Coburg Island at the entrance to Jones Sound.
g Northeast Pacific Ocean
Frontal probabilities for the northeast Pacific are presented in
Fig. 17. An extensive study of thermal fronts in the Pacific
Ocean and its marginal seas has already been published by
Belkin and Cornillon (2003). Major large-scale features high-
lighted in our study have thus already been described. To
avoid redundancy, we will limit ourselves to new features
found in our dataset that resulted from the higher resolution
(1.1 km versus 9.28 km) and different algorithm (VW-SIED)
used.
Fig. 17 Mean frontal frequency (19862010) for the northeastern Pacific Ocean. The 1000 m isobath (shelf break) is shown. Here, the states of Washington (WA),
Oregon (OR), and California (CA) are also shown.
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Fig. 18 Details of the Gulf of Alaska from Fig. 17. The 100 and 1000 m isobaths are shown. Hecate Strait is identified with the acronym HS.
Fig. 19 Chl-a concentration climatology (19982010) for the Gulf of Alaska. The 100 and 1000 m isobaths are shown. Hecate Strait is identified with the acronym
HS.
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Possibly the most striking difference between our study and
the Belkin and Cornillon (2003) study is the relative absence
of a distinct front for most of the open water (i.e., far from
the coast; Fig. 17). This difference illustrates how the spatial
resolution and the algorithm used in our study affect front
detection by selecting fronts where SST gradients are stronger
and at a smaller scale.
Figure 18 presents frontal probabilities for the Gulf of
Alaska. A major oceanographic circulation feature of the
Gulf of Alaska is the Alaska Current (see Reed and Shumacher
(1986) for an extensive review of the physical oceanography
of the Gu lf of Alaska). It is a wide current (400 km) that
flows along the western Alaska coast from Queen Charlotte
Islands to the head of the Gulf of Alaska (the northernmost
part of the gulf). Although Belkin and Corn illon (2003)
found that the Self-Slope Front (SSF) is associated with this
current, this front is not clearly visible in our 25-year high-res-
olution dataset (Fig. 18). It is possible that the meandering,
seasonal evolution and the presence of eddies (Belkin & Cor-
nillon, 2003; Crawford, Brickley, Peterson, & Thomas, 2005;
Fig. 20 Details of Vancouver Island and its surroundings from Fig. 17. The 100 and 1000 m isobaths are shown. The locations of Hecate Strait, the Strait of Georgia
(SG), the Fraser River (FR), and Juan de Fuca Strait (JFS) are shown along with the states of Washington (WA) and Oregon (OR).
20 / Frédéric Cyr and Pierre Larouche
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Ueno et al., 2009; Whitney, Crawford, & Harrison, 2005) scat-
ters the signal at 1.1 km resolution. The highest frontal prob-
abilities are found in the northern Gulf of Alaska, between
Cross and Prince William sounds and are more or less con-
strained between the coast and the 100 m isobath. Coastal
runoff from the Copper and Alsek rivers and the presen ce of
inlets, bays, and nearby glaciers and land-based ice fields
may be partly responsible for these high frontal probabilities.
Climatological chl-a data show higher concentrations along
the coast (Fig. 19), consistent with a previous study (Brickley
& Thomas, 2004). There are, however, strong seasonal
changes in chl-a that are not captured in this climatology
because the reduction in downwelling-favourable winds in
summer generally lowers the SST and increases chl-a concen-
trations along the coast (Waite & Mueter, 2013). Both SST
and chl-a variables have been found to be related, although
high chl-a concentration measurements may also be influenced
by high turbidity and freshwater in this area (Stabeno et al.,
2004). Except along its western coast, the strong chl-a concen-
trations observed within the Alexander Archipelago are not
associated with higher frontal probabilities. Further south, in
Hecate Strait, high frontal probabilities are found along the
northern Queen Charlotte Islands, consistent with those pre-
dicted by the Simpson-Hunter stratification parameter in this
Fig. 21 Chl-a concentration climatology (19982010) around Vancouver Island. The 100 and 1000 m isobaths are shown. See Fig. 20 for acronyms.
