South African kelp forests

Dr Rob Anderson MCM

Visitors to South Africa's cold-water coasts, from the Orange River to Cape Agulhas, are often struck by the huge beds of kelp that are visible at low tides. In the area around Gansbaai, the sea-bamboo, Ecklonia maxima, dominates inshore kelp beds, with a smaller kelp species, Laminaria pallida, forming a sub-canopy under the water, a metre or two above the rock.

Ecklonia is attached by a tough holdfast, and has a flexible stipe or stem up to about 8 m long, the top of which is swollen into a gas-filled bulb. This bulb bears numerous fronds, and floats at the surface of the water to keep them near the light.

Laminaria, by comparison, has a stipe up to about 2m long, and a flat, hand-like blade that is often split into many smaller fronds.

Both species are endemic to southern Africa. Like all kelps, their life-history involves the large, visible plants (sporophytes), producing millions of microscopic spores that swim through the water to settle and grow into tiny filaments. These filaments are the sexual phase (gametophytes), and males produce sperm that fertilise the eggs on the female filaments. The eggs become embryos that grow up into the large kelp plants that we see. It takes about 2 years for a kelp plant to grow from an embryo to 2-3 metres in length, and plants are estimated to live for 5-7 years.

Kelp beds are often called forests because of similarities with their terrestrial counterparts. On the rock under the kelps are a variety of smaller seaweeds, from the paint-like coralline crusts, through filaments and all sorts of shapes, to larger red, green or brown, leafy species. In among them are dozens of types of animals. Larger, more recognizable commercial species like rock-lobster and abalone (perlemoen) are well known, but in fact the kelp forests sustain complex food-webs involving hundreds of species. Kelps are eaten directly by a few fish species (e.g. strepies, Sarpa salpa), and pieces of kelp are the main food of abalone and many limpets in the subtidal.

Sunlight provides the energy that drives the whole system. The kelps and other primary producers (other seaweeds and the plant-or phytoplankton) use this energy to produce organic matter from inorganic chemicals. This organic matter (now plant material) enters the food web, either directly when it is eaten by herbivores, or indirectly as the algae decay and are broken up by bacteria. Much of the broken-up matter is filtered out of the water by mussels, sponges and other filter-feeders. Higher up the food-chain, these mussels may be eaten by a rock-lobster or a fish. Even higher, the fish or lobster may fall prey to a seal, or that ultimate predator, a human.

Kelp is used commercially, and the beds around Gansbaai annually contribute a large part of the more than 5000 tonnes of Ecklonia fronds that go to feed farmed abalone.

As the natural abalone stocks have been poached out of commercial existence, the onshore farming of abalone has become a massive industry in South Africa. Harvesting of these kelp beds is controlled by a system of annual allowances (Maximum Sustainable Yields) and permits, and only a small proportion of the biomass is removed. Research has shown that harvesting at these levels has a negligible effect on the ecology of the beds. These kelps are highly productive and grow fast, and the kelp beds, which annually lose about 10-12 % of their biomass naturally in storms, are very robust to disturbance.