Wetlands Ecology and Management 8: 29–36, 2000. c  2000 Kluwer Academic Publishers. Printed in the Netherlands. 29 Salinity intrusion and rice farming in the mangrove-fringed Konkoure River delta, Guinea Eric Wolanski 1 & Bernard Cassagne 2 1 AIMS, PMB No. 3, Townsville MC, Qld.4810, Australia; 2 FRM, Parc du Millenaire BAT 23, 34036 Montpellier Cedex 1, France Received 1 March 1999; accepted 7 June 1999 Key words: acidification, mangroves, rice farming, salinity, West Africa Abstract Field and model studies were undertaken of the hydrodynamics and salinity intrusion in the mangrove-fringed, 320 km 2 Konkoure River delta, Guinea. The mangroves directly fringing the main channel of the Konkoure River are best flushed, while those mangroves draining smaller coastal streams are least flushed. Large seasonal and spatial variations of salinity exist in the delta. Hypersalinity and slow flushing (residence time ≈ 14–30 days) occur in the dry season in some areas of the delta. These areas appear unsuitable for rice farming and may be best managed on a sustainable basis for fishery and forestry resources. Introduction The Konkoure River is one of the largest Rivers of Guinea, West Africa. It forms a delta of 320 km 2 , which is vegetated by mangroves (Figure 1). Peak River discharge in the wet season fluctuates widely typically between 800 and 3,000 m 3 s −1 , in the dry sea- son the mean discharge is 100 m 3 s −1 (CERESCOR, 1991). The delta of the Konkoure also receives the discharge of the Soumba River, this is a small coastal stream and its discharge essentially vanishes in the dry season. The Sankine River is a natural flood overflow channel of the Konkoure River. The delta fronts San- gareya Bay which is shallow with mean depth of about 5 m. Semi-diurnal tides prevail with a peak tidal range of about 3.5 m. Over the last 15 years there has been increased human encroachment in the mangrove forest. This en- croachment has been largely unregulated as a result of political benign neglect. Elsewhere in Guinea, for example in the mangroves south of Conakry, this has resulted in serious degradation, for instance adult trees have largely disappeared. In the Bay of Sangareya an EEC-financed project has been implemented in the early 1990’s to manage the mangroves in a sustain- able basis whereby artisanal and commercial fishing as well as selective harvesting of mangrove wood is permitted but regulated. However the mangrove land is also increasingly targeted for rice farming (see Fig- ure 1). Rice farming is carried out by clear-felling an area, constructing dikes to retain rainfall water, us- ing pesticides to remove crabs and generally yields only one crop per year. In the dry season generally the dikes are not removed. As a result the soils are not tidally flushed and they acidify. This progressively reduces the area where rice can grow (Figure 2a) until the area becomes a wet desert of salt water, mud and weeds (Figure 2b). At that time the area is abandoned by the farmers who then slash and burn another man- grove area. Mangrove trees seem unable to re-establish themselves in these areas except where tidal drain- age is re-established. Re-establishing tidal drainage is expensive and is essentially never carried out. As a result the rice farms more commonly then not remain a wet desert for 10 years or more. This scenario has been repeated throughout Guinea over the last 20 years (Bertrand, 1993; Rue, 1993, 1995; Rue and Fontana, 1995) and mimics the failure of rice farming in man- groves of Indonesia (Claridge, 1994). Essentially rice farming has devastated mangroves in Guinea in the same way that shrimp farming did in South East Asia (Primavera, 1991, 1993; Flaherty and Karnjanakesorn, 1995).