Download The Ocean Environment

Oceanography 10, T. James Noyes, El Camino College
2A-1
The Ocean Environment
Living in the ocean is very different from living on land. In this section, we will explore some of
the physical conditions in the ocean which marine organisms must be able to tolerate and take
advantage of to survive and prosper in the ocean.
Density
The density of water is about 1000 time greater than the density of air. This means that water
provides a great deal more support than air. On land, we must use strong muscles (attached to
strong, heavy bones) and spend lots of energy to fight the force of gravity so that we can move
around – even to keep our blood circulating. In the ocean, the water itself lifts organisms up;
they spend much less energy to move around, because they do not need to be able to support
their own weight. Thus, the largest whales are larger than the largest land animals that ever
lived, the dinosaurs. Another good example is gelatinous animals (“jellies”): without the support
of water, their bodies collapse into a pile of goo in the air. Even if an organism’s density is
greater than the density of water, the water still provides a strong upward push, so even
organisms with weak muscles can still move around 1. The support is helpful in other ways, as
well. Our heart muscle has to be very strong to pump blood upwards through our body against
the force of gravity, something that ocean animals do not have to worry about.
Most ocean animals and algae are plankton: they try to float
The word “pelagic” usually refers
and are poor swimmers, so they drift with the ocean currents.
to plankton. In other words, it
Water’s high density makes the pelagic (planktonic) lifestyle – means “planktonic.” It can also
the floating lifestyle – possible. On land, everything lives on
be used to indicate the part of the
ocean far away from the coasts.
the ground 2, but in the ocean most of the life is above the
ocean floor. (In other words, on land we only have “benthos,”
no nekton or plankton). Most benthos and nekton start life as plankton (drifting larvae), so it is
safe to say the pelagic lifestyle is important for almost all life in the ocean. If water did not have
a high density, organisms would not be able to float, and they would need to be radically
different to survive.
Benthos
Nekton
Do Not Float
Poor Swimmers
Do Not Float Well Good Swimmers: can swim against waves and currents
Try to Float
Plankton
Poor Swimmers: waves and currents push them around
(“Drifters”) (low-density body)
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Water’s high density (and viscosity) make it more difficult to move in the ocean than on land in some ways.
For example, it is harder to push aside water than air. Thus, many ocean organisms are streamlined so that
they can “cut” through the water.
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We do not have “nekton” on land. The closest animals (birds and insects) typically land frequently, so even they
are more like “benthos” than “nekton.”
Oceanography 10, T. James Noyes, El Camino College
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Examples of Plankton. All of the organisms except the foram (left) can swim, but not well enough to resist ocean
currents. National Oceanic and Atmospheric Administration, Department of Commerce.
Examples of Nekton. All of these animals are strong swimmers.
National Oceanic and Atmospheric Administration, Department of Commerce.
Examples of Benthos. All of these animals live on the bottom of the ocean.
National Oceanic and Atmospheric Administration, Department of Commerce.
Oceanography 10, T. James Noyes, El Camino College
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Organism’s have a variety of adaptations which help them float, or at least sink slower. Many
organisms have gelatinous bodies (in other words, they are “jellies”), which are 95% water, so
they have nearly the same density as water. Another common adaptation is to fill one’s body
with low density substances like oils, fat, and even air. Marine mammals like whales have lots
of blubber which not only helps them float (very important since they get oxygen from air, not
water), but also stores food and provides insulation 3. Some diatoms (a kind of phytoplankton)
have a drop of oil in them 4. Kelp have bladders 5 filled with carbon dioxide, helping them reach
up towards the surface of the ocean to get sunlight. The low density air trapped in the shell of a
nautilus helps balance the high density of its shell.
Some fish have special organs called “swim
bladders.” They pump air into or out of their bodies
to lower or raise their density. (Air adds more space
than weight, so they inflate themselves like a child’s
Swim Bladder
“water wings” or a “beach ball.”)
Pneumatocysts
Kelp. (NOAA)
Nautilus Shell. Courtesy of
Jitze Couperus (CC BY 2.0).
Some organisms slow their fall with bulky, wide bodies that “catch” the water, like a parachute.
