SANDY BEACH AND WRACKLINES WHAT IS A BEACH? A coastal beach is the zone of unconsolidated material extending from the lowest low water landward to the point of permanent vegetation or change in topography. Unconsolidated means "not cemented together." The beach is a constantly shifting, dynamic area. Many of the beaches around Boston Harbor, particularly in East and South Boston, are artificially nourished with sand- sized sediment from sand and gravel quarries.
ORIGINS The beaches of Boston Harbor are composed of loose, broken rock material. Their color and composition varies depending on whether the material is delivered by streams or erosion of the coastal rocks nearby. In New England, the sand is mainly quartz, but also contains bits of most minerals found in the till brought here by the glaciers. Look carefully at a handful of sand with a hand lens and you will see black, green, red and brown grains in with the white or clear grains of quartz. As the sand ages, more of the colored grains dissolve and leave the quartz behind. Our sands are very young. None is more than about 15,000 years old. The sands of the Caribbean are white because they are made of calcium carbonate pieces of algal and animal skeletons. In Hawaii some beaches are made of black volcanic sand. Beaches everywhere reflect the sediment of the area.
BEACH STRUCTURE The size of the particles of rock influence the structure of the beach. The zone from the lowest low water to the highest high water is called the beach face. This zone is controlled by the energy of the waves that attack the beach. In the high energy areas of the outer islands and headlands the beaches tend to be made up of gravel, pebbles and cobbles. On some islands with slate or schist bedrock the beaches are made of flat rocks called shingles. However, in the quiet inlets and bays the sediment is finer sand and mud. There is a relationship between sediment size and the slope of the beach face. The coarser the sand, the steeper the face angle. Watch for this effect from a distance, then check it out close up. There is usually a change in slope at the top of the beach face. This is called the beach berm. It marks the beginning of the back beach. The back beach extends to the permanent vegetation or to the sea scarps (a cliff or steep slope) so common to New England. It is the width of this zone that changes throughout the year. As winter begins, the storm waves excavate the beach and store the sand in near shore bars. This makes the beach narrow or even disappear completely. When the wave energy drops between storms or as summer comes on, these bars migrate onto the beach, making it wider. This process can be observed especially well at Hammonassett Beach, and many others along Connecticut's Long Island Sound shoreline.
LIFE ON THE BEACH Most people think nothing lives on the beach, but this is far from true. The animals that live on the beach include the many birds that feed there and the very small animals that live burrowed into the sand or between the sand grains. Look at any beach and you will see lines of dead seaweed and other flotsam, called wracklines. Under and within these there is a microcosm of animals feeding on the decaying seaweed. Most common is the beach flea, or beach hopper (Americhor, chestia sp.). In the lower parts of the beach, soft-shelled clams (Mya arenaria) and Atlantic razor clams (Ensis directus) burrow into the substrate. Feeding on these are a number of birds including the great black-backed gull, the herring gull, the ring-necked plover, the sanderling, as well as various terns and sandpipers. Raccoons, skunks, white-footed mice, rats and possums also venture onto the beach to scavenge. While the beach face and back beach are so dynamic that few plants can grow there, the dunes a few meters landward and above the highest storm waves are the home of pioneer plants such as dune grass (Ammophila), dusty miller (Artemisia stelleriana), beach pea (Lathyrus japonicus var. glaber) and cocklebur (Xanthium echinatum). These plants stabilize the dunes and enrich the sand base, making possible the typical coastal communities of salt spray rose (Rosa rugosa), bayberry (Myrica pensylvanica), seaside goldenrod (Solidago sempervirens), beach plum (Prunus maritima) and poison ivy (Rhus radicans). If undisturbed, the dunes will often become covered with poverty grass (Hudsonia sp.), which is actually not a grass but a heather. As soil develops on the dunes, ecological succession tends to produce the typical Eastern decidious hardwood forest of the mature climax communities of New England.
SPECIAL ADAPTATIONS/WHAT TO LOOK FOR Animals that live in sandy shores have adaptations that allow them to live in this ever-shifting, unstable environment. Some animals, like the bivalves, have a foot that can be inserted in the sand and inflated with body fluids to secure them in the sand. To allow them to breathe while buried, they have a set of siphons or tubes that extend up into the water column when they are submerged. Many beach-dwellers are camouflaged to look like sand, such as crabs and small 54 New England Aquarium crustaceans. Birds that nest on the beach such as plovers and sandpipers have protective coloration. Their nests are little more than depressions in the sand and their eggs are the color of sand. Between the grains of sand live a group of tiny worms called interstitial fauna. These are largely microscopic. Mole crabs (Emerita talpoida) and ghost crabs (Ocypode sp.) are active burrowers on the beaches south of Cape Cod. Sandy soil does not hold moisture well, and the salt spray in the air can draw out the fresh water in plants. Beach peas curl their leaves during intense sun exposure to retain moisture. Beach grasses have adapted to these low moisture conditions by developing deep roots that tap into fresh water, and rhizomes that hold the sand in place when winds come, stabilizing the beach. Some plants, like beach peas, are legumes and "fix" their own nitrogen.
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)