Distribution and Sources of Carbon, Nitrogen and Their Isotopic Compositions in Tropical Estuarine Sediments of Mtoni, Tanzania

Elemental (C, N) and isotopic (delta C-13, delta N-15) signatures were used as proxies to identify seasonal changes in proportions of sedimentary organic matter (OM) in the Mtoni estuary and its Kizinga and Mzinga tributary rivers. Depth-averaged values of TOC and TN in the Kizinga River were low in the wet season (TOC: up to 1.0 +/- 0.3%; TN: up to 0.1 +/- 0.1%) compared to the dry period (TOC: up to 1.9 +/- 0.2%; TN: up to 0.2 +/- 0.1%), and vice versa in the Mzinga River (wet: up to 5.7 +/- 2.8% for TOC and 0.3 +/- 0.03% for TN; dry: up to 3.6 +/- 0.7% for TOC and 0.2 +/- 0.01% for TN). C/N ratios in rivers showed no clear seasonal trend. At all sampling stations, delta C-13 values were higher in the wet season (up to - 23.9 +/- 0.8 parts per thousand) and low in the dry period (up to - 26.3 +/- 1.0 parts per thousand) whereas delta N-15 values were low in the wet season (up to 6.6 +/- 0.4 parts per thousand) and high in the dry period (up to 8.1 +/- 1.4 parts per thousand). Spatial gradients of OM during the dry period were observed from upstream to the estuarine mouth for TOC and TN. FRUITS model results revealed that OM in the Mtoni estuary was dominated by anthropogenic sources in both seasons. Total contribution of mangrove, phytoplankton, and macroalgae to the Mtoni OM was relatively high in the wet season. It is concluded that the Mtoni estuary sediment OM was a mixture of different sources. The contribution of OM from phytoplankton and macroalgae was low while the contribution from mangrove OM was restricted to the Mzinga River.