Porewater nutrient profiles and nutrient sediment–water exchange in a tropical mangrove waterway, Mapopwe Creek, Chwaka Bay, Zanzibar

Sediments were examined in the Mapopwe Creek, a tidally dominated mangrove waterway in the Chwaka Bay mangrove forest, Zanzibar, to assess their significance in the nutrient dynamics of the mangrove forest and the adjacent bay. Porewater concentrations of dissolved ammonium and that of soluble reactive phosphorus (SRP) were generally higher during the dry season than during the wet season. NO₃^? plus NO₂^? concentration averaged 1 µm and did not vary much between the two periods. Fluxes of ammonium ranged from ?575 to 523 µm m^?2 h^?1 and those of SRP from ?55.7 to 69.5 µm m^?2 h^?1. Measurements of NO_x did not show any consistent fluxes of this dissolved nitrogen species. Variations of flux rates between the two seasons were not significant even though there were small variations in the flux direction in both nutrients. Results imply that Mapopwe sediments are a source of NH₄⁺ but act as a sink for SRP.