The South African west coast is characterised by the tall, beautiful brownish green sea plants called kelp. These plants thrive in cold (below 20 degrees celcius), nutrient-rich, highly aerated water, and the strong wave action of the Atlantic is thus a feature of their ideal habitat.
Much of the water at the Two Oceans Aquarium is filtered through the kelp forest exhibit before being transferred to other tanks in the facility. The kelp plants do a remarkable job of cleaning the water of ammonia and other waste products. The fact that there is a kelp forest exhibit in the TOA is remarkable – it’s a non-trivial undertaking to grow live kelp plants successfully in a confined environment. Read more about it here.
Three main species of kelp dominate the South African coast: sea bamboo (which is what we see mostly in Cape Town), split-fan kelp, and bladder kelp. Kelp plants are made up of a holdfast (which looks like a tangle of roots, but actually just clings to the rocks), a long stipe or stalk reaching upwards towards the surface, and the fronds, leaves or (most accurately) blades, which absorb nutrients from the water.
Kelp plants like rocky surfaces to anchor onto, so the dive sites that feature kelp forests are often rocky reefs and outcrops such as Fisherman’s Beach, Shark Alley, and most of the Atlantic sites. Sandy bottoms are no good for kelp – nothing to grip onto – which is why there’s not much of it at Long Beach.
At low tide you can often see the tips of the kelp stems sticking just out of the water. There are small air-filled floatation devices called pneumatocysts attached to each kelp plant, which keeps the blades close to the surface of the water where they can take advantage of the sunlight. Some species (like sea bamboo, which is in most of the photos I have here) have a single large pneumatocyst at the end of each stipe, and others have one at the base of each blade.
Kelp forests provide a habitat for countless creatures, from the fish that shelter among their blades, to the kelp limpets that are specially shaped to fit snugly against the kelp stipes, to the crabs that love to hide in the waving forests. Abalone feed on kelp, and sea urchins use drifting pieces of kelp blades as hats to shelter from the sun.
During our training at the Two Oceans Aquarium, we dissected a kelp holdfast, and the number of creatures that live in that tightly-packed root system is astonishing. We found tiny brittle stars, kelp crabs the size of your fingernail, hundreds of worms, isopods, mussels, and tiny limpets. There’s a whole ecosystem that subsists entirely within the holdfast.
There are also many creatures that call the higher-up portions of kelp plants their homes. Cape rock crabs often shelter in the fronds, and orange-clubbed nudibranchs feed on the bryzoans (moss animals) that form pretty lacy patterns on the kelp leaves.
Top shells (there are a couple of varieties) live and feed on the kelp fronds. They’re really hard to photograph (and they’re SO pretty) because the movement of the kelp in the water confuses my camera (and the photographer).
The kelp stipes themselves are a habitat for other creatures. Coraline algae encrusts them, and tiny barnacles, hydroids and sea plants form beautiful, detailed colonies that reward close examination.
These photos were taken at Fisherman’s Beach, which boasts particularly gorgeous kelp stems. Inside broken stipes, we’ve found isopods, sea lice and other creatures that shelter inside the hollow tubes.
Diving in a kelp forest for the first time can be scary – I was terrified I’d get wrapped in the kelp and be stuck there forever. In fact, if you move slowly, it’s very easy, and it’s REALLY hard to get anything wrapped around you to the extent that your movement is hindered. Kelp blades are smooth and just stroke over you gently. There’s no sinister thrashing or wrapping like a giant squid grasping you in its tentacles. Kelp is your friend!
In surgy conditions, kelp is useful to hang onto (this may not be good advice). The holdfasts attach to the rock unbelievably firmly, in order to withstand the buffeting that the kelp gets from the waves, so the stems can generally support 65 kilograms of diver as well!
Kelp is hugely useful to humans – it’s used in the production of plant fertilisers (mmm – breathe in the smell!). Alginate, a substance derived from kelp, is used to thicken custard, toothpaste, salad dressings, mayonnaise, ice cream and jelly. Kelp grows incredibly fast, so it’s an ideal crop. Sometimes you can see strips carved out of the kelp forests between Kommetjie and Misty Cliffs – that’s where one of the kelp product manufacturers has been harvesting. They move down the coast taking a strip at a time, and by the time they get back to the beginning the forest has recovered.