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area (Perryet al.,1983).Tidal mixing over the shallow regionsof
thestraitis the likely generationmechanism responsiblefor these
fronts that are linked to high chl-a (Fig. 19), nutrient, and plank-
ton concentrations (Perry et al., 1983). Humpback whales are
also present in this area (Dalla Rosa, Ford, & Trites, 2012).
High frontal probabilities are also observed west of Vancou-
ver Island (Fig. 20). These are probably driven by seasonal
wind-driven upwelling (Freeland & Denman, 1982) and are
associated with very high chl-a concentrations (Fig. 21). The
highest frontal probabilities near Vancouver Island are,
however, found at the entrance to Juan de Fuca Strait and in
the Strait of Georgia. The former is the location of a seasonal
eddy (Freeland & Denman, 1982) leading to cold surface
waters. Strong tidal mixing in the strait also leads to enhanced
nutrients in the surface layer. This area is known for the presence
of birds, fish, and whales (Dalla Rosa et al., 2012; Healey,
Thomson, & Morris, 1990; Vermeer, Butler, & Morgan,
1992), and a strong association was found between many
seabird species and temperature gradients in the coastal zone
along a transect extending off Juan de Fuca Strait (OHara,
Morgan, & Sydeman, 2006). In the Strait of Georgia, the high
frontal probability observed likely results from the strong fresh-
water outflow from the Fraser River (3800 m
3
s
1
). The pres-
ence of large amounts of dissolved and suspended matter in
that area makes comparison with climatological chl-a maps dif-
ficult. The high frontal probability observed along the Washing-
ton coast results from the presence of the California Current
System. The coastal upwelling typical of southward flowing
eastern boundary currents produces a strong thermal contrast
between warm offshore and cold inshore waters creating a per-
manent front. This is a well-known oceanographic feature pre-
viously discussed in Belkin (2002).
4 Conclusions
Using 25 years of satellite data, the results of this study show
the location of fronts in Canadian coastal waters. Many of
these fronts were detected for the first time because of the
use of increased spatial resolution imagery combine d with a
variable window-size detection algorithm. The results show
that many fronts are associated with increased biological
activity (chl-a concentra tion). However, some very prominent
fronts showed no signature on chl-a concentration images.
This shows that the front-generation mechanism is important
in determining the role of fronts on ecosystems. Because
fronts generated by processes such as upwelling and tidal
mixing inject nutrients in the surface layer, they may be
more efficient than density-driven fronts in sustaining
primary production and the upper trophic levels of the
marine food chain.
The aim of this study was to provide a reference document
which can be consulted in preparation for physical and bio-
chemical studies in Canadian coastal waters, or to interpret
results of previous campaigns using a broader perspective.
Results of this study can also be used for validating numerical
modelling studies. For example, many fronts described in this
paper likely correspond to currents (e.g., in Halibut Channel
and off Cape Canso) or gyres (e.g., in Foxe Basin) that were
only identified in numerical results.
Finally, the datasets used in this study are enormous and can
still be exploited. In particular, the 25-year time series can be
used to study the temporal evolution of fronts in the context of
environmental changes. Also, the daily images of fronts allow
tracking of their seasonal evolution. These studies could be
carried out on a broad scale (e.g., Canadian waters) or in
specific subregions.
Acknowledgements
This work was made possible by a grant from the Canadian
Space Agency and is a contribution to the scientific program
of Québec-Océan and Fisheries and Oceans Canada. Thanks
to Alexandre Livernoche, Jonathan Graveline, Guillaume Des-
biens, Julien Laliberté, and Audrey Patry-Quintin for their help
with data processing. We also acknowledge the helpful com-
ments from Dr. Igor M. Belkin and an anonymous reviewer.
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Canadian Meteorological and Oceanographic Society
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... Therefore, this area appears to have long-term consistency in bloom dynamics. Overall, blooms dynamic in bioregion #1 generally follows wind-driven upwelling events that resupply nutrients into the euphotic zone through vertical advection (Sackmann et al., 2004;Foukal and Thomas, 2014;Cyr and Larouche, 2015;Jackson et al., 2015). ...
