Monday, September 3, 2012
Fall Turnover of Water in the Great Lakes
Here is an excerpt from my book The Dynamic Great Lakes
available on Amazon's Kindle and Barnes & Noble and many other stores.
Fall Turnover of Water
On a cold November night with no clouds, the reflections of the
moon and stars sparkle on the calm surfaces of the lakes, and the
silhouette of a vee shaped string of geese migrating southward
crosses the salmon colored moon. Cold north winds have cooled the
Great Lakes waters.
As the air cools, the water becomes cooler and cooler. When
water reaches 39.2/ F, it reaches its greatest density. Waves rolling
in on the beach look heavier, almost like boiling sugar water as it just begins to thicken. The fall turnover of water is about to occur,
an important event in the natural cycle of the Great Lakes. When
surface lake water reaches 39.2/ F, its maximum density, the water
sinks since the surface water is heavier than the water below.
The sinking top layer of water causes the lake water to turn over.
The fall turnover of water in the Great Lakes is important because
oxygen poor water in the deeper areas of the lakes mixes with
surface water containing more dissolved oxygen (DO). This keeps
the bottom from becoming depleted of oxygen.
Bottom dwelling fish and plankton need dissolved oxygen in
water just as we need oxygen in air. When the layers of water turn
over, there are no longer three layers of water since mixed water
results in uniform temperatures.