Impact of pollution on cyanobacterial diversity, nitrogen fixation and microalgal biomass on selected mangrove ecosystems of Dar es salaam, Tanzania

This study aimed at analyzing the pollution impact of sewage on the cyanobacterial diversity, N2-fixation rates and microalgal biomass in the sediments at Mtoni Kijichi (sewage impacted) and Rasi dege (relatively prestine) mangrove ecosystems in Dar es Salaam. A total of 15 cyanobacterial genera were identified from the two study sites, of which the non heterocystous were the dominant group. N2-fixation was assessed by the acetylene reduction technique while the Shannon-Wiener Index was used to asses cyanobacterial diversity. Compared to Mtoni, Rasi dege mangrove sediments were found to support significantly higher ^-fixation rate and cyanobacterial diversity. However, the microalgal biomass as well as nitrate, nitrite and phosphate concentrations were significantly higher at Mtoni compared to Rasi dege. ^-fixation at Mtoni showed a significant negative correlation with inorganic nutrients, but there were no significant correlations between N2-fixation and the nutrient concentrations at Rasi dege. Although microalgal biomass at Mtoni showed no significant correlations with the nutrient levels, there were significant positive correlations with temperature and salinity, respectively. At Rasi dege the microalgal biomass showed no significant correlations with the nutrient levels. Cyanobacterial diversity showed significant positive correlations with N2-fixation and nutrient concentrations at Rasi dege, but correlated negatively with the nutrients and N2-fixation at Mtoni. The elevated nutrient levels observed at Mtoni, which are probably of anthropogenic origin may have hindered cyanobacterial diversity and N2-fixation rate but have increased microalgal biomass. Careless disposal of sewage should be stopped to ensure sustained water quality and healthy aquatic ecosystems.