Fate of organic carbon in a tropical mangrove ecosystem

Sammanfattning: The aim of this thesis was to study the budget and fluxes of organic carbon in a tropical mangrove forest. Maruhubi forest is located in the Island of Zanzibar, East Africa. Aspects of structure and functions of the ecosystem are investigated. Litterfall, litter standing stock and the effect of macrofauna and the tidal regime on litter turn-over, are key factors in the present study.The inventory of the forest in terms macrofauna and flora was taken. Avicennia marina forms an almost pure stand at the terrestrial edge of the forest, nevertheless, the species is ubiquitous in the area. Sonerratia alba grows exclusively at the marine fringe and the shoreline areas that are frequently inundated. Grapsid crabs, mainly Neosarmatium meinerti occupy the terrestrial edge mangrove area which has a hard mud substrate. Ocypodid crabs, mainly Uca spp occur seaward in the marine fringe and shoreline zones with a sand/muddy bottom. The mollusc Cerithidea decollata is entirely abundant in the terrestrial mangrove fringe sub-area. The marine fringe and shoreline sub-areas are colonised by several species of snails.Lateral fluxes of macrodetritus, suspended particulate matter and dissolved organic carbon showed a significant export of mangrove litter to the adjacent marine area during spring tides. However, in the terrestrial fringe litter was not effectively transported by the tide. Net organic carbon export for the entire forest was 79 x 106 g C y-1, dissolved organic carbon accounted for 78% of the total export.Distribution of sedimentary organic carbon in this mangrove forest which is partly contaminated with raw sewage, shows that a high proportion of carbon in the landward extending zone is organic. Relative content of inorganic carbon increases seaward with carbonate shells of gastropods, oysters and coral sand dominating. Vertical profiles of organic carbon showed a general surface enrichment and subsurface depletion. Interstitial water in the terrestrial mangrove fringe with sewage was acidic (pH 4 - 5) and strongly reducing (Eh approx. -300 mV). pH in the terrestrial fringe sub-area without sewage was close to neutral values (pH > 6). There was no significant difference between the rates of organic matter mineralization in the forest zones. Relative values, however, indicate mineralization was slowest in sewage contaminated areas followed by areas with high litter loading on the sediment floor. The rate was high in the frequently inundated mangrove areas.An empirical model of the fate of organic carbon in Maruhubi forest was constructed. Results from the model supports the field observation that outwelling of dissolved organic carbon was the dominant component of the mangrove forest which influences litter dynamics. Grapsid crabs are important for litter turn-over in the intermittently flooded zones of the forest. Further, stable isotope data confirmed that crabs in the terrestrial fringe zone fed almost exclusively on mangrove litter (d13C = 27.97±0.25). In addition to that, the sedimentary organic matter (d13C = 22.50±3.54) was also to a large extent, of mangrove origin.