Sediment sources and their Distribution in Chwaka Bay, Zanzibar Island

Abstract

This work establiShes sediment sources, character and their distribution in Chwaka Bay using (i) stable isotopes compositions of organic carbon (OC) and nitrogen, (ii) contents of OC,nitrogen and Caco" (iii)CIN ratios, (iv) distribution of ,sediment mean grain size and sorting, and (v) thickness of unconsolidated sediments. The Chwaka .Bayis located on the eastern coast of Zanzibar Island and is about 50 km'.lt is a shallow partly sheltered lagoon with large intertidal area and fringing mangrove forest The stable isotope values of nitrogen and OC, which averages 15:1:0:9%0and -17.5:f:2.3%crespectively, increases offshore indicating decrease in the influence of terrestrial material Furthermore, .nitrogenisotape values ranging from 0.3 to 3.9% suggest that the values result from nitrogen fixing plants. Because .thebay has high abundance of calcareous .greenalgae Halimeda plants, most likely low 61 'N values reflect the influence of this photosynthetic plant to the total sediment'budget. This inference is supported by low ClNratio values that averages 7;9 Z 1.4 and with highest values being confined close to the mangrove forest. The contents of OCand .nitrogen, which averages 0:75:1:0.60% and 0.11:1:0.08%respectively, are highest close to the mangrove forest The sediment thickness in the Bay is higher in the northempart.than in the southern part. The character of sediments in terms of grain size .differs 'between eastern and western parts of the Bay. The eastern part is characterized by medium to fine white carbonate sand with mean grain size higher than 241, whereas coarse sediments with mean values less than 2l/l characterize the western part. The content of calcium carbonate is very .highwith values being higher than 90% for the whole Bay owing to high concentration of carbonate sand-forming Halimeda.