Nutrient Dynamics and Exchanges Between a Mangrove Forest and a Coastal Embayment: Chwaka Bay, Zanzibar

Abstract: Nutrient dynamics and exchanges were studied in the Mapopwe Creek, a tidally dominated mangrove lined waterway, and a shallow lagoon in Chwaka Bay, Zanzibar, in order to assess whether there was any ecological coupling between the mangrove forest and adjacent ecosystems. In this study it was found that seasonal terrestrial input sources and benthic microalgal uptake strongly influenced the concentrations and distribution of dissolved nutrients in the system. The waterway had low inorganic nutrient levels and concentrations showed considerable spatial variability with values decreasing towards the mouth. The low water column concentrations were attributed principally to low input from the sediment. The sediment showed strong porewater gradients of ammonia and soluble reactive phosphorus (SRP) while nitrate plus nitrite (NOx) was limited to the top 1 cm of sediment in low concentrations. Generally, both water column and porewater nutrients showed strong seasonal fluctuations in concentrations. Mapopwe sediment showed complex and inconsistent sediment-water exchange patterns. Generally though, ammonia dominated benthic fluxes and fluxes of ammonia were mainly directed towards the water column. By contrast SRP showed low levels of exchange and the creek sediment acted as a sink for the nutrient. No fluxes of NOx were detected. Unlike the mangrove waterway, there was no significant sediment-water exchange of nutrients in the bay.The sediment in the creek showed low denitrification rates. This was attributed to low water column nitrate concentrations, low rates of in situ nitrification and competition for nitrate between nitrifiers and other benthic microbial populations. Strong spatial differences in the denitrification rates in the creek were attributed to unequal supply of organic matter between sites, and spatial differences in the distribution of bioturbating organisms and microautotrophs in the sediment.The study showed that there was a strong coupling between the forest and adjacent seagrasses and algal communities. Although measurements of tidal exchanges of dissolved nutrients between the mangrove forest and the bay did not show any exchanges between these biotopes, however the study showed that the forest exported a significant amount of particulate organic matter to the bay. This material was however trapped and utilised in production by seagrass beds just outside the mangrove forest; the communities growing farther from the forest were only indirectly affected. Apparently, mangroves may support productions in other systems but not directly. However, the seagrass and algal communities growing adjacent to the forest were vital in utilising and conserving mangrove outputs. Cosnsequently, nutrients are efficiently conserved within the bay and the associated mangrove forest implying that if the forest were to receive an increased nutrient load, they will not be removed and may result in eutrophication of the system.