Porewaternutrientpro¢lesandnutrientsediment^water exchange inatropical mangrove waterway, Mapopwe Creek,ChwakaBay,Zanzibar SalimM. Mohammed 1 and RonW. Johnstone 2 1 Institute of Marine Sciences, University of Dar es Salaam, PO Box 668, Zanzibar,Tanzania; 2 Formally of Department of Zoology, Stockholm University, S-10691Stockholm, Sweden Abstract Sediments were examined in the Mapopwe Creek, a tid- ally dominated mangrove waterway in the Chwaka Bay mangrove forest, Zanzibar, to assess their signi¢cance inthe nutrient dynamics of the mangrove forest and the adjacent bay. Porewater concentrations of dissolved ammonium and that of soluble reactive phosphorus (SRP) were generallyhigherduring the dryseasonthan during the wet season. NO 3  plus NO 2  concentration averaged1 m m and did not vary much between the two periods. Fluxes of ammonium ranged from 575 to 523 mm m 2 h 1 and those of SRP from 55.7 to 69.5 mm m 2 h 1 . Measurements of NO x did not show any consistent £uxes of this dissolved nitrogen species. Variations of £ux rates between the two seasons were not signi¢cant even though there were small variations inthe£uxdirectioninbothnutrients.Resultsimplythat MapopwesedimentsareasourceofNH 4 þ butactasasink forSRP. Key words: benthic, £uxes, mangroves, nutrients, pro¢les, tropical Re¤sume¤ Onaexamine¤ les se¤ diments dans laCrique de Mapopwe, une voie d’eau soumise aux mare¤ es, de la fore“ t de man- groves de Chwaka Bay, a' Zanzibar, pour e¤valuer leur importancepourladynamiquedesnutrimentsdelafore“t de mangroves et de la baie voisine. Les concentrations d’ammonium dissous et de phosphore re¤actif soluble (SRP) dans l’eau e¤ taient ge¤ ne¤ ralement plus e¤ leve¤ es pen- dant la saisonse' che qu’ensaisondes pluies. Les concen- trations de NO 3  et NO 2  s’e¤ levaient en moyenne a' 1 m M et ne variaient pas beaucoup entre les deux pe¤ riodes. Les £ux d’ammonium variaient de ^ 575 a' 523 mM/m 2 / h, et ceux de SRP de ^ 55.7 a' 69.5 mM/m 2 /h. Les mesures de No x ne montraient pas de £ux conse¤ quent de ce type d’azotedissous.Lesvariationsdes£uxentrelesdeuxsai- sons n’e¤taient pas assez signi¢catives me“ me si l’on a observe¤ de petites variations dans la direction des £ux desdeuxnutriments.Lesre¤ sultatsmontrentquelesse¤ di- ments de Mapopwe sont une source de NH 4 þ maisqu’ils absorbentleSRP. Introduction Sediments in shallow coastal waters are known to be important sites for the accumulation of organic matter and the subsequent remineralization and recycling of nutrients (Bonanni et al.,1992). In contrast to their role assources,sedimentshavealsobeenreportedtobesinks for nutrients (Alongi, 1994a; Tam & Wong, 1995) and may thus act as nutrient traps (Carman & Wul¡,1989). Whilsttherehavebeensomestudiesexaminingdi¡erent aspects of near-shore sediment nutrient dynamics in the tropics (e.g. Johnstone, Koop & Larkum,1988; Corre- dor & Morell,1989; Clough,1992), including those that have speci¢cally focused on mangrove areas (Alongi, Christo¡ersen & Tirendi,1993; Alongi,1994b), thereare comparativelyfewstudiesthathavelookedatthisaspect in EastAfrican mangrovesareas. Given the high production rates attributed to man- groves, mangrove forests have been considered a major source of nutrients to nutrient-limited ecosystems such 172 # 2002EastAfricanWildLifeSociety, Afr. J. Ecol., 40, 172^178 Correspondence: Salim M.Mohammed. E-mail:salim@zims.udsm.ac.tz