Late Holocene Trends of Phytoplankton Productivity and Anoxia as Inferred From Diatom and Geochemical Proxies in Lake Victoria, Eastern Africa

Abstract

Lake Victoria ecosystem has undergone major ecological changes in the recent decades. Sedimentary diatom analysis and Fe/Mn determined by Energy Dispersive X-ray Fluorescence (EDXRF) have provided phytoplankton (diatom) productivity 5 and the resultant anoxia (Fe/Mn) in Lake Victoria at Napoleon Gulf during the late Holocene (1778 cal yrBP (calibrated years before present) to 2008 AD) with radiocarbon dates determined using Accelerator Mass Spectrometry standard method. The results showed that increased total diatom counts in Napoleon Gulf during the late Holocene correspond with increased Fe/Mn ratio (anoxia) in some of the pro10 files and not in others and in most cases those that correspond correlate very well with increased eutrophication from nitrate input (Total Nitrogen, TN). Therefore, slightly increased anoxia not related to increased diatom productivity was recorded in Lake Victoria at Napoleon Gulf from the period 1778 to 1135 cal yrBP. There was slightly increased diatom productivity at Napoleon Gulf from the period 857 to 758 cal yr BP 15 but it did not increase anoxia in the lake. The period 415 cal yrBP to 2008 AD recorded increased anoxia at Napoleon Gulf related to high diatom productivity especially from 415 to 390 cal yrBP and 191 cal yrBP to 2008 AD.