Report of the thermal variability with waters of Fathom Five
National Marine Park between the 19th of July and the 19th of
October 2006.
Mathew Wells
Department of Physical and Environmental Science
University of Toronto
Email: wells@utsc.utoronto.ca
Measurements of the thermal stratification within the Fathom Five National
Marine Park between 19 July and 19 October showed the presence of strong
oscillations in the position of the thermocline. These oscillations moved the
thermocline as much as 20 m vertically and had a period of approximately 15-17
hours. These oscillations were initially forced by strong wind events but as they
persist for several days they were a ubiquitous feature of the water column during
the summer stratification. Hence when the water column is stratified these
oscillations may be the dominant physical mechanism responsible for dispersion of
biota and nutrients within the Fathom Five National Marine Park.
Three temperature moorings were deployed in the Fathom Five National Marine
Park during summer and early autumn in order to measure the temperature fluctuations
and the frequency of upwelling events. The moorings were located at three sites near the
under water escarpment the runs through the Fathom Five National Marine Park. The
locations of the moorings are shown in Figure 1(a). Their position relative to the
underwater escarpment is shown in bathymetry map of Figure 1(b). The first mooring
(labeled NW) was located to the west of the escarpment and north of Flowerpot island in
35 m of water, the second mooring (labeled SW) was located near the top of the
escarpment near Middle island in 40 m of water and the third mooring (labeled SE) was
located east of the escarpment in water 85 m deep.
The three moorings were deployed on the 19th of July. The moorings were anchored
at the base with 100 kg of gym weights. On each mooring, seven temperature data–
loggers were attached along a rope that was kept afloat by a sub-surface buoy. To alert
boating traffic to the underwater moorings, flashing yellow surface buoys were tethered
to the subsurface float. These were generously provided by Environment Canada. The
temperature data-loggers used were HOBO Underwater Temperature Data Loggers.
These have an accuracy of ±0.2°C and recorded temperature every 12 minutes for the 3
months of the field deployment. There are photographs of how the data-loggers were
attached to the rope in Figure 2(a) and 2(b). The surface floats initially used to warn boats
are shown in Figure 2(c), these were later replaced with the flashing warning buoys
shown in 2(d).
The three moorings were deployed to measure the frequency of upwelling events
and determine the temperature variability near the escarpment. I arranged the dataloggers so that there was adequate coverage around the depth of 15 m, where the
thermocline was expected. The previous observations of water temperature made by
Paolo Travaglini found that the average thermocline depth was at 15 m, with fluctuations
between 10-25 m depth (as shown in Figure 6a). Paolo’s observations of temperature
profiles were made to calibrate the speed of sound in water for the acoustic instruments
she was using as part of the ongoing bathymetry survey by the DFO. The SE mooring
had temperature data-loggers at depths of 5, 10, 15, 20, 40, 60 and 80 m. Unfortunately
on retrieval of the mooring the data logger at 20 m was broken and lost. This was a
shame as the strongest temperature variability was expected to be at 15-20 m depth. The
two other moorings (NW and SW) were located above the escarpment in water of 35 m
depth, and the data loggers were fixed on the rope at depths of 1, 6, 11, 16, 21, 26, 30 and
31 m.
The temperature time-series for the NW and SE moorings are plotted in Figure 3
and show strong variability in the position of the thermocline. The surface waters are
seen to warm to a maximum of 24oC in late July, after which there is a gradual cooling to
10oC by mid October. During this whole period the water column is stratified, and there
are frequent upwelling events. The timing of these upwelling events is well correlated
between the NW and SE mooring. The temperature time-series for the SW moorings are
plotted in Figure 4. The SW mooring moved at least twice, so that three different depths
are plotted in Figure 4. The first time the mooring moved was between August 4th and 5th,
when it was dragged from depths of 35 m to 45 m, when Eric Johnson, Lisa Tutty and
Alex Ragosnig were attaching the new flashing warning buoys. The second time the
mooring moved was during a storm that occurred sometime before September 25. The
strong wind driven currents dragged the mooring into deeper waters, and the surface buoy
was no longer visible. This mooring was later recovered after the heroic efforts of Scott
Parker, Glen Dunham and Jeff Truscott, who scuba dived for the mooring on October
4th.
