Estuarine, Coastal and Shelf Science (1997) 45, 697–701 The E ffect of Increased Sediment Accretion on the Survival and Growth of Rhizophora apiculata Seedlings J. Terrados a , U. Thampanya b , N. Srichai b , P. Kheowvongsri b , O. Geertz-Hansen c , S. Boromthanarath b , N. Panapitukkul b and C. M. Duarte a a Centro de Estudios Avanzados de Blanes, C.S.I.C., Camı ´ de Santa Ba ´rbara, s/n. 17300 Blanes (Girona), Spain b Coastal Resources Institute, CORIN, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand c Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK 3400 Hillerød, Denmark Received 23 January 1997 and accepted in revised form 6 May 1997 The effects of experimental sediment accretion on the survival and growth of Rhizophora apiculata seedlings planted on an expanding mud flat in Pak Phanang Bay (south-east Thailand) were assessed. Seedling mortality rates increased linearly ( R 2 = 0·87, F = 75·9, P<0·0001) with increasing sediment accretion, at a rate of 3% per cm of sediment deposited, and implied a 96% increase in mortality at the highest sediment accretion applied (32 cm). Similarly, seedling growth declined linearly with increasing sediment accretion ( r= 0·95, P<0·01) with the seedlings receiving 32 cm of sediment showing no significant growth. These results clearly show that Rhizophora apiculata seedlings will not be efficient colonizers of coastal areas exposed to sudden events of high (>4 cm) sediment accretion and, therefore, afforestation programmes based on this species are unlikely to be successful in such areas.  1997 Academic Press Limited Keywords: mangrove seedlings; sediment accretion; Rhizophora apiculata; south-east Asia Introduction Deforestation, a widespread problem in south-east Asia ( Panayotou, 1993 ), increases sediment delivery to the coastal zone, which in south-east Asia ranks amongst the highest on the planet ( Milliman & Meade, 1983 ). Widespread deforestation leads to an increased frequency of flooding and sediment dis- charge during heavy rains, when as much as 10 cm of sediment may be deposited during a single event ( Bamroongrugsa & Yuanlaie, 1995 ). In addition, aquaculture activities, which have boomed in the region, further increase the sediment inputs to the adjacent coast ( Gujja & Finger-Stich, 1996 ). The increased sediment discharge to the coastal zone is expanding the mud flats in the area, thereby creating new habitat for mangrove colonization ( Thom, 1984 ; Aksornkoae, 1993 ). As vegetative propagation is uncommon in mangroves ( Tomlinson, 1994 ), the colonization of newly formed mud flats depends on the survival and growth of the established seedlings. Knowledge of the abiotic and biotic factors that con- trol the mortality of mangrove seedlings is therefore essential to understand the development of mangrove forests ( McKee, 1995 ), whether resulting from natural colonization or re-afforestation programmes. In par- ticular, the establishment of mangroves over newly accreted mud flats may be limited by their ability to withstand sediment accretion, which may represent an obstacle to the success of natural and man-made mangrove colonization. However, the influence of in- creased sediment accretion on the survival and growth of mangrove seedlings remains to be investigated. The aim of the present study was to examine the effects of experimental sediment accretion on the survival and growth of Rhizophora apiculata seedlings planted on an expanding mud flat in Thailand (Pak Phanang Bay, south-east Thailand). The authors focused on R. apiculata because it is one of the domi- nant species in south-east Asian mangrove forests ( Tomlinson, 1994 ), and because it is widely used in afforestation programmes in this area ( Aksornkoae, 1993 ). Mangrove trees are usually planted as young (<1 year) seedlings which may be quite sensitive to disturbance ( McKee, 1993, 1995 ), such as that induced by an abrupt sediment discharge. The observed responses can be used to infer the likelihood of success of this species in habitats under contrast- ing sediment accretion, and help design successful afforestation programmes. 0272–7714/97/050697+05 $25.00/0/ec970262  1997 Academic Press Limited