Even small spikes and hair-like structures help. In some cases, animals spread out their bodies
while they rest, and the pull in the structures to become more streamline when they want to
swim. In all of these cases, the key is that the organisms have more surface area which produces
more “friction” with the ocean water and slows their fall.
Illustrations of Radiolarians (left) and Copepods (right). Haeckel (public domain).
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Greatly reducing the amount of body heat that they lose to the cold ocean water.
If their bodies pile up on the bottom of the ocean, they can become a source of petroleum oil (over a long time).
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The bladders are called “pneumatocysts.” “Pneuma” means “air” or “wind” or “spirit.”
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Oceanography 10, T. James Noyes, El Camino College
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Water’s density affects other aspects of organisms’ lifestyles as well, like feeding and
reproduction. Just as water provides support for organisms, it also provides support for potential
food. This means that animals do not have to go in search of food; it can drift to them. This
helps small, drifting larvae survive on their own in ocean water. However, it also works against
them as well: many larger organisms have special adaptations for capturing tiny plankton drifting
in the water. We call this filter feeding or suspension feeding. Some grab small plankton out of
the water with “arms,” tentacles, legs, etc. (e.g., sea anemone, barnacles, jellyfish) as they pass
by. Others “weave” and cast sticky mucus “nets” to catch small plankton (e.g., some mollusks,
larvaceans), and then pick them off net – or eat the entire net. Some draw water into their bodies
and put it through a “strainer” (e.g., sponges, some mollusks, baleen whales). Sessile 6 animals
take this to an extreme: they live most of their lives stuck in one spot and wait for the food to
come to them. Many tide-pool animals like sea anemones and barnacles are sessile, filterfeeders.
Filter / Suspension Feeders: Jellyfish (NOAA), humpback whale (NOAA), and basking shark (Chris Gotschalk,
public domain). National Oceanic and Atmospheric Administration, Department of Commerce.
Sessile Filter/Suspension Feeders: These animals live their lives stuck in one spot. From left to right: sea
anemone, tube worms, and a sea fan. National Oceanic & Atmospheric Administration, Department of Commerce.
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The word “sessile” is pronounced like the English name “Cecil.”
Oceanography 10, T. James Noyes, El Camino College
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Water’s density also affects
organisms’ reproductive strategy.
On land, males and females must find
one another and physically touch to
mate 7. In many ocean species, males
and females gather in large groups and
release their gametes (eggs and
sperm) directly into the water where
they meet and join. The fertilized
eggs develop into larvae
(zooplankton) that drift in the water
and feed on phytoplankton and
smaller zooplankton.
For this method of reproduction to
work well, the adults must be close
to one another: the closer the gametes
(eggs and sperm) are to one another,
the more likely that they will meet.
It can be dangerous to gather in large
Corals releasing gametes into the water. National Oceanic and
Atmospheric Administration, Department of Commerce.
groups, so often many adults die. In
some species (e.g., salmon, some squid). , this is part of
Suppose that humans reproduced
their reproductive strategy. The adults spend all of their
like this. Ladies, would you go to
energy and resources, even their bodies (i.e., they die), to
a public swimming pool?
insure the survival of the next generation Timing is also
Just imagine the accusations:
important in this method of reproduction: all the organisms “Which one of you guys…?”
need to release their gametes at the same time or they will
Imagine this: you mate once, and
not meet. (“Everybody on a count of three: 1-2-3then die. From our perspective as
reproduce!”) Often ocean organisms use natural cycles
humans, it may seem like an awful
(e.g., tides) to determine when to gather and reproduce.
choice: mate and die, or remain
celibate and live to a ripe old age.
Some ocean organisms use a variation on this method. For
example, a female animal might collect sperm from the
water and use it to fertilize her eggs, releasing them later
after they’ve had time to develop and become a bit larger
(and therefore less likely to get eaten). A benthic animal
might release eggs and sperm into the water to meet, but
then fertilized egg might drift for a while and then settle on
the ocean floor to develop into an animal instead of
becoming a zooplankton. However, most ocean animals use
the reproductive strategy described in the first paragraph at
the top of the page.