The interesting thing about all the temperature times-series in Figures 3 and 4 is the
intense variability in the position of the thermocline, which moves by as much as 20 m in
a 16 hour period. I initially thought this would correlate well with the winds but the
upwelling of cold water was much more frequent than the strong wind events. In Figure 5
(a) I have plotted the winds (measured at the Environment Canada station on Cove
Island) above the temperature profile measured at the NW mooring. There are two
pictures, one showing the whole three months, and the other just the first 16 days. I
identified the strong winds events and marked these with red dot in the top figure, and a
red line in the temperature plot. The strong winds occur about every 3-4 days, while the
vertical movement of the thermocline is about every 15-18 hours. These movements in
the position of the thermocline represent internal seiche motions that can persist for
several days, before being excited again by the wind. The period of these seiche modes
in determined partially by the strength of the thermal stratification, the size of the Lake
Basin and, perhaps surprisingly by the Earth’s rotational period. I will explain this in
more detail when I discuss Figure 8.
The mean temperature profiles of the water column of Fathom Five National
Marine Park are plotted in Figure 6. In Figure 6 (a) I have plotted the data collected by
Paolo Travaglini in August 2005 and May 2006. Her data shows that the water column is
largely unstratified in May and has become stratified by August. The depth of the
thermocline is between 10-20 m, with warm water of temperature 22oC overlying colder
water of temperature 4oC. In Figure 6 (b) I have plotted the mean temperatures recorded
by my data loggers at the three moorings and plotted the mean profiles as red, blue and
black lines. This data for 2006 is very similar to Paola’s data from 2004, and shows
essentially the same water temperatures and the same mean depth of the thermocline. For
reference, I have also plotted in Figure 6 (b) the bottom water temperatures that Lisa
Tutty measured at her 60 sampling sites. When I offset her temperatures by 2 oC there is
very good agreement with my observed temperatures. I believe her temperatures were
systematically higher due to the warm weather that she did the experiment in. Lisa and
Alex took 2 litre water samples at every sampling site using a Nansen bottle. This was
hauled back onto deck, where the water temperature was measured using a large
electronic thermometer. As the air temperature was in the mid 20’s, I believe the
thermometer was much warmer than the water samples leading the systematic error.
The vertical movements in the thermocline resulted in strong temperature
fluctuations in the epilimnion. In Figure 7(a) I have plotted the mean temperature profiles
and the range of observed temperatures for each data logger, during the month of August.
In all cases there is very little variability in temperature below 30 m. There are
considerable departures from the mean temperature between 20 m depth and 5m below
the surface, indicative of the strong upwelling events of cold water that occurred almost
every day. This variability is quantified in the frequency histograms shown in Figures 7
(b-d), where I have plotted the relative frequency of a given temperature occurring at
each data logger during August. These figures show the same information as Figure 7(a),
and the tight distribution in temperature range at depth can be clearly seen. For depths
between 10 and 20, the distributions are much broader, reflecting the moving
thermocline. For instance at the NW mooring the temperature record at 11m depth shows
that the percentage of the time that the temperature is at 20oC is the same as when it is at
6oC.
I hope that the information in Figure 7 can be used to help define the suitable
habitats for fish and marine life in Fathom Five. This information should be useful to
estimating where the threatened short-jawed Cisco could live as an example. I recall Scott
Parker telling me that these fish will only live in environments that are between 4-6oC
and that experience minimal variations in temperature. At Fathom Five this means the
suitable habitat will be in the waters below 35 m depth. The influence of cold upwelling
of water is recognized as biologically important in a number of papers, for example
Forney (1971) showed that a short-term effect of an upwelling was the mortality of fish
and crayfish due to an internal seiche in Georgian Bay. In the long-term temperature
variability can lead to increased biodiversity, as was shown by McCabe and Cyr (2006)
who found that environmental variability in the water temperature promoted increased
diversity of algal species. Strong upwelling events have also been observed at Cape
Croker (Figure 9) where surface water temperatures fluctuated between 4 oC and 22oC
during the summer. These fluctuations caused a high level of stress in caged aquaculture
fish and ultimately lead the Nawash to abandon an aquaculture facility at this site.
