The few seaweeds that most divers will notice on the reefs of Sodwana are the ten to twenty relatively large, showy species. They are unusual in two respects: they are a small minority of the species present, and they escape grazing by fish and invertebrates, either by producing nasty chemicals or hard carbonate walls, or by growing fast or in places where grazers can’t reach them.
In fact Maputoland is home to at least 400 species of seaweeds, but most are very small (their lengths measured in millimeters) and grow twined together in compact carpets (“turfs”) that cover reef edges and hard surfaces like dead coral. These inconspicuous turfs are incredibly rich in species. On the Sodwana reefs, we found 104 species of small seaweed in a total of 1.56 m2 of turf – an area not much bigger than a door, containing 20% of the seaweed species found in the whole of KZN! Remarkably, this number of species per area is almost exactly that found in turfs in Hawaii. The turfs are usually being constantly grazed, which keeps them low and keeps many of the seaweeds small, like “bonsai algae”. They are in fact highly productive and grow fast, but most of their production is entering the food chain through grazers.
As with the fish and invertebrates on the Sodwana reefs, the seaweeds are mostly tropical species that reach the southernmost extent of their distribution on these reefs. Their affinities are overwhelmingly “tropical western Indian ocean”, and they are bathed in the warm Mozambique Current that flows very close to the Maputoland coast.
By Dr Robert Anderson - Rhodes University