Nutrient dynamics in the sediments and water column on selected coral reefs on the West Coast of Unguja Island

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University of Dar es Salaam
Nutrient dynamics in the water column and the sediment, primary production, sediment organic load, benthic cover and sediment grain size were analysed and compared between two coral reefs, i.e., Chapwani and Bawe close to Zanzibar Town. The selected coral reefs are receiving sewage effluent which is characterised by suspended solids, organic substances, nitrogenous compounds, phosphates and other inorganic nutrients. The mean levels of ammonium concentration in the water column were 0.60 ± 0.18 at Bawe and 0.60 ± 0.16 µM at Chapwani, while the levels for soluble reactive phosphate were 0.21 ± 0.03 and 0.23 ± 0.04 µM for Bawe and Chapwani respectively. Pore water ammonium concentration differed significantly between the sites with 342 ± 68.4 µM of ammonium measured at Chapwani and 690 ± 81.2 µM at Bawe. Concentrations of soluble reactive phosphate for Chapwani and Bawe were 35 ± 11.6 µM and 66 ± 26.2 µM respectively. Observations on nutrient concentrations in the water column showed no significant differences between dry and wet seasons. The only differences in nutrient concentrations between the sites were found between neap and spring tides for ammonium in the water column. Measurements of nutrient fluxes revealed that there were movements of ammonium and soluble reactive phosphate from the sediment into the water column whereas nitrate/nitrite moved from the water column into the sediment. These fluxes were attributed to the gradient differences between the water column and the sediment and wether the sediment is reduced or oxidized. Other parameters that were measured included community production, biomass production and sediment organic load where the two sites showed no significant differences. This study has indicated that the waters off Zanzibar town are influenced by natural processes rather than anthropogenic disturbances from the Zanzibar municipality.
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Coral reefs and islands, West coast, Unguja Islands, Tanzania
Mohammed, M. S. (2000) Nutrient dynamics in the sediments and water column on selected coral reefs on the West Coast of Unguja Island, Masters’ dissertation, University of Dar es Salaam. Available at (