Does kelp make a break more glassy?

Though kelp seems determined to turn your paddle out into a tangled mess, with its grubby little kelp hands holding onto your leash for dear life, it’s also actively turning choppy waves into a glassy session for you.

A recent study by Mathilde Lindhart and colleagues, titled "Short wave attenuation by a kelp forest canopy," sheds light on how kelp forests affect wave energy. This research, conducted in the Point Loma kelp forest, reveals some fascinating insights into how these underwater forests shape our surf.

This project was cool. Admittedly, I helped with some of the dive operations, so I am biased, but the statement still stands. The kelp forest that the instruments were deployed in was dense and full of life when we were there, and the dives ran like clockwork due to the planning and organization that the leads on this project prioritized.

The main finding of this study is that kelp, specifically Macrocystis pyrifera or Giant Kelp, significantly reduces the energy of high-frequency waves. These are the small, choppy waves that make our surf sessions less than ideal. Essentially, kelp acts like a natural wave filter, letting the good stuff through while blocking out the chop.

Figure 1 of the paper, showing why the surface density of kelp decreases as the tide goes up. Credit: Lindhart et al. 2024.

To really nail down the relationship between kelp density and wave smoothing (does more kelp mean that less chop gets through?), the researchers needed a clever way to measure different amounts of kelp. Since the kelp field didn't shrink or grow significantly during the study, Dr. Lindhart and coauthors used the tide to their advantage.

At high tide, less kelp reaches the surface, with shorter strands submerged and longer ones more vertical. Low tide, however, allows more kelp to spread out horizontally at the surface, creating a denser canopy. By analyzing data at different tide levels, they showed that more surface kelp equals more wave smoothing.

The results are clear in their spectral analysis, which is shown below here. For the waves we surf (less than 0.2 Hz, or periods over 5 seconds), kelp doesn't make much difference. You can tell this by the fact that the energy at these frequencies inside and outside of the kelp forest is the same. But for those higher frequency waves, the kelp forest significantly reduces their energy. And this effect gets stronger as the kelp canopy gets denser due to the low tides.

Figure 3 of the paper shows that at high tide in panel (a), there is no difference in high-frequency energy at the far right of the plot. But at low tide in panel (f), the gap between the lines at high frequencies is significant.

The exact way that the energy is sapped from these waves and where it goes is still unknown, but this is a really good start. And it helps inform us of something that anyone in Santa Barbara and north can tell you: if it’s windy out, you can still snag some clean waves at the breaks with loads of kelp.

It should also be noted that kelp doesn’t actually have hands, in case anyone was wondering about that opening.

Further Reading:

The Paper

Giant Kelp