Processes Governing the Retention of Phosphorus and Nitrogen in Nyashishi Wetland

One of the important functions of the wetland is the retention of catchment nutrients and improving lacustrine water quality. This study analyzed how much nutrients were retained in the Nyashishi wetland, southern part of Lake Victoria, and went further to analyze the processes which were responsible in the reduction of nutrients. Three major processes were analysed in this study, namely nutrients uptake by three macrophyte species (Eichhornia crassipes, Cyperus papyrus and Typha domingensis) dominating the Nyashishi wetland, nitrification and denitrification processes. The study demonstrated high nutrients retention especially phosphorus nutrients of which up to 98% were retained. In some occasions, particularly for nitrogen nutrients, there was 0% retention especially during wet season. In some other instances, the inflow exceeded the outflow meaning that, more nutrients were generated by the wetland itself. Among the three processes analyzed, nutrients uptaken by macrophytes were more efficient in reducing nutrients in wetland water. Biological nitrification and denitrification which are believed to be the major pathway for ammonia removal in both natural and constructed wetlands were less important in this study. Among the macrophyte species, Eichhornia crassipes demonstrated higher uptake rate than the other two species possibly due to its high turnover rate. This ability can be exploited in removing excess nutrients from runoff by frequent harvesting of the mature plants.

Assessment of Sediment and Benthic Macrofauna Distribution at Dar es Salaam Harbour Channel

Sediment and macrobenthic fauna distribution at Dar es Salaam harbour channel were investigated to attest the influence of sediment grain size distribution and total organic carbon contents on benthic macrofauna dominance and diversity. Sampling campaign was conducted from January to April, 2019 in 25 sampling stations along the channel. Sediment grain sizes ranged between 0.7405 and 0.00273 mm with an average of 0.0804 mm classified as medium silt. Sediment distribution showed an onshore-offshore pattern consisting of fine dark clay-silt sediments in the southern part of the harbour close to Mwalimu Nyerere Bridge, medium (sandy) at the harbour and coarse sandy offshore. Five classes of macrofauna were identified with sedentary Polychaetes (Diopatra cuprea) found in tubes and free forms being dominant in most of the sediment types, however more abundant in clay-silt. High species diversity index values were encountered in sediments with medium total organic carbon (TOC) and sediment grain sizes (sand-silt). Apparently, low species similarity indices were observed in all sediment type indicating significant dissimilarities in species composition among sediment grain sizes. Thus, the distribution of benthic macrofauna species along Dar es Salaam harbour channel is strongly influenced by the sediment grain sizes and organic carbon contents in sediments.