Juvenile Crustaceans begin life as plankton,
and later become benthos (live on ocean floor).
National Oceanic and Atmospheric
Administration, Department of Commerce.
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In most animal species, at least.
Oceanography 10, T. James Noyes, El Camino College
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Viscosity
Viscosity refers to how easily a fluid “flows:”
a high viscosity fluid flows poorly (it oozes
slowly), while a low viscosity fluid flows easily
(it is “runny”). For example, toothpaste and
honey have higher viscosities than water and
oil. Viscosity is often related to density (a
“thick,” dense fluid is often more viscous, that
is, has a high viscosity), but they are not the
same thing, so you must be careful not to
confuse one with other. Viscosity is related to
the amount of “friction” or “drag” in the fluid.
The viscosity of the water makes it more
difficult for organisms to move, particularly
microscopic ones, but it also helps many
organisms by slowing their fall. (See the box on
the next page.)
Phytoplankton need to float to survive, but
most of them are more dense than water
because of their heavy shells, so they sink.
Their strategy is to sink as slowly as
possible, waiting for a passing wave to reach
down and lift them up. If they drift too deep,
though, waves cannot reach them, so they
will continue sinking and die. This is why
phytoplankton prefer high density and high
viscosity water: the more support they get
from the water and the higher the drag, the
slower they sink. There are some benefits to
falling, though. For example, falling exposes
phytoplankton to more water, helping them
obtain more nutrients.
Like density, viscosity is also affected by temperature: the higher the temperature, the lower the
viscosity. (Think of the syrup that you put on your pancakes, French toast, etc.: when you heat
it, does it flow more easily or less easily than cold syrup?) This is the same rule as density, so it
is not too hard to remember: cold water (low temperature) has a high density (“it’s thicker”) and
a high viscosity (“more drag”).
Before
After
warm syrup
Warm syrup has a lower viscosity than cold syrup,
so warm syrup “runs” (flows) more easily than cold syrup.
I suspect that the one time that you encounter the word “viscosity” in your
own life is when you are adding oil to the engine of your car. Oil lubricates
an engine; it keeps the metal parts from coming into contact and damaging
one another. If your oil’s viscosity is too low (“thin”), the metal parts will
bang into one another, and if your oil’s viscosity is too high (“thick”), the
moving parts get stuck. Since the viscosity of the oil changes with
temperature, you need a different kind of oil when starting your car (when
it is cold) than you do after the engine has been running for a while (when it
is hot). Oil companies have gotten around this problem by developing
special “multi-viscosity” oils that do not change viscosity as they warm up.
Oceanography 10, T. James Noyes, El Camino College
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The Beauty of Being Small
Why are so many ocean algae and animals very small? What is the advantage?
It may seem like being small makes you vulnerable, but there is one big advantage: if two
objects have exactly the same density, the smaller one will sink slower than the heavier one, so
a small organism has to spend less of its energy to stay near the surface of the ocean than a
larger one.
It may be obvious to you that the smaller object sinks slower based on your own experience,
but it does not hurt to try and understand why. Perhaps it is easiest to imagine that a sinking
object must push water “out of the way” to sink deeper into the ocean. A larger object has
more weight, making it easier for it to push water out of the way. However, it also has to push
more water “out of the way.”
The real key is that it experiences less drag – less resistance – relative to its weight. Falling is
a balance between several forces: gravity pulls an object down (weight) while the water pushes
it upward (the buoyancy force: the larger the object, the stronger the upward push) as does
friction with the water (“drag”). Drag is affected by the water’s
4π 3
viscosity and the area of the falling object: more surface area =
Volume = 3 r
more friction or drag. If you have a spherical phytoplankton and
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double its size, then you double its weight, but its area only goes up
Area = 4 π r
by about 8/5 (1.6). Thus, the weight factor increases 25% more
Area ∝ Volume2/3
than the drag factor, and therefore a larger phytoplankton falls
faster than a small one.
Here’s another way to look at it: Suppose that you stuck a bunch of smaller plankton together
in a big ball. Those on the inside would not be in touch with the water, so their surface area
would not contribute to the drag. As a group their weight would stay the same but they would
experience less drag than they would individually, so they sink faster.
