Here's a scenraio; It is your first time surfing the Outer Kom, South Africa’s most historic big-wave spot. The swell is ten to twelve feet, the tide is low and the locals are calling it as good as it was during the 1985 Spur Steak Ranch Surfabout. Without thinking about it too much, you jump off the rocks and start paddling towards the lineup.
Before you know it, you are trapped in an impenetrable carpet of thick brown leaves. You are a sitting duck, at the mercy of the next set looming on the horizon. The first broken wave hits you.
To your surprise it has already backed off, reduced from what was a booming ten-foot grinder to a weak dribble. It lifts you up, releases you and lets you paddle a few strokes before you get stuck again. After about five or six of these you are clear, you make it to the line-up and start trying to make sense of those giant shifting peaks.
The thick carpet of vegetation is, of course, kelp. Kelp is a major factor at reef breaks in South Africa and around the world. It has evolved to live in or near the surf zone and relies on the waves in order to survive.
So, what exactly is kelp? Strictly speaking, kelp is not a plant. It is a chromist. It is similar in appearance to a plant, but the parts work slightly differently. The stipe is analogous to the stem of a plant, and the blades are analogous to the leaves.
The blades are responsible for both nutrient uptake and photosynthesis. The holdfast resembles the roots of a plant, but is only used to anchor the kelp to the rocks, not to obtain nutrients. Some types of kelp also have gas bladders, which serve as flotation devices to keep the blades near the surface so that they receive light.
For kelp to grow it needs a specific set of environmental conditions. For example, it will only grow on coastlines where there is something hard to hold onto, such as rock. It also needs a lot of nutrients.
Coastlines where there is upwelling are good for kelp because the surface waters contain an abundance of nutrients. Upwelling is where winds blow the surface water away from the coast allowing cold, nutrient-rich water to well up from underneath (see my article HERE).
Importantly, coastlines with large, consistent surf favour the growth of kelp, because the constant water movement stirs up the nutrients and helps those nutrients to be absorbed.
The way kelp affects your surfing experience can be quite different according to your location and the kind of kelp you are dealing with.
If you happen to be surfing one those world-class reefs in Northern Scotland, the type of kelp you will see is probably Laminaria hyperborean. This is quite short in length, and sits below the water level at mid or high tide, where a mattress of kelp covering the flat reef will cushion your wipe-outs. At low tide, however, it will make walking out to the break treacherous, particularly if it is frozen.
The way kelp affects your surfing experience can be quite different according to your location and the kind of kelp you are dealing with
In southern Africa, the dominant species of kelp, ecklonia maxima, grows in extremely dense patches, right there in the surf zone. From the kelp’s point of view, the extra turbulence produced by the breaking waves helps it to feed by stirring up the nutrients.
From a surfer’s point of view, kelp can make paddling out a comical experience, especially if you are not used to it. It can also be very annoying on a small swell or low tide, because the kelp will stick up right in the middle of the wave and can stop you dead in the water or even break your fins. In fact, some spots can only be surfed properly on bigger swells or higher tides. Once the waves break, the kelp absorbs a lot of the energy – useful on big days when you get caught inside.
If you are in California, kelp is practically all good news. The giant kelp found here, macrocystis pyrifera, tends to grow in large forests beyond the breakpoint. Therefore, most of the time, it doesn’t directly affect your surfing. Instead, it filters out the short-period chop, resulting in a cleaner, smoother wave at the line-up.
People have also hypothesised that the kelp releases an oily substance that increases the surface tension, further reducing any ripples on the surface.
The major effect of kelp on the waves is to dampen them down, to reduce their energy. Whether just filtering out the short-period chop from behind the breakpoint as in California, or dampening the broken waves as in South Africa, right?
If you are a surfer that’s pretty obvious, but scientists are struggling to prove it. The problem is simply that the effect of kelp on waves hasn’t been studied enough to explain our observations as surfers. In fact, some textbooks contain sweeping statements asserting that kelp has no effect on the waves whatsoever.
One of the better studies on the effect of kelp on wave energy was published in 1995 by Hany Elwany and colleagues from Scripps Institute of Oceanography. They measured the change in wave energy as the waves propagate towards the coast, first through an area containing a large kelp bed, and then through a ‘control’ area just down the coast with no kelp.
The hypothesis was simple: if the energy loss through the kelp area was significantly greater than the energy loss through the no-kelp area, we can say that kelp attenuates the waves. It turned out that their instruments did not find any difference between the energy losses through the different areas; therefore kelp doesn’t take energy away from the waves.
But of course we know it does. So I had another look at the paper and found that their instruments were only able to measure waves with periods longer than about three seconds. They should have concluded that kelp doesn’t affect waves with periods longer than three seconds.
They should have concluded that kelp doesn’t affect waves with periods longer than three seconds
In other words, waves with periods shorter than three seconds might have been attenuated by the kelp, but this wouldn’t have been detected in their measurements. And it might just be that waves with periods of less than three seconds make all the difference to the choppiness of the water surface.
So, to explain the observation made by hundreds of surfers in California every day, namely that kelp cleans up the waves, you would have to repeat that experiment, but with instruments that can measure waves with periods of less than three seconds.
For more articles around surf science, go HERE.
Cover shot: Josh Kerr wrangles Mavs by Tucker Patton.