Wrack zone

The wrack zone or "wrack line" is a coastal feature where organic material (e.g kelp, seagrass, shells) and other debris is deposited at high tide. This zone acts as a natural input of marine resources into a terrestrial system, providing food and habitat for a variety of coastal organisms.

Wrack line reveals last high tide mark near the dunes
Wrack line on a sandy beach

Physical characteristics

The wrack zone is most commonly associated with a sandy beach habitat but can also be present in rocky shores, mangroves, salt marshes, and other coastal systems.[1] Debris is carried up the intertidal zone as the tide comes in, and is deposited on the sand when the tide goes out. The zone can be recognized as a linear patch of debris toward the upper part of a beach running parallel to the water's edge. The location of the wrack zone varies geographically and temporally. It is found at a higher elevation during spring tides compared to neap tides. The size of a beach and its intertidal zone will influence the location of wrack deposition. Additionally, storms will often increase the volume of debris that is deposited.

The wrack zone may be composed of a variety of materials, both organic and inorganic. A common organic component is seaweed, such as kelp, which easily floats to coastal waters after being dislodged by its holdfast or otherwise torn by wave action and animal activity. Other organic components may include seagrasses, terrestrial plants, driftwood, and stranded animal remains. Common inorganic components include plastics, fishing line, and other manmade materials.


Sanderling (Calidris alba) (6)
Sanderling (Calidris alba) feeding in the wrack zone

Role in coastal food webs

Organic debris that accumulates in the wrack zone is considered a cross-boundary subsidy, linking the marine system to the terrestrial system by providing resources that form the base of coastal food webs.[2] Terrestrial invertebrates such as isopods, amphipods, polychaetes, and shore flies feed on seaweed and other dead material.[3] These invertebrates provide food for shore birds and other predators on the beach. In addition, when organic debris decomposes, it delivers nutrients to the soil, promoting the growth of coastal vegetation.[1]

Role in habitat formation

The wrack zone adds structure to the beach landscape, providing habitat for animals that live there. For example, rove beetles burrow in the wet sand below the wrack zone, benefiting from moist conditions and the availability of herbivorous invertebrate prey species[3]. Additionally, the wrack zone plays a role in the formation of dunes by promoting the accumulation of wind-blown sand.[4]

Human impacts

Albatross carcass and marine debris
Albatross carcass with marine debris at Eastern Island, Midway Atoll

Inorganic debris

Manmade objects are often washed ashore in the wrack zone, posing a threat to coastal animals. Plastics in particular are the most common form of litter found on beaches,[5] and it is estimated that 46% of shorebirds ingest plastic in their lifetime while 26% experience entanglement.[6] A variety of effects have been observed in animals that ingest plastic, including reduced reproductive success, changes in immune function, and increased mortality.[6] There is also growing evidence suggesting that plastic bioaccumulates through the food web, so predators may be affected by the accumulation of plastic in their prey's diet.[6]

Beach raking

Sandy beaches are often groomed for aesthetic and recreational value. The removal of organic debris limits habitat and food availability for wrack-associated animals and inhibits the formation of dunes.[4]

Shoreline hardening

Sea walls and other coastal armoring structures can affect the location of a wrack zone and reduce the accumulation of organic material.[1] This can negatively impact the structure and diversity of coastal habitats.

