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 CaCO₃, (iii) C/N ratios, (iv) distribution of sediment mean grain size and sorting, and (v) thickness of unconsolidated sediments. The Chwaka Bay is located on the eastern coast of Zanzibar Island and is about 50 km². It is a shallow partly sheltered lagoon with large intertidal area and fringing mangrove forest. The stable isotope values of nitrogen and OC, which averages 1.5±0.9‰ and -17.5±2.3‰ respectively, increases offshore indicating decrease in the influence of terrestrial material. Furthermore, nitrogen isotope values ranging from 0.3 to 3.9% suggest that the values result from nitrogen fixing plants. Because the bay has high abundance of calcareous green algae Halimeda, plants, most likely low δ¹⁵N values reflect the influence of this photosynthetic plant to the total sediment budget. This inference is supported by low C/N ratio values that averages 7.9 ± 1.4 and with highest values being confined close to the mangrove forest. The contents of OC and nitrogen, which averages 0.75±0.60% and 0.11±0.08% respectively, are highest close to the mangrove forest. The sediment thickness in the Bay is higher in the northern part 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 2φ, whereas coarse sediments with mean values less than 2φ characterize the western part. The content of calcium carbonate is very high with values being higher than 90% for the whole Bay owing to high concentration of carbonate sand-forming Halimeda.

Keywords: Halimeda, stable isotope, sediment distribution, sediment thickness

West Indian Ocean Journal of Marine Science Vol. 4 (2) 2005: pp.185-198