How kelp protects coasts

Some protection from waves crashing on the shore could be found in what lies beneath the surface.

Kelp beds are well-known for the ecosystem services they provide, but do they do more? A recent study by Rebecca Morris and colleagues looked at the kelp beds of Port Phillip Bay, in Australia, and studied what effect they had on wave amplitude.

Dr Morris said that Port Phillip Bay was an excellent place to look at how kelp beds attenuated waves for a few reasons. “Kelp beds have suffered significant decline in Port Phillip Bay due to overgrazing by the native urchin, there is an effort to restore kelp beds, so a complete knowledge of the services they provide will help to drive restoration efforts. It’s important to understand as lots of people live around Port Phillip Bay and there is an increasing interest in nature-based coastal defence to manage erosion and flooding. Also, due to the urchin barrens, there were a few places that we know had incipient barrens (areas of barrens next to kelp beds) where we could have control-kelp treatments.”

The waves were measured using special loggers. These were deployed for a couple of weeks at a time. That way the team could compare the average wave height by kelp beds with places where waves ran over barrens to hit the shore. Here, working in the bay was a help, Dr Morris said. “The access to Port Phillip Bay is easier than on the open coast, we have to work around the weather to get boats and divers into the water to deploy and retrieve the equipment. Within Port Phillip Bay there is still a lot of variety in wave exposure and the health of kelp beds around the bay. For future work, it would also be interesting to deploy wave loggers on the open coast though as it will be exposed to both wind and swell waves and the kelp tends to grow much bigger.”

“The loggers are pretty inconspicuous on the reefs (they are only about 20 cm big), we used the boat to make a good GPS mark to find them again! So there was no problems with leaving them on the reefs.”

If you can’t think of much work that had been done on kelp’s impact on waves, there’s a good reason for that. “The coastal protection provided by kelp is a surprisingly understudied service,” Dr Morris said. “The little literature to date points to kelp having a pretty insignificant effect on surface waves, thus providing little shoreline protection. Our study supports this idea, and from what we know about the conditions that promote wave attenuation in other coastal vegetation, this may not be surprising i.e. that the vegetation needs to take up a large percentage of the water column to have an effect on wave transmission. However, I do think that because of this, wave transmission through a kelp bed would be very variable depending on the site, especially the depth of the reef, but also the width of a kelp bed e.g. a very wide kelp bed might have more of an effect even if it is deeper.”

In order to understand the kelp better Dr Morris is thinking about taking it to a lab. “The intricacies of kelp and waves can be hard to tease out using field experiments as we are limited to the sites available, thus a useful next step would be to do some flume testing to determine whether there are any conditions that favour wave attenuation by kelp. We could then look for these conditions in the field to validate it.”

Another issue Dr Morris identified is the sheer variety of kelp. “Kelp (the Laminariales) is very diverse, not only between species but also morphologically within a species, which is driven by the environment. In Australia, Ecklonia radiata is our dominant kelp species, however, the diversity of kelp beds varies worldwide, some are dominated by one species whereas others will have multiple species. The species and morphology of kelp is likely to have an effect on kelp attenuation, thus this is an interesting avenue for future research.”

Kelp. Image: Canva.

If you’re looking for another angle on coastal protection then the effects of kelp is a field with plenty of opportunity for further research. “We have summarised the current state of knowledge and identified the significant research gaps in the coastal protection service of kelp, thus this paper provides a good place to start for anyone doing research in this field,” Dr Morris said. “More broadly, the risk of coastal erosion and flooding is increasing globally, and nature-based coastal defence when used appropriately can provide an effective and sustainable approach to addressing these challenges. We need to fill the research gaps in this area to provide coastal mangers and policy makers with the information needed to implement nature-based coastal defence on a wider scale. This paper is contributing to a broader understanding of the tools available within nature-based solutions to shoreline protection.”

In their conclusions, Morris and colleagues note that kelp is in decline around the world. As well as the obvious loss in biodiversity, it may be that the disappearance of kelp will be felt physically too.

Further reading

Morris R, Graham T, Kelvin J, Ghisalberti M, Swearer S. 2019. Kelp beds as coastal protection: wave attenuation of Ecklonia radiata in a shallow coastal bay. Annals of Botany. https://doi.org/10.1093/aob/mcz127