Dive Dry with Dr. Bill

095: First the Tide Rushes In

My columns haven't dealt much with the physical aspects of our marine environments, so I thought I'd spend a little time talking about one very important factor today. I'm referring to the tides which are of importance to marine biologists, fishers and boaters alike... not to mention many marine critters!

I wonder if any of our island residents are unaware of the tides that affect the waters and shoreline of Catalina? For some reason I doubt it, with the possible exception of the very young and the very old who may not often venture beyond their homes. During winter storms we can be made very aware of them by all the sandbags if a storm peaks during a high tide in town. Extreme low tides may fill the air with the fragrance of seaweeds and other exposed marine life (your nose may respond differently than mine).

Most know that tides are related to the phase of the moon. Fewer know that they are also related to the Earth's position relative to the Sun. And only a small number realize that it is not just the gravitational pull exerted by these two heavenly bodies on our planet's ocean, but the centrifugal force due to our slightly wobbly revolutions about one another (not you and me, the heavenly bodies). Although less massive than the Sun, the Moon has an affect on our tides about twice as strong as the Sun's because it is much closer to the Earth (240,000 vs 93,000,000 miles).

When the Sun and the Moon are aligned on the same, or on opposite sides of the Earth (new moon or full moon); both exert forces in the same direction and we have what are known as spring tides. These occur twice every month year-round and are not limited to the spring season. During spring tides, the high tides are higher and the low tides are lower resulting in a difference between the highest high and the lowest low tide of up to nine feet (9') on our coast. We're fortunate since this is much lower than the world's greatest tidal range (about 50 feet) in Canada's Bay of Fundy between Nova Scotia and New Brunswick. If we had such tides, the stores near Front Street would have been wiped out long ago (if they were ever built)! When the Sun and the moon are at 90 degree angles to one another, we experience neap tides which have less amplitude than spring tides.

The height of the high and low tides also depends to a lesser degree on how far the Sun and the Moon are from Earth. The Moon revolves around the Earth, and the Earth around the Moon, in orbits that are not perfectly circular. Thus the distance between the Sun or the Moon and Earth changes constantly. The tide generating forces are strongest when the Moon is closest to the Earth and the Earth is closest to the Sun, and weakest when the orbits are at their farthest points.

The time of the high and low tides varies daily because of the moon's rotation about the Earth. Every night the moon rises 50 minutes later than it did on the previous night. Since the tides follow the position of the moon relative to Earth, the tides peak 50 minutes later each day (and thus have a period of 24 hr 50 min vs 23 hr 56 min for Earth's rotation about its axis).

How many are aware that tides differ significantly depending on where you live on our planet? Because the tides are strongly aligned along the orbital plane of the Moon (which is tilted 23 degrees to our own equator), equatorial regions have different tidal patterns than those at mid and higher latitudes (north or south of the equator). Regions which receive only one high and one low tide each tidal period have diurnal (every "24" hours) tides. Regions with two nearly equal highs and two nearly equal lows experience semidiurnal (every "12" hours) tides. Regions like ours experience two unequal highs and two unequal lows every tidal period, known as "mixed tides."

In addition, differences in coastline configuration and water depths create different tidal patterns even at sites in "close" proximity to one another. Thus the tidal patterns in Los Angeles harbor occur at roughly the same time as Catalina's but do not exactly mirror one another. Of course we utilize the mainland tide tables because such tables are not published for every location on Earth!

Marine animals which live between the lowest low tide and the highest high tide are known as intertidal organisms. They often form different "zones" on the rocks because certain species are only able to tolerate so much exposure (the upper limit) or so much predation from subtidal predators (the lower limit). Looking at our rocks during extreme low tides you can often see these well delineated zones which may be dominated by barnacles in one zone and seaweeds in another.

Through sea walls and sand bags, we humans have isolated ourselves somewhat from the impact of the tides unlike these intertidal species whose lives are so strongly influenced by them. However, for those visitors to and residents of our fair island who are savoring "sex on the beach," they might have more reason to pay attention!

© 2004 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Tide chart showing orientation of Earth and Moon at two different times and the resulting tidal patterns.

This document maintained by Dr. Bill Bushing.
Material © 2004 Star Thrower Educational Multimedia