Seaweeds for coastal defense

Seaweeds can be an effective nature-based solution to defend coastal regions against storm damage and soil erosion. We’ll look at the science underlying the idea, as well as the organisations working to implement solutions.

The science

What’s wrong with traditional storm defense systems?

Traditionally, hard structures like seawalls and breakwaters have been used as coastal defenses. However, these structures can cause habitat loss for coastal species, aggravate soil erosion and inhibit the natural accumulation of sediments.

Additionally, these conventional hard engineering defenses need continual and costly maintenance. With rising sea levels and more powerful storms, these costs will continue to rise.

Natural and nature-based infrastructure may be a viable alternative or a supplement to hard shoreline protection systems. Living breakwaters automatically adapt to sea level rises, and they bring added economic and ecological benefits.

How coastal vegetation absorbs storm impacts

(We leaned heavily on Morris et al., 2019 and Denny, 2021 to write this bit. If you spot any mistakes, please let us know!)

Coastal vegetation like seagrass and oyster beds, kelp forests and mangroves acts as a barrier to destructive storms by dissipating the energy of waves.

Kelp beds may also create onshore currents, which can promote the movement of sediment onshore (see figure 1 here).

Of course, it’s a bit more complicated than that. Flexibility, height, density, buoyancy, place in the water column and individual stiffness of the algae as well as the hydrodynamic properties of the sea, such as wave height and period, wave–current interactions and water depth all play a role in determining if, when and how much stands of seaweeds will buffer the impact of destructive waves.

One also has to keep in mind that the size and composition of kelp forests changes with the seasons. On some temperate shores, seaweeds grow large in the calm summer months, but are pruned back or dislodged when drag exceeds their strength in fall and winter storms.

So, a lot of variables to keep in mind. Sadly, for now there have only been a limited number of studies that have investigated the coastal protection provided by kelp beds, with only a few species examined. Some species have performed better than others, and certain species can even have the opposite effect, with waves bigger than an empty patch of seabed.

Does seaweed aquaculture have a similar effect?

According to a 2020 paper by Zhou et al., “Incorporating suspended aquaculture farms offshore significantly enhances the coastal protection effectiveness of (…) living shorelines and extends the wave attenuation capacity over a wider wave period and water level range. The combination of suspended aquaculture farms and traditional living shorelines provides a more effective nature-based coastal defense strategy than the traditional living shorelines alone.”

If you zoned out during that paragraph: yes, it works!

Zhou et al. followed up in 2021 with tips to increase a seaweed farm’s buffering capacity: “install the kelp farms in shallower water, expand the farm size by adding more longlines, locate the kelp in a higher position of the water column, grow the kelp more densely, and choose the kelp species with more rigid, wider, and longer blades/biomass.”

The companies

Two startups are currently working on putting the science into practice. ReShore is building a living breakwater which combines coastal defense with shellfish and seaweed aquaculture, while Emerald Tutu is engineering floating wetlands with marsh grass above the waterline and seaweeds below to deploy within city harbours.