Article
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Classifying the ocean into regions with distinct biogeochemical or physical properties may enhance our interpretation of ocean processes. High-resolution satellite-derived products provide valuable data to address this task. Notwithstanding, no regionalization at a regional scale has been attempted for the coastal and open oceans of British Columbia (BC) and Southeast Alaska (SEA), which host essential habitats for several ecologically, culturally, and commercially important species. Across this heterogeneous marine domain, phytoplankton are subject to dynamic ocean circulation patterns and atmosphere-ocean-land interactions, and their variability, in turn, influences marine food web structure and function. Regionalization based on phytoplankton biomass patterns along BC and SEA's coastal and open oceans can be valuable in identifying pelagic habitats and representing a baseline for assessing future changes. We developed a two-step classification procedure, i.e., a Self-Organizing Maps (SOM) analysis followed by the affinity propagation clustering method, to define ten bioregions based on the seasonal climatology of high-resolution (300 m) Sentinel-3 surface chlorophyll-a data (a proxy for phytoplankton biomass), for the period 2016-2020. The classification procedure allowed high precision delineation of the ten bioregions, revealing separation between off-shelf bioregions and those in neritic waters. Consistent with the high-nutrient, low-chlorophyll regime, relatively low values of phytoplankton biomass (< 1 mg/m 3) distinguished off-shelf bioregions, which also displayed, on average, more prominent autumn biomass peaks. In sharp contrast, neritic bioregions were highly productive (>> 1 mg/m 3) and characterized by different phytoplankton dynamics. The spring phytoplankton bloom onset varied spatially and inter-annually, with substantial differences among bioregions. The proposed high-spatial-resolution regionalization constitutes a reference point for practical and more extensive implementation in understanding the spatial dynamics of the regional ecology, data-driven ocean observing systems, and objective regional management.
... ~ 10 cm s -1 ) and a low sedimentation rate (e.g. 5 m d -1 , i.e. 16 days of sedimentation between the surface and 80 m depth,Turner et al. 2015) would place POM sampled at 80 m depth during the High Stratification Period ~ 130 km from surface waters where it originated. However, primary production and stratification are generally homogeneous around the Newfoundland Shelf(Craig and Colbourne 2002, Cyr and Larouche 2015, Pepin et al. 2017. Therefore, although b-POM may have undergone some advection during the High Stratification Period, this would not influence POM quality and quantity since the impact of stratification on primary production and vertical sedimentation rates are likely homogeneous across the Newfoundland Shelf. ...
Thesis
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Under climate change, Arctic and sub-Arctic Coastal Systems expérience one of the largest increases in stratification at the global scale due to warming and/or freshening of their surface waters. However, the subséquent impacts of these environmental changes on the functioning of Coastal benthic ecosystems is still poorly understood. This thesis aims to study how future increases in stratification in these ecosystems could affect the organic matter quality, pelagic-benthic coupling intensity and benthic food web structures. Two Coastal Systems subject to strong seasonal variations in sea surface température and salinity were studied: a high- arctic fjord (Young Sound, NE Greenland) characterized by strong haline stratification and a sub-Arctic archipelago (Saint-Pierre-et-Miquelon, Newfoundland continental shelf) exposed to strong thermal stratification. In the first part of this PhD we show that strong stratification reduces the quality of pelagic organic matter sources and intensity of organic matter transfers from surface waters toward the benthic compartiment. On the other hand, no impact was observed on the quality of benthic organic matter sources. In the second part we show that stratification does not alter benthic food web structures thanks to the high trophic plasticity of primary consumers and high levels of omnivory in the community. In addition, benthic primary production in Coastal environment could potentially provide an alternative source of organic matter to pelagic primary production for primary consumers during high stratification conditions. Through these results, we propose several conceptual models describing the potential évolutions of these ecosystems under climate change and we show the importance of considering the singularity of coastal ecosystems as well as their small-scale spatial variations.
... Our inflow velocities at Mooring F contained some component of cross-strait flow, differing from the eastern Hudson Strait section occupied by Drinkwater (1988). The crossstrait flow here is potentially driven by the frontal system described by Cyr and Larouche (2015). ...
