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Natural Disturbances to Estuaries

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before ivan   after Ivan

This pair of images illustrates the destructive power that hurricanes can have on estuarine environments. This barrier island in Pine Beach, Alabama, was severed following hurricane Ivan’s landfall in late 2004. The image on the left was taken on July 17, 2001. The image on the right was taken on September 17, 2004, soon after Hurricane Ivan reached the Alabama mainland. Click on either image for a larger view.

Estuaries are fragile ecosystems that are very susceptible to disturbances. Natural disturbances are caused by the forces of nature, while anthropogenic disturbances are caused by people. Natural disturbances include winds, tidal currents, waves, and ice. Anthropogenic disturbances include pollution, coastal development, and the introduction of non-native species to an area.

microscopic view of zooxanthellae

Dead floating plant material, called wrack, is often deposited on salt marshes by high spring tides, smothering all of the plant life beneath it. Click on image for more details and a larger view. (Photo: Weeks Bay NERRS site)

We like to think of natural places as being stable over time, but, in fact, they are not. Natural habitats are continually disturbed by natural processes, followed by periods of recovery. When a natural disturbance is followed by an anthropogenic disturbance or vice versa, a habitat may become so damaged that it never recovers.

One type of natural disturbance is the continual pounding of ocean waves. In many estuaries, barrier beaches protect inland habitats from wave erosion. If these beaches are destroyed, salt marshes and inland habitats adjacent to the estuary may become permanently damaged. Waves can also dislodge plants and animals, or bury them with sediments, while objects carried by the water can crush them. Large storms are especially destructive to estuaries, particularly in areas like Florida and the Carolinas, where barrier beaches are common.

A common disturbance to estuaries in nontropical regions is winter ice (Bertness, 1999). Ice can freeze on an estuary’s shoreline, or float freely in the water. When slabs of free-floating ice make contact with the shore, they have a scouring effect, dislodging and killing the plants and shoreline animals that lie in their path. When sheets of ice form on the shore, especially in salt marshes, they can trap plants and grass stalks inside them. During high tides, these ice sheets are lifted up, or rafted, inland to the high marsh. These rafts carry both ice and tufts of plants inshore. When the rafts settle down at low tide, they can smother inshore vegetation or scrape it from the soil. Further damage is caused as these sheets of ice and vegetation are rafted and dragged across the marsh with the ebb and flow of the daily tides.

Another natural disturbance in salt marshes is the burial of vegetation by rafts of dead floating plant material, called wrack. Wracks can be quite large—up to hundreds of square meters, and up to 30 centimeters thick. The spring high tides often move these wracks into the high marsh, where they become stranded (Bertness, 1999).


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