CHANGES IN INFERRED PRIMARY PRODUCTIVITY FROM RECENT SEDIMENTS FROM LAKE MALAWI, EAST AFRICA

We have used a multi-proxy approach to examine recent changes in primary productivity and nutrient regimes in Lake Malawi, East Africa. Biogenic silica accumulation in sediments is considered by some to be a useful indicator of past changes in primary production. These changes can be attributed to changes in climate (e.g. wind patterns and intensity, precipitation), which can cause increased nutrient availability through upwelling or runoff. Land use changes within the watershed may also accelerate nutrient and sediment delivery to the lake. Biogenic silica percent and mass accumulation rates were determined for nine multi-cores from four basins in Lake Malawi. Lead-210 was used to date the multi-cores. Cores from the south-central and southern basins show high biogenic silica mass accumulation rates peaking in the late 1970’s and decreasing towards present. Biogenic silica mass accumulation rates are as high as 81 g/m²/yr (1.35 mol/m²/yr) in the south-central basin. Sedimentation rates range from 0.14 cm/yr in the north basin, to 1.71 cm/yr in the south-central basin. Results of a whole-lake silica burial model will be presented. Diatom analyses were also conducted on several of these cores. These analyses provide insight to changes in nutrient availability and relative diatom productivity at the core sites. The presence and relative abundance of specific diatom species are indicators of nutrient availability and often lake turbulence, which can be the results of climatic and/or anthropogenic processes. Diatom species abundance and taxonomic ratios will be presented to support evidence of changes in primary productivity in regions of Lake Malawi.