Following the motion of the ocean

A beach at high tide

A beach at high tide

There are a lot of things in the world that are hard to predict. For example, the weather; we can never figure out whether we should bring an umbrella or wear a sweater for tomorrow. However, we can predict and rely on the tides.

What are tides?

Have you ever been to the beach and found that the water line was so high that there was barely any room to lay a beach towel? Meanwhile, six hours later there is suddenly plenty of sandy beach available? This rise and fall of water levels from the sea is referred to as a tide.

Tides are one of the most reliable phenomena in the world. It always occurs at its predicted time and it always lasts for a certain amount of time, about 6 hours.  According to the National Oceanic Atmospheric Administration (NOAA), tides are caused by waves that move through the oceans in response to the forces exerted by the moon and the sun. It is also termed as the rise and fall of the levels of the ocean that in turn, affects water levels in a body of water or at a beach. 

The Sun, Earth, and Moon

Tides are caused by the gravitational pull of the sun and moon as well as the rotation of the earth. As the earth rotates and the moon orbits around the earth, the gravity of the moon causes sea levels to fluctuate between high and low states. In most locations, these changes occur twice a day, producing two high tides and two low tides every 24-hours.

HIGH_LOW_TIDE.350122338_std.jpg

Knowing where the moon is located in relation to the earth, we can predict where in the world high and low tides will occur. Low tides occur on the sides of the earth, 90 degrees away from the moon, while high tides occur on the earth directly below the moon and on the opposite side of the Earth. The diagram below does a better job explaining this. 

Following the flow

As the sea level is either rises or falls, it makes sense that these changes result in water flowing to or from the ocean. This flow causes currents called tidal currents.

A flood current occurs as the sea level is rising towards high tide. Water is flowing towards the shore and away from the ocean.

An ebb current occurs as the sea level is dropping towards low tide. Water is flowing away from the shore and towards the ocean.

At the exact time of high tide or low tide there is no current. This time is called slack water.

Strong currents pushing against an anchored buoy.

Strong currents pushing against an anchored buoy.

Understanding these currents can be extremely important. For those who live on or near an ocean, your life practically revolves around the tides and currents. For some of us at FTDM and a lot of other researchers who do field work out on the water, we have to know what tide it is and when high or low tide is for us to work safely out on the water, especially swimming or diving. If you are swimming/diving with current, especially in an area known for strong tidal currents, you run the risk of exerting a lot of energy swimming in the opposite direction of a current or being carried away by a current. SCUBA divers also have to time their dive cautiously.

Types of Tides

Depending on what time of month and the positions of the earth and moon in orbit around the sun, tides may be referred to as spring or neap tides. Spring tide occurs when the Sun and the Moon are aligned; spring tides create the largest tidal range: tides will be their highest and lowest. A neap tide is when those tidal ranges is at its smallest; this occurs during the first and third quarters of the Moon. 

http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT307/HTML/AT30706.HTM Figure 7.23

http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT307/HTML/AT30706.HTM
Figure 7.23

Living with the rhythm of the tides 

We follow a rhythm of life, so to speak. Most of us humans follow a day-night cycle: we are awake when it is light/day out and we are asleep or inactive when it is dark/night.

Horseshoe crabs spawning at high tide.

Horseshoe crabs spawning at high tide.

Some marine animals also follow this day-night cycle, but animals that live in the intertidal or in places that are heavily influenced by tides also follow a tidal cycle. For example, my study animal, the horseshoe crab. During their mating season, horseshoe crabs come on to shore to mate and spawn. However, they only come into the shallow waters and beaches during high tide. Why might this be? Scientists believe they spawn during high tide because the water line comes higher on the beach, allowing horseshoe to exploit more of the beach for suitable nesting sites. 

Faced with strong tidal currents and fluctuating tides on a daily basis, living in a tidally-influenced area has its pros and cons. Check out some of the benefits and the stresses of the high tide and low tide conditions.

Benefits of high tide = protection by being submerged, more constant temperature from being submerged, food, higher water line to exploit more of a beach (i.e. for spawning)
Stresses of high tide = being submerged all the time, possible marine predators to come in, wave action, less food, less sun (algae conduct photosynthesis)

Benefits of the low tide = protection from predators (less swimming predators coming in), sun (benefiting algae) more food
Stresses of low tide = exposure to sun and heat risk being dried out, exposure to rain risk of freshwater (marine animals need salt water for physiological reasons), wind, freezing, lack of food, susceptible to predators (birds).

Hermit crab in its shell

Hermit crab in its shell

How do animals adapt to the motion of the ocean?

During low tide, bivalves (two shelled animals, like clams and mussels) are able to clamp their shells closed so that they retain water and avoid drying out. Hermit crabs and other crabs either stay in their shell to prevent drying out or hide within and underneath rocks.  Larger animals may just follow the tides and head out to deeper water until the water comes in again.

 

During high tide, sea stars and mussels have special attachment appendages where they are able to anchor themselves during times of high water waves. Animals like pipefish are able to camouflage themselves in sea grass to hide from predators when it is high water.

Mussel byssal threads attached to rocks

Mussel byssal threads attached to rocks

You can learn more about the animals that live in the intertidal from this past blog post by Sabah.

Tides not also affect the lives of marine animals but they also affect us. From extracurricular activities, water safety, and potential coastal flooding, tidal patterns are hugely important for those of us who live near the water. Luckily for us, tides will always remain are predictable so we know what tide it will be, at what time, and how long it will last.