The Ocean Basket Kelp Forest Exhibit is an underwater forest where kelp plants grow like tall trees, fish swim through the fronds like birds and abalone, sea urchins and rock lobsters feed and take cover among the root-like holdfasts.
An enchanted underwater forest
Living kelp forests are currently only displayed in a handful of aquariums in the world – Monterey Bay Aquarium in California in the United States and here at the Two Oceans Aquarium in Cape Town, among others.
Here, three species of giant kelp provide shelter for an array of local fish, which drift amongst the kelp fronds. Our visitors are mesmerised by the hypnotic sway of the tall kelp plants and the play of dappled sunlight on silver fish. Central to the "two oceans" theme, this enchanting forest remains one of our biggest drawcards and many visitors return time and time again to its beauty and tranquility.
Kelp forests thrive off southern Africa where the icy Atlantic Ocean washes over rocky and shallow reefs from Cape Agulhas, at the southern tip of Africa, to central Namibia. The southeasterly that buffets the Cape Peninsula in summer causes upwellings of rich nutrients from the cold, dark depths of the ocean. These nutrients fertilise the kelp plants, allowing them to grow into giant forests, the canopies of which can be seen from the shore.
Four species of kelp grow off our coast, but you are most likely to relate to the sea bamboo, Ecklonia maxima, which is washed onto our beaches by rough seas.
Are you an advanced diver? You can dive in this exhibit.
Fascinating facts about the exhibit
- It contains 800 000 litres of seawater
- The tank it is housed in is 12m x 12m x 6m deep
- The water temperature is between 12 and 15º C
- The windows are made of acrylic (a type of plastic) and are about 180mm thick
- The rocks are made from fibreglass. Sand and crushed shells were blasted onto the fibreglass to make the rocks look real
- We pump seawater from the harbour to all our exhibits, including the Kelp Forest. Over and above the water we add every day, we filter 800 000 litres of seawater into the kelp forest every two hours
- Three species of kelp are often displayed in our kelp forest – sea bamboo, Ecklonia maxima, split fan kelp, Laminaria pallida, and bladder kelp, Macrocystis angustifolia
The Kelp Challenge – grow kelp in an aquarium
Like land plants, kelp plants also require certain conditions in which to grow, including sunlight, cool water temperature, wave action, or water movement, and nutrients.
This exhibit is open to the sky so that the kelp plants can receive sunlight to photosynthesise. We have also added strong lights over the exhibit so that we can increase the amount of light on very overcast days and during winter when the sun is lower in the sky.
The water temperature in this exhibit must be kept between 12 and 15°C to ensure optimum temperature levels for growth. This is a challenge in summer and especially when the Cape experiences unusual heat waves. We have installed large chillers to keep the water temperature within a suitable range.
Wave action and water movement
In the wild kelp plants receive their nutrients from upwelling. During summer, the strong south-easterly wind, which buffets the Cape Peninsula, blows surface water away from the coastline, allowing icy-cold water from the ocean depths to replace it. Wave action and water movement are also vital for kelp growth because they stir up the nutrients and enable the plants to absorb these nutrients through their fronds.
We have installed three systems to increase water movement and the circulation of nutrients in the Kelp Exhibit. The plunger in the far right of the exhibit creates a surge while a streamer pump delivers approximately 70 000 litres of water per hour to create circular water movement within the exhibit. ‘’Dump boxes’’ tip large volumes of water into the exhibit at intervals – this stirs up the nutrients in the exhibit.
A giant water purifier
Kelp plants thrive on waste products (for example, ammonia) produced by fish. Together with the biofilter (aerobic bacteria living in the subgravel filter of the exhibit), the kelp purifies the water. As a result the Kelp Forest Exhibit has the best quality water in the Aquarium.
In fact we use the Kelp Forest Exhibit as a water purifier when algal blooms in the harbour die, turning the water toxic (anaerobic). When this happens, we reduce the amount of water we pump from the harbour and pass it through the Kelp Forest before distributing it to our smaller exhibits.
Collecting kelp at sea
Kelp grows incredibly fast and must be replaced regularly. We often go out to sea to collect kelp from the rocky reefs off Robben Island and Bantry Bay. Divers carefully remove the kelp holdfasts from the rock and swim the plants to the boat.
Once on the boat, the plants are placed in a specially designed fibreglass box that has a spray bar and a reflective covering to keep them wet and cool. Up to 15 plants are collected and transported back to the Aquarium where the divers tie them down to rocks in the exhibit.
A forest home for fish
South Africa’s kelp forests are home to many fish species found nowhere else in the world. Some fish, such as southern mullet, strepies and hottentots, live permanently in kelp forests where they find food and shelter, while others, such as Cape salmon, giant kob and even yellowtail, move in and out of kelp forests in search of food.