The Vanishing Tide

[HenryC]

Florida Wildlife, Mar-Apr, 2011

Every schoolboy knows what causes the tides. The Moon's gravity pulls on the waters of the ocean and as the Moon orbits the spinning Earth a heap of water is dragged across the planet.  The Moon pulls on the rock of the continents too, but being stiffer, stone doesn't "give"  as much as water, so a great wave follows the Moon as the world rotates beneath it. Actually, there isn't just one wave, there are two of them!  The lunar gravity distorts the world ocean into an egg or football-shaped ovoid.  The far part of this water world is further from the Moon, and less affected by its gravity, so there is a matching bulge there too and both of these chase each other around our planet as the combination of  Moon motion and terrestrial spin cause them to circle the Earth. 

But our world is not a smooth sphere of deep ocean.  There are continents scattered across its surface and the oceans have varying depths.  The shorelines are complex, twisted, and shoals and channels further disrupt and guide the  paths of the waters.   It takes time to move mountains of fluid and friction and the Earth's spin play their  role as well.  Neither does the Moon travel  uniformly above the equator or circle the Earth at the same distance and speed. The Sun causes tides too, not as strong as the Moon's because of its great distance from Earth, but they sometimes help, other times impede, the Moon's.  The water sloshes across ocean basins in great slow waves that reinforce or damp out the tides themselves–in short, the tide is ever changing, never the same, either in space or time.  It is the breath and the heartbeat of the planet, but it does not repeat itself, and the  living things in the sea are well aware of it and take it into account. 

In most places on Earth the high tide comes twice a day, answering the Moon's pull but delayed
somewhat due to local conditions.  Between the flows come the ebbs and at the times of  high and low is slack water, where the sea pauses briefly between breaths. Tides along the East Coast of Florida come twice a day,  they are said to be semidiurnal.  In other places, such as  the Florida panhandle,  resonances and oscillations cause the tide to adopt a daily or diurnal rhythm; one high and one low per lunar day, the amount of time it takes the Earth to rotate completely and catch up to the moving Moon, roughly 25 hours.  The other high and low may be there, but so spread out and damped so as to not be noticeable. There is an intermediate case as well; the Florida Gulf coast south of Apalachicola to the Florida Keys is said to be of mixed tides, a combination of the diurnal and semidiurnal which may vary from place to place and time to time.  At full and new Moon the Sun's gravity aids the Moon's and we experience extra high and low spring tides; at first and third quarter the two bodies interfere and we get the milder neap tides.  And sometimes, in some places, the high and low are so gentle as to merge imperceptibly–the so-called "vanishing tides".  As the Sun and Moon appear to travel east and west about the Earth, and as they move north and south in the sky, the direction and strength of their combined gravitational pull constantly varies so no two tides are ever the same.

For the mariner, the tide determines where his vessel can go and what currents act on it, since close ashore, the height of the tide dominates the flow. Much of Florida is characterized by relatively shallow waters, so even a tide range of a few feet can greatly expand or restrict a ship's operations. On the Gulf coast, tidal ranges are on the order of a foot, on the southeast coast, somewhat higher, about 2 or 3 feet.  From mid-state northwards on the Atlantic the tidal range increases from 3 to 6 feet.  But in any one place, although the tides may have astronomical causes, they are not easily predictable.  Both the time and range of the tides may differ substantially between two locations only a few miles apart.  Fortunately, long experience and familiarity with our harbors have provided our maritime authorities with a good understanding of local tidal conditions and the official predictions are usually right on target. The navigator must  pay close attention to tide and tidal current tables and should use them extensively in his planning.

The same advice should be heeded by the sportsman and naturalist.  The angler may understand that at certain states of the tide some fish are biting and others may be running but it is often much more subtle than that.  At best, all we can really say is that things happen and that the tide affects them.  Beyond that, we know little.  In deeper waters just offshore or in tidal estuaries, the tide is the prime mover.  As the Moon pulls on the sea, currents race across the  surface and the bottoms, rushing through straits and slowly filling slough and bayou;  the lazy flow of rivers reverses and salt water invades deep inland as bay and sound empty and fill.  In Florida, this movement of sea water into the broad estuaries and up into slow streams allows marine life to penetrate far into the state, slowly acclimating to the lower salinity conditions away from the ocean.  It is not unusual to see mullet and blue crab miles from salt water, alongside many other marine species as well, such as manatee.

Along the littoral, the intertidal zone divides itself into a series of parallel habitats, each with its own distinctive living communities..  The area alternatively submerged and exposed by the tide lies at its center, but on either side are the sections of coastline underwater or dry at all but the most extreme tides. On sandy shores, the swash, that area underneath the pounding surf, moves landward and seaward in response to the tide, while the constant motion of water and current sculpts and sorts the sand into distinctive landforms: bars and flats, dunes and beaches, barrier islands and lagoons, capes and bays.  Between  islands are the mouths and points, often still called by their old Spanish names, Boca and Punta.    Often, these features can appear or vanish overnight during hurricanes, hence colorful names on the charts like Hurricane Pass or Midnight Pass.

Different species of plants and animals will specialize to thrive in zones with a specific fraction of submersion time, while more mobile organisms will exploit the movement and follow the water as it advances and retreats. Even in an area of small tidal range like Florida, these belts can still be quite extensive because of the lack of relief; a small change of water depth can cover or reveal a large amount of bottom.  Combining this with the variety of Florida marine habitats, sandy beach, mud flat, coastal marsh, mangrove and so on, the possibilities are limitless.  Farther offshore, the grass flats and coral reefs also respond and adapt to the tide. The edge of the sea is the richest and most productive of all marine habitats and most sea life spends at least part of its time there.

The living things along the coast depend on the tides to regulate their feeding, reproductive and migrational behavior.  Marine organisms are sensitive to them, and their life-cycles as well as their daily habits are tuned to them.  The state of the tide and its resulting currents is communicated from species to species, between predator and prey, both by sensory experience as well as by the behavior of each member of the biological community.  To a human being, it is all a great mystery, but to the creatures of the sea the tide is as closely sensed as the cycles of day and night, or the pulse of the seasons and the state of the weather.  Together, these great natural cycles mesh and interact with the life of the sea to create an ever-changing natural system of immense complexity and profundity, one which we humans can exploit, perhaps destroy, but scarcely understand and certainly not control.