Article
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Hudson Strait is the main pathway of heat, mass, and freshwater exchange between Hudson Bay and the Arctic and North Atlantic oceans. The outflow along the southern coast of the strait, a fresh, baroclinic jet directed toward the North Atlantic, has received more attention due to its potential impact on deep convection in the Labrador Sea. However, details about the westward, more barotropic inflow along the northern coast of Hudson Strait remain unknown due to a scarcity of observations. Hudson Strait inflow waters affect the physical and biogeochemical systems of the bay, as well as the marine ecosystem, which supports the livelihoods of many Indigenous communities surrounding the Hudson Bay region. Here, we address this knowledge gap by analyzing data from two hydrographic surveys and four moorings deployed across the strait from 2008 and 2009. Three moorings were deployed on the northern side of the strait to map the inflow, and one was deployed on the southern side of the strait to capture the outflow. Along the southern side, a stratified, fresh outflow was observed, consistent with earlier studies. Along the northern coast, the inflow is weakly stratified and more saline, with seasonal changes distributed throughout the water column. Source waters of the inflow stem mainly from Arctic Water in the Baffin Island Current. A comparison with historical data suggests that after modification in western Hudson Strait, part of the inflow enters northern Hudson Bay to become deep water in the bay.
... Nutrients, phytoplankton, and zooplankton which were trapped in the convergence zone when water masses from different origins met, had an impact on increasing productivity (Sholva et al., 2013). Thermal fronts could be induced not only by upwelling and tidal mixing (Cyr and Larouche, 2015), but also by various forces such as tides, river flow, current convergence, wind, solar heating, and bathymetry (Valavanis et al., 2009). ...
Article
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Indonesia is a region that is directly adjacent to the Pacific Ocean and the Indian Ocean which allows a thermal front phenomenon. The purpose of this study was to identify the presence of thermal fronts based on seasonal variations and inter-annual variations in Indonesia waters. The data used in this study were Sea Surface Temperature (SST), Ocean Nino Index (ONI), and Dipole Mode Index (DMI) from January 2007 - December 2017 (11 years). The SST data was a level 3 Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) satellite image data with a resolution of 4 km. SST data was analysed using remote sensing techniques and Geographic Information System (GIS). The results of this study indicated that the distribution of SST and thermal fronts were influenced by seasonal variations and inter-annual variations. The highest average thermal front event in Indonesian waters occurred in a combination of El-Nino and Positive Indian Ocean Dipole (IOD) conditions. The highest average thermal front incidence in Indonesian waters also occurred during the East Season, while the smallest average occurred during the Transition Season II. During West Season, Transition Season II, and East Season, the largest number of thermal fronts was found in Western Indonesian Waters. Meanwhile, in the Transition Season I, the largest number of thermal fronts was found in Central Indonesian Waters. Thermal fronts were often found when the SST of Indonesian waters were cooler than normal conditions. The cooler condition made the SST more heterogeneous and formulated the thermal front phenomenon.
... ~10 cm s − 1 ) and a low sedimentation rate (e.g. 5 m d − 1 , i.e. 16 days of sedimentation between the surface and 80 m depth, Turner, 2015) would place POM sampled at 80 m depth during the High Stratification Period ~130 km from surface waters where it originated. However, primary production and stratification are generally homogeneous around the Newfoundland Shelf (Craig and Colbourne, 2005, Cyr and Larouche 2015, Pepin et al. 2017. Therefore, although b-POM may have undergone some advection during the High Stratification Period, this would not influence POM quality and quantity since the impact of stratification on primary production and vertical sedimentation rates are likely homogeneous across the Newfoundland Shelf. ...
Article
In response to ongoing global climate change, marine ecosystems in the northwest Atlantic are experiencing one of the most drastic increases in sea surface temperatures in the world. This warming can increase water column stratification and decrease surface nutrient concentrations, in turn impacting primary productivity and phyto-plankton assemblages. However, the exact impacts of these changes on sources and quality of organic matter as well as its transfers to the benthic compartment remain uncertain. This survey characterized organic matter sources and quality within a highly-stratified sub-Arctic coastal system (Saint-Pierre and Miquelon) and described its transfer towards a biomass-dominant primary consumer, the sand dollar Echinarachnius parma. This study analyzed fatty acid and stable isotope (δ 13 C and δ 15 N) composition of surface and bottom Particulate Organic Matter (s-POM and b-POM, respectively), Sedimentary Organic Matter (SOM) and sand dollar tissue along a near shore to offshore gradient during two contrasting seasons associated either with sharp or weak water column stratification (i.e. High vs Low Stratification Periods). Results revealed high relative abundances of polyunsaturated fatty acids (notably macro-and microalgae markers) in POM during the Low Stratification Period while the High Stratification Period was characterized by elevated relative abundance of saturated fatty acids indicating a higher organic matter degradation state. In addition, strong seasonal differences were also observed in food availability with four-fold higher concentrations in total suspended solids during Low vs High Stratification Periods. These results suggested thus multiple negative effects of stratification on pelagic-benthic coupling and POM quality. Lower nutrient repletion of surface waters during period of sharp stratification diminishes pelagic-benthic coupling by reducing food availability, POM quality and vertical transfer of organic matter. By contrast, the sediment-based diet of E. parma showed a low spatiotemporal variability reflecting the homogenous composition of the SOM. This study suggests that intensified water column stratification due to increasing sea surface temperatures may modify the pelagic-benthic coupling and future quality and composition of POM pools.
