The diatom £ora of the lake is currently dominated by several species, such as Aulacoseira baicalensis, A. islandica, Cyclotella minuta and Stephanodiscus binderanus v. baicalensis. All these species, except for C. minuta, have become more common in the lake in approximately the last 130 years, and we hypothesize that these changes may be attributed to a number of di¡erent processes linked to an ameliorating climate after the end of the Little Ice Age. The results presented here have important implications for this recently designated World Heritage Site, with regard to future pollution controls and catchment management policies.

Lake Baikal is the world's largest freshwater lake and is internationally famous for its rich and largely endemic biota. Concern about this unique ecosystem has grown since the late 1970s but whether recent biological changes result from natural fluctuations or pollution is unclear. One way of discriminating between these processes is to examine records of recent change in radiometrically dated deep-water sediment cores. Here we use high-resolution diatom analysis of one core to show that abundances have not changed significantly over recent decades. By contrast, we demonstrate that the lake is contaminated by atmospheric pollutants and has experienced a small qualitative change in soil derived magnetic minerals. Sedimentary lead concentrations show an increasing trend in the c. 150-year core sequence and spheroidal carbonaceous particles (SCPs) contaminate post-1930 sediment. Although we provide no evidence that twentieth-century pollution has affected the endemic planktonic diatoms in the central western region of southern Lake Baikal, longer trends in species abundances could be related to naturally occurring climatic cycles or to global warming.

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