 |
| Ecosystem
Page 1
2
3
4
5
6
7
|
|
The Shifting
Sands of Prince Edward County
Since the last ice age, waves
driven against the shores of
prevailing southwesterly winds
in eastern Lake Ontario have
created some of the most extensive
baymouth sandbar and coastal
sand dune systems in the world.
The Sandbanks area is located
in Prince Edward County, an
island (formerly an isthmus,
before the construction of the
Murray Canal in 1889) that juts
into the Lake just south of
Belleville. Now a popular Southern
Ontario recreational destination,
the area includes 2 specialized
ecosystem features:
• Open dunes, 12 -
25 metres high, with juniper heaths,
and pine, maple, hemlock and cedar
forests
• Pannes, or shoreline
fens, meadows situated on the
moist sand of the lake plain.
Land-clearing by early Prince
Edward County farmers contributed
to the erosion of the dunes,
but recent efforts to stabilize
the sands have revived unusual
arid plant species such as bluet,
hoary puccoon, sea rocket, sand
spurge and butterfly weed.
|
|
Great Lake Genesis
To the sailor scanning the limitless horizons
of Lakes Superior, Huron, Michigan, Erie or
Ontario, it may seem impossible to believe
that North America's vast freshwater seas
have not always existed. But today's lobe-like
constellation of Great Lakes waterways is
barely more than 10,000 years old, born only
in the last few seconds of Earth's geological
history.
Ancient Pre-Cambrian rock is the basement
foundation laid under the Great Lakes over
3 billion years ago, forming a giant granite
basin. Beginning about 600 million years
ago,
during the Paleozoic Era, the basin was flooded
by a broad, shallow sea filled with primeval
life forms such as corals, brachiopods and
mollusks. When the inland ocean drained
away
(about 300 million years later), a thick
sedimentary layer of lime silts, clays,
sands and salts
of marine life left deposits of limestone,
shale, sandstone, halite and gypsum on top
of the Pre-Cambrian shield.
Early
Erosion: Through the millennia, as
the earth heaved and the Appalachian mountain
range emerged, massive rivers rushed through
the heart of the continent, carving valleys
and etching patterns into the soft upper sediments
and penetrating the harder limestone. The
basins of the Great Lakes may have begun to
form during this period. Some researchers
have suggested that large scale movements
in the Earth's crust, combined with the powerful
water currents that they created, along with
the effects of the Earth's rotation, were
responsible for most of the excavation of
the Great Lakes.
Most geologists, however, believe that the
lake-making process began in earnest about
one million years ago, when the continental
glaciers of the Pleistocene Epoch began their
slow but relentless cycle of advance and retreat.
With each southward extension of the northern
polar cap, sheets of ice up to 2,000 metres
thick scoured the earth's surface, grinding
away hills and enlarging the valleys that
had already been created by the river systems.
(At a maximum depth of 405 metres, Lake Superior
extends deep into Pre-Cambrian rock, while
the shallower basins of Lakes Huron, Michigan,
Erie and Ontario are eroded into the upper
Paleozoic sedimentary layer.)
| The
Big Flip
As the surface of Lake Ontario
warms during the summer months,
a water layering process known
as thermal stratification takes
place. The deepest water (244
metres at its maximum, 86 metres
on average) is insulated from
the sun and stays cool, while
the water closest to the surface
forms a warmer, less dense surface
layer. At the height of the
summer, an in-between layer
forms, dividing the surface
layer from the cold, deep bottom.
These layers are known as:
•
Hypolimnion -
The cold bottom layer, with
low oxygen levels that become
even lower as summer progresses.
• Thermocline -
The thin middle layer between
the warm upper water and cold
bottom water. Water temperature
and oxygen levels within this
layer change dramatically from
upper to lower levels.
• Epilimnion - The
warm upper layer, rich in oxygen,
that supports most of the life
in the Lake, including algal
growth.
Four
Degrees and Falling:
In late fall, when the epilmnion,
or surface layer, cools to its
maximum density at a temperature
of 4 degrees Celsius, it begins
to sink. The deep waters of
the lake are displaced, causing
a mixing, or turnover, of the
entire lake. The same process
occurs again in spring, when
temperatures and densities of
deep and shallow waters change.
The turnover process is vital
to the health of the Lake, allowing
circulating water to come into
contact with the surface and
distributing much-needed oxygen
evenly throughout the lake.
Bobbing
Bacteria: Recent die-offs
of fish and fish-eating waterfowl
in Lake Erie, has been blamed
on the effect of invasive species
and the natural turnover process.
Deadly botulism bacteria, normally
confined to the lake bottom,
have been disturbed, circulated,
and biomagnified by zebra mussels
and round gobies. As the numbers
of both species increase in
Lake Ontario, similar bacterial
outbreaks may occur. |
|
Iroquois
Ancestor: About 12,000 years ago, as
the "Wisconsin" ice age drew to
a close, the melt water along the front of
the retreating glacier pooled into the depressions
that had been formed by the weight of the
ice. Enormous bodies of water - much larger
than the present Great Lakes - began to form.
Wherever it was freed from the weight of the
glacier, the land began to rise or rebound,
causing dramatic changes in the size, distribution
and drainage patterns of the glacial lakes.
With each new shift, or uplift, the lakes
spilled out in new directions, to the southwest,
to the south, and to the east.
Present-day Lake Ontario was born from the
giant glacial lake known as Lake Iroquois.
Water from the higher-level Lake Erie basin
drained over the Niagara Escarpment, to the
Lake Iroquois basin below. The ancient Lake,
shaped like Lake Ontario, was larger and deeper.
Whoosh! :
Glacial ice had retreated first from the western
end of the Lake Iroquois basin, sending its
outlet flow south toward New York's Hudson
River Valley. But when a massive plug of ice
finally pulled away from the Thousand Islands
(where it had been blocking flow in the basin's
eastern end), Lake Iroquois surged through
the newly-opened channel of the St. Lawrence
River. The Lake began to drain east toward
the Atlantic, and the eastern end of the Iroquois
basin sprang back from the weight of the glacier.
Water levels in Lake Iroquois began to drop
and the Lake's shoreline receded, beginning
the long, slow birth of present-day Lake Ontario.
|
|
|
|
|
|
|
|
|
