Abstract
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.
Highlights
Post 1999 Dar es salaam harbour dredging, the harbour basin experienced a significant sediment deposition at rates between 13 and 43 cm/year caused by agitated bottom sediments after dredging, change of channel alignment and erosion of sediments at Rasi Makabe and scouring in at the harbour entrance and some areas in the harbour basin (Sanga & Dubi, 2004)
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
Polychaetes were the most highly distributed benthic macrofauna especially Diopatra cuprea, and the highest percentage was found in a mixture of silt-sand sediments which is probably due to higher interstitial spaces for polychaetes burrowing (Musale & Desai, 2011; Sasikala, Saravanakumar & Balasubramanian, 2017) and medium contents of organic matter associated with low water velocity in the harbour channel which is an enclosed area like a sheltered bay
Summary
Post 1999 Dar es salaam harbour dredging, the harbour basin experienced a significant sediment deposition at rates between 13 and 43 cm/year caused by agitated bottom sediments after dredging, change of channel alignment and erosion of sediments at Rasi Makabe and scouring in at the harbour entrance and some areas in the harbour basin (Sanga & Dubi, 2004). The distribution of surface macrobenthic fauna in coastal marine sediments is influenced by various factors including sediment texture, salinity, organic matter content and water movements (Edgar & Barrett, 2002; Teske & Wooldrridge, 2003). Water movements such as tidal currents, waves, river discharge greatly govern the deposition and distribution of sediments and organic matter which determine the distribution and diversity of macrobenthos. Understanding sediment and benthic macrofauna distribution and their relationships in hotspot areas like this became important for the conservation and management of aquatic ecosystems
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