In addition to falling slower, small organisms don’t need complicated systems (e.g., our blood
circulation) to transport necessary materials (e.g., nutrients) into the center of their bodies and
remove wastes. Many phytoplankton just let nutrients and wastes naturally drift into and out
of their bodies. (Much like a small child in a public swimming pool…ick.)
Sometimes bigger is not better.
small ones
clumped together
also sink faster
Sinks Slower
Sinks Faster
Sinks Faster
Oceanography 10, T. James Noyes, El Camino College
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Temperature
Open ocean temperatures range from about 30oF to 90oF, while temperatures on land range from
about -125oF to 136oF! The temperature changes less in the ocean, making it is easier to survive
in the ocean than on land. (The ocean has a more stable environment than the land.) However,
since many ocean organisms are adapted to this kind of environment, many ocean organisms
cannot survive what you and I consider to be a small change in temperature (e.g., coral or kelp
can die owing to the 1-4oC warming during an El Niño).
There are several reasons for the relatively stable temperature of the ocean. When heated or
cooled, water’s temperature changes very little compared to other substances. (Think about a
hot afternoon at the beach: Which is warmer, the sand or the water?) In addition, when water
molecules become too hot, they evaporate, leaving the cooler molecules behind in the ocean.
Also, energy from the sun is distributed over a lot of water, not just the first inch of soil at the
top, and waves and other phenomena mix cold water from below with warm water at the top.
Later in the course we will discuss how
water has a “high heat capacity,” meaning
Very Hot
Nice & Cool
that it holds more heat than another
substance with the same temperature. If
the other substance holds less heat, then
less heat was put into the substance to
raise its temperature originally. If this
smaller amount of heat was put into the
water, the water’s temperature would be
less: in other words, its temperature
On a summer afternoon, beach sand is hotter than the nearby
would not have changed as much as the
water, even though both have received the same amount of
other substance’s temperature did.
sunlight.
Recall that the temperature of the water affects the density of the water, making it easier or
harder the float: the higher the temperature, the lower the density. When warmed, water expands
(gets larger): the water molecules spread out, reducing the water’s density. Thus, changes in
temperature do not just the warm or cool organisms’ bodies: they also help the organisms float or
cause them to sink more rapidly.
The satellite images clearly show that the colder waters near the Poles are “greener” than the
warmer waters closer to the Equator. Like plants, most phytoplankton are green (due to the
molecules of chlorophyll they use to capture sunlight for photosynthesis.) This often confuses
my students, because they (correctly) reason that warmer water is warm due to more sunlight and
sunlight is needed for photosynthesis. What they forget, though, is that warm water has a lower
density, so phytoplankton sink more easily in warm water, and the deeper they sink away from
the surface, the less sunlight they get. More importantly, if the surface water is too warm,
waves have difficulty stirring up nutrient-rich water from down deep. (The deeper water is cold
and dense, so when the waves try to bring the water up, it immediately sinks back down.)
Oceanography 10, T. James Noyes, El Camino College
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Salinity
Many different substances are dissolved in sea water, not just salt. Salinity includes nutrients
(need by phytoplankton to perform photosynthesis) and gases like oxygen and carbon dioxide.
Many ocean organisms’ bodies are quite exposed to their environment; substances in the water
can easily drift in and out. Often this is a good thing for them. For example, phytoplankton
shells are quite “holey,” allowing carbon dioxide and nutrients to drift in and wastes like oxygen
to drift out. However if ocean salinity changes too much, they may get too little 8 of what they
need, which can, of course, be harmful.
In general, randomly-moving water molecules tend to move from the place where they are most
concentrated to where they are less concentrated; in other words, their movement “evens out”
differences. This phenomenon is called osmosis.
If ocean water is too salty for an organism (saltier than the cells of its body), water molecules
will leave its body, moving outside to where there is relatively more salt and less water. The
organism then suffers from dehydration (lack of water). This is why freshwater fish cannot live
in the ocean 9 and we cannot drink seawater. If you drink too much seawater, water from your
body’s tissues will move into your stomach (where the salt is), and then be lost from your body
when you urinate. As a result, you become dehydrated.
