A 140-year record of recent changes in aquatic productivity in a remote, tropical alpine lake in the Rwenzori Mountain National Park, Uganda

Environmental change in many tropical, alpine habitats remains poorly resolved due to an absence of proximate and sustained observations. In the Rwenzori Mountains of East Africa, glaciers have receded rapidly over the last century, and here we assess the impact of this recession through palaeolimnological analyses of a 45 cm sediment core (Buju3) from Lake Bujuku which is closest to the ice-fields and partly supplied by melt water in-flows. 210Pb and 137Cs suggest that Buju3 has an average sedimentation rate of 2.9 mm yr-1 and the base of the core can be dated to 1864 ± 20 years. Contemporary diatom taxa found in the lake are dominated by Tabellaria flocculosa and Synedra spp, but also include Achnanthes minutissima and Fragilaria pinnata. However, the diatom flora for Buju3 is less diverse and dominated by small, tychoplanktonic species of Fragilaria. Over the period associated with glacial recession, organic carbon isotope analysis (δ13C) suggests a small but distinct increase in within-lake productivity, which increases in rate since the mid 1970s up to the present day, in line with a shift towards increased algal productivity (as highlighted by C/N ratios). However, the diatom and pollen records appear rather insensitive to changes in glacier recession since the late 19th century. Key words: Rwenzori Mountains, East Africa, recent environmental change, diatoms, pollen, organic carbon isotope