Phytoplankton and allochthonous matter are important sources of dissolved organic carbon (DOC) for planktonic bacteria in aquatic ecosystems. But in small temperate lakes, aquatic macrophytes may also be an important source of DOC, as well as a source or sink for inorganic nutrients. We conducted micro- and mesocosm studies to investigate the possible effects of an actively growing macrophyte, Vallisneria americana, on bacterial growth and water chemistry in mesotrophic Calder Lake. A first microcosm (1 L) study conducted under high ambient NH4+ levels (NH4+ > or = 10 microM) demonstrated that macrophytes had a positive effect on bacterial densities through release of DOC and P. A second microcosm experiment, conducted under NH4+-depleted conditions (NH4+ < 10 microM), examined inter- active effects of macrophytes and their sediments on bacterial growth and water chemistry. Non-rooted macrophytes had negative effects on bacterial numbers, while rooted macrophytes had no significant effects, despite significant increases in DOC and P. A 70-L mesocosm experiment manipulated macrophytes, as well as N and P supply under surplus NH4-+conditions (NH4+ > or = 10 gmicro), and measured effects on bacterial growth, Chl a concentrations, and water chemistry. Bacterial growth and Chl a concentrations declined with macrophyte additions, while bacterial densities increased with P addition (with or without N). Results suggest that the submersed macrophyte Vallisneria exerts a strong but indirect effect on bacteria by modifying nutrient conditions and/or suppressing phytoplankton. Effects of living macrophytes differed with ambient nutrient conditions: under NH4+-surplus conditions, submersed macrophytes stimulated bacterioplankton through release of DOC or P, but in NH4-+depleted conditions, the influence of Vallisneria was negative or neutral. Effects of living macrophytes on planktonic bacteria were apparently mediated by the macrophytes use and/or release of nutrients, as well as through possible effects on phytoplankton production.