Nitrogen fixation in seagrass ecosystems along Dar es salaam coastal waters, Tanzania

Abstract

Cyanobacterial diversity, percentage cover and nitrogen fixation rates were studied in intertidal seagrass meadows at Ocean Road K)R) and Mjimwema (MM), Dar es Salaam, Tanzania. Four seagrass genera Thalassia, Halodule, Cymodocea and Thalassodendron were found common at both sites throughout the study period, September 2002 - August 2003. MM had less disturbed intertidal seagrass beds with higher biomass (average 208 +20 g dw/m2) than OR (138 + 16 g dw/m2). Nutrient concentrations were significantly higher at OR than MM (P = 0.001 for nitrate and P= 0.025 for phosphate). Higher nutrient levels at OR could be due to higher anthropogenic activities such as waste release, fishing, collection and harbor activities, which were evident during this study. A total of 19 different cyanobacterial taxa were found, among which Oscillatoria, lyngbya and Spirulina species were the most dominant. The cyanobacterial cover were higher at MM (H ' = 0.28 - 0.98 and 31-100%, respectively) as compared to 0R (H ' = 0.0 - 0.45 and 0 - 60%, respectively). Cyanobacterial diversity and percentage cover were higher during the South East Monsoon (SEM) than in the North East Monsoon (NEM). Significant correlation between cyanobacterial diversity, percentage cover and nitrogen fixation rates were observed at both sites. Nitrogen fixation rates measured by acetylene reduction technique were higher, average of 6.7 ± 6.2 µ mo1 of N-fixed/m2/hr in the sediments within the seagrass beds, than bare sediments that average of 0.036 ±0.0001 µmo1 of N-fixed/m2/hr. The fixation rates were higher at MM ranged from 0.7 ± 1: 1 . 1 to 7.2 ± 13.5 µmo1 of N-fixed/m2/hr than at OR that ranged from O.1 ± O.15 to 2.1± 3.6 µmo1 of N-fixed/m2/hr. There were no significant differences in nitrogen fixation rates between NEM and SEM. This study revealed that apart from diazotrophic cyanobacteria there are other factors that may influence the nitrogen fixation rates. The study also clearly showed that seagrasses support higher cyanobacterial diversity, in which most of them are said to be important nitrogen fixers. Increasing anthropogenic activities and inputs to the coastal waters could be the source of the elevated nutrient levels in Ocean Road, which in tum hinder the occurrence of the diazotrophic cyanobaclerial and their activity.