Beach wrack ormarine wrack isorganic material (e.g.kelp,seagrass,driftwood) and other debris deposited athigh tide on beaches and other coastal areas. This material acts as anatural input of marine resources into a terrestrial system, providing food and habitat for a variety of coastal organisms.
The wrack zone is most commonly associated with a sandy beach habitat but can also be present inrocky shores,mangroves,salt marshes, and other coastal systems.[1] Debris is carried up theintertidal 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 duringspring tides compared toneap 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 may be composed of a variety of materials, both organic and inorganic. A common organic component isseaweed, such askelp, which easily floats to coastal waters after being dislodged by itsholdfast or otherwise torn by wave action and animal activity. Other organic components may includeseagrasses, terrestrial plants,driftwood, and stranded animal remains. Common inorganic components includeplastics, fishing line, and other manmade materials.
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Organic debris that accumulates in the wrack zone is considered across-boundary subsidy, linking the marine system to the terrestrial system by providing resources that form the base of coastalfood webs.[2] Terrestrial invertebrates such asisopods,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]
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]Kelp flies likeCoelopa pilipes rely on washed up kelp in wrack zones for food, shelter, andoviposition. Additionally, the wrack zone plays a role in the formation ofdunes by promoting the accumulation of wind-blown sand.[4]
Manmade objects are often washed ashore in the wrack zone, posing a threat to coastal animals. Plastics in particular are themost 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 plasticbioaccumulates through the food web, so predators may be affected by the accumulation of plastic in their prey's diet.[6]
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, although the deleterious effects are ameliorated by the removal of plastics and the like.[4]
Sea walls and othercoastal armoring structures can affect the location of a wrack zone and reduce the accumulation of organic material.[1] This can negatively affect the structure and diversity of coastal habitats.
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