Fig. 1. Distribution of the anoxic chlorophyll maximum (ACM) in the oxygen-deficient zone of Lake Tanganyika.

Data shown are from the April–May 2018 campaign, while a return expedition was made in April–May 2019 to better resolve the vertical structure and nitrogen transformation processes in the ACM, shown in Figs. 2 and 3. a In situ chlorophyll a fluorescence. The oxygen-deficient zone is delimited at an O₂ cutoff of 1 μM indicated by the dotted line. The main sampling stations 2 and 7 are marked by red triangles, while additional sampling stations are indicated by black triangles. b Temperature distribution. c Vertical temperature gradient. d Bathymetry of Lake Tanganyika and the sill that separates the North, Central, and South basins, based on the bathymetry map by Cohen et al.⁵⁷. The lake water column is denoted in blue, while the surrounding mountain terrain is indicated in brown.