Tides

Twice a day along the Puget Sound the water seems to drain and then refill again. When it “drains” far enough, we are able to learn about some neat animals that call the Puget Sound their home. How does the water level change? Where does the water disappear to? Does this happen in other parts of the world? How do the animals survive out of water for a time? By the end of this post, you will be able to answer all of these questions yourself!

We call the rise and fall of the water in the Puget Sound and in the ocean the tide. The tide happens all over the world but may look a little different depending on where you are. The movement of the water is caused by the gravitational pull of the moon and the spin of the Earth. Jump up into the air. Gravity is the force that brings you back down. That is the gravitational force the Earth has on you. Earth’s gravity is also what keeps the Moon orbiting around the Earth. Because the Moon is also very large and close to Earth, it’s gravitational field effects the things on earth as well. The Moon’s gravity pulls on the water that is on the Earth. So, when the Moon is overhead, the water is pulled slightly towards it. This is high tide. That water is pulled from other parts of the ocean that would then be experiencing low tide. Therefore, when there is less water at low tide, the water does not disappear. It is being pulled toward an are of the earth that is experiencing high tide.

The Bay of Fundy in Eastern Canada has the biggest difference between high tide and low tide Photo: amusingplanet.com

Photo: openstax.org

In this picture, you may notice that there are two high tides on opposite side of the Earth. This is because there are a lot of forces involved that ultimately create another high tide, or tidal bulge, directly across from the first one. If you want to know more about how this works, I recommend: https://scijinks.gov/tides/

Photo: gotbooks.miracosta.edu/ocean

The Sun also affects the height of the tides. When the sun and moon are aligned (New Moon and Full Moon phases). Their gravitational forces combine and we get something called Spring Tides. This has nothing to do with the season we call Spring. During Spring Tides, the difference between high and low is the largest so the tide line “springs” back and forth. The opposite of this is called Neap Tides (Waxing and Waning Quarter Moon phases). This is when the difference between the high and low tide is at its smallest.

Tides happen in lakes as well but lakes are so small and shallow compared to the ocean, we are unable to see the tide difference. In the Great Lakes, scientists have shown that during spring tides, the water level only changes by 5 cm (but it does change!)

Many other factor affect the tide changes around the world including: Wind and weather, geography, and the time of year (seasons).

Tides and the beach

Where we mainly see the the effect of tide is at the beach. When the water is out at low tide, we are able to explore what gets left behind. This also means that some animals are left without fresh sea water for hours. This is also an opportunity for predators, like many shore birds, to find a meal Some animals have shells to protect them from drying out and from predators, others hide under rocks, algae and sand. Because different animals tolerate the loss of water in different ways, we get different intertidal zones. Animals in the spray zone survive better with less water and ones in the low tide zone need to be in the water most of the time. The tidal changes help these areas fresh by taking waste away as the tide goes out and bringing in new nutrients when the tide comes back in.

Intertidal Zones Photo: pathwayz.org