Our Maine Street's Aroostook Issue 16 : Spring 2013 | Page 28

Spring on the Lake
What Drives Lake Turnover?
Branching Out With STEM

It’s springtime in Aroostook. The sun has blazed back into
our lives. The seeds of May flowers are soaking up the
April showers. The snow and ice are melting, and lakes are
turning over.

above freezing, at 4 degrees Celsius (39 degrees Fahrenheit).
It then becomes less dense (lighter) as its temperature
increases or decreases from 39 degrees. All things equal,
buoyancy and density are inversely proportional, which
means that buoyancy increases as density decreases, or
Like most natural process, lake turnover doesn’t just happen lighter objects float higher in the water. For water itself, that
without a good reason. It’s the culmination of physical relationship means that water that is warmer or cooler than
processes and properties including ice melt; heat transfer; 39 degrees is more buoyant than, and will float on, water
water density (its mass
that is exactly 39 degrees. In
divided by its volume); and
lakes, the dense, deep water
buoyancy (the force that
is called the hypolimnion,
dictates whether an object or
and the lighter, higher water
liquid will float on another
is called the epilimnion.
liquid). Let’s take a closer
(A thermocline, which
look at these concepts and
is a distinct change in
their effect on our lakes and
temperature, often forms
ponds.
where the two layers meet. If
you’ve ever been swimming
Water is strange stuff: it’s one
at your favorite swimming
of the only substances that is
hole, and your feet hit the
less dense as a solid than it is
“cold spot,” you’ve found a
as a liquid. If it wasn’t, ice
thermocline.)
would sink to the bottom of
ponds instead of floating on
Toward the end of the
top. Give that a quick thought . . . In cold, deep ponds, winter, before the ice melts, the hypolimnion is close to 4
sunken ice would likely never thaw because there wouldn’t degrees Celsius, and the epilimnion is closer to zero degrees
be enough sunlight to warm it. In fact, some lakes might Celsius. (It’s true- the water at the bottom of the lake is
remain frozen solid year-round except for the top few feet warmer than the water at the top of the lake!) This is a
shallow enough to be warmed by the sun. It’s not hard to stable condition, with the less dense water floating on the
imagine the impact that would have on aquatic ecosystems more dense water, and the ice floating above it all. When
and fishing in Northern Maine!
the ice melts, however, the increased air temperature and
light penetration warms the epilimnion. At the point that
Water’s density also changes with temperature even when the epilimnion warms to 4 degrees Celsius, it matches the
it stays in a liquid state. It is most dense (heaviest) just density of the hypolimnion. Because the water column is
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SPRING 2013