The oscillations in the thermocline shown in Figure 3 have a period of about 17
hours. You can calculate this roughly by counting the number of oscillations that occur
over a given period. A more precise method is shown in Figure 8 where we plot a Fourier
transform of the NW mooring data. This shows a peak near the frequency of 1/17.1
hours. What is so special about a period of 17.1 hours is that it corresponds to the
frequency of an “inertial oscillation”. This type of movement of water in a large lake or
in the Ocean is due to the Coriolis force, which describes the tendency for moving objects
to be turned to the right in the northern hemisphere due to the Earth’s rotation. Due to
thermal stratification most wind driven currents in the ocean or the Great Lakes are not
quickly slowed down by drag from the bottom. So when a wind blows in a large lake the
resulting currents persist for several days and are turned by the action of the Coriolis
force. These inertial currents are the most common currents in the ocean and they have
been observed at all depths in the ocean and at all latitudes. The motions are transient and
decay in a few days, so that if the wind blows every 3-4 days, the observed inertial effects
will dominate the water column. The inertial period is one half the time required for the
rotation of a local plane on Earth's surface, which is described mathematically by this
formulae for the inertial period of Ti = 1 day /(2 sin(latitude)). At the latitude of
Tobermory (44o North) the inertial period is 17.1 hours, which is almost exactly what is
observed in the fluctuations of the temperature records.
The strong oscillations in the position of the thermocline are not unique to Fathom
Five National Marine Park; they are also observed at many sites around the Great Lakes,
such as 70km south of Tobermory at Cape Croker. In Figure 9 I have plotted the thermal
evolution of the thermocline between June 26 2002 and December 17 2002, at the
proposed site of an aquaculture facility owned by the Nawash. These measurements were
made by Professor Rich Moccia, of the University of Guelph. The oscillations in the
depth of thermocline are perhaps even more striking than at the Fathom Five National
Marine Park as they are much more periodic. This is seen in the much stronger signal in
the Fourier transform, shown in Figure 9 (d). The period of these oscillations is again
close to 17.1 hours.
There are several published observations of thermocline oscillations in the Great Lakes,
such as in Lake Michigan by Hawley & Muzzi (2003) and Hawley (2004), and the
observations in Lake Huron by Murthy & Dunbar (1981) and Lake Ontario by Rao &
Murthy (2001). The measurements by Hawley in Lake Michigan were made in water of
55 m depth, near Grand Haven and Muskegon in Michigan. During the month of August
the surface water temperature was 22oC, overlying colder water of temperature 4oC, with
a thermocline at a located at approximately 10 m. There were strong oscillations in the
depth of the thermocline between depths of 0 and 15 m, which has an oscillation period
of 17.6 hours. Associated with these movements of the thermocline, there were strong
oscillating currents in the water column, which had maximum amplitudes at depths
between 5 and 25 m and reached maximum speeds of 30 cm/s. The currents were out of
phase with the thermal oscillations, so that maximum currents occurred while the
thermocline was rising or falling, and minimum currents occurred at the limits of the
thermocline movement. At the base of the water column at 55 m depth these oscillating
currents had mean speeds of 2-5 cm/s s. Resuspension of sediments at the base of Lake
Michigan would periodically occur when these currents were at their strongest. Hawley
(2004) attributed these intertial waves as the main source of energy to form and maintain
the benthic nepheloid layer, and hence these currents played a significant role in the
transport of both suspended sediments and their associated pollutants. The recorded
velocity time-series of the stratified coastal regions of Lake Huron and Lake Ontario by
Murthy & Dunbar (1981) and Rao & Murthy (2001) also show a strong period of 17
hours. These two studies measured the time variation of temperature and velocity in the
coastal region during summer stratification and in both cases found that the dominant
mode of variability was due to near-inertial oscillations. The first study was made in Lake
Huron near the Bruce nuclear power station and the second in Lake Ontario near the
Darlington nuclear power station. All of these studies show that strong oscillations in the
thermocline depth are a ubiquitous feature of summer stratification in the Great Lakes.
