In George Begg’s article on the Kosi system, mention is made of the five species of mangrove which occur there, two of which (Ceriops tagal, the Indian mangrove, and Lumnitzera racemosa, the Tonga mangrove) are at the southernmost limit of their distribution at Kosi. The three other species are the red mangrove (Rhizophora mucronata), the black mangrove (Bruguiera gymnorrhiza) and the white mangrove (Avicennia marina). Keith Cooper, now The Wildlife Society’s Director of Conservation, surveyed South Africa’s mangroves for a report published in 1968 (reference below) and estimated that there were 32 ha of mangrove community at Kosi.

This may not seem to be much, but mangroves in fact play a very important role in the ecological health of the systems of which they are a part. Firstly, they are important spawning and nursery areas for many species of marine fish (and marine invertebrates). Secondly, they provide organic material in the form of leaves and detritus, which forms the food base for the complex society of estuarine life. (The rotting leaves are coated with bacteria, which makes them an attractive, protein-rich food source for invertebrate animals from nematode worms to shellfish, which in turn provide a delectable menu for juvenile fish.) Thirdly, they play a vital role in stabilising estuarine sediments and preventing the undercutting of riverbanks by flood waters and wave action. Fourthly, they have direct uses to local people – for example at Kosi the mangroves provide materials for the construction of fish traps.

“In the mangrove swamp, the habitats of land and sea overlap,” says the World Wildlife Fund’s Yearbook for 1982. (That peculiar fish, the African mudskipper, Periophthalmus cantonensis, which inhabits the mangroves, lives almost entirely out of water, providing a clue perhaps to the way aquatic creatures first left the water for the land.) The Iternational Union for the Conservation of Nature and Natural Resources (IUCN) has drawn up a report on the status of the world’s mangroves and has urged every country with a mangrove resource to develop a “National Mangrove Plan” to safeguard mangroves and to provide for their rational utilisation if necessary.

So far we aren’t doing too well in South Africa. Mangrove swamps exist in only about a dozen estuaries and Bruguiera gymnorrhiza, and several of these have been severely damaged by man’s activities (for example the Bruguiera gymnorrhiza stand at Sodwana, and the mangroves at Richards Bay, Durban and Isipongo).

Mangroves can be severely affected by alteration of the water level. At Sodwana the building of a small bridge raised upstream water levels by 80 cm, which had the effect of drowning the younger mangroves by preventing their pneumatophores (aerial breathing roots) from obtaining air. At Kosi Bay in 1965 the natural closing of the estuary mouth for five months resulted in flooding of the mangrove swamp (aided by Cyclone Claude) and caused a mass mortality of the mangroves from which they are at present slowly recovering.

Development at Kosi Bay will almost certainly harm the mangroves there. And if so, the whole estuarine ecosystem will be the poorer.

References:

Anon. 1982. Why save mangroves? The World Wildlife Fund Yearbook 1982:

84-95.

Cooper, K.H. 1968. A report on mangroves in South Africa. Unpublished

report. The Natal Branch of The Wildlife Society of Southern Africa,

Durban.

Moll, E.J., Ward, C.J., Steinke, T.D. & Cooper, K.H. 1971. Our mangroves

threatened. Afr Wildlife 25 (3): 103-107.