... Le plateau continental de Terre-Neuve a fait l'objet de nombreuses études depuis les années 1980. Ces études utilisent des mesures in-situ (Han et al., 1993;Schwing, 1992a;Thiebaut and Vennell, 2010;White and Hay, 1994), satellitaires (Cyr et al., 2015;Han et al., 1997), ainsi que des modèles (e.g. Daifuku and Beardsley, 1983;Dupont et al., 2002;Schwing, 1992a;Sheng et al., 2006). ...
Thesis
Full-text available
Saint-Pierre-et-Miquelon (SPM) est un archipel français au large de la côte est du Canada.Cette région peu connue a fait l’objet de récentes observations. Elles ont révélé des oscillations inattendues des courants et des températures à des périodes de quelques heures à quelques jours.L’objectif de cette thèse est d’étudier les principaux processus physiques à l’origine de ces oscillations en hiver et en été.Dans une première partie, nous avons étudié les oscillations à des périodes de ~2-4 jours mesurées par deux courantomètres durant l'hiver 2014, qui pouvaient temporairement dominer les courants de marée. Un modèle numérique 2D a été implémenté à l'échelle régionale pour étudier ce processus. Les résultats des simulations montrent le rôle des tempêtes dans le forçage de ces oscillations, par le biais d’ondes piégées par la topographie.Ces ondes se propagent sur le plateau à l'échelle régionale vers l’équateur. Elles excitent des ondes locales qui font le tour de l’archipel dans le sens horaire, en approximativement une période.Dans une seconde partie, des oscillations diurnes des températures et des courants ont été révélées par les observations pendant les étés 2015-2017. Un modèle numérique 3D forcé par la marée a été implémenté à l’échelle locale avec des conditions de stratification estivale. Le modèle montre que ces oscillations résultent d’une amplification des courants de marée diurnes. Elles sont la signature d'une onde piégée par la topographie qui tourne autour de SPM dans le sens horaire en deux jours, ce qui correspond à deux longueurs d’onde. Le modèle montre également que la zone de génération de ces ondes se situe au nord-ouest de SPM.
... Nutrients, phytoplankton, and zooplankton which were trapped in the convergence zone when water masses from different origins met, had an impact on increasing productivity (Sholva et al., 2013). Thermal fronts could be induced not only by upwelling and tidal mixing (Cyr and Larouche, 2015), but also by various forces such as tides, river flow, current convergence, wind, solar heating, and bathymetry (Valavanis et al., 2009). ...
Conference Paper
Indonesia is a region that is directly adjacent to the Pacific Ocean and the Indian Ocean which allows a thermal front phenomenon. The purpose of this study was to identify the presence of thermal fronts based on seasonal variations and inter-annual variations in Indonesia waters. The data used in this study were Sea Surface Temperature (SST), Ocean Nino Index (ONI), and Dipole Mode Index (DMI) from January 2007-December 2017 (11 years). The SST data was a level 3 Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) satellite image data with a resolution of 4 km. SST data was analysed using remote sensing techniques and Geographic Information System (GIS). The results of this study indicated that the distribution of SST and thermal fronts were influenced by seasonal variations and inter-annual variations. The highest average thermal front event in Indonesian waters occurred in a combination of El-Nino and Positive Indian Ocean Dipole (IOD) conditions. The highest average thermal front incidence in Indonesian waters also occurred during the East Season, while the smallest average occurred during the Transition Season II. During West Season, Transition Season II, and East Season, the largest number of thermal fronts was found in Western Indonesian Waters. Meanwhile, in the Transition Season I, the largest number of thermal fronts was found in Central Indonesian Waters. Thermal fronts were often found when the SST of Indonesian waters were cooler than normal conditions. The cooler condition made the SST more heterogeneous and formulated the thermal front phenomenon.
