Two main species occur at Macquarie Island, the leathery bull kelp Durvillaea antarctica which occupies the bottom of the rocky shore, and the giant kelp Macrocystis pyrifera which forms dense forests at depths greater than 6 m. Both species can reach lengths of over 20 m. At Heard Island, which lies below the Antarctic Convergence, and thus experiences much lower water temperatures, only bull kelp is present. In the kelp zone at Heard Island, one square metre of rocky shore may support up to 47 kelp plants, and over 200 kg of plant tissue. The bull kelp Durvillaea antarctica is thought to be the strongest kelp in the world and is able to withstand the massive seas that are characteristic of the Southern Ocean.Kelp has one of the fastest growth rates of any of the marine plants (up to 60 cm per day or 3 kg of dry weight per square metre of habitat per day recorded for the Giant Kelp) and this makes it an important primary producer supplying food into nearshore and intertidal marine habitats. Some animals, such as sea-urchins, shells and small crustaceans, feed directly on kelp. However, it is through the breakdown of detached pieces of kelp and whole plants that most consumers benefit. Although kelp plants have a strong "root" system, known as a holdfast, that attaches them to the substrate, wave strength in the Southern Ocean is such that plants are often torn from the bottom and flung onto adjacent beaches. Smaller pieces of kelp also break off during heavy seas and these may strand on local beaches or sink into deeper waters. Wherever they end up, detached kelp becomes a food source for scavenging animals such as amphipods (beach hoppers) and are gradually broken down liberating nutrients back into coastal waters. As well as providing an important supply of food to local marine communities, kelp provides a refuge from the harsh physical environment for many organisms. The long fronds of bull kelp, that are often packed tightly around the lower shore, absorb much of the energy from the waves crashing on the coast, creating a lower energy environment where marine life abounds. Small plants and animals can be found living between the large holdfasts of kelp and many animals also live within the holdfast itself.
Despite the fact that the holdfast of the bull kelp is a solid hemisphere of tough tissue, a species of small isopod crustacean (Limnoria) makes its living consuming this tissue. In the process, the holdfast becomes riddled with tunnels and chambers which are used as living space by other small invertebrates. Different species of Limnoria are the primary space providers at Macquarie Island (L. stephenseni) and Heard Island (L. antarctica).
Studies of the kelp holdfast communities at Macquarie Island and Heard Island have revealed that they support a higher diversity than any other shore habitat with over 90 species of invertebrate finding shelter in the tunnels and chambers that riddle their large masses. A typical holdfast community includes worms of many varieties, molluscs, mites, sea-stars, sea-cucumbers and a large variety of crustaceans.The diversity of organisms inhabiting the holdfast makes this habitat a valuable tool for monitoring changes in shore communities under the influence of both natural, and human induced impacts. Indeed, the holdfast community of bull kelp was the most sensitive indicator of the effects of, and subsequent recovery from, a small oil spill at Macquarie Island in 1987.
Both bull kelp and giant kelp have very buoyant fronds that are capable of supporting whole plants at sea for considerable periods of time. Bull kelp, in particular, is exceptionally buoyant and has been documented to carry boulders weighing up to 75 kg from the lower to upper shore at Macquarie Island. It has been hypothesised that animals associated with floating kelp (kelp rafts) can be transported between the far-flung islands across the subantarctic. This potentially explains the observation that many kelp-associated species are found throughout the islands of the subantarctic.There are plenty of rafts available to animals in the Southern Ocean with recent surveys estimating that, between the latitudes of 46-53°S, over 70 million plants are afloat at any one time. In addition, some recent studies have indicated that animals can live attached to floating kelp for long periods of time. This idea of invertebrate transport by kelp is the subject of continuing research.