Anoxic in situ production of bacterial GMGTs in the water column and surficial bottom sediments of a meromictic tropical crater lake: Implications for lake paleothermometry

•BrGMGTs are produced within the water column and surface sediments of Lake Chala.•Aquatic production of brGMGTs responds to seasonal mixing and oxycline depth.•BrGMGT production is greatest in the lake’s anoxic bottom sediments.•Sedimentary production complicates the use of brGMGTs as an air temperature proxy.