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On land, we are used
to rapidly changing
temperatures.
The air is cool in
the morning, warm in
the afternoon, and
cooling again as
evening comes.
We are used to this,
and don't think
about it, but think
of what this must be
like for other life
forms.
For ectotherms, that
are the same
temperature as the
environment, the
rate of metabolism
and growth varies
over the course of
the day. For
endotherms (animals
here) that heat
their bodies by
burning energy, the
organism must
constantly work to
keep the internal
temperature
steady. When
we hear of people
suffering
hypothermia from
exposure to cold, or
dying of heat stroke
in the desert, we
know that their
bodies are not able
to adapt to the
changing temperature
of their
environment.
As
we think about life
forms in the ocean,
it is helpful to
think about
temperatures in the
water.
Temperatures affect
living organisms,
because life
processes are
chemical, and
chemical reactions
occur more quickly
in warm water than
in cold water.
Life forms in the
ocean are exothermic,
which means that
their cells are the
same temperature as
the water that
surrounds
them.
(Exceptions to this
are whales, seals,
and dolphins, which
are animals who have
returned to the
ocean after their
ancestors evolved as
land animals).
The temperature of
the water in which
these life forms
live is an important
factor for
them.
Respiration
is the word that
describes the
processes that
living organisms do
to stay alive.
It includes using
the food that they
eat and breathing
the gases that they
get from the
environment.
As respiration is a
chemical process, it
works more slowly in
cold organisms than
in warm ones.
This is especially
important for
exothermic (cold
blooded) animals,
who can get along
with very little
food in a cold
environment.
For organisms in the
water, temperature
fluctuations do not
really present a
problem. Water
has a high Specific
Heat,
which means that you
have to put a lot of
heat into water to
warm it up, or take
a lot of heat out of
water to cool it
down. On a hot
day the surface of
the ocean may get a
tiny bit warmer --
but it will not get
hot like the sand,
or the blacktop on
the path down to the
beach. On a
cold night the
surface of the water
may cool a little --
but not as much as
the land beside
it. The
temperature in an
aquatic environment
tends to be stable,
and to change very
slowly if it does
change.
However, as we
learned in The
Transfer of Heat:
Conduction,
Convection, and
Radiation,
warm air, water,
and melted rock tend
to rise, and cold
air, water and magma
tend to sink.
So it is reasonable
to wonder if the
great depths of the
oceans are also
cold.
Yes,
they are.
The chart below
tells you about
temperatures in the
ocean. On the
surface of the ocean
the sun warms the
waters, and winds
and water turbulence
stir the warm water
and mix it with the
cooler water below.
The mixing zone goes
down to somewhere
between 100 to 400
meters, depending on
the latitude and the
weather.
As you would expect,
the surface sea
water temperatures
are close to
freezing near the
poles.
You have seen
pictures of ice
sheets and ice bergs
in the high
latitudes. In
summer the ice
sheets break up and
the icebergs float
into waters in the
temperate zones and
melt
there.
In the tropics,
surface waters are
warm, sometimes
coming close to
human body
temperatures.
Over the whole
earth, the average
temperature of the
surface waters is
about 63o
Fahrenheit (17o
Celsius).
Below this mixing
zone, the waters
begin to get
steadily
colder. The
area in which they
are cooling is
called the thermocline.
No matter what the
surface temperatures
were, the water
cools down to nearly
0 o Celsius
(32o
Fahrenheit) and
stays at that
temperature.
About 90% of the
ocean water is below
the thermoclime..
The cold water below
the thermocline is stratified
(lies in layers,
like the sedimentary
rocks) by density,
which is dependent
on the water's
temperature and salinity
(saltiness).
Here
is a chart modified
from Windows to the
Universe which shows
the temperature
pattern in the
mid-latitudes.
Temperatures
are Fahrenheit or
Celsius,
The
Photic
Zone is the
area into which
sunlight
penetrates. It
might be this deep
in the open ocean,
but would be
shallower closer to
land.
This
is the part where
the water gets
colder very
quickly. It is
already very dark.
Photosynthesizers
cannot live here,
but fishes and other
animals can spend
time here.
Some of them come up
closer to the
surface to feed.
Down
here the water is
just a little above
freezing. It
is stratified in
layers by density. The saltiest water is at the bottom
of this zone.
Most
of the abyssal
plain is at
about this level. The
deepest point in the
ocean is 6.8 miles
(10.9 Kilometers)
deep.
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