If the ocean is too fresh for an organism, extra water will enter its body, causing its cells (and the
organism) to bloat – and if the cells bloat too much, they burst! Our bodies cannot be so open to
our environment (because we are surrounded by air so we would quickly lose too much water).
However, some water can move through our skin. Since our bodies are much saltier than fresh
water, our bodies absorb water and we get “wrinkles” in our skin from the extra water when we
are in fresh water too long (e.g., bathing, washing dishes).
Many animals’ bodies actively try to maintain the right salinity in their body. The tissues of fish
are saltier than freshwater, but not as salty as the ocean. Since saltwater fish live in salty water,
water tends to leave their bodies, so saltwater fish excrete highly concentrated urine to limit how
much water they lose. Freshwater fish live in fresh water, they tend to absorb water, so their
urine is mainly water, a way for them to get rid of all the extra fresh water. Because we live in
air, our bodies are much better at holding in water and at actively maintaining the right salinity
inside us than the bodies of most ocean animals. As you probably know, our tears and blood are
“salty;” we literally carry our own ocean inside ourselves.
Like temperature, salinity also affects the density of sea water, making it easier or harder to float:
the higher the salinity, the higher the density. Salt atoms are heavier than water molecules, so
adding salt to water adds more weight than space, increasing the density of the water. Thus,
changes in salinity do not just cause organisms’ bodies to bloat or dehydrate, they also help the
organisms float or cause them to sink more rapidly.
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Getting too much of what you need can be harmful as well in some cases. Also, too many wastes can stay inside
your body, getting in the way.
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A freshwater fish dies of thirst in the ocean!
Oceanography 10, T. James Noyes, El Camino College
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Oxygen
Ocean animals need oxygen to breathe just like you and I (respiration is the process by which
you convert the food you eat into usable energy to power your body), and have special
adaptations like gills for extracting oxygen from water. When ocean animals are out of the
water, they cannot breathe 10. For example, when the tide is down, many “tide-pool” animals are
left above the water, and may have to hold their breath for hours until the tide rises again.
Animals living in tide pools are not safe: they can use up all the oxygen in the pool’s water!
Phytoplankton make most of the world’s oxygen, and the ocean provides us with over half the
oxygen that we breathe. However, there are places in the ocean with very little oxygen (we say
that the water is “anoxic”). Unlike the atmosphere, where oxygen molecules move around
freely, oxygen moves slowly through ocean water (liquids are “thicker” than gases; the water
molecules “get in the way” of the oxygen molecules). Most of the oxygen in the ocean is near
the top, where it leaks into the atmosphere. Oxygen levels are often quite high near the bottom
of the ocean (but not as high as at the surface), because when surface water becomes very cold
and salty, it sinks to the bottom, carrying its oxygen with it. Typically the lowest oxygen levels
are found somewhere in the middle, between the top and bottom of the ocean.
Nutrients
Nutrients 11 are atoms and molecules that are needed to get the food chain started: plants and
algae need nutrients to carry out the process of photosynthesis (they make their own food using
the energy of sunlight) and build some body parts (e.g., shells). The nutrients are not “used up”
during photosynthesis: instead, they are used to build molecules needed to carry out
photosynthesis. (In other words, nutrients are needed to make molecular “tools,” like a hammer
is needed to build a house or spatula is used to make a hamburger. The “tools” but are not
actually used up in the building process, but can be used to build again and again.) Examples of
nutrients include nitrogen and phosphorus compounds, and silica.
On land, plants extract nutrients from the soil via their roots. When the plants die, their bodies
are broken down, and the nutrients are returned to the soil for the next generation of plants. This
is why we need to use fertilizers when we farm: since we remove the plants, we are removing
nutrients from the soil as well.
In the ocean, nutrients are not recycled as efficiently as on land. Dead, decaying material tends
to sink downwards, leaving the sunlit waters at the surface where photosynthesis is possible. As
the bodies slowly sink, the bodies are decomposed by bacteria, which releases nutrients into the
deep, dark water. Thus, ocean algae often have difficulty obtaining the nutrients that they need
to make their own food from sunlight, so they are most abundant in places where nutrients are
brought to the surface of the ocean: near the coasts where nutrients are washed off the land by
rainwater runoff and in special places where winds and currents bring deep, nutrient-rich water
up towards the surface (“upwelling zones”).