... In Atlantic Canada, studies documenting SST patterns have been prolific, especially in recent years. As a result, the basic patterns of spatial and temporal variation in SST are reasonably well understood for oceanic waters off Nova Scotia, particularly on the Scotian Shelf (Hutt et al., 2002;Cyr and Larouche, 2015;Loder and Wang, 2015;Hebert et al., 2016;Larouche and Galbraith, 2016;Richaud et al., 2016;Thomas et al., 2017;Greenan et al., 2018). ...
Article
Full-text available
Sea surface temperature (SST) is a frequently monitored variable in marine sciences because it is related to various oceanographic processes and influences marine species distribution. Thus, spatial and temporal patterns in SST are well understood for many regions of the world. In Atlantic Canada, surveys have often covered the Scotian Shelf, but there is a scarcity of data for intertidal environments along the Nova Scotia coast. Using in-situ data loggers, we measured SST every day between 1 May 2014 and 31 October 2018 at nine wave-exposed intertidal locations spanning the full Atlantic coast of mainland Nova Scotia, nearly 415 km. The resulting data set is freely available through this publication. Overall, the expected warming towards the summer and cooling towards the winter occurred along the entire coast every year. However, the latitudinal SST trend exhibited a winter-to-summer reversal, as SST generally decreased northwards in winter but decreased southwards in summer on this cold-temperate coast. In summer, cooling events were frequent and often pronounced in the south, SST sometimes dropping by 10 °C in 5-10 days to reach values below 5 °C, but weakened progressively towards the north, SST never dropping below 10 °C at our northernmost location. These SST patterns are suggestive of alongshore differences in summer coastal upwelling. Because of such a seasonal reversal in the alongshore SST trend, the annual SST range increased from south to north. This data set represents the first baseline on intertidal SST generated for the Nova Scotia coast. Further monitoring is encouraged to understand how decadal-scale oceanographic variability and anthropogenic climate change may influence these patterns.
Article
The spatiotemporal distribution of phytoplankton biomass drives the marine ecosystem. Chlorophyll-a concentration (Chla) is a biomass index for microalgae in seawater that is commonly used to study phytoplankton by means of satellite remote sensing. The St. Lawrence Estuary and Gulf (SLEG), in Eastern Canada, is a highly dynamic subpolar environment characterized by complex marine optical properties that make it difficult to distinguish Chla from the background signal caused by a strong freshwater discharge. In this study, we implement an inverse model based on a set of in situ Chla processed through a principal component analysis, making it specifically designed for the local marine optical conditions. We used this model to convert a multi-mission remote sensing reflectance dataset to daily Chla between 1998 and 2019 at a 4 km spatial resolution. From the resulting Chla time series, we computed the climatology, phenology and trend over the SLEG. The Chla climatology reveals the presence of relatively high Chla in the Gaspé Current, along the Gulf's North Shore and in areas of strong tidal mixing. Important differences in the phytoplankton phenology between the various subregions are found, with a prevailing shift towards earlier spring blooms of larger intensity. Finally, we found a positive mean Chla increase of 1.1% yr⁻¹ over the SLEG, with strong positive trends in the Magdalen Shallows and west of the Anticosti Island. This description of the surface Chla in the SLEG provides important baseline information for the marine ecosystem.
