Labile carbon supplement induces growth of filamentous bacteria in the Baltic Sea

A 3 wk experiment with 9 mesocosms (51 m 3 ) was carried out at a coastal site of the brackish northwest Gulf of Finland (Baltic Sea) to study the effects of nutrient limitation and labile carbon (C) amendment on the plankton community. A 5 d period with daily inorganic nitrogen (N) and phosphorus (P) additions boosted algal biomass increase and the demand for mineral nutri- ents. In the following 2 wk, the supply of 1 nutrient was cut, while keeping or increasing the other, thus inducing a gradient of nutrient-limitation regimes. Labile organic C (glucose) was added to increase bacterial growth and induce mineral nutrient competition between bacteria and algae. Here we report the effects of the treatments on bacterial abundance, production, and community composition. Addition of labile organic C led to a significant increase (p < 0.001) in the biomass of large filamentous bacteria and in bacterial productivity but not in the biomass of small coccoid bacteria. Addition of inorganic N and P did not have any clear-cut effect on bacterioplankton. Our results suggest that bacterial assemblages have the capacity to respond to enhanced substrate availability, and that glucose strongly enhances the development of filamentous bacteria, with implications for food-web structure, biodiversity, and biogeochemistry.