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Testing
the waters
Photo by Chris Hammond
Dr. Brad de Young
By
Alex Dalziel
SPARK Correspondent
The ecological situation of St. John’s harbour has been
a contentious issue in both the city and province for many years.
Recently, physical oceanographers from Memorial have waded in
and developed some pertinent insights.
St. John’s harbour is a place where human activity and the
marine ecosystem meet. A major factor in assessing the human
impact on the harbour is how quickly water circulates into and
out of the Narrows. A research team, consisting of Dr. Don Deibel
and Dr. Ray Thompson from the Ocean Sciences Centre, and Dr.
Len Zedel and Dr. Brad de Young from the Department of Physical
Oceanography, investigated how this circulation actually works.
Financial support came from Environment Canada via the Atlantic
Coastal Action Plan.
“Basically, we were looking at the exchange of water in
the harbour, trying to understand how water moved through the
Narrows, and what the ecological implications were for this,”
commented Dr. de Young. “(Local municipalities) dump biological
materials that use up oxygen in the water. If that water is not
replenished, then bottom water in the harbour could become anoxic.”
Such anoxic water is depleted of oxygen, and thus damages the
ecosystem. However, according to Dr. de Young, “This does
not seem to be happening.”
What they found, surprisingly, was that a powerful cycle of circulation
effectively replaces the harbour’s water. “The good
news here is that the exchange rate is reasonably substantial,”
Dr. de Young informed the Gazette. “The timescale
for the exchange of water is roughly five to 10 days, which is
not long.”
The work of Dr. de Young and his colleagues is an excellent example
of science in action. It involved postulating hypotheses, disproving
them, then attempting new theoretical solutions and creating
new approaches to test them. The researchers combined techniques
from physics, oceanography, and biology to establish a multidisciplinary
perspective on a practical problem in human-environment interaction.
Their first experiments showed their initial hypothesis on the
circulatory regime in the harbour to be too simplistic. “In
1999, we put a single current meter in the Narrows. The harbour
is kidney-shaped with a narrow channel entrance that is about
800 metres long and 200 metres across. We got some information,
but discovered that the channel was more complex than we had
thought originally,” Dr. de Young said.
Thus, they had to devise a new method to unravel the complexities
of the harbour’s circulatory system. “The following
year, we put five current meters across the Narrows, a couple
of hundred feet apart,” Dr. de Young explained. “With
these data in hand we were able to look at the exchange in and
out of the harbour.”
Two patterns can be discerned in the team’s data. “The
exchange in the harbour is not solely regulated by the tides
and the winds in the harbour. You expect the tides to be important
— they slosh back and forth, moving a volume of water equivalent
to the surface area of the harbour times the range of the tide,
roughly twice a day. This you would expect to see.
“However, in addition, during the summer, roughly every
four to five days, large exchange events take place,” he
said. “Water at mid-depth is coming in, and water at the
surface and bottom is going out. There is this big pumping going
on, driven by an external force. That was unusual – we did
not expect to see such energetic exchange during the
summer. ”
The researchers needed to posit a mechanism to explain this.
“That forcing of water is driven, we think, by wind,”
Dr. de Young said. “There are winds in Conception Bay and
outside St. John’s harbour that generate waves known as
Kelvin waves, that propagate by the harbour and basically lift
water up into the harbour basin.”
As with much good science, these findings have opened new vistas
for study. According to Dr. de Young, “There are some subtleties
in the data that we want to explore. Doing some numerical modelling
on these data is our next step.”
Dr. de Young’s research both extends the scope of physical
oceanography and contributes to the discussion on environmental
policy in Newfoundland and Labrador. Such work is an important
example of Memorial’s dedication to learning and its special
relationship to its home province.