See also


  1. ^ a b c Strain, E.M.A.; Heath, T.; Steinberg, P.D.; Bishop, M.J. (March 2018). "Eco-engineering of modified shorelines recovers wrack subsidies". Ecological Engineering. 112: 26–33.
  2. ^ Schooler, Nicholas K.; Dugan, Jenifer E.; Hubbard, David M.; Straughan, Dale (2017-07-01). "Local scale processes drive long-term change in biodiversity of sandy beach ecosystems". Ecology and Evolution. 7 (13): 4822–4834. doi:10.1002/ece3.3064. ISSN 2045-7758.
  3. ^ a b "Wrack Community | Explore Beaches". explorebeaches.msi.ucsb.edu. Retrieved 2018-03-29.
  4. ^ a b Martínez, M.L.; Gallego-Fernández, Juan B.; Hesp, P. (2013). Restoration of coastal dunes. Springer. ISBN 9783642334450.
  5. ^ Law, Kara Lavender (2017-01-03). "Plastics in the Marine Environment". Annual Review of Marine Science. 9 (1): 205–229. doi:10.1146/annurev-marine-010816-060409. ISSN 1941-1405.
  6. ^ a b c Worm, Boris; Lotze, Heike K.; Jubinville, Isabelle; Wilcox, Chris; Jambeck, Jenna (2017-10-17). "Plastic as a Persistent Marine Pollutant". Annual Review of Environment and Resources. 42 (1): 1–26. doi:10.1146/annurev-environ-102016-060700. ISSN 1543-5938.
Coastal geography

Coastal geography is the study of the constantly changing region between the ocean and the land, incorporating both the physical geography (i.e. coastal geomorphology, geology and oceanography) and the human geography (sociology and history) of the coast. It includes understanding coastal weathering processes, particularly wave action, sediment movement and weather, and the ways in which humans interact with the coast


The Coelopidae or kelp flies are a family of Acalyptratae flies (order Diptera), they are sometimes also called seaweed flies, though both terms are used for a number of seashore Diptera. Fewer than 40 species occur worldwide. The family is found in temperate areas, with species occurring in the southern Afrotropical, Holarctic, and Australasian (which has the most species) regions.


Heterocheila is a genus of acalyptrate true flies (Diptera). They are placed in their own family, Heterocheilidae, in the superfamily Sciomyzoidea. They are not widely familiar outside entomological circles, but the common name "half-bridge flies" has been associated with them. They are medium-sized flies occurring mainly in temperate regions on seashores of the Northern Hemisphere, where they and their larvae typically feed on stranded kelp in the wrack zone. In this, they resemble kelp flies, which are members of a different family, though the same superfamily.

The family Heterocheilidae was established by McAlpine in 1991. He distinguished it from other families to which Heterocheila had hitherto been referred at various times and by various authorities – Helcomyzidae, Dryomyzidae and Coelopidae.


An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. A grouping of geographically or geologically related islands is called an archipelago, such as the Philippines.

An island may be described as such, despite the presence of an artificial land bridge; examples are Singapore and its causeway, and the various Dutch delta islands, such as IJsselmonde. Some places may even retain "island" in their names for historical reasons after being connected to a larger landmass by a land bridge or landfill, such as Coney Island and Coronado Island, though these are, strictly speaking, tied islands. Conversely, when a piece of land is separated from the mainland by a man-made canal, for example the Peloponnese by the Corinth Canal or Marble Hill in northern Manhattan during the time between the building of the United States Ship Canal and the filling-in of the Harlem River which surrounded the area, it is generally not considered an island.

There are two main types of islands in the sea: continental and oceanic. There are also artificial islands.


Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera.Kelp grows in "underwater forests" (kelp forests) in shallow oceans, and is thought to have appeared in the Miocene, 5 to 23 million years ago. The organisms require nutrient-rich water with temperatures between 6 and 14 °C (43 and 57 °F). They are known for their high growth rate—the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching 30 to 80 metres (100 to 260 ft).Through the 19th century, the word "kelp" was closely associated with seaweeds that could be burned to obtain soda ash (primarily sodium carbonate). The seaweeds used included species from both the orders Laminariales and Fucales. The word "kelp" was also used directly to refer to these processed ashes.

Kelp fly

Kelp fly is one common name of species of flies in a number of families of "true flies" or Diptera. They generally feed on stranded and rotting seaweed, particularly kelp in the wrack zone. When conditions are suitable they are very numerous and may be ecologically important in the turnover of organic material on the coast. In this role they also may be an important item in the diet of beach-dwelling animals and birds. The flies most generally referred to as kelp flies are the widely distributed Coelopidae. In popular speech however, they are not clearly distinguished from other flies with similar feeding habits, such as the Heterocheilidae, the Helcomyzinae and sundry members of the Anthomyiidae.


Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A recent global analysis suggested they are as extensive globally as mangroves. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of estuarine silts, clays and marine animal detritus. Most of the sediment within a mudflat is within the intertidal zone, and thus the flat is submerged and exposed approximately twice daily.

In the past tidal flats were considered unhealthy, economically unimportant areas and were often dredged and developed into agricultural land. Several especially shallow mudflat areas, such as the Wadden Sea, are now popular among those practising the sport of mudflat hiking.

On the Baltic Sea coast of Germany in places, mudflats are exposed not by tidal action, but by wind-action driving water away from the shallows into the sea. These wind-affected mudflats are called windwatts in German.

Piping plover

The piping plover (Charadrius melodus) is a small sand-colored, sparrow-sized shorebird that nests and feeds along coastal sand and gravel beaches in North America. The adult has yellow-orange-red legs, a black band across the forehead from eye to eye, and a black stripe running along the breast line. This chest band is usually thicker in males during the breeding season, and it is the only reliable way to tell the sexes apart. The bird is difficult to see when it is standing still, as it blends well with open, sandy beach habitats. It typically runs in short spurts and stops.

There are two subspecies of piping plovers: the eastern population is known as Charadrius melodus melodus and the mid-west population is known as C. m. circumcinctus. The bird's name is derived from its plaintive bell-like whistles which are often heard before the bird is visible.

Total population is currently estimated at about 6,510 individuals. A preliminary estimate showed 3,350 birds in 2003 on the Atlantic Coast alone, 52% of the total. The population has been increasing since 1999.

Their breeding habitat includes beaches and sand flats on the Atlantic coast, the shores of the Great Lakes, and in the mid-west of Canada and the United States. They nest on sandy or gravel beaches or shoals. These shorebirds forage for food on beaches, usually by sight, moving across the beaches in short bursts. Generally, piping plovers will forage for food around the high tide wrack zone and along the water's edge. They eat mainly insects, marine worms, and crustaceans.

Surf zone

As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface called surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves (now reduced in height) continue to move in, and they run up onto the sloping front of the beach, forming an uprush of water called swash. The water then runs back again as backswash. The nearshore zone where wave water comes onto the beach is the surf zone. The water in the surf zone, or breaker zone, is shallow, usually between 5 and 10 m (16 and 33 ft) deep; this causes the waves to be unstable.

This (fly)

This is a genus of kelp fly in the family Coelopidae. As of 2017, it is monotypic, consisting of its type species This canus. This and T. canus were respectively circumscribed and described in 1991 by the Australian entomologist David K. McAlpine. It is endemic to southern Australia.

Wrack (seaweed)

Wrack is part of the common names of several species of seaweed in the family Fucaceae. It may also refer more generally to any seaweeds or seagrasses that wash up on beaches and may accumulate in the wrack zone.It consists largely of species of Fucus — brown seaweeds with flat branched ribbon-like fronds, characterized in F. serratus by a saw-toothed margin and in F. vesiculosus, another common species, by bearing air-bladders. Another component of sea wrack may be seagrasses such as Zostera marina a marine flowering plant with bright green long narrow grass-like leaves. Posidonia australis, which occurs sub-tidally on the southern coasts of Australia, sheds its older ribbon-like leaf blades in winter, resulting in thick accumulations along more sheltered shorelines.

"Bladder wrack", Fucus vesiculosus

"Channelled wrack", Pelvetia canaliculata

"Knotted wrack", Ascophyllum nodosum

"Spiral wrack" or "flat wrack", Fucus spiralis

"Toothed wrack" or "serrated wrack", Fucus serratusHistorically wrack was used for making manure, and for making "kelp", a form of potash.


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