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There are exceptions, of course. Mammals like dolphins and reptiles like sea turtles take oxygen out of the air
with their lungs. However, very few animals can take oxygen from both air and water (have both lungs and gills).
11
Another word for nutrients is “minerals;” however, they are not “vitamins.”
Oceanography 10, T. James Noyes, El Camino College
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Pressure
Pressure is, of course, caused when something presses against something else. n this class, we
will primarily discuss atmospheric pressure and hydrostatic ocean pressure which are caused by
the weight of the air and/or ocean water above. The basic rule is this: the more stuff (air or
water) there is above, the higher the pressure (because the more stuff there is, the heavier it will
be).
Air is very light, but there are miles of air above your head. The atmosphere
is quite strong because all this air really adds up: atmospheric pressure can
actually hold up a column of water over 30 feet high!
When you go up a mountain or in an airplane, your ears
start to feel uncomfortable until they “pop,” because
you are an airhead. (Don’t take this the wrong way:
you have cavities – open spaces – in your head that
contain air.) The air pressure outside your ear drums
gets lower (there is less air above, trying to push in),
allowing the air inside your head to press outwards
against your ear drums (ouch!!!), literally pushing them
out of your head.
air
inside
Air
Molecules
ear
air
outside
The lower pressure up in the
mountains is related to why it takes
longer to boil food (e.g., pasta) up in
the mountains. The water can
evaporate more easily (there is less
air pressing down on it, so it is easier
to “expand” into a gas), that is, at a
lower temperature. So, the water
cannot get as hot, and therefore it
takes longer for it to cook your food.
We hardly notice the effects of air pressure, because our
bodies are built to tolerate it: no more, no less; it is
“normal” for us. Water, of course, is much heavier than
air. Every 10 meters (33 feet) of water that is above
your head is equivalent to the weight of the entire
atmosphere. To keep from being crushed by the
pressure in the deep ocean, ocean animals fill their
bodies with water and other liquids which cannot be squeezed easily (unlike air). High pressure
on your body squeezes nitrogen gas (from the air in your lungs) into your blood. If you swim to
the surface too quickly – that is, if the pressure outside you decreases quickly – then the nitrogen
gas changes (e.g., forms bubbles) inside your body; there is no time for it to be squeezed back
out of your body and into your lungs. This may cause nitrogen narcosis (“rapture of the deep”)
in which the nitrogen gas seems to act something like nitrous oxide (“laughing gas”) and thus
makes the diver feel good and impairs their reasoning, followed by decompression sickness (“the
bends”) in which nitrogen gas joins together to form bubbles in cavities like those in your joints
and pinches nerves when you try to move. Humans can safely spend about 30 minutes at a depth
of about 100 feet (30 meters) using SCUBA gear. The deeper you go, the quicker nitrogen is
Oceanography 10, T. James Noyes, El Camino College
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squeezed into your body, so you cannot (safely)
spend as much time there. Deep-diving marine
mammals have lungs like you and me, so they can
also suffer from these effects, though they typically
have adaptations to make them less harmful. For
example, whales will squeeze air out of their lungs
before diving.
Recompression Chamber. (NOAA)
If you experience the bends or decompression sickness, then your body
is behaving like a carbonated beverage (e.g., a can of Coca-Cola).
Carbon dioxide was pushed into the liquid under high pressure
(= when you dived). When you open the can (= go back up), you are
reducing the pressure on the liquid, allowing the gas to form bubbles.
The only difference is that the gas cannot escape your body. The more
gas that escapes the can, the “flatter” the beverage gets. This is what
needs to happen inside your body for you to feel better.
Bubbles in Coke (Public Domain.)
In space, there is no outside pressure, so a human body would undergo “explosive decompression.”
