The West Coast Forests

The West Coast of southern Africa is dominated by dense beds of sea bamboo or kelp which are almost like underwater forests and extend from the shore to as much as 3km offshore.  These kelps are extremely productive.  Not only are they a major source of food for animals, but their dense growth breaks the force of waves, thus providing shelter for calm-water animals and plants.

The kelp forests on our West Coasts are the only ones in Africa, since they only occur in the Benguela Great Marine Ecosystem of South Africa and Namibia. Circumstances in this area are ideal for kelp growth - the availability of hard substrates to attachment of their tangled mass of haptera, enough light to be able to photosynthesize, an excess volume in the upwelling of nutrient-rich water and a water temperature of lower than 18°C.  Kelp can grow here by 13mm per day.

Camps Bay.  Picture credit: Derek Keats

Four kelp species occur in the South African kelp forests. The most familiar of the kelp species is Ecklonia maxima, which is a large plant with a cylindrical stipe reaching up to 12m in length and forming a flat, palm-like “hand” bearing strap-shaped fronds.  Ecklonia maxima has a hollow gas-filled stipe with a buoyant swollen bulb at the top which holds the plant upright, allowing its fronds to float near the surface of the water.  This fact is of great importance in the ecology of the system.  Firstly it means that Ecklonia always forms the top canopy of the kelp bed, shading other species.  Secondly it means that the plants float when torn free and can thus be washed ashore where they are of central importance to the ecology of sandy beaches on the west coast, as explained in a previous post.

Picture credit: Flickr

Laminaria pallida grows under Ecklonia's foliage cover. It can grow up to 5m in length and in depths of up to 30m depending of light penetration.  It has a short, stiff stipe with fan shaped foliage curling at the edges. The foliage can curl down to the bottom of the sea where it sweeps the ocean floor when swept around by the ocean current. It removes sea urchins and sea cucumbers that feed on kelp spores and sporelings, helping the new kelp to grow where the ocean floor is clean. North of St. Helena Bay, Laminaria pallida, is replaced by a similar species, Laminaria schintzei.  The fourth west coast species is Macrocrystis angustifolia , a slender plant with a rope-like stipe that forms blades all along its length, each with a small gas-filled float.  Macrocrystis is only found in sheltered water and contributes little to the total amount of kelp.

Macrocrystis angustifolia. Photo credit: Flickr

As with rain forests, kelp forests are biologically diverse and is home to a lot of animal species, included the economic important abalones (Haliotis midae) and the West Coast rock lobster (Jasus lalandii).  One of the dominant species, the urchin Parechinus angulosis, feeds mainly on broken pieces of kelp that sink to the floor of the kelp bed.  However, it also scrapes the surface of rocks and in the process removes all young plants including kelp sporelings.  If the urchins are experimentally removed about three times as much kelp can develop.  Animals thus reduce the settlement and growth of young kelp.  On the other hand, once they are established, the adult kelp plants are largely immune to herbivores, for water movement prevents the animals from climbing the kelp stipes.

Picture credit: reefdiversity.org.za

Apart from the sea urchin, there are three more types of herbivores that are common in the kelp forest.  The kelp limpet is specially adapted to live and graze on Ecklonia stipes, each defending its territory by pushing against intruders until they move off.  The abalone or perlemoen secures its food by trapping a drifting piece of kelp or the end of a kelp frond as it sweeps by in the current, lifting up part of its shell and foot and then clamping down.  The alikreukel or giant periwinkle sometimes trap-feeds on drifting kelp, but mostly grazes on other seaweeds in the kelp bed.

The most important group of animals in the kelp forest are the filter-feeders, such as sponges, mussels, red bait and sea cucumbers, which typically comprise 70-90% of the animal community.  The mucus released from kelp fronds is rich in organic compounds that encourage the growth of bacteria, which are in turn a food source for protozoans such as flagellates and ciliates.  Filter feeders consume these micro-organisms, as well as phytoplankton (microscopic algae), kelp spores and tint fragments eroded away from the tip of the growing kelp fronds.

The most important carnivore, at the top of the food chain, is the rock lobster.  It feeds on many species in the kelp forest, but prefers mussels, crushing the shells with its strong mandibles.  Other carnivores of the kelp forest are starfish, octopus, anemones, whelks, Hottentot fish, dogfish and seals.

Kelp forests are an important marine resource, with both direct and indirect value.  Apart from supporting commercially valuable species such as rock lobster and abalone, kelp beds help to buffer the shore from strong wave action, thereby reducing erosion and the risk to coastal developments.  Washed up kelp is collected by licensed concessionaires from beaches and processed for alginate, used as a gelling agent in thousands of everyday products such as toothpaste, cosmetics, flavoured milks, salad dressing and pizza toppings.  Limited harvesting of fresh kelp is permitted for production of an agricultural growth stimulant and as a food source for cultured abalone.  The harvest is carefully managed to ensure that these extremely productive ecosystems are safeguarded.