Detritus cycling in a shallow coastal lagoon in Natal, South Africa

Abstract

Heavy siltation is at present characteristic of most of Natal's estuaries as a result of agricultural mismanagement. A reduced freshwater input, invasion by the reed Phragmites australis (Cav.) Trin ex Steudel, and anaerobic conditions, caused by an inability to cope with the detritus load, are frequently concomitant factors. The Siyaya system is a typical case and was chosen for a catchment rehabilitation study to establish whether restorative action would effect an improvement in the lagoon. Conditions related to the detritus food chain were studied in the lagoon as part of the programme, as a response to improvements in the system are expected to be manifested first at this level. Measurements were made on various physical conditions, the rates of leaf litter input and labelled leaf litter decomposition, and the detritus and microbial levels. The results provide baseline data for future comparative studies and reveal that litter input (1.61 kg·m −2· a −1) exceeds the estimated mineralization rate. Anaerobic conditions prevent the development of a population of secondary consumers to utilize the excess detritus and the reeds prevent it from being flushed from the system by floods. Organic matter thus accumulates in the lagoon and this is portrayed in a simple budget of carbon flux through the detritus food chain. Eradication of the reed beds appears to provide a key to rehabilitation, but this action would initially exacerbate the problem by increasing the detritus load. Aeration is suggested as a means of overcoming this problem.