Tides

Posted: November 30, 2011 by Bill Baldwin

Tides

By: Dianne Faucette, Master Naturalist

 

Our coast experiences two high and two low tides daily. They are caused by the gravitational pull of the moon and (to a lesser extent) the sun and the earth's rotation. One high tide occurs when our location rotates past the moon. The other high tide occurs when we are on the opposite side of the earth from the moon, and is caused by centrifugal force.

A "spring tide" occurs at every full moon and every new moon, and this is when the high tides are higher than normal. Halfway between new and full moons, the sun and moon are at right angles working against each other, and cause less tide movement. Tides at this time are called "neap tides."

South Atlantic Bight

Our location on the Atlantic coast provides a unique tide system. We have an average 8-foot difference between high and low tides (10 feet during "spring tides"). The South Atlantic Bight is a dramatic curve in the coast from Cape Hatteras to the Florida Keys. The Georgia-South Carolina border is the center of this curve, and receives the greatest tide effects as the rising water is pushed into the smallest point (like a funnel). The diagram will show a comparison of differences between high and low tide at several points in the South Atlantic Bight.

On the Atlantic coast, our area has the second highest tidal difference after the Bay of Fundy, Nova Scotia.

Intertidal Zone

The section of the beach that is between low and high tides is home to burrowing mollusks and crustaceans. They are provided oxygen by the tide action. Coquina clams, ghost shrimp and jackknife clams leave their trademarks of siphons and holes they use for breathing. Sand dollars are prolific just under the sand. They are filter feeders, eating the sediments brought in by the waves. The burrowing mole crabs, butterfly clams and surf clams are eaten by willets, sanderlings and red knots (shorebirds). So tides create a habitat for these creatures which, in turn, provide food for shorebirds.

Sand Flats

Underwater sand flats beyond the low-tide line provide habitat for many of the crabs, whelks, sea stars, sea cucumbers and marine worms-which wash ashore with the incoming tides.

Green Soup

The drab green nutrient-rich ocean water on our coast is perfectly described by Todd Ballantine in his book Tideland Treasure as "nature soup. Teeming with the essential elements for life, the brackish bouillabaisse provides the  foundation for a broad foodweb that supports shrimp, crabs, oysters, fish, dolphins, shorebirds and so much more."

Receding Tides

Receding tides leave interesting tidepools in small depressions in the sand. In the tidepools, I have seen sea stars (starfish), jellyfish, small fish, live whelks, sea pansies, a variety of birds feeding and bathing, and children using them as baby pools.

Incoming Tides

Incoming tides carry shrimp and red drum larvae into the tidal creeks and estuaries which provide protection from the predators in the ocean. We refer to these estuaries as "nurseries" for this reason.

Who would have thought that tides could have such an effect on life?

• High tides are 12 hours and 25 minutes apart.
• Two high tides and two low tides occur every 24 hours and 50 minutes.

For more photos and information about local nature, visit Dianne Faucette