The internal pressure exerted by the air, water transforming into vapor, and other things inside our
body – which normally balance the inward pressure of the outside air – would push our body
Atmospheric pressure can have a big impact on sea-level. Storms are associated with low
atmospheric pressure, and big ones like hurricanes can raise sea level by over 3 feet. The winds
of storms can push this mound of water against land, piling it up until it is over 20 feet above sea
level and floods miles of the coast. (This is called “storm surge” and is how Hurricane Katrina
overtopped the levees, pouring water into New Orleans.) The ocean is pushed down strongly
beneath the higher air pressure outside the
storm, so the ocean rises where the downward
push is weaker. (The water is that is pushed
Air
down has to go somewhere. This is somewhat
Molecules
like a “see saw” or “teeter-totter”: if one end
goes down, the other must come up.) Storm
systems are associated with low atmospheric
pressure, because the air in a storm system is
warm, rising air. Warm air expands
(get “bigger”), so there is less air above each
location (it has spread off to the side), and
therefore less pressure.
As the warm air
rises, it cools, and
water vapor in the
air condenses into
Land
Ocean
clouds and rain 12.
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Of course, you probably associate storms and rainy weather with cooler conditions, not warm air. We experience
cooler conditions, because cooler, higher-density air comes in and lifts up the warmer, lower-density air.
Oceanography 10, T. James Noyes, El Camino College
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Light
Ocean water is somewhat transparent to light; in other words, light can travel through ocean
water for some distance before being absorbed. This makes eyesight valuable in the ocean,
particularly near the surface where most of the sunlight is. However, deep ocean animals need to
be able to see 13 too, because most deep sea animals are bioluminescent (they can create their own
light chemically).
Ocean organisms use different color schemes to avoid being seen, whether they wish to hide
from predators or to sneak up on prey. Some, like jellies, are transparent, allowing them to blend
in with ocean water; other animals see right through them. Some animals match the color of the
ocean bottom where they live. Some, like orcas (“killer whales” like Shamu at Seaworld), have
a darker-colored top and a lighter-colored bottom. This is called “countershading.” When their
prey looks down at them, they blend in with the darker water below, but if their prey looks up at
them, their white bellies help them blend in with the sunlight streaming down from above. As
you have probably noticed, the natural color of ocean water is blue, and many ocean animals
have a blue or grey color that helps them blend in. (Ocean water can be other colors depending
upon the amount of sunlight and what is in the water. For example, if there are lots of algae in
the water, it appears greener, and if there are lots of sediments in the water, it appears browner.)
National Oceanic and Atmospheric Administration.
Looking Up From Below
Looking Down From Below
White Belly/Bottom
Black Top
How Countershading Works
13
Many animals do not have eyes which show them a picture. Instead, their eyes can sense which direction light is
coming from and maybe the strongest color in the light.
Oceanography 10, T. James Noyes, El Camino College
2A-14
Re
Water is “blue,” because
dL
igh
different colors of light interact
t
Y
differently with water. Sunlight
ello
wL
consists of a combination of all
igh
t
of the colors of the rainbow (red,
B lu
eL
orange, yellow, green, blue,
ig h
t
violet – they’re magically
delicious ): this is why
Sunlight contains all the colors of the rainbow. When sunlight strikes a
sunlight can make objects look
red shirt, all the colors but red are absorbed. Red light bounces off the
any color, and why you can see
shirt and into your eye, so the shirt appears red to you.
little “rainbows” near the edges
of sunlight going through glass (the glass “splits” the colors). Sunlight appears yellow to us,
because our eyes only tell us about the strongest color that we see. A red shirt appears red,
because the red part of the sunlight interacts differently with the shirt’s material than the other
colors: the red light is reflected (it “bounces”) off the shirt and into our eyes, while the other
colors of light are absorbed by the shirt (warming it). A red shirt appears red, because red is the
strongest color of light that is reflected off the shirt. When there is very little light reflected off
an object, our eyes translate that as the color “black;” in other words, “black” is the absence of
light. (Just turn off the lights and see for yourself.) Our eyes report that something is “white” if
there is no strongest color, if all the colors of light are about equally mixed.
en
re
G
Bl
Ocean water appears blue, because
Very little blue light (dashed line) is absorbed by water;
colors of light on the blue side of
blue light mainly "bounces back" (" scatters").
