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Tidal Flats

While there are no extensive tidal flats on Outer Island, elsewhere they are an important habitat for estuarine life.

WHAT ARE TIDAL FLATS? In many locations along Connecticut's Long Island Sound shoreline there are large expanses of sand and mud that are exposed at low tide—these are called tidal flats. They are formed as a result of low-wave energy in estuaries and harbors, which allows sand and mud to collect into shallow deposits or banks. These banks are composed of rather fine sediment which was brought here first by the glaciers of the last ice age and then redistributed by waves and running water. Where wave energy is high, such as on headlands and the windward sides of islands, the sand has been removed and boulders and cobbles are left to form a rocky shore. However, in the quiet corners of the harbor, in the lee of islands and in estuaries where waves do not reach, the sand and mud accumulates into the characteristic flats we see (and smell) exposed at low tide. Since tidal exchange in Long Island Sound is a hefty 9.5 feet (3 m), the low slope and flat relief of the banks makes for large areas of tidal flats.

PHYSICAL FEATURES The particle size in a given tidal flat is a function of how the sediment deposit was created. In areas where rivers and streams deliver extremely fine sediments and decomposed organic matter, there are true mud flats. Where the flats are formed by waves redistributing glacial till, the flats may be more sandy than muddy. Flats are just that, flat. Relief is usually confined to the adjacent beach face and to tidal channels that dissect the flat. They have a typical slope of about 1:500. Occasionally, we see large boulders protruding from the surface of the sand or mud.

Characteristic of all flats is the fact that the sediments are loose and unconsolidated. They shift with every storm. This lack of a firm substrate makes it impossible for most organisms of the rocky shore to find a footing.

CONDITIONS FOR LIFE As noted above, the substrate in the flats is constantly shifting. Thus the algae which so easily attaches to rocks and floats cannot grow here. Instead the area is bathed by a mixture of microscopic planktonic (free floating) algae brought in with the incoming tide. When conditions are calm we see a greenish-yellow sheen on the surface of the mud. This sheen is caused by algae growing on the sand surface. Diatoms and dinoflagellates (two phyla of algae) dominate these films. They disappear with the next storm and reestablish when conditions allow. The only other photosynthesizer is eel grass (Zostera marina), a true flowering plant that grows in thick beds in the subtidal areas adjacent to some exposed flats.

As with other intertidal habitats, tidal flats experience the stress of daily variation in temperature, desiccation and salinity. Animals who live in them are equipped with adapta- tions for burrowing or anchoring themselves in the mud. Most clams have an inflatable foot which can be inserted into the mud, inflated and used to anchor the clam in place. The soft-shelled clam (Mya arenaria) is one of the most common burrowing clams. The Atlantic razor clam (Ensis directus) burrows even deeper and faster. These clams breathe and ingest nutrients through a set of siphons, long tubes that extend through the mud up to the water. When the tide is out these siphons store water to keep the clam moist while exposed. Look carefully at the surface of the sand and you may see the holes through which these siphons extend at high water. Step close to them and a squirt of water tells you the clam is retracting its siphon deeper into the hole.

Many other holes are formed by worms of the polychaete group. These are segmented worms that feed actively at high tide, sticking their heads out of the holes. A worm's gut constantly processes food from the grains of sand and sediment that it ingests. The indigestible sand grains pass out of the animal's anal opening to the surface forming the coils of sand, mud castings or mounds. For this reason, many of the marine worms live in a vertical U-shaped position.

Worms may be either sedentary or mobile. The mobile worms leave their burrows at high water to hunt other small worms, fish and crustaceans. These often have large paddle-like "legs" or parapodia on the sides of their bodies. They have small simple eyes and large well-formed jaws that can give a painful bite. The sedentary worms tend to have smaller parapodia that are modified for anchoring the worm in its burrow. Some of these worms, like the bamboo worm (Clymenella torquata) and the parchment worm, construct linings for their burrows that stick out of the mud.

While the water cover changes dramatically over tidal flats, the composition of the soil remains relatively stable. The sediments retain moisture and buffer temperature and salinity fluctuations, making the flats an easier place for burrowers to live. Many of the mud flat worms are deposit feeders, eating organic material (detritus) and single-celled algae in the sediments. Worms are constantly eating and eliminating waste, which can look like mounds of sand at the surface or straw-size sand grain chimneys. Three strange denizens of the flats include the ribbon worm (Cerebratulus lacteus) a strap-like worm that can grow to 3 feet long and an inch wide. This worm catches its food with a modified proboscus. In sandier flats, you can find peanut worms (Golfingia gouldii) and glass worms (Leptosynaptera sp.), the latter of which are not true worms, but sea cucumbers. In addition to worms, tidal flats are also home to small crustaceans, crabs, snails and a variety of other mollusks. Although the tidal flats may look relatively uninhabited, a little digging will prove that the density of life is tremendous. In Barnstable (on Cape Cod), studies indicate that between 7,000 and 355,000 animals live in each square meter of flat.

EELGRASS Eelgrass is a submerged aquatic plant from the seagrass family that inhabits the shores of most of the world's north temperate and arctic coastlines. It is very productive, growing up to a couple meters in length, and provides a valuable nursery habitat for many animals, including commercially important species such as shellfish and cod. Common in some areas, eelgrass will occasionally be exposed on intertidal flats during extremely low tides. Eelgrass beds provide important habitat for a wide variety of fish such as juvenile winter flounder and cod that use the sea grass for a nursery. Invertebrates such as isopods, amphipods and small crustaceans also forage in the eelgrass on organic material that settles between the blades of grass. Predatory fish, such as the striped bass, also use eelgrass beds as hunting grounds.

* To grow and thrive, eelgrass requires shallow protected waters, good water quality and abundant light. Historical records show that at one time eelgrass beds in Long Island Sound were widespread. However, with development, pollution and disease, fewextensive eel grass beds remain. With the recent improvements in water quality, due mainly to the new sewage treatment planst and other pollution control initiatives, environmental conditions may favor the return of eelgrass beds to Long Island Sound*

The New England Aquarium has been working on eelgrass in Massachusetts since 1993, investigating the importance of eelgrass beds to local fisheries. More recently, Aquarium researchers have focused on the relationship between lobsters and eelgrass.

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Conservation Notes Explore the wrackline for shells, molts, algae and one evidence of intertidal life left behind the receeding tide. The popularity of beaches for recreation can interfere with many of a beach's natural functions. Cars, trucks, dog-walk- ers and joggers can kill areas of beach grass, contributing to erosion. Trash and plastic can threaten beach habitats and wildlife. Development and foot traffic can force nesting shorebirds, including the threatened piping plover and least tern, out of their habitats. Certain areas are now cordoned off during nesting season. Beaches cycle through seasonal changes, often losing sand in harsh winter storms, and rebuilding in the summers. This natural, periodic waxing and waning of beaches is at odds with the human desire to build permanent structures. So, to stabilize beaches, people have built jetties and groins out of rocks and concrete. Instead of solving the problems, these armored beaches cause adjacent beaches to lose sand. Neighboring beaches then build up protections, too, resulting in miles and miles of armored beaches. Instead of providing a solution, these artificial structures interfere with the natural equilibrium of these environments and prevent the sand that would naturally nourish the beaches from reaching them.

The above text is adapted from New England Aquarium's Boston Harbor Seaside Educator's Guide
(illustrations after Cindy Lydon - NEAQ Boston Harbor Seaside Educator's Guide)