Kelp

First published in The Surfers Path

© Tony Butt 2010 - Please be decent enough to contact me before plagiarizing my stuff

You are in Cape Town, South Africa, there is an eight to ten foot west swell running, and the Outer Kom, perhaps South Africa’s most historic big-wave spot, is going off. You jump off the rocks and start paddling towards the line up. 

The first ten minutes of your surf session are not quite the same as at most other surf spots. Instead of jumping off into a channel or perhaps punching through one or two lines of whitewater to get a clear route to the line-up, you jump into an impenetrable carpet of thick brown leaves. Progress seems impossible. Your leash gets caught in the vegetation and you are trapped – 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. The wave lifts you up, releases you from the vegetation and lets you paddle a few strokes before you get stuck again. This process is repeated five or six times, and, before you know it, you’re in the line-up, making friends with the local crew and surfing some epic waves.

The thick carpet of vegetation is, of course, kelp. Kelp is a major factor at reef and point breaks in South Africa and around the world. Its effects on your surfing experience can vary quite radically according to your location and the kind of kelp you are dealing with. In California, for example, the kelp ‘canopies’ tend to lie offshore and clean up the waves before they reach the line-up. In Scotland, however, the kelp is a soft, slippery mattress covering the flat reefs, cushioning your wipeouts but making walking across the reef at low tide treacherous.

 

What is kelp?

Strictly speaking, kelp is not a plant. It is a chromist. The anatomy of a kelp is similar in appearance to a plant, but the parts work differently. The stipe is analogous to the stem of a plant, and the blades or fronds 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 take up nutrients. Some kelps also have gas bladders or pneumatocysts, which serve as flotation devices to keep the blades near the surface so that they receive sufficient light.

In order for kelp to grow, you need a specific set of environmental conditions. 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 a process whereby strong winds blow the surface water away from the coast allowing cold, nutrient-rich water to well up from underneath). Also, coastlines with large, consistent surf favour the growth of kelp, because the constant water movement around the blades stirs up the nutrients and helps the kelp absorb those nutrients. 

Not surprisingly, different types of kelp are found in different parts of the world. To keep things simple and to avoid getting lost in the world of biological taxonomy, we’ll just split them into three categories:

Laminaria:

Found around Scotland and Ireland, Norway, Japan, on the Kamchatka Peninsula and on the north-east coast of North America;

Relatively short and has no gas bladder;

 Exists mainly below the water level, lying close to the sea bed

Ecklonia:

 Found in South Africa, southern Australia, Tasmania and New Zealand;

Grows up to three of four metres in length;

Contains a gas bladder which allows the blades to float on or near the surface in a couple of metres of water (often right in the path of the broken waves);

 Can form very dense ‘forests’

Macrocystis:

Found on the west coast of North and South America and around Argentina, plus on many Southern Ocean islands;

Largest of the three categories;

Can grow up to 40 metres in length at a speed of up to 60 centimetres per day;

 Has a gas bladder;

Grows in depths between about eight and 20 metres (most of the time beyond the breakpoint)


How kelp can affect your surfing experience

Now let’s have a look at the effect of kelp on your surfing experience. The fact that it removes energy from the waves is common to all types of kelp, but, of course, the consequences of this are not always the same.

If you happen to be surfing one those world-class reefs in Northern Scotland, you will probably see Laminaria hyperborean. This is quite short in length, so it sits below the water level at mid or high tide. A mattess of kelp covering the flat reef will cushion your wipeouts at high tide, but will also make walking out to the break at low tide very slippery, especially if it happens to be frozen.

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 stirs up the nutrients and helps it to feed. From a surfer’s point of view, it can be very annoying on a small swell, because the kelp will stick up right in the middle of your wave. If it doesn’t stop you dead in the water, it will at least force you to take a higher line than usual. If the swell is bigger, the faces of the waves will be relatively kelp-free. Once the waves have broken and the whitewater starts to hit the kelp bed, a huge amount of power is absorbed, which can be a tremendous advantage when paddling out on big days.

If you are a surfer in California, kelp is practically all good news. The giant kelp found here, macrocystis pyrifera, tends to grow in huge ‘forests’ out beyond the breakpoint. Therefore, most of the time, it doesn’t actually affect your ride. Instead, it filters out the short-period chop (in scientific terms, the high-frequency end of the wave-energy spectrum), resulting in a cleaner, smoother wave at the line-up. People have also hypothesized that the kelp releases an oily substance that increases the surface tension, further reducing any ripples on the surface.

So, 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?  Well, if you are a surfer that’s pretty obvious, but if you are not a surfer you might be forgiven for thinking otherwise. 

In some textbooks you will find sweeping statements asserting that kelp has no effect on the waves whatsoever. For example: “Complementary flow data gathered using moored instruments indicate that surface gravity waves are not noticeably damped in passing through kelp forests”.  Some even suggest a reason why kelp has no effect on the waves. For example: “Kelp has evolved a hydrodynamically streamlined form that enables them to bend with the flow”.

The problem is simply that the effect of kelp on waves hasn’t been studied enough to explain our observations as surfers.

One of the best studies 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. Instruments were placed in deep water outside each area, and in shallow water inside each area. To obtain the change in wave energy, the shallow-water measurements were simply subtracted from the deep-water ones.

The hypothesis was this: 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. On the other hand, if the energy loss through both areas was more or less the same, kelp doesn’t attenuate the waves.

Elwany and co-workers concluded that “The similarity of the wave field at the onshore kelp and control sites shows that this typical southern California kelp bed…does not have a significant effect on waves”.

Now, their instruments were only able to measure waves with periods longer than about three seconds.  So, their conclusions should have been that kelp does not have a significant effect on waves with periods longer than three seconds.

In other words, waves with periods shorter than three seconds might have actually been filtered out 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, we would have to repeat that experiment with more sensitive instrumentation.