We can expect that the currents associated with the oscillating thermocline at
Fathom Five National Marine Park will be of the same order of magnitude. As the
thermocline oscillations have a period of approximately 17 hours, we could expect to see
strong sustained currents with speeds up to 30cm/s for 7 hours, before the flow slows
down and changes directions. In 7 hours a current with a speed of 30 cm/s will travel 7.5
km, which is a very large fraction of the Fathom Five National Marine Park. For instance
the distance from the Cove Island lighthouse in the west to the Bears Rump Island in the
east is 14 km. These strongest currents are expected to occur close to the thermocline at
depths between 10-20 m. Based upon the bathymetry shown in Figure 1(b), I calculated
that approximately 20% of the seabed in the Fathom Five park is located in this depth
range (the exact percentages are shown in Figure 10). The presence of such strong
currents will presumably have strong implications for where fish prefer to live within the
park and how sediments are deposited.
This summer I will hope to use an Acoustic Doppler current profiler to measure the
currents within the Fathom Five National Marine Park. This instrument measures velocity
profiles by using acoustic pulses. It can be used from a moving boat to survey the water
movements. The use of such an instrument would allow us to measure the currents within
the Fathom Five National Marine Park and determine if the strongly correlate with the
oscillations of the thermocline, as has been observed at other sites. The instrument can
also estimate how turbulent the water column is, and so we can estimate the vertical
transport of nutrients within the water column. I hope that the data on water circulation
and temperature could then be related to the spatial distribution of marine biota, which
should be a very interesting scientific paper and useful for the ecological management of
Fathom Five National Marine Park. The only previous studies of the physical limnology
of Georgian Bay were made during the 1972-1973 International Field Year for the Great
Lakes, and are discussed in a book chapter by Bennett (1988). This report only describes
the mean water currents and does not recognize the important role that fluctuations in the
thermocline will have on horizontal dispersion of marine biology near the boundaries of
Georgian Bay.
Acknowledgements
This work would not have been possible without the extensive help of Scott Parker,
Eric Johnson, Glen Dunham and Jeff Truscott of Parks Canada, Paola Travaglini of the
DFO, and Lisa Tutty and Alex Ragnosig from the University of Toronto.
References
Bennett, E.B. (1988) On the physical limnology of Georgian Bay. Hydrobiologia, 163:
21-34
Emery, A.R. (1970) Fish and crayfish mortalities due to an internal seiche in Georgian
Bay, Lake Huron. J. Fish. Res. Board Can., 27:1165-1168.
Hawley, N. (2004) Response of the benthic nepheloid layer to near-inertial waves in
southern Lake Michigan. J. Geophysical Res. 109 (C04007):14
Hawley, N. and Murthy, C.R. (1995) The response of the benthic nepheloid layer to a
downwelling event. J. Great Lakes Research, 21, 641-651.
Hawley, N. and Muzzi, R.W. (2003) Observations of nepheloid layers made with an
autonomous vertical profiler. J. Great Lakes Res.h 29:124-133
McCabe, S.K. and Cyr, H. (2006) Environmental variability influences the structure of
benthic algal communities in an oligotrophic lake. Oikos 115: 197-206
Murthy, C.R. and Dunbar, D.S. (1981) Structure of the Flow within the Coastal
Boundary Layer of the Great Lakes J. Phys. Ocean. 11: 1567–1577
Rao, Y.R. and Murthy, C.R. (2001) Coastal Boundary Layer Characteristics during
Summer Stratification in Lake Ontario. J. Phys. Oceano. 31:1088–1104
Wuest, A. and Gloor, M. (1998) Bottom Boundary Mixing: The Role of Near- Sediment
Density Stratification Physical Processes in Lakes and Oceans Coastal and Estuarine
Studies 54:485-502
* There are some nice field measurements of the upwelling and downwelling events
measured by Hawley in Lake Michigan on this GLERL website
http://www.glerl.noaa.gov/res/Task_rpts/2002/pphawley02-2.html