Grazing of the copepod Diaptomus connexus on purple sulphur bacteria in a meromictic salt lake

A meromictic lake ecosystem (Mahoney Lake, BC, Canada) was investigated to elucidate the significance of chemocline bacteria in the total carbon cycle under natural conditions. In this lake, primary production by oxygenic pho totrophs was insufficient to support the observed net secondary production of the calanoid copepod Diaptomus connexus and the rotifer Brachionus plica tilis, indicating the presence of additional food sources for consumers. Ma honey Lake harbours the densest population of phototrophic sulphur bacteria ever reported in a natural body of water. This layer is located at the int erface between oxic and anoxic water layers and is dominated by the purple sulphur bacterium Amoebobacter purpureus. The transfer rates of A. purpureu s carbon to D, connexus determined in stratified mesocosms were very low (0 .71 ngC copepod(-1) day(-1)) and accounted for only 0.6% of the observed ne t biomass increase in the zooplankter. Stable stratification within the mes ocosms prevented an upwelling of A. purpureus into the oxic part. However, measurements of carbon fluxes, infrared fluorescence microscopy and stable carbon analysis provided cumulative evidence that, under in situ conditions , the cell carbon of purple sulphur bacteria indeed enters the aerobic food chain via the grazing activity of D. connexus. Based on a two-source isoto pic mixing model, A. purpureus represents at least 75-85% of the diet of D. connexus. Autumnal upwelling into oxic water layers and aggregation of A. purpureus cells appear to be the main factors determining the high carbon f lux from purple sulphur bacteria to zooplankton under natural conditions, a nd most probably also play a key role in other aquatic ecosystems. Through this pathway, over 53% of the reduced organic matter of purple sulphur bact eria trapped in anoxic bottom waters is returned to the oxic realm.