OXYGEN CONCENTRATION PROFILES IN SEDIMENTS OF 2 ANCIENT LAKES - LAKE BAIKAL (SIBERIA, RUSSIA) AND LAKE MALAWI (EAST-AFRICA)

Oxygen concentration profiles have been measured with microelectrodes in sediments of two major ancient Rift lakes: Lake Baikal (Eastern Sib eria) and Lake Malawi (East Africa), along depth transects in the cons titutive basins of the lakes including the deepest point, 1680 m, in L ake Baikal. Sediment oxygen penetration depths (SOPs) display very dif ferent patterns in the two lakes. In Lake Baikal, SOPs are variable, s how no significant relationship with bathymetric depth and are surpris ingly deep on Akademichesky ridge (> 50.0 mm), emphasizing the distinc tive feature of this region in the lake. While the Selenga river is an important source of eutrophication, the similarity of SOP-values in t he Selenga shallow with those of most other sites suggests either a di lution of organic material by allochthonous matter, or a strong south- to-north transport of particles. In Lake Malawi, available oxygen is r estricted to a maximum of three millimetres of the sediment, and there is a negative relationship with bathymetric depth, as a result of a s teady decline of oxygen concentration with depth through the water col umn. Amongst the few parameters known to affect SOPs, the oxygen consu mption by the sediment seems the most significant in both lakes. SOP-v alues furthermore confirm differences in the trophic status of Baikal and Malawi, respectively. The importance of oxygen as a factor likely to create ecological segregation for benthic organisms is discussed. L ake Malawi offers possibilities of bathymetric segregation but no vert ical segregation in the sediment. In contrast, no bathymetric segregat ion related to oxygen is possible in Lake Baikal, but vertical segrega tion in the sediment is very likely.