Sources and diagenetic alteration of organic matter in coastal waters at the vicinity of Dar es Salaam, Tanzania.

Abstract

The stable isotope compositions of organic carbon (OC) and nitrogen, contents of OC and nitrogen, and C/N ratios for four short cores, which were recovered from Msansani Bay (WO 57), off Msimbazi River (WO 76) and Dar es Salaam harbour (WO 62 and WO 74), are used to document temporal variation in the sources and diagenetic alteration of organic matter (OM). The δ15N and δ13C values at sites WO 57, WO 62 and WO 74 show a trend of down-core decrease to the base of the core, and such a down-core trend could be related to diagenetic loss of compounds enriched in 13C and 15N. At site WO 76, the δ13C values show two zones that are depleted in 13C. These two zones correspond to black-greasy layers that were observed during sampling, and low δ13C values probably is an indication that the material was derived from the continent. Furthermore, these two zones have elevated values of OC, nitrogen and C/N ratios. The stable isotopes of OC and nitrogen show inter-sites differences with enrichment in 15N in the following order: WO 57 < WO 74 < WO 62 < WO 76. A similar inter-core difference is observable for the 13C, but the enrichment show a reversed order. The inter-site differences in 13C and 15N could be related to the sources of OM. In the Msansani Bay the material is derived from seagrasses while in the Dar es Salaam harbour and Msimbazi micro-bay, large proportion is derived from the continent. Other parameters of OC, nitrogen and C/N ratios for these three sites show a slight down-core decrease to the base of the core, probably the result of diagenetic alteration of OM. Highest contents of OC and nitrogen are observable at site WO 62 owning to preferential preservation of OM due to high input of more refactory material derived from the continent and lack of tidal flushing.




Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 39-55