Wrack line reveals last high tide mark near the dunes

Beach wrack or marine wrack is

organic material Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...

(e.g.

kelp Kelps are large brown algae or seaweeds that make up the order (biology), order Laminariales. There are about 30 different genus, genera. Despite its appearance and use of photosynthesis in chloroplasts, kelp is technically not a plant but a str ...

seagrass Seagrasses are the only flowering plants which grow in marine (ocean), marine environments. There are about 60 species of fully marine seagrasses which belong to four Family (biology), families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and ...

driftwood Driftwood is a wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves. It is part of beach wrack. In some waterfront areas, driftwood is a major nuisance. However, the driftwood provides ...

) and other debris deposited at

high tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

on beaches and other coastal areas. This material acts as a natural input of marine resources into a terrestrial system, providing food and habitat for a variety of coastal organisms.

Physical characteristics

The wrack zone is most commonly associated with a sandy beach habitat but can also be present in

rocky shore A rocky shore is an intertidal area of seacoasts where solid rock predominates. Rocky shores are biologically rich environments, and are a useful "natural laboratory" for studying intertidal ecology and other biological processes. Due to th ...

mangrove A mangrove is a shrub or tree that grows mainly in coastal saline water, saline or brackish water. Mangroves grow in an equatorial climate, typically along coastlines and tidal rivers. They have particular adaptations to take in extra oxygen a ...

salt marsh A salt marsh, saltmarsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. I ...

es, and other coastal systems. Debris is carried up the

intertidal zone The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various ...

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 Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables c ...

compared to

neap tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables c ...

s. 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 is

seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of ''Rhodophyta'' (red), '' Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...

, such as

, which easily floats to coastal waters after being dislodged by its

holdfast Holdfast most often refers to: *Holdfast (biology), a root-like structure that anchors aquatic sessile organisms to their substrate *Holdfast (tool), a tool used to secure a workpiece to a workbench or anvil Holdfast or hold fast may also refer t ...

or otherwise torn by wave action and animal activity. Other organic components may include

es, terrestrial plants,

, and stranded animal remains. Common inorganic components include

plastic Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...

s, fishing line, and other manmade materials.

Ecology

Role in coastal food webs

Organic debris that accumulates in the wrack zone is considered a

cross-boundary subsidy Cross-boundary subsidies are caused by organisms or materials that cross or traverse habitat patch boundaries, subsidizing the resident populations. The transferred organisms and materials may provide additional predators, prey, or nutrients to r ...

, linking the marine system to the terrestrial system by providing resources that form the base of coastal

food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...

s. Terrestrial invertebrates such as

isopods Isopoda is an Order (biology), order of crustaceans. Members of this group are called isopods and include both Aquatic animal, aquatic species and Terrestrial animal, terrestrial species such as woodlice. All have rigid, segmented exoskeletons ...

amphipods Amphipoda () is an order (biology), order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphip ...

polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...

s, and shore flies feed on seaweed and other dead material. 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.

Role in habitat formation

The wrack zone adds structure to the beach landscape, providing habitat for animals that live there. For example,

rove beetle The rove beetles are a family (biology), family (Staphylinidae) of beetles, primarily distinguished by their short elytra (wing covers) that typically leave more than half of their abdominal segments exposed. With over 66,000 species in thousand ...

s burrow in the wet sand below the wrack zone, benefiting from moist conditions and the availability of herbivorous invertebrate prey species. Kelp flies like ''

Coelopa pilipes ''Coelopa pilipes'' (common name kelp fly or seaweed fly) is a common European species of kelp fly. It was species description, described by Alexander Henry Haliday, A. H. Haliday in 1838. Their appearance differs greatly from that of other ''Co ...

'' rely on washed up kelp in wrack zones for food, shelter, and

oviposition The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...

. Additionally, the wrack zone plays a role in the formation of

dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat ...

s by promoting the accumulation of wind-blown sand.

Human impacts

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, and it is estimated that 46% of shorebirds ingest plastic in their lifetime while 26% experience entanglement. A variety of effects have been observed in animals that ingest plastic, including reduced reproductive success, changes in immune function, and increased mortality. 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.

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, although the deleterious effects are ameliorated by the removal of plastics and the like.

Shoreline hardening

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

References

{{coastal geography Coastal geography