the rainbow are absorbed more
Red light (dotted line) is mainly absorbed;
slowly by ocean water and because
a little red light goes more-or-less straight through.
these colors are scattered (“reflected”)
Some green light (solid line) is absorbed;
more easily by the ocean water. In other
Water
some green light "scatters."
words, ocean water looks blue, because the
Molecule
other colors (e.g., red, yellow, green) are not
there any more: they are quickly absorbed
(“removed”) more by the ocean water 14. If this is all
that happened, then ocean water would look black as
Blue
Blue
Red
Red
you looked to your left or right when underwater, because the
sunlight would only go straight down. Blue light also scatters
(“bounces off”) water molecules and other stuff in the water
more easily than the other colors, so it does not go straight
down, but takes a crazy path through the water, scattering again
and again, until it is finally absorbed. So, the scattering of blue
light by water causes blue light to come at you from all
directions when you’re underwater. Thus, ocean water appears
“blue,” because unlike the other colors of light, blue light has
not been absorbed (removed) by the water, and instead bounces
off water molecules into our eyes. Thus, we see mainly see
blue light when we look around beneath the water, not the other colors.
ue
d
Re
14
There is less violet in sunlight than blue. Even though violet light is absorbed more slowly than blue, there is a lot
more blue in the sunlight to begin with.
Oceanography 10, T. James Noyes, El Camino College
2A-15
Experiment: Take a flashlight, and shine it through a glass of water with a drop of milk in it (to
increase the number of “scatterers”) Notice that the light coming out the other side of the glass is
more “reddish” than the light that went in. The flashlight emits a mixture of all the colors of the
rainbow. The red light tends to go straight through, but the blue light is “scattered” (knocked off
course). Thus, we see more red light than blue on the other side, making the light overall
“redder.”
Light from the flashlight
Light after going through water
Glass
of
Water
Flashlight
Oceanography 10, T. James Noyes, El Camino College
Sound
2A-16
Experiment: Knock on a table, desktop, or
wall. Then, put your ear against the surface, and
knock on it again. In which case is the sound
louder? Does this suggest that sound travels
better through air or solid objects? Will sound
travel better in the atmosphere or in the ocean?
On land, an animal can see another animal in the
distance long before it can hear the other animal.
In the ocean, organisms will hear other organisms coming long before they can see them.
Sound is transmitted much faster and better
(goes farther before being absorbed) by solid objects.
Water is more “solid” than air, so sound travels faster
and better in ocean water, making hearing more
useful in the ocean than on land. For example, whale
o
songs can travel thousands of miles in the ocean,
ech
allowing whales to communicate over vast distances.
(There are certain depths in the ocean where the
water carries sound better, and the whales use these
layers to their advantage.) Since light does not travel
as well through water as it does through air, eyesight Sound bounces ("reflects") off
the fish, and the predator hears
is less valuable in the ocean, making good hearing
the "echo"of the sound that comes back
even more important. In many cases, ocean
organisms’ entire bodies are sensitive to sound, not just their ears. Some 15 have developed
strategies for actually “seeing” using sound. For example, dolphins will emit high-pitched
noises, and then listen for the echoes that bounce off objects. By listening for where the sound
comes back from (its direction) and how long it takes for the sound to return (the farther away
the object is, the longer it will take for the sound to return), dolphins can determine where
objects around them are and what they look like, to “see” using sound instead of light. This is
called “echolocation,” and is how our SONAR 16 technology works.
National Oceanic and Atmospheric Administration,
Department of Commerce.
15
At first, whalers (men who hunted whales) thought that the large amount of white, milky stuff in the heads of
sperm whales was “sperm” and called it “spermaceti.” (Therefore, they called the whales “sperm whales.”) In fact,
the spermaceti is used to direct sound inside the whale’s heads to the right place to process it so that they can “see”
underwater using sound. The whalers should have figured out that their idea was wrong pretty quickly, because they
should have noticed that female whales have “spermaceti” too!
16
Dolphin “SONAR” is much better than our best SONAR technology.
Our navy is very interested in understanding just how dolphins do it.