What are Barrier Islands?

Barrier islands are long, narrow, offshore deposits of sand or sediments that parallel the coast line. Some barrier islands can extend for 100 miles (160 km) or more. The islands are separated from the main land by a shallow sound, bay or lagoon. Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets, such as the Outer Banks of NC.

The formation of barrier islands is complex and not completely understood. The current theory is that barrier islands were formed about 18,000 years ago when the last Ice Age ended. As the glaciers melted and receded, the sea levels began to rise, and flooded areas behind the beach ridges at that time. The rising waters carried sediments from those beach ridges and deposited them along shallow areas just off the new coast lines. Waves and currents continued to bring in sediments that built up, forming the barrier islands. In addition, rivers washed sediments from the mainland that settled behind the islands and helped build them up.

The structure of a typical barrier island consists of the following zones from the ocean side toward the sound:

Barrier islands serve two main functions. First, they protect the coastlines from severe storm damage. Second, they harbor several habitats that are refuges for wildlife. In fact , the salt marsh ecosystems of the islands and the coast help to purify runoffs from mainland streams and rivers.

Barrier-island Ecology
Even though barrier islands are narrow, they have several distinct habitats:

Each habitat has varying conditions and wildlife. We will examine some of them according to each habitat.

Beach Habitat
On the ocean side is the barrier island's beach habitat. The beach is much like a desert in that it lacks fresh water, but a large portion of the beach gets covered almost entirely with salt water twice daily (the entire beach gets covered to the dune base during storms). Animals and plants in this environment (known as the intertidal zone, between tides) must endure long periods of exposure to salt water and drying air. On the beach, the only plant life you'll see is some algae that get washed ashore. Bacteria live in the spaces between the sand grains where water from the surf percolates through. The animals on the beach itself include burrowing animals like mole crabs and clams that filter feed during high tides, burrowing worms that feed on bacteria in the sand, scavenging crabs (ghost crabs) and various shorebirds (sandpipers, seagulls and pelicans) that eat the crabs, burrowing animals and offshore fish.

Dune Habitat
The dunes receive moisture from rain and surf and are occasionally flooded during severe storms. The dunes are still a relatively hostile environment with high salt content, sandy soil and little fresh water. Plants such as sea oats and bitter pancum provide stability to the dunes. Their root systems hold the sand in place and their shoots slow the winds, thereby allowing sand to be deposited. Along the dunes, you will find many crabs, particularly ghost crabs. Again, you will find birds (gulls, terns) that feed on the animals that inhabit the dunes.

After the dunes, some islands may have maritime forests with shrubs and trees (Sand Live Oak, Myrtle Oak, Slash Pine and Magnolia). Animals in these forests include various snakes, opossums, skunk, raccoon and fox.

Barrier-flats Habitat

On other islands, the barrier flats come after the dunes. The primary vegetation includes cordgrass and sawgrass. These areas are often flooded daily during high tides. The muds and sediments are full of anaerobic bacteria (there is little oxygen in the sediments). The bacteria decompose the rich organic material in the sediments and from dead plants and animal. Animals that live in the wet muds filter feed bacteria and plankton from the tidewaters or feed on bacteria in the muds; these animals include clams, mussels, snails and worms. Various fish come and go with the tides. Fiddler crabs feed on the bacteria in the muds. Ghost crabs and blue crabs feed on the bacteria, small invertebrates and small fish. Various birds (seagulls, egrets, pelicans) feed on the fish, crabs and invertebrates.

Salt-marsh Habitat
The sound-side of an island is usually dominated by the salt marsh. The salt marshes that you find on the sound sides of barrier islands are similar to those found on the coastal mainland. Like the barrier flats, salt marshes are regularly flooded with seawater during high tide and the animals and plants that you find are similar to those in the barrier flats.

Barrier Island Review:

1. What are the 2 functions of a barrier island?  _________________________________

__________________________________________and ________________________

_____________________________________________________________________

2. What are the 5 distinct habitats found on a barrier island? _______________________  ______________     _______________     _______________   ___________________

3. List the 3 other names that a barrier flat might be called. ________________________

_____________________________     and  __________________________________

4. What does the intertidal zone describe? _____________________________________

_____________________________________________________________________

5. Name 2 plants that help provide stability to the sand dunes.  _____________________

________________________________  How do these plants help hold the sand in place?  _______________________________________________________________

6. What is anaerobic bacteria?  ______________________________________________

      7.    Put a circle around the animals that feed on bacteria and plankton; put an X on animals 

             that feed on bacteria, small invertebrates and small fish.

 mussels            ghost crabs                   clams                snails                worms 

 blue crabs        fiddler crabs                 

Make a Difference

As the old man walked the beach at dawn, he noticed a young man ahead of him picking up starfish and flinging them into the sea. 

Finally, catching up with the youth, he asked him why he was doing this.  

The answer was that stranded starfish would die if left until the morning sun.

 “But the beach goes on for miles and there are millions of starfish,” countered the old man.  

“How can your effort make any difference?” 

The young man looked at the starfish in his hand and carefully threw it to safety in the waves. 

“It makes a difference to this one,” he said.  

-Anonymous

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