Recent trends in accumulation rate, elemental and isotopic composition of organic matter in the Magu bay, Lake Victoria

Abstract

Stable isotope composition of organic carbon (OC), 14C data, OC and nitrogen content of recently deposited sediments in Magu Bay of Speke Gulf, Lake Victoria are used to document accumulation rates, sources and distribution of organic matter in the bay. The stable isotope values of both organic carbon and nitrogen decrease towards offshore, and this trend is associated with increase in the contents of organic carbon and nitrogen. The isotope results show that the organic material deposited in the bay are generally derived from C4 type of plants and are not transported far from the river mouth. Also the results show that preservation of organic matter is high in open water probably due to high primary productivity owing to high concentration of nutrients and low in turbidity. Low isotope values in offshore sites indicate that the available stock of nutrients far exceeds the demand. Sedimentation rates in the bay are not high and range from 5 to 54 cm/ka. This could be attributed to trapping efficiency by shoreline aquatic macrophyte. Furthermore, the mean accumulation rates of organic carbon and nitrogen for the bay, range from 6.92 to 57.25 gC/m2/yr and 0.51 to 4.37 gN/m2/yr, respectively, and show that Magu Bay is currently experiencing eutrophication.