What is Wrack?

Each year, you may notice a lot of “weedy” stuff washed up onto the beach, especially after big storms. This weedy stuff is called beach wrack, and many people want to keep it off our shores. Although it can be smelly and it takes up room in the sand near the water, it plays an important role in coastal ecology. But before we get into the serious stuff, let us talk about what wrack is and how it comes to be sitting on beaches all over the world.

Wrack is dead marine plants such as seaweeds and seagrasses that get washed up onto beaches all over the world (Figure 1). The amount of wrack washed up on beaches varies a lot around the world, between as little as 0.1 kg up to a huge 325 kg of wet wrack per meter of coastline. This wrack originates in the seagrass meadows and seaweed forests offshore, which regularly shed old leaves—just like trees lose their leaves in autumn. These leaves are then carried to the shore by tides, waves, and wind and it then drifts on and off the beaches and may even travel back offshore, completing a wrack lifecycle. At every point along this journey, the wrack provides important ecosystem functions.

Ecosystem Functions And Services

Ecosystem functions are the interactions between living and non-living things that maintain a healthy environment. These can include things such as the production of food by plants through photosynthesis as well as pollination, in which bees visit plants to collect nectar and in turn help the plants complete their life cycles. Ecosystem services are the group of ecosystem functions that benefit people. Marine plants that become wrack along the coastline also perform important functions and provide ecosystem services that benefit humans. Next, we will describe some of these functions and service, illustrating why it is important to share the beaches with wrack and the creatures that live in it, to maintain a healthy environment.

Little Creatures Call Wrack Home

The coastal habitat can be very harsh but beach wrack makes an excellent home for many animals, providing safety from predators and other dangers such as heat (Figure 2). Most of the animals that make beach wrack their home are invertebrates (animals without backbones), including beetles and tiny crustaceans called isopods and amphipods (kind of like a shrimp). Some of these animals are only found in beach wrack and do not live anywhere else, making wrack very important for the survival of these species. Beach wrack is also a fantastic nursery for many invertebrates, such as adorable baby maggots.

Food Webs Within Wrack

Most of the animals that live in wrack are detritivores, meaning they love to eat dead plant material (detritus) just like roly-polies in your garden. But not all marine plants are equal when it comes to being food! The detritivores in wrack love to eat kelp, a type of seaweed, because it is more nutritious than seagrass. But the ability to eat even their favorite food depends on how moist it is, as dry food is hard to eat. It also depends on the amount of microbes (e.g., bacteria) on the detritus, which are very tasty! These microbes improve the food quality for detritivores by breaking down the detritus. Microbes themselves are also a food source.

Greater amounts of food on the beach leads to larger numbers of invertebrates, which are eaten by fish in the water or beetles and spiders on the beach. Many fish in wrack are young and benefit from feeding on amphipods in the wrack. These fish can then be eaten by larger fish or birds in, around, or above the wrack. Similarly, emerging kelp flies and their maggots become food for birds. In this way, wrack supports a whole food web on the beach and in the shallow waters (Figures 2, 3) [1].

Nature’s Fertilizer

Similar to a compost bin at home, beach wrack is a slow-release fertilizer in the ecosystem as it gradually breaks down (Figure 3). The nutrients in wrack—for example, nitrogen—provide the energy needed for life—, just like carbs, fats, and proteins do for us. Tiny microbes feeding on the wrack help to break it down and release its nutrients through the process of decomposition [2]. You may have noticed a funky smell at the beach; this is caused as the wrack breaks down and releases toxic “rotten egg” gas (called hydrogen sulfide). Similarly, invertebrates feeding on the wrack digest the plant material and their poop also adds to the nutrients. Altogether, this maintains a healthy environment where plants, algae, and animals grow, die, and decompose in the wrack cycle of life.

Connecting Land and Sea, The Wrack Circle of Life

Wrack is a temporary feature on beaches but plays a vital role in the connection between land and sea (Figure 3). Above, we highlighted the importance (for invertebrates and juvenile fish) of wrack on the beach and in the surf zone (area of the beach where waves crash). The nutrients taken up by the juvenile fish are taken out to deeper waters where the fish eventually become adults and reproduce [2]. In this way, nutrients in wrack connect the beach with the open ocean. Similarly, invertebrates in wrack can be eaten by land animals that then transfer nutrients inland [3]. You will also remember we discussed dissolved nutrients being recycled through decomposition of wrack. Once released, these nutrients can be washed back into the ocean where they are used by ocean plants to grow, or they can be used by shoreline vegetation.

Wrack Off!

Despite the clear ecological importance of beach wrack, it is often unloved by people. Our desire to lounge on clean, white beaches has led to frequent removal of wrack which then get dumped in landfills (Figure 1e). That is truck load after truck load of habitat and food being removed! This practice also leads to the release of greenhouse gases, such as carbon dioxide and methane, as the wrack decomposes in the landfill.

Coastal development (think marinas and jetties) can have a massive impact on wrack. These developments can destroy the seaweed forests and seagrass meadows that create the wrack, and can trap large amounts, preventing it from moving along the shoreline. This can cause marinas to be clogged up with wrack, or mountains of it to build up on beaches (Figure 1f), releasing toxic gases. This trapping also breaks the cycle of wrack shown in Figure 3, starving other areas that need wrack to function properly.

Questions remain about the impact that climate change will have on wrack. In many colder locations, seaweeds and seagrasses are being lost as the oceans warm. Will these cool-region seaweeds and seagrasses simply be replaced by other, warm-water species that provide the same ecosystem functions and services? Climate change may also alter where and how much wrack accumulates on coasts due to altered currents, sea-level rise, and the number and intensity of storms.

Many people may view wrack as a smelly nuisance, but it is essential for keeping our coasts healthy and productive. Changing people’s perspective on wrack is a critical first step in ensuring that we manage it, and our coasts, sensitively and sustainably. It is important that we continue to learn about beach wrack, the creatures that call it home, and the impacts of its removal, so we can ensure correct management for healthy beaches into the future. Where possible, it is encouraged that wrack be left on the beach so it can be buried by sand and add to the structure of the beach face, helping to limit erosion, or be washed back into the ocean where it will break down and continue to add to the food web.

Glossary

Ecosystem Functions: ↑ Different jobs that nature does to keep everything healthy and working well.

Groin: ↑ Low wall or barrier built out into sea from the beach.

Ecosystem Services: ↑ Helpful things that nature gives us to make our lives better.

Detritivores: ↑ Animals that eat dead plants and animals.

Food Webs: ↑ A connection of food chains that involve animals feeding on each other.

Decomposition: ↑ Breakdown of organisms once they die.

Landfill: ↑ Big holes in the ground where people bury rubbish.

Erosion: ↑ When wind and waves cause the beach to wear away.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.