Dynamics of bacterioplankton in a mesotrophic French reservoir (Pareloup)

Bacterioplankton abundance, biomass and production were studied at a central station (35 m depth) from April 1987 to September 1988 in a mesotrophic reservoir. Bacterial production was calculated by the (³H) thymidine method.For the water column, integrated estimates of bacterioplankton abundance ranged from 2.3 10⁹ to 4.6 10⁹ cells l^–1, and carbon biomass from 0.037 to 0.068 mg C l^–1; the thymidine incorporation rates ranged from 0.8 to 17.2 picomoles l^–1 h^–1, leading to net bacterial production estimates of less than 0.7 µg C l^–1 d^–1 in winter to 18 µg C l^–1 d^–1 in summer. About 55% of the production occurred in the euphotic layers.Over the year, the bacterial carbon requirement represented 90% of the autotrophic production for the whole lake. It was five times lower than autotrophic production in spring, but twice as high in summer. This important temporal lack of balance suggests that not all the spring primary production products are consumed immediately and/or that other carbon sources probably support bacterial growth in summer.