Lecture 23
Tidal Marsh Ecosystems I

Read reserve reading: Our Restless Tides. These will be on reserve in Earth Sci. library, or you can go to the website and download your own copies directly. Web address is: http://www.co-ops.nos.noaa.gov/restles2.html and read also http://www.co-ops.nos.noaa.gov/restles3.html and http://www.co-ops.nos.noaa.gov/restles4.html -- basically 3 short chapters covering this lecture material Very helpful.

KEY WORDS: Lunar cycles, hydroperiod, salinity, zonation, Neap and Spring tides

Tide marsh ecosystems: Geography and Biogeography
Geographical extent: essentially temperate phenomenon. Mangroves in the tropics, very little salt marsh in polar latitudes.
Salt marshes are where the ocean meets the land.

They experience daily cycles of inundation:
Hydroperiod: the duration, frequency, and depth of inundation - various parts of the marsh are submerged for varying lengths of time each day;
One result of this daily inundation and subsequent exposure is :
Salinity: a gradient of salinity can be found across salt marshes with the most saline areas flooded the least frequently;
And thus only specially adapted plants can survive in salt marshes and these plants often are distributed in an organized fashion with respect to the extent of the tides and the salinity gradient across the marsh surface.
Salt marsh vegetation zonation: the plants are distributed largely in zones determined by the hydroperiod and salinity of the local marsh.

Why do we have tidal marshes

Back to orbital parameters: this time the Earth-moon relationship, primarily.

The center of revolution of this motion of the earth and moon around their common center-of-mass (Barycenter) lies at a point approximately 1,068 miles beneath the earth's surface, on the side toward the moon, and along a line connecting the individual centers-of-mass of the earth and moon.
The center-of-mass of the earth describes an orbit around the center-of-mass of the Earth-moon system just as the center-of-mass of the moon describes its own monthly orbit around this same point.

The Effect of Centrifugal Force. One of the two force components creating the tides. As the earth and moon orbit their Barycenter, (common center-of-mass) the centrifugal force produced is always directed away from the center of revolution.
Because the center-of-mass of the earth is always on the opposite side of this common center of revolution from the position of the moon, the centrifugal force produced at any point in or on the earth will always be directed away from the moon.

Note: that the centrifugal force produced by the daily rotation of the earth on it axis (coriolis) must be completely disregarded in tidal theory.


The tide raising force of the moon is insufficient to "lift" the waters of the earth physically against the far greater pull of Earth's gravity. Instead, the tides are produced by that component of the tide-raising force of the moon which acts to draw the waters of the earth horizontally over its surface toward the region directly below the moon, and on the opposite side of Earth from this point. Since the horizontal component is not opposed in by gravity , the particles of water move freely over the earth's surface.

Sun also pulls, but as strong as the moons because of distance
The tidal force exerted by the sun is a composite of the sun's gravitational attraction and a centrifugal force component created by the revolution of the earth's center-of-mass around the center-of-mass of the earth-sun system, in an exactly analogous manner to the earth-moon relationship.
The position of this force shifts with the relative orbital position of the earth in respect to the sun. Because of the great differences between the average distances of the moon (238,855 miles) and sun (92,900,000 miles) from the earth, the tide producing force of the moon is approximately 2.5 times that of the sun.

Neap and Spring tides
The gravitational attractions (and resultant tides) produced by the Moon and Sun reinforce each other at times of new and full moon to increase the range of the tides, and counteract each other at the first and third quarters to reduce the tidal range.

This can be accentuated by Parallax inequalities:
Both the Moon and the Earth revolve in elliptical orbits and the distances from their centers of attraction vary. Increased gravitational influences and tide-raising forces are produced when the Moon is at position of perigee, its closest approach to the Earth (once each month) or the Earth is at perihelion, its closest approach to the Sun (once each year). When perigee coincides with perihelion tides of augmented range are produced.
When perigee, perihelion, and either the new or full moon occur at approximately the same time, considerably increased tidal ranges result. When apogee, aphelion, and the first- or third-quarter moon coincide at approximately the same time, considerably reduced tidal ranges will normally occur.