Inputs, outputs, and internal cycling of silica in a large, tropical lake

A silica budget was constructed for Lake Malawi by measuring major cycling processes, including river input/output, atmospheric deposition, sinking, burial, and vertical flux within the water column. A large proportion (> 80%) of the annual input to the lake occurred in the December to March rainy season. During this period, biogenic Si (BSi) made up 61 % of Si input from rivers. Microscopic examination indicated that a large fraction of BSi was phytoliths, which likely originated from cultivated maize and terrestrial grasses. Over an annual cycle, river input made up approximately 25% of total Si input to the epilimnion, while 75% resulted from vertical exchange with Si-rich deep water. Due to its long hydraulic residence time, Lake Malawi retains nearly all of the Si that enters it. Despite high temperatures, a relatively high proportion (7 to 11%) of diatom production is permanently buried. A Si flux model is used to demonstrate that river discharge and vertical mixing can have similar effects on BSi burial rate over short time scales (one to two decades), but very different effects over longer time periods. This temporally dependent response must be taken into account when interpreting sediment records.