Hello marine biology students.
In this video we're going to talk about waves
and tides and how they impact both human and
non-human life on the shores.
[Intro Music]
In this last section of notes for this week
we will be discussing waves and tides.
Waves are the results of wind blowing over
the water's surface.
The size of the waves depend on how long and
how fast the wind blows, with longer and faster
winds creating a larger wave.
The size of waves is also larger when the
fetch or the amount of open water is large.
So when we look at a diagram of wave formation
over a water basin, again the larger the area
the wind can blow over, the larger the waves
that will end up being produced.
And when we look at waves, the highest point
of the wave is known as the crest, whereas
the lowest point of the wave is the trough.
The distance between two crests or two troughs
is known as the wavelength.
The time it takes for a wave to pass by a
set point is known as the wave period.
So when we look at a series of waves and the
wave length is from crest to crest or trough
to trough and the wave period is the amount
of time it takes to move one wavelength.
As the waves are arriving near the shore,
the bottom of the wave drags against the bottom.
This forces the wave to slow down and the
waves start moving closer together they have
a shorter wavelength but this also drives
the wave to be taller and taller.
This is why waves start increasing in height
as they start coming into shore.
Eventually the drag causes the wave crest
to fall over the wave itself.
We call this a wave break and the resulting
surf caused by the breaking waves can displace
a lot of sand and end up affecting the organisms
living at the seashore.
For most waves, it's the same water molecules
moving in a circular pattern and moving back
and forth and it's the energy that's transferred
so that the water that's making up the waves
as it approaches the shore is the water that
had been near the beach in the first place.
Now we don't often think of it, but tides
are actually a form of waves as well, but
instead of being driven by the wind, these
tides are driven by the gravitational pull
of our moon.
So, tides play a role in ocean circulation.
Circulation is driven by tides, which is the
rhythmic rising and falling of sea surface
levels.
The tides are caused by the gravitational
pull
of the moon on ocean waters and to a lesser
degree by the Sun as well.
Waters on the side of the earth closest to
the moon are pulled towards the moon, causing
a high tide.
The opposite end of the planet will also have
a high tide due to centrifugal force.
Low tides will be perpendicular to high tides.
So, as we look at the diagram of the earth
and the moon, we can see the gravitational
force makes a high tide in the direction of
the moon and centrifugal force makes a high
tide on the far side of the planet.
Most locations on earth have two high tides
and two low tides each day.
This is known as semidiurnal tides.
The numbers and size of tides varies geographically
and can be affected by bottom features and
geographic features, such as islands, canyons
and reefs.
The amount of difference 
between high tide and low tide is known as
the tidal range and this can also be affected
by bottom features in the surrounding area.
An important thing to realize is that the
tidal cycle happens every 24 hours and 50
minutes, because the orbit of the moon and
the earth around their center of gravity is
a bit slower than that of the rotation of
the planet Earth on its axis.
So here we see a diagram of high tides at
a specific point on the surface of the Earth.
We'll alternate from high tide to low tide
to high tide to low tide again, but again,
at 24 hours it hasn't quite reached back to
its high tide yet due to the movement of the
moon.
Instead it takes 24 hours and 50 minutes.
When the Sun and Moon, both of which effect
tides, are in line at times of the new and
full moon, the tidal range is even greater.
These are called the spring tides, not because
of the season of spring, but because of the
concept of the spring of water.
So despite their name, they happen each month
and it's a sudden bursting of water or spring
of water, which is how it got its name and
not due to the season.
Conversely, at the 1/4 moon and 3/4 moon,
where the moon and the Sun are not aligned
with each other, these produce the neap tides.
During the neap tides, you're going to have
a lower difference between high and low tide
and neither will be as extreme as during the
spring tides.
So when Sun and Moon are aligned, high tide
will be taller and low tide will be lower,
whereas during a neap tide the difference
between high tide and low tide will be reduced.
Now organisms that live in areas exposed during
a low tide will be especially affected during
the spring tides.
Movement of water during tidal changes results
in significant mixing of water, which also
affects organisms.
And lastly, many marine organisms time their
reproduction according to the tides.
In the case of the California grunion shown
here, the males and females actually beach
themselves as high as they can during the
highest of tides of one month to reproduce
and lay their eggs within the sand, then one
month later, at the next spring tide, those
eggs will be hatching and the young fish will
be able to swim back to the ocean.
This is the reproductive behavior of these
fish.
So that takes us to the end of the videos
for this week.
Next week we're going to be talking about
biology and life and how that relates to the
marine environment.
Thanks for your attention and I'll see you
in the next videos.
