Lake Turnover
By R. Karl
Water is quite possibly the most
fascinating substance on earth. This is true for a number of
reasons, not the least of which is that without water, life as we
know it would not exist. In terms of humans, I recently heard of a
new way to calculate how much water one should consume each day:
Take one's body weight in pounds and divide it by two, and that is the
number of ounces of water required daily! I am unaware of the
requirements for other warm-blooded animal forms, but I do know that the
cold-blooded species so cherished by sports enthusiasts -- fish like
walleye, bass and northern pike -- require a lot of water:
fresh, clean and oxygen-laden H2O in order to survive.
And a simple process referred to as lake turnover plays a very critical
role in that survival process. Did I say simple? Well... it
is, and then again it isn't. But I'll try to describe the process
in as simple and non-technical a way as possible.
I began the article by stating that
water is fascinating, for a number of reasons. One of the more
interesting reasons is due to a water molecule's amazing structure - its chemistry
if you will. In nature, heat and cold change the physical
properties of all substances in predictable ways. In general, as
things get colder, they contract and become more dense/heavier; as they
get warmer, they expand and become less dense/lighter. This
principal holds
for solids, liquids and gases. Water (able to exist in all three
forms) as a liquid, contracts and becomes more dense as it gets
colder... at least to a point. And that point is where the oddity
owing to water's special chemistry comes into play, the one that keeps
your favorite lake -- and your favorite gamefish -- healthy and happy.
Something strange and wonderful
happens when water reaches a temperature of approximately 390
Fahrenheit (40 Celsius). Well, it is wonderful for
lakes and the life in them, not so for the roads on which we drive --
more on that later. In any case, as water cools to that
temperature, it does, as predicted, contract and become more dense,
ultimately sinking to the bottom of the lake and pushing the water it
has displaced to
the surface, where it too can cool. With continued cooling at the
surface, the
decreasing temperature should eventually cause all water to eventually freeze solid
and sink to the bottom... meaning that the lake would freeze from the
bottom up -- eventually destroying all life beneath the waves. Why
then does this not happen? The chemistry of the water molecule
dictates that at 390 (39.20 to be precise) Fahrenheit, water actually expands and
becomes less dense, allowing it to float above the
warmer water! The water that cools below that temperature, to 320, freezes
and stays on the top, effectively
capping the lake. it also stops further energy loss from the lake.
Everything beneath the surface of the ice never gets any colder than 390.
So how does all of this relate to
lake turnover (which by the way can take place once or several times per
season, depending on many additional factors)? Let's first
consider the fall turnover. Starting in the spring and over the
course of the summer, surface waters absorb a lot of the sun's energy
and can heat extensively, causing them to become quite buoyant.
Winds and storms can cause some mixing and do add some oxygen;
atmospheric oxygen is added by the air-water interaction to the oxygen
produced within the water by aquatic plants. But there is too much
difference in temperature between the surface water and that at depth to allow for complete mixing of all the water in the lake.
Because of the density-temperature relationship, many lakes in temperate
climates tend to stratify, that is, they separate into distinct layers.
The middle layer, known as the thermocline, acts as an effective barrier
to any mixing of the deeper waters. Toward the end of summer, the
deep water becomes quite depleted of oxygen because no mixing has taken
place.
As the days get shorter and cooler,
and energy is transported away from/out of the lake, mixing becomes
easier. At about 500, the cooler water at the surface
begins to sink into and through the thermocline, forcing warmer and less dense
water to the surface and erasing the temperature stratification built up
over the summer. At some point, the majority of the water in the
lake reaches an approximately uniform temperature. Now storms and
sustained high winds can begin to perform the task of overturning
and mixing all of the water in the lake -- referred to as fall
turnover. The deep water contains an abundance of decaying
matter and sulfurous gases; when it reaches the surface, it produces a
telltale odor that indicates the process has begun. Eventually the
turnover mixes fresh oxygen into the entire lake mass, replenishing the
deep waters with the life-giving stuff and cleansing the sulfurous fumes
from the water, allowing fish to return to the depths where they will
spend the winter months.
As winter approaches, the water that
has now reached 390 sinks to the bottom, allowing colder and
less dense, buoyant water to remain at the surface to freeze.
The ice thickens because it is not a good insulator; water in
contact with the underside of the ice cools further and freezes, adding
to the surface layer.
A stratification similar to that of
the summer months will occur in the water column during the winter months,
but not to as great an extent.
With the advent of spring, the warming/melting of the ice layer at the
surface and the much smaller temperature differences in the water
column, winds and storms are able to create a spring turnover
with little difficulty. As the waters continue to warm,
stratification begins again and the endless cycle continues.
Mother Nature has performed her timeless and never-ending task, adding
additional life-giving oxygen to the lake and readying it for the
onslaught of another season of fishermen.
Oh, yes -- remember the roads I
mentioned earlier? Well, the same temperature and density changes
that cause the expansion and contraction of water as it freezes and
thaws -- part of the natural and positive process for lakes -- works the
same way for the water that seeps into and under the roads on which we
drive. As it freezes and thaws, expanding and contracting as the
temperature continually moves up and down past 390,
ultimately causes the potholes and cracks that are headaches for drivers
and road crews alike. Just remember to take that bottle of beer
out of the freezer before it explodes!
As always, I hope to
see you On
the Lake!