Article
This paper describes the physical oceanography of Fortune Bay, a broad, mid-latitude fjord located in Newfoundland (Canada). Fortune Bay is subject to a strong seasonal stratification (0–16 °C sea-surface temperature range with up to 1 °C/m vertical gradient) influenced by local freshwater runoff, wind forcing and shelf inputs. Sea-ice is seldom present in the bay and unlikely to be of importance on the seasonal stratification and mixing processes. Fortune Bay is warmer than its adjacent shelf both at the surface (by about 2 °C) and at intermediate depths (by about 1 °C from 50–150 m). While the former is likely due to local freshwater runoff stratification influence, the latter is probably related to the warm, deep water input occurring in winter below sill depth and subsequently mixed with the intermediate layer via the input of a colder water mass flowing in summer and which eventually reaches the bottom as well. Currents are dominated by the ‘weather band’ (2–20 d) and characterized by energetic pulses associated with downwelling and upwelling events. Mean circulation is rather weak and the seasonal pattern obtained her e did not reveal either the presence of a distinct estuarine circulation nor a strong influence of the main coastal current. Tidal currents are weak also and no inertial signal was observed. Estimates of water exchange between the inner and outer part of the bay were calculated using several methods and led to residence times of the order of a few to several months for the upper layers and of the order of a year for the bottom layer with a probable strong seasonal variability (larger residence time in summer for the upper layers). The “baroclinic pumping” processes, which include the downwelling/upwelling events, appear to be important players but more work is needed to better understand their nature and actual contribution.
Chapter
This chapter discusses tides in Hudson Bay that are classified as semidiurnal and only a small region along the eastern shore is classified as mixed but mainly semidiurnal. The semidiurnal tidal amplitude ranges from 1.25 m along the western shore to 0.10 m along the eastern shore. The maximum tidal currents are observed at the entrance to Hudson Bay where they reach velocities of 90 em s−1; smaller maximum values of 30 em s−1 occur within Hudson Bay. Tidal and wind-generated currents determine the location where strong mixing occurs and where the strong stability of freshwater plumes breaks down. The modeling research addresses not only the ice cover on the tide, but also the discrepancy in the offshore data relative to model results. Year-round offshore data within Hudson and James Bays are required to obtain reliable amplitudes and phases of offshore tidal heights and currents.
Chapter
Mesoscale features found in the St. Lawrence estuary range from those encountered in much smaller estuaries to those occuring on continental shelves. The influence of freshwater input, wind forcing and tidal processes on the distribution of temperature, salinity and current fields are discussed. Mechanisms responsible for variability of frontal phenomena, eddy formation and coastal jet dynamics are considered as a function of seasonal changes in the forcing variables. Comparisons of features in the St. Lawrence and in similar regions elsewhere are presented
Article
Several thermal images taken by the HCMM satellite between May 1978 and April 1979 were analysed in order to identify the surface circulation pattern of the marine estuary of the St. Lawrence River. Digital analysis consisted primarily in accentuating the contrast and then filtering and producing the equidensity of the resulting image.The images revealed interesting and new characteristics of the surface circulation in this region. Particularly in the image of August 7, 1978, two eddy systems were evident: one cyclonic, the other anticyclonic. Previous oceanographic studies had only been able to surmise their existence. Other phenomena were evident on the image, such as a resurgence at the mouth of the Saguenay River, a major tributary, a density front off Pointe des Monts at the contact with the Gulf of St. Lawrence, and two small eddies related to that front. These two phenomena have never been observed before. The HCMM images have provided, for the first time, an overview of the surface circulation and its evolution in time. Furthermore, different patterns observed on thermal images of 1978 agree with oceanographic data of 1979.
Article
Vertical mixing of the water column in estuaries and other coastal environments requires an input of mechanical energy that is mainly provided by the tides, the wind stress on the water surface and the fresh water runoff. Variations in these three sources are known to have a marked influence on the phytoplankton. At the seasonal scale, river runoff has been identified as an important driving force of phytoplankton dynamics. For example, Gilmartin (1964) has shown that the increased river runoff during the winter in Indian Arm (a fjord of Western Canada) destabilizes the water column which favors the replenishment of the surface mixed layer in nutrients. These nutrients are subsequently used for the initiation of the phytoplankton spring bloom, when the water column stabilizes following the decrease in runoff. At the time scale of a few days, physical transient phenomena (wind storms, periodic upwelling, etc.) associated with the passage of frontal disturbances (Heath, 1973; Walsh et al., 1977) are also strong destabilizing agents of the water column. Iverson et al. (1974), Takashi et al. (1977), Walsh et al. (1978), Walsh (1981) and Legendre et al. (1982) have reported intermittent phytoplankton blooms following stabilization of a water column previously destabilized by strong winds.