Browsing by Author "Mohammed, Mohammed Suleiman"

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    Distribution and abundance of diseases on reef corals in Tanzania
    (University of Dar es Salaam, 2016) Mohammed, Mohammed Suleiman
    Diseases affecting reef corals are on the rise in the world oceans. Warming of the seas, deteriorating water quality and increased human activities in the nearshore waters are considered to play a major role in the prevalence of coral diseases. The goal of this study was therefore to characterize and investigate the abundance of coral diseases and its environmental drivers in reefs off Zanzibar and the main coast of Tanzania. To achieve these, 20 sites were selected along the whole coast of Tanzania and nearby islands. Rapid assessment surveys, line intercept transects and belt transects were used to monitor the selected study sites. In addition, the effect of temperature and nutrients on white syndrome (WS) disease on table Acropora and bleaching on Porites australiencis was assessed through sets of experiments done both in laboratory and in the field. The study has revealed seven coral diseases with significant difference between localities (p < 0.001). The mean prevalence was relatively low ranging from 2.24±1.6% in Pemba to 0.09± 0.3% in Zanzibar Stone town reefs. Prevalence of WS on table Acroporids was significantly higher during warm season 5.63±3.7 than cool season 3.55±2.2% (p<0.05). The progression rate was higher (p<0.05) during cool period where 80.42±15.42 cm2/day was recorded compared to 16.14±8.79 cm2/day recorded during warm period. These results show that there is no relationship between temperature and WS prevalence but the virulence of WS is more pronounced during cool season. When temperature and nutrients were manipulated over bleached Porites australiensis to check the progression rate of bleaching over time scale, the bleached corals did not show any significant difference between the treatments. Similarly, the treatments could not result in bleaching on healthy corals. These results show that temperature and nutrients at the levels used in this study may not have direct impact on corals but may delay coral recovery from any unforeseen impact. Disease prevalence in Tanzania water is low compared to other regions in the world. Low prevalence may be due to low levels of anthropogenic influence as compared to the areas with higher disease prevalence. Coral bleaching is the major threat to coral reefs as when it occur it cover large geographical area and involves many coral species.
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    Nutrient dynamics in the sediments and water column on selected coral reefs on the West Coast of Unguja Island
    (University of Dar es Salaam, 2000) Mohammed, Mohammed Suleiman
    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.